Sensory scientists usually refer to smelling through the nostrils as “orthonasal perception”, whereas the aroma compounds that gain access to the olfactory epithelium through the nasopharynx (i.e., molecules released in the mouth) are referred to as being perceived retronasally. The latter is often mistakenly referred to as taste by laymen. It should perhaps more correctly be referred to as flavor, although we prefer to think of flavor as the combination of the perception of taste in the mouth and retronasal aroma in the nose (see section below). It is one of the challenges for Molecular Gastronomy to develop an appropriate language that can be used by chefs, the general public, as well as the scientific community to describe the various ways we interpret the signals from our chemical senses.

When eating a food the initial olfactory stimulation takes place as we smell the aroma of the food before the food is in the mouth. Thus, orthonasal perception is often said to be of the external world. In contrast, the aromas perceived retronasally are said to be of the oral cavity (the interior world).

Small and colleagues(35) compared these two distinct pathways of delivering odorants and found different patterns of neural activation depending on whether the aroma compounds are delivered ortho- or retronasally. Further, a few experiments examined differences in perception of aromas delivered by the two pathways; these have rather variable results. In one study, Aubry and colleagues(36) found no overall difference in the ability of trained sensory panelists to describe a set of Burgundy wines. By contrast, other research examining the dose−response behavior of flavor molecules ortho- and retronasally have revealed differences which depend strongly on the physical characteristics of the aroma compounds.(37) Much further work is needed before we will be able to understand the extent to which individuals perceive odor differently depending on whether they are delivered ortho- or retronasally; at his stage, all we can do is to note that it is likely that there will be a range of where the initial smell (the orthonasal stimulus) may be rather different from their “flavor” (the combination of the taste and retronasal stimulus). One such example that is well known to gourmets is that of the pungent smelling Durian fruit, which has, for most people, a very unpleasant (toilet-like) aroma when smelled orthonasally but, for many, a very pleasing flavor when in the mouth and the aroma is detected retronasally.

When subject to a constant stimulus, the senses become less responsive. When holding a solution of a tastant (e.g., sucrose) motionless in the mouth, the solution will become completely tasteless after a while.(61)

This phenomenon is well known to us all, although we tend to ignore it. Whenever we leave our homes for a prolonged period, to go on holiday or to a week-long conference, we find on our return that as soon as we walk in the front door our home has a slightly “musty” smell. Thus, we open the windows and “air” the house. The odor quickly goes away. Of course, in actual fact, our homes always have that smell, it is what our friends and neighbors perceive as the smell of our home. However, because it is always present in our environment we rapidly become adapted to it and simply do not notice it at all. When eating, we will quickly become “bored” with a dish which appears to lose its flavor if we are subject to the same taste or aroma continuously for a prolonged period. Variety becomes the spice of life.

If we are aware of this phenomenon we can make all meals more interesting simply by increasing variety, a large number of small differently flavored or textured dishes (such as the Spanish Tapas) will provide greater interest than one single, larger item. Many restaurants provide a diverse range of elements on every plate; some very small items can break up a larger item to provide the necessary changing stimulus to retain the diner’s interest and enjoyment.

In the gastronomic kitchen, some chefs, having recognized the adaptation phenomenon, have tried to create dishes that continually provide a diverse range of stimuli so as to retain (and hopefully enhance) the diner’s interest. One such example comes from the Fat Duck, cauliflower risotto.(62) The central idea here was to take a vegetable, regarded by many as essentially rather uninteresting (i.e., one that in which diners might quickly lose interest as they start to ignore the flavor), but to prepare a dish that has a wide variety of different textures and flavors that constantly stimulate the brain, so preventing any adaptation and (hopefully) turning a plain cauliflower into a very tasty and exciting dish. The result was “cauliflower risotto” (for a photograph see page 323 of The Big Fat Duck Cookbook(62)) a dish that was for some time one of the signature dishes at the Fat Duck.

To achieve the desired effect, Heston Blumenthal used a combination of many different cauliflower preparations (dried cauliflower, cauliflower cream, foamed cauliflower, raw cauliflower, and a risotto made with a cauliflower stock together with other contrasting ingredients such as cocoa jellies) to create a spectacular dish of which no diner is likely to tire.

Mixture suppression is the phenomenon that individual taste and smell characteristics are perceived as less intense in mixtures than alone.(63) Thus, when preparing a complex recipe and mixing several foods with different flavors or tastes, the perceived intensity of the flavors of the separate ingredients is decreased relative to that of the same tastes or aromas of the ingredients on their own. There is a very interesting and useful exception to this phenomenon, Release from suppression; when adapting to one component in a mixture, other components are less suppressed and will then be perceived as more intense.(64) This is routinely used by flavorists and perfumers to analyze competitors’ blends.(65)

Once again, awareness of the issue can quickly provide new ideas to improve cooking. Rather than mixing all the ingredients together in a single pot, it can be much better to present them separately. For example, there may be several different sauces or dips associated with a single dish; if these are presented in separate pots and used individually, rather then in combination, their impact is greater.

Microbial reactions due to yeasts and bacteria have been essential to the development of our whole cuisine. Historically the yeasts that cause alcoholic fermentations have been used to make some of the most important staples of our diet, especially breads, beers, and wines. The main reaction of the fermentation(92) process is to convert sugars, including short-chain carbohydrates, to yield carbon dioxide gas, which makes bread rise and gives beer its fizz, and ethanol, which adds flavor and preserves beers and wines as well as providing them with an active effect on the consumer. However, there are many additional side reactions that produce other volatile compounds.^93–95 It is these compounds that impart the distinctive, and appealing, flavors to the various cheeses, breads, beers, and wines with which we have become familiar over the centuries.

Initially all these fermentation reactions were produced using wild yeasts carried in the air, on the skins of fruits, or on seed cases of the grains. Not only did this make the processes somewhat hit and miss, it also led to great variability in both the flavor and the quality of the resulting products. Today most manufacturers rely on cultured yeasts to provide consistent products. Those who have tried to make beer at home will almost certainly at some time have encountered the problem that some batches simply do not have the same flavor as others and occasionally are quite undrinkable. While some wild yeasts do make pleasant beers, it is generally best to stick to one of the specially cultured yeasts designed over the years to produced high-quality beers such as S. carlsbergensis.

There is another slightly different fermentation process that occurs when lactic acid bacteria (e.g., Oenococcus oeni) convert malic to lactic acid;(96) this is known as malolactic fermentation. In wine making the malolactic fermentation can be particularly advantageous since lactic acid has a softer, rounder taste than malic acid, which is perceived as rather “tart”. Thus, wines that have undergone malolactic conversion tend to appear softer, have a more rounded mouth feel and have a buttery note that comes from diacetyl produced during the malolactic fermentation.^97,98 Conversely, since malic acid has the taste of apples, wines that have not been subject to malolactic conversion tend to have a green apple note.

Malolactic fermentation sometimes occurs unintentionally after the wine is bottled. The result is a slightly carbonated wine that usually tastes bad as the wild bacteria that can cause the malolactic fermentation produce a wide range of “off flavors”. Accordingly, many wine makers tend to inoculate their wines in the vats with a culture of desirable bacteria to avoid any risk of accidental malolactic fermentation in the bottle and thus prevent the possibility of any such off flavors occurring.

Of course, by far the largest application of bacteria that produce lactic acid is in the fermentation of dairy products to produce yogurts, soured creams, acidified milks, and of course cheeses. In general, the bacteria convert lactose in milk to lactic acid, thus decreasing the pH of the milk and causing it to aggregate. Different bacterial strains follow somewhat different routes and can produce a range of side products, including diacetyl (giving a buttery taste), acetaldehyde (characteristic of yogurts), as well as carbon dioxide and ethanol. Some Swiss cheeses owe their characteristic holes to the use of carbon dioxide producing bacteria.

As with the use of fermentation to produce breads, beers, and wines, much of the fermentation of dairy products relied initially on strains of wild bacteria which were generally cultivated in local and domestic production by the simple process of adding a little of the previous batch to the next one to be made (backslopping). Today, with the centralization of production of most dairy products many of these individual strains have been lost along with their unique flavors. This perhaps accounts for the increasing popularity of small “boutique” producers who continue to use more traditional methods and thus provide a range of different and distinctive flavors in their products.

The flavor of fermented dairy products comes from a wide variety of molecules which can be formed during reactions. However, in most of the fermentation processes a range of small oxo compounds are produced. For example, in butter production the difference between sweet butter and acidified butter is the presence of diacetyl resulting from fermentation.^93,95,98 Sweet butters are often preferred for certain dishes due to the milder flavor. Another example is that yogurt often develops a rather tart flavor due to the production of acetaldehyde. This is often disguised in fruit yogurts by the addition of sugars, causing the sweet flavor to become dominant.

A similar process has occurred with a range of meat products. In Southern Europe sausages were often fermented with added spices and then air dried to yield specific flavors and good long-term storage properties. In Northern Europe smoke was more often used to dry the sausages giving other flavors while also imparting excellent preservative characteristics. In the intermediate regions such as Hungary fermented sausages were matured for longer times at lower temperatures and covered by molds to help preserve them. Once again, modern centralized production is inevitably leading to the loss of some of the bacterial strains used in these traditional processes and with that the potential loss of some unique flavors.

While raw ingredients carry with them a vast array of naturally occurring flavors, think of fragrant strawberries, sweet carrots, mildly acidic apples, and sharp limes, many of our most favored flavors today actually do not occur directly in nature but are created through chemical reactions within and between the food ingredients after harvesting. Without chemistry there would be no chocolate, no coffee, and no “meaty” flavors. There are so many different chemical reactions that go into generating these flavors, and they differ subtly in every kitchen with slightly different ingredients, temperatures, and tools, making it quite impossible to list them all.

Instead, in this section we will simply attempt to extract from the vast literature some of the more salient issues of flavor development from the wide variety of chemical reactions that occur as we prepare our foods. Some of these occur before we start any cooking; they happen either in the mouth as enzymes in saliva react with molecules in the raw ingredients or when the raw ingredients are cut open, bringing enzymes into contact with the substrates upon which they can act. We briefly discuss such reactions in the first part of this section.

In the remainder of the section we briefly review a range of different types of chemical reactions that break larger molecules into small volatile compounds that can (and do) greatly affect the flavor of our foods when we process them further (usually under the influence of heat). These include hydrolysis, oxidation, and the Maillard and caramelization reactions, each of which is discussed separately even though in real cookery all can (and do) occur simultaneously.

4.1.2.1. Physical Processing. The first stage of processing of most fresh ingredients involves cutting or otherwise breaking them into smaller pieces. In many cases the very fact of fracturing the cell walls leads immediately to the release of enzymes and the start of enzymatic reactions that alter the flavor.

In many plants enzymes are separated from their substrate so as to avoid reactions occurring when the plant is growing. However, when the cellular structure is destroyed by, for example, cutting or grinding by the cook or chewing by the diner they come into contact and flavor-producing reactions may occur.

The pungent taste of several plants in the Cruciferae (Brassicaceae) family, including, for example, mustard, horseradish, and wasabi, occurs via breakdown products from glucosinolates. The glucosinolates are hydrolyzed to isothiocyanates when the enzyme myrosinase (thioglucoside glucohydrolase) is released due to plant tissue being mechanically disrupted or injured by, for example, chewing/crushing/grating.^99–103

A similar process in Allium species such as onions, leek, and garlic provides the characteristic flavor formed when plant tissue is disrupted, releasing alliinase. The enzyme released in this process breaks down odorless sulfur-containing amino acids (S-alk(en)-yl-l-cysteines) and their sulfoxides. These are cleaved to yield pyruvate, ammonia, and sulfur-containing volatiles (Figure 4), giving the characteristic pungency and lachrymatory effect.^104–109

It is important to note that thermal processing of the vegetables denatures the enzymes responsible for these reactions;^102,111 hence, preparing, for example, onions and garlic whole in the oven gives a much milder and rather different flavor since the enzyme will never become active. The alliinase can be inactivated irreversibly at low pH (below 3), preventing these reactions to occur. Thus, acid marinating directly after cutting and crushing would result in similar mild flavors.

4.1.2.2. Hydrolysis. Many of the nutritionally important molecules in our food are large polymers, e.g., starches and proteins; these are neither particularly soluble or volatile and so have little if any perceivable taste or aroma. We can only detect their presence in foods though the textures they create and not by any specific taste. However, when degraded into smaller molecules they generate a wide range of tastants and aromatics that we can and do enjoy in the taste and smell of our foods. While there are several different routes through which such degradation can occur, hydrolysis is probably the most important of these and the subject of this section.

The three macronutrients carbohydrate, proteins, and lipids are all able to react with water so that the macromolecules are degraded into smaller pieces which are likely to possess aroma or flavor (which may be pleasant or in some cases unpleasant). Carbohydrates are polymers in which the monomers (sugars) are linked together by glycosidic bonds, which may hydrolyze as a result of acid catalysis or via enzymatic reactions. For example, malting, the initial stage in brewing beer, is an enzymatic hydrolysis of starch which produces smaller carbohydrates, making them available for the yeast to convert to alcohol and carbon dioxide. The sweet taste of malted bread is similarly produced from the small carbohydrates or oligosaccharides produced by hydrolysis.

Proteins may, in addition to the enzymatic and acid-catalyzed hydrolysis seen in carbohydrates, also hydrolyze via base catalysis since the amino acids are linked together by amide bonds. For example, proteins can be hydrolyzed during the ripening of cheese or boiling of meats. In this process shorter peptides are formed which eventually can be cleaved to individual amino acids. In general, the more hydrophobic amino acids taste bitter,(112) while the more hydrophilic amino acids have a neutral or sweet taste.^112,113 Thus, peptides with specific combinations of hydrophobic amino acids can be extremely bitter and may ruin the flavor of cheeses if formed in significant quantities. The amino acid glutamic acid (and small peptides with glutamic acid) are often formed during cooking of meat, in soups, and in savory cheeses. These are particularly important as they excite the umami taste sensation, giving these foods their characteristic rounded flavors. Most foodstuffs are either acidic or neutral, so base catalysis is relatively uncommon. However, in fish treated with lime or other alkaline substances (as is traditional in some Scandinavian and Asian countries) hydrolysis through base catalysis plays a significant role in flavor development.

Lipolytic enzymes can hydrolytically cleave the lipids to form free fatty acids; these reactions are particularly important when considering rancidity in oils and fats. For example, olive oil is produced from fruit pulp which has a high lipid content; these lipids are easily hydrolyzed by enzymes so that the oil can contain significant quantities of free fatty acids.(114) A high content of free fatty acids in an oil lowers its thermal stability during frying which may impair its flavor. Similarly, butter may go rancid due to lipid hydrolysis of butterfat to produce short-chained fatty acids such as butyric acid, ruining the flavor.

In some sausages lipid hydrolysis during fermentation and ripening actually provides a more soapy flavor, which is not necessarily negative for the overall flavor.(115) Partially hydrolyzed lipids are surface active and find use as emulsifiers, for example, in bread to reduce staling and starch crystallization.

Mold ripening of cheese yields very complex flavors. The initial ripening occurs through protein hydrolysis and is followed by lipolysis, while for blue cheeses like Danablue and Roquefort free fatty acids make significant contributions to the flavor.

4.1.2.3. Oxidation. A second process that can degrade food molecules is oxidation. Unlike hydrolysis, in nearly all cases oxidation leads to flavors that are not desirable, so we normally strive to reduce or avoid oxidation during storage and processing of fresh ingredients.

Oxidation of foods may be initiated enzymatically, for example, in vegetables post harvest, by catalysis by transition-metal ions, which can occur, for example, when using copper pans for frying, or by exposure to light, as happens with green plant oils. Most food components are vulnerable to oxidation, and oxidation may change their flavor, color, and nutritive value. In the kitchen we are familiar with oxidation especially in relation to lipids; vegetable oils and lard may go rancid during storage, while pork and poultry often develop a so-called “warmed-over flavor”(116) upon reheating. Among the lipids, the degree of unsaturation is crucial. Oils and fats from fish are extremely sensitive to oxidation, while animal fats from ruminants are less sensitive; lard and plant oils have intermediate sensitivity. The propensity to oxidation depends on the degree of unsaturation of the lipids and on the level and nature of any antioxidants present. A convenient illustration can be seen in the best choice of various plant oils for cooking. Heavily unsaturated oils such as pumpkin oil and walnut oil oxidize easily on heating and should never be used for frying, while they are perfect for dressings with their mild and unique taste. More saturated oils such as palm oil, rapeseed oil (canola), and olive oil are almost perfect for frying having high monounsaturated content, while grape-seed oil and soy oil should be used with care due to the increasing amounts of polyunsaturated fatty acids present in the lipids. Olive oil adds a unique flavor to fried vegetables or meat, while canola oil is more neutral and accordingly preferred by some.

The reuse of oil for frying, as is often the case in deep frying, can generate new flavors that are characteristic of the specific oil used; often these flavors can be unpleasant and are classed as “off flavors”. Fatty acids belonging to the Ω-9 family such as oleic acid have the major oxidation product nonanal, while hexanal dominates the oxidation of the Ω-6 fatty acids and propanal the Ω-3 fatty acids. Hexanal has been found to be an important compound in warmed-over flavor.^116,117

Another important aspect of oxidation of foods in the kitchen is the oxidative polymerization of polyphenoles which causes the enzymatic browning in avocados and apples. Control of enzymatic browning is also important for producing tea. Green teas are obtained by deactivating polyphenoloxidases by heating freshly picked leaves, black teas get their color from polyphenoloxidase activity, while oolong teas obtain their light brown color by strictly controlled heat deactivation of the enzymes. Notably, co-oxidation of carotenoides in the tea leaves is crucial for the development of the flowery notes characteristics for the fermented tea.

4.1.2.4. Lipid Oxidation and Antioxidants. Lipid oxidation is a chain reaction with free radicals as reactive intermediates.(118) Lipid oxidation depends on oxygen activation or attack by free radicals on the lipids, leading to formation of lipid hydroperoxides without flavor as the primary oxidative products as illustrated in Figure 5. Oxygen activation by metal-ion catalysis may produce the hydroxyl radical capable of initiating the chain reaction (A in Figure 5). Lipoxygenase (D in Figure 5) results in direct formation of the lipid hydroperoxides, and also photosensitization by chlorophylls and other pigments (C in Figure 5) is important. Secondary lipid oxidation products are formed by cleavage of the hydroperoxides to form aldehydes and ketones with low sensory threshold values.

Metal catalysis cleavage of the hydroperoxides may be due to the redox activity of heme pigments. The traditional use of thyme to spice blood sausages in traditional Nordic cooking is an example of the beneficial effect of a natural antioxidant. Thyme prevents any oxidized flavor as phenolic compounds such as thymol and carvacrol present in thyme provide antioxidant protection. Phenolic antioxidants, to which class vitamin E, α-tocopherol, also belongs, are chain breaking as they donate a hydrogen atom to the lipid radical in the reaction marked by A in Figure 5. Another example of a traditional source of natural antioxidants is rosemary; many traditional dishes spiced with rosemary are inherently protected against warmed-over flavor by the phenols from rosemary.(119)

Interestingly, the lipoxygenase enzymes can be active even at the low temperature of frozen storage.(120) This is the reason why blanching of vegetables prior to frozen storage to inhibit such lipid oxidation being initiated by lipoxygenase is important (D in Figure 5).

Carotenoids which are present together with the chlorophylls in the photosynthetic apparatus in plant cells yield protection against photo-oxidation(121) (C in Figure 5). Photo-oxidation is a comparatively slow process and seldom a problem in the kitchen. However, changes due to photo-oxidation will occur in some foods when stored in the light. For example, virgin olive oil and other green vegetable oils should never be stored in light as they will slowly oxidize. Thus, even though a bottle of olive oil in the kitchen window can be very decorative it is not a good idea as the oil will slowly deteriorate.

A third antioxidative mechanism depends on the complexation of pro-oxidative metals by flavonoides(122) (B in Figure 5). Onions, which are rich in quercetin, may be active by this mechanism when added to high-fat fresh sausages and pâtés. Green tea rich in catechin holds the potential for a similar use in other dishes.^123,124

The quality of raw material is important for the oxidative stability. For nonruminants the feeding regime is crucial. A large supply of unsaturated lipids in the feed increases the risk of oxidation in the meat, while a rich supply of antioxidants from herbs or other sources such as acorns yields protection. On the Iberian Peninsula pigs for the highly praised dry cured hams finish their lives roaming in the cork oak grooves feeding on acorns rich in antioxidants. Lipid oxidation in such products is characterized by a lag phase, the duration of which depends on the feeding regime. As long as the antioxidants are available at a sufficient level, the lipids are protected, while when they are depleted, oxidation wins and the meat can become rancid very rapidly.

4.1.2.5. Maillard, Strecker, and Caramelization Reactions. We all know from experience that flavor develops when we cook (heat) our food. Thus, the most important reactions, from the perspective of flavor development, are those that are thermally activated. Among these, the Maillard, Strecker, and caramelization reactions are largely responsible not only for the characteristic flavors of cooked meats; but also for the flavors of chocolate and coffee as well the caramel flavors of cooked sugars in sweets and on the crust of freshly baked breads, etc. All of these flavors are produced in chemical reactions and as such are not natural, although most people would regard these flavors as completely natural while considering some extracted essential flavors (from, for example, citrus fruits) which may be added back into food products as artificial. It will be one of the challenges of Molecular Gastronomy to educate people (domestic cooks, chefs, and the wider public) to understand better how flavors arise and appreciate the very subtle differences between so-called “natural” and “synthetic” products.

When foods are heated, reducing sugars and other carbonyl compounds together with amino acids (or other amines including peptides and proteins) undergo a complex network of reactions that produce both volatile and higher molecular weight compounds including colored pigments and heterogeneous polymers. The formation of brown products on heating aqueous mixtures of amino acids and sugars was first described by Louis Maillard in 1912. Since then, Maillard reactions have been subject to many studies in foods and model systems. Maillard and related reactions are complex and difficult to understand fully or predict. However, we do now have a fairly detailed understanding of the reaction pathways involved as well as the role of the reaction products in flavor, browning, and nutrition. Further, the antioxidative, toxicological, and antimutagenic properties of many of the products of these reactions are now reasonably well documented.(125)

Of course, most foods are complex systems with many components that can react when heated. Accordingly, the Maillard reaction often occurs in parallel with other reactions such as the Strecker reactions and caramelization of sugars. Maillard reactions have also been implicated with thermal degradation of lipids, leading to the lack of the desirable flavor generation by producing heterocyclic compounds with aliphatic side chains.(126)

4.1.2.5.1. Principal Mechanisms of Maillard and Associated Reactions. The Maillard reaction has been divided into three stages, consisting of the condensation of an amine with a reducing sugar, to produce an N-substituted glycosylamine. The subsequent Amadori (for aldoses) or Heyns (for ketoses) rearrangement then produces isomeric compounds. These unstable compounds give neither browning nor aroma but serve as important precursors for subsequent reactions (Figure 6). The second stage involves the enolization of the Amadori and Heyns products and elimination of the amino compound under formation of 1-deoxyosones or 3-deoxyosones. These compounds subsequently undergo dehydration and fragmentation reactions when sufficient heat is provided.

The Amadori product can be broken down by either of two pathways. The 1,2-enolization is promoted by protonization of the N atom in the Amadori compound, which is favored under acidic conditions and leads to formation of a 3-deoxyosone. In a more alkaline environment, protonization of the Amadori compound is less effective and, thereby, indirectly stimulates the 2,3-enolization.^127,128 Alkaline conditions thus favor the formation of hydroxyfuranones via the 2,3-enolization and formation of a 1-deoxyosone (Figure 7).

In the dehydration of the deoxyosones, furfurals and/or furanones are formed. Furthermore, the fission of deoxyosones, e.g., by retro-aldolization, may produce a range of carbonyl compounds such as acetaldehyde, 2-oxopropanal, hydroxypropanone, and butanedione.(128) An example of the degradation of the deoxypentosonone is given in Figure 8. It is at this stage of the Maillard reaction that many different flavor compounds develop in a network of competing reactions.

The formation of reductones and fission products in the Maillard reaction is associated with the Strecker degradation of amino acids. This reaction involves condensation of α-amino acids with conjugated dicarbonyl compounds to form a Schiff base, which enolizes into amino acid derivatives that are easily decarboxylated. The new Schiff base with one atom less is then split hydrolytically into an amine and an aldehyde which correspond to the original amino acid with one carbon atom less. The net result of the Strecker degradation is a transamination which could be an important reaction for the incorporation of nitrogen into melanoidins.(129) Secondary amino acids such as proline and hydroxyproline resist Strecker degradation because of the blocked transamination. However, in heating reactions with sugars, proline is an important precursor for compounds with cereal-like flavor notes. The Strecker reaction of cysteine results in the generation of ammonia, ethanal 2-mercaptoethanal, and hydrogen sulfide, which are important precursors for the hetrocyclic flavor compounds. Methionine can also undergo further degradation by forming ammonia, methional, methanethiol, and dimethyl disulfide.(130) The aminoketones formed during Strecker degradation can through condensation reactions form different kinds of alkylpyrazines, which are typically found in toasted/roasted foods such as nuts, bread, and meat.(131)

The intermediate stage of the Maillard reaction and related reactions provides a complex pool of reactive compounds, which are subjected to rearrangements and further reactions producing several classes of heterocyclic (volatile) products, several of which are important for cooked flavor. This can be characterized as the final stage of the Maillard reaction, which also comprises the irreversible conversion of carbonyl compounds, e.g., by retro-aldolization with or without involving amines into high molecular weight (brown) heterocyclic (co)polymers, the melanoidins.

Caramelization of sugars also gives rise to browning and generation of volatile flavor compounds and involves enolization, dehydration, and fragmentation pathways. Dehydration of sugars producing 2-furfural (from pentoses) and 5-hydroxymethyl-2-furfural (from hexoses) occurs at temperatures greater than 150 °C. At higher temperatures colored pigments and a number of volatiles are generated, including furans, carbonyl compounds, etc.(132) In the Maillard reaction, amines allow such reactions to take place under milder conditions and in addition serve as a source for other heterocyclic compounds to be generated.(133)

In addition to the classical pathways in the Maillard reaction, alternative routes have been proposed. Some of these reaction schemes involve the early stage of the reaction such as the double substitution of the amino-deoxy-ketose with subsequent regeneration of the amino acid.(134) Other pathways include an early cleavage of the sugar moiety of the Schiff base prior to Amadori rearrangement. This route seems to occur at neutral and alkaline pH and involves formation of C₂ and C₃ carbonyl-alkylamine fragments that may condense to N-heterocyclic polymers.^135,136 Mechanistic studies using [¹³C]-labeled sugars have provided more details of possible fragmentation routes of the sugar moiety in the Maillard reaction.^137–139

4.1.2.5.2. Interactions with Lipids. Several studies have pointed out the participation of compounds from lipid degradation in the Maillard reaction. A number of volatiles have been identified, with the majority known as long-chain alkyl-substituted heterocycles with nitrogen and/or sulfur in the ring. The mechanisms for the formation of these lipid−Maillard interaction compounds from foods and model systems have been reviewed elsewhere.(126) The flavor characterisitics of some of these alkyl-substituted heterocycles have been described by ‘fatty’, ‘tallow-like’, ‘green’, ‘vegetable-like’ terms;(140) however, their odor threshold values are generally much higher than the Maillard compounds contributing with desirable flavor notes. Thus, when competing for reaction intermediates, lipid degradation products may decrease the Maillard flavors during cooking. The levels of unsaturated fatty acids and conjugated alkadienals may be important for these reactions to occur.

An example of the reaction of 2,4-decadienal with ammonia or hydrogen disulfide leading to the formation of 2-pentylpyridine, 2-hexylthiophene, and 2-pentyl-2H-thiapyran is given in Figure 9. In baked cereals n-ethyl, n-butyl, and n-pentyl 2H-thiapyrans have been identified when levels of secondary lipid oxidation products were relatively high.(141) In cooked beef from cattle fed diets supplemented with fish oil or linseed oil Elmore et al.(142) found many alkylthiazoles and alkyl-3-thiazolines. The substituents in positions 4 and 5 were methyl or ethyl groups, while the 2-position contained isopropyl, isobutyl, and C4−C9 n-alkyl groups. The concentrations of 2-n-alkyl-3-thiazolines were much higher in the steaks from the cattle fed with fish oil supplements than in the control samples. These 3-thiazolines may also form from the interaction of intermediates of the Maillard reaction with aldehydes derived from lipid degradation.

4.1.2.5.3. Factors Affecting the Rate and Direction of the Maillard Reaction. The Maillard reaction has been shown to be strongly dependent on the reaction conditions and availability of reactants throughout the food. The most important parameters affecting the generation of aroma volatiles are combinations of temperature−time, moisture content, pH, and type of amine and carbonyl precursors present.^{129,141,144,145} Many foods are heterogeneous materials, and the reaction may be favored or inhibited locally in the product. The reaction often is most severe at the surface of a product, where the water content locally can be reduced and concentrations of precursors increased rapidly. Furthermore, the product surface temperature may locally exceed 100 °C. Water transport from the inner part of the product to the surface during cooking may also contribute to transport of Maillard precursors, such as monosaccharides and amino acids, which decrease due to their reactivity at the product surface. Changes in the physical phases in the food, where Maillard reactions take place, also contribute to the degree of color and flavor generation in the product. Phase transitions from liquid to rubbery and glassy states significantly affect the course of the reaction. At temperatures below the glass transition temperature (T_g) the rates of browning are generally low and increase at temperatures closer to and above T_g. Different reactivity for monosaccharides has been observed, and the rate of browning is not clearly related to the rate of loss of amino acid.^146,147 At temperatures below T_g, the limited mobility of small reactant molecules in the glass material may be a limiting factor for the Maillard reaction to occur efficiently.(148)

The pH is also an important factor in controlling the Maillard reaction, influencing the reactivity of free amino acids (both thiols and amine groups) and enolization of the Amadori compounds. In cooked meat with relatively small changes in the initial pH (4.0−5.6) rather large changes in the aromas and aroma volatiles have been reported. The total quantity of volatile compounds increased as the pH decreased. A number of furanthiols and their oxidation products were preferentially formed at acid pH; some of these compounds had strong meaty aromas. The formation of other heterocyclic compounds such as thiazoles and pyrazines were favored by higher pH.(149) In extrusion cooking of cereals, changing the pH by addition of sodium hydroxide in the feed also showed significant changes in the range and levels of aroma volatiles; however, these changes were less marked than those achievable by temperature and moisture combinations.(141) A summary of changes occurring during the different stages of the Maillard reaction is given in Table 3.

4.1.2.5.4. Maillard-Derived Flavor Compounds and Their Presence in Foods. Flavor formation constitutes quantitatively only a minor pathway in the Maillard reaction, since the majority of the reactants are converted to carbon dioxide, melanoidins, and numerous intermediates rather than volatiles. However, a great number of volatile and nonvolatile compounds may be generated in the Maillard reaction, and many of them have been identified and characterized sensorial. The aroma volatiles from the Maillard reaction may be classified into three groups to provide a convenient way of viewing the origin of the complex mixture of compounds. The classes of compounds in these groups are organized according to their stage of formation and origin and include (1) simple sugar dehydration and fragmentation products such as furans, cyclopentenes, and carbonyl compounds, (2) amino acid degradation products such as Strecker aldehydes, and (3) products from further interactions.(133)

A number of these compounds have relatively high flavor threshold values and need to be generated in excess in order to contribute to flavor. However, the compounds belonging to the third group include a wide variety of small, often branched, heterocyclic flavor compounds including pyrroles, pyridines, pyrazines, imidazoles, oxaxoles, thiophenes, thiazoles, thiazines, furanthiols, and sulfides. Several of these compounds have rather low odor threshold values, and their aromas are often characterized by ‘nutty’, ‘roasted’, ‘toasted’, ‘cooked vegetable’, ‘caramel’, and ‘meaty’ flavor notes typically present in heat-treated foods as in coffee, chocolate, roasted seeds and nuts, vegetables, meat, and cooked cereals.^140,150 The mixture of flavor compounds created in the Maillard reaction is usually complex. The contribution of individual compounds to the flavors of the Maillard reaction is difficult to predict, since they can give different kinds of perceptual effects in mixtures, such as suppression, addition, and in few cases even synergy. Furthermore, the compounds can both contribute to odor impressions as well as taste sensations.

In a few instances Maillard compounds have been attributed as major contributors to the typical flavor of foods.(151) For example, 2-furanylmethanethiol is an impact compound in the aroma of freshly roasted coffee,(150) whereas 2-acetyl-1-pyrroline and to a lesser degree 6-acetyl-2,3,4,5-tetrahydropyridine have been identified as key aroma compounds in bread crust(152) and popcorn.(153) The chemistry and occurrence of these and other related potent ‘roasty’ smelling compounds have been reviewed elsewhere.(154) Alkylpyrazines are often an indication of the degree of Maillard flavor development during cooking, although very few of them are generated in sufficiently high amounts to give a contribution to the flavor.

In meat flavor it has been difficult to identify key impact compounds, since meat can be cooked in different ways and originates from different species. Maillard flavors in beef, chicken, and pork flavor have been investigated to a greater extent relative to other species. The Maillard flavors have been shown to depend on the raw meat quality and cooking method, and many different heterocyclic compounds have been identified.(155) The flavor precursors in meat are different from those in plant materials. This is partly due to the fat content and fat distribution throughout the meat, which can interact with Maillard reactions, but also due to enzymatic reactions post mortem, which often increase the levels of important Maillard precursors. Enzymatic hydrolysis of ribonucleotides including adenosyl monophosphate (AMP) post mortem can lead to relatively high levels of ribose. This aldopentose is an effective Maillard precursor for typical ‘meaty’ flavor compounds with very low flavor thresholds such as 2-methyl-3-furanthiol (MFT) and bis(2-methyl-3-furanyl) disulfide (MFT-MFT)(156) (see Figure 10). Although these compounds have long been known from model reactions of ribose and cysteine, they have also been identified in cooked beef.(157) Further, phosphorylated monosaccharides and peptides are hydrolyzed post slaughter and can yield significantly higher levels during post mortem conditioning of beef.^157,158 In pork it has been shown that ribose is rather unstable and glucose and fructose are the most likely important Maillard precursors.(159)

Studies on the key aroma compounds in beef have shown the importance of the Maillard reaction. In boiled beef, MFT, 2-furanylmethanethiol, 2-acetyl-1-pyrroline, MFT-MFT, and methional were ranked with a relatively high sensory impact together with some lipid degradation products.(157) In roasted beef other Maillard compounds were indicated as sensorially more important and included 2-acetyl-2-thiazoline, 4-hydroxy-2,5-dimethyl-3(2H)-furanone, 2-ethyl-3,5-dimethylpyrazine, 2,3-diethyl-5-methylpyrazine, and methional.(160)

Although many volatile compounds have been identified in Maillard reaction systems and implicated in the aromas of many foods, their impact to bitter tastes should also be mentioned. Sugar dehydration products such as 2-furfural, 5-hydroxymethyl-2-furfural, and 2-methyl-3-hydroxy-4H-pyran-4-one are known to possess bitter tastes as well as a number of alkylpyrazines(161) and some thiazoles and thiazolidines.(162)

Products derived from the reaction of carbonyl compounds with proline provide the specific bitter compounds cyclo[b]azepin-8(1H)-ones and pyrrolidinyl-2-cyclopenten-1-ones as identified in model proline reaction systems and beer.(139) Cyclo[b]azepin-8(1H)-ones and a number of 2,3-dihydro-1H-pyrrolidines have also been found in bread crust from dough enriched with proline.(163) In addition to the volatile compounds from the Maillard reaction, which as well as their aroma also possess a bitter taste, a number of nonvolatile bitter compounds have been identified, e.g., diketopiperazines in cocoa.(164) More recently, bitter-tasting compounds have been identified by taste-dilution analysis, and potent bitter compounds from the Maillard reaction with xylose, rhamnose, and l-alanine have been characterized including 1-oxo-2,3-dihydro-1H-indolizinium-6-olates.(165) In particular, the thiophene derivatives show extraordinarily low bitter taste thresholds down to 6.3 × 10⁻⁵ mmol/L water.

The Maillard and caramelization reactions provide an excellent means of generating a variety of desirable flavors during cooking of foods. Although many Maillard precursors and conditions have been identified to influence the course of the reaction in different foods, the applications in gastronomy will still depend on experimenting with these to control the desirable outcomes. Although the positive formation of flavors in the Maillard reaction has received much attention, control of the generation of toxic compounds, e.g., imidazoquinolines and acrylamide, also deserve consideration. For example, tryptophan is effective in preventing the formation of imidazoquinolines, and cooking methods can be optimized with regard to acrylamide formation from asparagine. The toxicological aspects of the reaction should deserve attention among gastronomers both from a consumer standpoint and from a chef’s standpoint during the preparation of the food. Some toxic compounds may not enter the food, since they are evaporated during the cooking process, but need to be removed efficiently from the cooking environment in protection of the chef’s health.

It is clear from the foregoing sections that the chemistry of cooking is extremely complex, making it very difficult to predict with any certainty how flavor will develop in any real cooking process. Not only will the ingredients themselves vary from batch to batch but also the actual processing of these ingredients will never be fully consistent in a real kitchen nor will the temperatures and times of the cooking processes. Thus, while we may understand some of the general principles it is unlikely we will ever be able to master cooking simply from the underlying chemistry; this is one reason why chefs always continually taste samples from their dishes as they prepare them. However, it is a worthwhile exercise to examine at least one apparently standard cooking process to see how many of the above reactions occur and attempt to understand how knowledge of the chemical changes occurring during cooking can be translated into the kitchen environment. To this end, we have chosen the cooking of a basic meat stock as an exemple.

One of the most remarkable processes in the kitchen is the preparation of a stock; initially flavor molecules are simply extracted from the basic ingredients (meat, vegetables, etc.) in water (or sometimes in wine), but something quite remarkable happens as the liquid is kept simmering: the flavor changes and apparently new flavors develop. More astonishing is the fact that as the simmering continues, the stock is reduced (by evaporation of water), and the kitchen is filled with the aromas of the cooking stock, even more flavor develops. One would naturally expect that the flavor molecules, being volatile, would evaporate, leading to a reduction in the overall flavor of the stock. However, in practice, the process of reducing a stock by slow evaporation actually concentrates and enhances the flavor.

Such a complex process is clearly too difficult to interpret fully, but it serves to illustrate several aspects of the development of flavors discussed above and, in particular, demonstrates the complexity of the nature of the chemistry of cooking. In the various studies we discuss below different recipes were used; some authors use the ingredients of a typical stock, meat, bones, and vegetables all cooked together, while others prefer to try to isolate the importance of particular ingredients and so tend to prepare a stock using only the meat (or the meat and bones) without the vegetables.

When studying a recipe for a stock a lot of things can be questioned: does it matter when the salt is added? Is the initial temperature of the water important? What is the effect of the time of cooking? How does the ratio of the meat to the bones affect the final product? And so on. When the stock is further boiled down (reduced) to make a concentrated liquid, further issues arise including which flavor compounds are lost through evaporation and which are formed in the pot during cooking.

Recipes will often tell you to start with cold water and bring the pot to simmer, regularly skimming off fat and scum. According to McGee,(166) starting with cold water allows soluble proteins to escape the solids and coagulate slowly, forming large aggregates that either rise to the surface and are easily skimmed off or settle to the bottom and sides. A hot start produces many separate and tiny protein particles that remain suspended and cloud the stock, and a boil turns particles and fat droplets into a cloudy suspension and emulsion. The reason for recommending to leave the pot uncovered is according to McGee:(166) it allows water to evaporate and cool the surface, which makes it less likely that the stock will boil. It also dehydrates the surface scum, which becomes more insoluble and easier to skim. Further, it starts the process of concentration that will give the stock a more intense flavor.

A lot of research has been concerned with the flavor of beef and attempted to identify the key aroma components constituting the beef stock aroma.^167–169 There has been considerable similar research on other types of meat such as chicken and seafood.^170,171 To identify the important aroma and taste compounds in a given food it is necessary to have appropriate techniques for separating the volatile and nonvolatile compounds (chromatography) and identifying the components as well as method for determining the sensory impact of these (GC-Olfactometry, a method where a panel of trained subjects evaluate the quality and intensity of GC effluents). Less research has been carried out on the cooking process of the stock, although some studies have been concerned with the cooking procedures of the stock and the difference it makes for the flavor of the stock.

On the basis of cookery books, Seuss et al.(172) hypothesized that the temperature progress during stock cooking is important for the resulting flavor. They investigated the effect of temperature and cut size (minced meat vs small cubes) on the flavor of beef stock as well as on several nonvolatile compounds. They found that the best flavor, as determined by a sensory panel, was obtained when cooking at 85 °C. The authors recommend cooking the broth at a temperature below the boiling point since they find that the stock cooked at 95 °C becomes less strong in meat flavor and more bitter. In general, positive sensory assessments were related to a high concentration of inosine monophospahate, inosine, lactate, and free amino acids. The difference between minced meat and small cubes of meat (2 cm) was found not to be important for the flavor.

Cambero et al.(173) also studied the flavor of beef stocks as affected by cooking temperature and identified some of the compounds responsible for the brothy flavor. In correspondence with Seuss et al., they found that the brothy flavor was strongest when cooking at 85 °C (based on four trained judges). The chemical analyses showed that a wide mixture of free fatty acids, peptides of low molecular weight (<300 Da), and IMP all had an important role in the flavor intensity of the beef broth.

The relationship between beef stock components and the flavor developed at different cooking temperatures has been further studied by Cambero et al.(174) The combined sensory study (descriptive analysis and rank order test with 9 trained panelists) with chemical analyses showed a significant increase of small non-amino acid nitrogen compounds, creatine, GMP, IMP, and AMP with beef stock flavor intensity. The beef stock obtained at 85 °C was found by sensory analysis to have the most acceptable flavor, presumably related to the free sugars and amino acids and their reaction products. Stocks produced at temperatures higher than 95 °C displayed off flavors which were easily detected by the sensory panel.

Furthermore, Cambero et al.(175) investigated the flavor development of beef stock in more detail by studying the effect of the cooking temperature, cooking time, ratio of meat and water, and NaCl concentration. They found that the cooking temperature is important since less intense heat treatments generated raw meat, bloody, and metallic flavors whereas stocks prepared at higher temperatures generated sour, astringent, and warmed-over flavors (WOF). Stock obtained at higher temperatures needed a shorter cooking time to obtain a good flavor; however, it was concluded that temperature plays a more important role than cooking time in the generation of the sensory properties of a good beef stock. The best stock was obtained by cooking at 85 °C for 60 min with 7.5 g/L salt with a ratio of 1:2 (meat:water by weight).

Pereira-Lima et al.(176) performed a similar study on the flavor of beef stock, comparing the sensory results with chemical data (amino acids, free fatty acids, and the dipeptides carnosine and anserine) on stocks cooked at various conditions (cooking temperature and time). A positive relationship is seen between a good beef stock flavor and increased levels of Glu, Asn, Lys, and Met. An inverse relationship to beef stock flavor was found with Cys, Pro, Ser, M-His, Tyr, Val, Arg, and Asp, which could be interpreted as a positive relation of the reaction products (Maillard, Stecker) of these and stock flavor. The amounts of the dipeptides, carnosine and anserine, increase significantly with increasing cooking temperature but not with the cooking time. Sensory evaluation shows that increased carnosine and anserine levels provide an improved flavor.

In a later study by Cambero et al.(177) they investigated the effect of cooking conditions on the flavor of shrimp stocks. They studied various NaCl concentrations, shrimp−solutions ratios, cooking temperatures, cooking time, and shrimp preparations in order to determine the cooking conditions that yield the best stock flavor. Best stock was obtained by using whole shrimp in 0.5% NaCl solution (1:2) at 85 °C for 30 min. The stocks were evaluated by a sensory panel (11 trained panelists, rank order test), and the chemical composition was analyzed. Boiling was found to cause the formation of off flavors. The free fatty acids were found to be very important for sensory evaluation of the stocks (the best stocks had the highest level of FFA). Overall temperature was found to be more important than time, just as with the beef stock study by the same author.

In the kitchen the preparation of stocks is one of the most important operations; stocks form the basis of nearly all sauces, so that chefs devote a great deal of time and effort to their preparation. Cookery texts differ greatly on the best methods to use when preparing meat stocks; for example, some indicate it is essential to put the meat in cold water and heat up, while others permit meat to be put directly in hot water; some suggest meat should be browned before boiling whereas others do not. Most cookery tests suggest “simmering” rather than boiling, which indicates they may be suggesting using a low temperature to achieve a better result. As yet, it is very difficult to draw any definite conclusions about the relative merits of such diverse methods of stock preparation. However, a few general principles do emerge from the limited scientific studies to date. For example, the fact that the amount of dipeptides created depends more on the temperature than the time immediately suggests there may be benefits from cooking stocks in a pressure cooker where temperatures well in excess of 100 °C can be used; indeed, this is the technique used for most stock production at several restaurants(178) or for longer times at much lower temperatures (no higher than 90 °C), which is not a technique used in any kitchen that we are aware of.

In modern retail, the quality of meat is often judged by its immediate appearance; thus, any apparent discoloration of meat is, for many consumers, the primary reason for rejection of a specific product. During cooking color changes are further used to follow the progression of heat treatment, and when served, meat is expected to have a characteristic color appearance depending on the actual meat product, heat treatment, dish of which the meat is a part, and personal preference of the diner. For example, the internal color of a steak is used by many as an indicator of its “doneness”, a purple color indicating a rare steak, red a medium steak, pink well done, and gray or brown showing the steak is overcooked.

The color of meat is dominated mainly by myoglobin, the heme protein with the physiological function of oxygen storage in muscles. The attractive cherry red color of meat is due to a steady-state concentration of oxymyoglobin, which in a specific acid-catalyzed process is oxidized to the brown metmyoglobin, a process which is often termed autoxidation.(179) The steady-state concentration of oxymyoglobin is maintained as long as the metmyoglobin reductase enzyme complex can use the reducing cofactor NADH for reforming the iron(II) form of myoglobin in a process which notably is less dependent on acidity than the autoxidation.

Myoglobin, which is violet, is further converted back to oxymyoglobin, when oxygen is available. The steady-state concentration of oxymyoglobin is thus dependent on pH, as decreasing meat pH accelerates autoxidation more than it accelerates enzymatic reduction of metmyoglobin. The steady-state concentration of oxymyoglobin is also dependent on the presence of reducing cofactors together with enzyme activity, as depletion of reducing cofactors or inactivation of enzymes will block the reduction back to the iron(II) state.(180) An example of the absorption spectra of some different forms of myoglobin is presented in Figure 11.

Further, salting decreases the steady-state concentration of oxymyoglobin, as the acid-catalyzed autoxidation shows a positive kinetic salt effect in agreement with protonization of the positively charged iron center as rate determining.^179,180 The balance between the violet and red form of iron(II) myoglobin depends on oxygen pressure. At the meat surface, the red oxymyoglobin dominates, while in the interior of the meat, where metabolic activity depletes oxygen, the violet myoglobin determines the color.

The strong oxidant oxygen is coordinated to the reducing iron(II) center of the porphyrin, which invariably carries the risk of one-electron transfer, creating the superoxide radical anion and metmyoglobin, the brown and physiologically inactive iron(III) form of myoglobin.^180–182

Most people associate a brown color in uncooked meat with spoilage and will reject it. However, a brown color in cooked meat is associated with doneness; the browner the meat the “better” it is done and the “safer” it is to keep. Thus, we tend to rely on smell to detect spoiled cooked meat on storage.

In the kitchen, the spectacular color changes are easily observed by cutting a piece of beef. Once the violet interior is exposed to the air it soon turns cherry red, and a similar bloom on the meat surface may be seen when a pack of vacuum packed beef is opened, allowing oxygen to reach the surface.

A careful inspection of freshly cut beef shows that the three myoglobin forms can be located within the meat by their colors: in the interior the violet myoglobin is separated from the cherry red oxymyoglobin on the surface by a narrow brown ribbon of metmyoglobin. Formation of metmyoglobin has a maximal rate in meat where the partial oxygen pressure makes oxymyoglobin and myoglobin concentrations equal due to the dominating bimolecular electron transfer to oxygen to yield hydrogen peroxide:(183) It is the balance between oxygen diffusion into the meat matrix from the surface (which generally follows Ficks law) and the rate of oxygen consumption by residual metabolic activity in the meat that determines the depth at which the two iron(II) forms of myoglobin have equal concentration and at which the rate for metmyoglobin formation is highest.(183)

Meats are probably the most perfect and complete protein source for human nutrition and provide endless possibilities of creation of meals with superior eating experience; thus, the production of meats of the highest possible quality in an ethical and widely acceptable fashion is a matter with which we should all be concerned. In this context, it is notable that an animal which is unstressed at the abattoir prior to slaughter has a high level of muscle glycogen, which in turn provides high levels of reducing cofactors in the meat with a high glycogen content, known to have better color quality.

Thus, we can, in principle, use the color of meat before and during cooking and later during eating the meal as an indicator of the ethical and acceptable production of the meat. Free-ranging animals such as pigs in the Iberian oak grooves or grazing cattle in highland regions eat a forage with a high tocopherol content, which further protects pigments and lipids against oxidation in the meat during storage and cooking, again providing a direct link between the way animals are raised and the quality of the meat we experience during cooking and in the meal.

The production of hydrogen peroxide in meat as the result of the dynamics of the meat color cycle or from lactic acid bacteria present on meat surfaces further affects the proteins and lipids. Hydrogen peroxide can oxidize metmyoglobin to hypervalent iron forms of myoglobin, which are highly pro-oxidant. Perferrylmyoglobin, a formal iron(V) compound, can thus initiate lipid oxidation, while ferrylmyoglobin, an iron(IV) compound, can cleave preformed lipid hydroperoxides.(183) The peroxidation cycle of myoglobin, which is linked to color cycle as may be seen in Figure 12, accordingly provides a coupling between pigment oxidation and lipid and protein oxidation in meat.^184,185 Persisting brown discoloration of meat thus indicates that the reducing cofactors in the product are becoming depleted and that accumulating metmyoglobin is now available for conversion to hypervalent and prooxidative myoglobins. NADH and other reductants are also efficient scavengers of the hypervalent myoglobin in effect protecting the lipids and proteins against oxidation. Meats with such persistent brown discoloration are likely to be tough to eat and have unpleasant flavors. The depletion of reducing cofactors is a direct indication of initiation of lipid oxidation, leading to meat rancidity, and of oxidative protein dimerization, leading to decreased tenderness.

In Norway it was legal and a common practice until very recently to pack meat in an atmosphere with a low level of carbon monoxide, which binds very strongly to iron(II) myoglobin.(181) Carbonylmyoglobin is intensely red and does not oxidize to metmyoglobin under normal conditions, so the meat products have a remarkable color stability, which was appreciated in the country with its many remote and small societies and difficult transportation of foods.

This practice is now being introduced for master packs of retail meats in the United States in order to achieve better color stability of fresh meat. The formation of carbonylmyoglobin, however, blocks the color cycle of meat, and the color is no longer a direct indicator of the oxidative status of the product. Moreover, the color changes associated with cooking become different, and color as a doneness indicator becomes corrupted.(186)

In other countries, some meat products are now being packed for the retail trade in a controlled atmosphere with a high oxygen content of up to 80% in order to increase the depth of the oxymyoglobin layer of the meat and accordingly to improve the red appearance of the meat.(187) Such practice has, however, been shown to increase lipid oxidation in meat and also oxidative dimerization of myosin, a meat protein of importance for meat tenderness.(187) For a specific meat product the benefits of using a controlled packaging atmosphere should accordingly always be compared to any other effects introduced in relation to product quality.(188)

Over the years the curing of meats has developed from simple salting to preserve meat into a major industry. Along the way, the introduction of degraded organic material containing nitrate into the salt serendipitously led to the modern use of nitrite and nitrate in brine curing. This curing method not only preserves the meat but also creates an appetizing red color. The importance of the color aspect of the curing is recognized in the German term “umrotung” for the process. The pink color of cured meat is due to nitrosylmyglobin, an iron(II) form of myoglobin with nitric oxide coordinated, formed by reduction of nitrite by added ascorbate or inherent reductants like NADH present in the meat.(189) Nitrosylmyoglobin is an antioxidant,(190) in contrast to oxymyoglobin, which is a prooxidant. Discoloration of cured meat as seen upon light exposure in the presence of oxygen should accordingly be avoided, and a brown or gray discoloration of cured meats indicates that the oxidative protection by nitrosylmyoglobin has gone. As long as the meat has a reductive capacity, nitrosylmyoglobin may, however, be reformed and the cured meat still has resistance against lipid and protein oxidation. The antioxidative cycle of nitrosylmyoglobin as associated to the product color is seen in Figure 10.

Parma ham, by contrast, is produced without nitrite or nitrate, and oxymyoglobin is converted by an unknown reaction sequence to zinc−protophorphyrin, which is the principal colorant of this type of dry cured meat.^191,192 For dry cured hams made with the use of nitrite the pigment transformation to the zinc pigment is completely blocked by an unknown mechanism. In Parma ham from Italy and in the similar Serrano ham from Spain the iron liberated from myoglobin by zinc apparently becomes immobilized or otherwise inactivated, since these dry-cured meat products are surprisingly resistant to lipid oxidation, which is normally catalyzed by simpler iron compounds including Fe(II) and Fe(III) ions. The high gastronomic value of these dry cured hams depends on very complex chemical transformations in the meat matrix during the long maturation period of up to 18 months, which are only poorly understood but should be explored for use in other processed meats.

The pigments of fruit and vegetables play important physiological roles in the living plants. The chlorophylls match the spectrum of sunlight for optimal conversion of light energy to chemical energy in the plant. At the same time, we have adapted so that the green color provides an indication of freshness while a fading green color acts as a sign of withering. The blue, red, and yellow pigments in plants belong to several classes of chemical compounds of which the carotenoids and polyphenols are the most important.(193) Carotenoids are associated with the chlorophylls as auxiliary light-harvesting pigments in photosynthesis and are quenchers of singlet-oxygen and scavengers of free radicals for protection during high flux of light.^194,195 Polyphenols have functions as filters for UV light and as protectors against insect attack, form a part of the enzymatic wound protection system, and are antioxidants.(196)

As we have already noted, man’s appreciation of vegetables is strongly influenced by the green color. Chlorophyll, in either the so-called a or b form, may degrade by either of two types of reactions during storage and cooking. In an acid-catalyzed reaction, chlorophyll will lose the magnesium ion coordinated in the phorphyrin, resulting in a color change from bright green to dull brown(197) (Figure 14). In the kitchen this color change may be prevented by using slightly alkaline water for boiling of green vegetables, which may be obtained, for example, by the addition of baking soda. Chlorophyll is lipophilic and anchored in the lamellae of the chloroplasts. Enzymatic hydrolysis of the phytol ester converts chlorophyll to the hydrophilic chlorophyllide in a process which occurs post harvest and increases leaching of green color from leafy vegetables during cooking.

The lipophilic character of chlorophyll is recognized in the much appreciated green color of virgin olive oil. However, the separation of chlorophyll from the carotenoids of the chloroplast makes the oil sensitive to light exposure, since the chlorophyll acts as a photosensitizer generating singlet oxygen, which then oxidizes oleic acid, resulting in a hay-like off flavor. The (physiological) function of chlorophyll in vegetable oil becomes uncoupled from the function of carotenoids as protectors against radicals and singlet oxygen, a protection which is highly important in the fruits of the olive tree during sun exposure.

Carotenoids are only synthesized in plants and algae and in the kitchen provide the appetizing red color of ripe tomatoes and carrots. However, carotenoids are transferred along the food chain, providing color to other organisms.(195) For example, in the oceans, astaxanthin synthesized by phytoplankton is transferred to krill and eventually to shrimp and salmonoids, providing them all with their distinctive reddish pink coloration. In Japanese kitchens the red color of shrimp is highly admired and any white spots of ikaite, a calcium carbonate hydrate formed during frozen storage in the shell, is considered as a serious defect although it is without effect on flavor.(198)

In most cultures, a pink salmon color is likewise appreciated in dishes prepared from smoked or boiled salmon or trout. Trout and salmon are increasingly being farmed in order to meet demands of the world markets. In such farms the pink pigmentation is ensured by the addition of carotenoids to the fish feed. The physiological function of astaxanthin in the flesh is uncertain, but a uniform pigmentation is considered to indicate high quality.

Carotenoids are, however, distributed unevenly in various tissues in most organisms that absorb them. Indeed, the pigmentation of many foods is affected greatly by seasonal (and other) variations in the availability of carotenoids in the diet. Structures of some of these compounds are illustrated in Figure 15. A good example, which is likely to be noted by chefs as well as domestic cooks, is the variability of the color of the yolks of hens’ eggs. Depending on the diet of the hen the yolks may vary in color from pale yellow to bright red; generally, free-range hens that have access to a range of feeds tend to eat more colorful foods and produce darker and redder egg yolks.(195)

Flavonoids and anthocyanines are examples of polyphenols important for the color of flowers and fruits. Apples which have been damaged by insects will turn red faster, indicating the protective role of the polyphenols in the living plants. In the kitchen the yellow, red, and violet colors of berries are important for deserts of various types and fruit drinks. Normally, fresh fruits have acceptable color stability, but upon preservation colors made fade. During canning, tin from metal cans will sometimes dissolve and form complexes with anthocyanines from the fruits, in effect stabilizing the color.(193) Anthocyanines have color, which varies with pH, so that red colors will dominate under acidic conditions as in desserts or fruit smoothies with addition of lemon juice. Such color varations allow the chef the opportunity to change the color of such fruits simply by changing the level of acidity. For anthocyanins the effect of pH on color is due to the complex acid/base equilibria depicted in Figure 16. The acid form of the anthocyanin is in mutual equilibrium with the corresponding base and with a pseudobase, the latter formed by addition of a hydroxyl ion. The equilibrium between the pseudobase and the base is independent of pH but depends on water activity. The heterocycle of the skeleton may open for both the base and the pseudobase. The equilibria are further complicated by molecular stacking phenomena, which further affects color. In the kitchen pH is easily controlled, and colors of fruit-based desserts or drinks may accordingly be adjusted.(199)

At first sight it would appear that it ought to be a simple matter to relate the perceived texture of food in the mouth with measurable physical properties of the food (for example, tensile and shear modulus and fracture stress for solid foods and viscosity for liquids). However, as with all things matters are far from this simple. First, very few if any foods have mechanical properties that can be characterized by single-valued properties. Foods are complex substances and have correspondingly complex physical properties. They are at best viscoelastic, so that knowledge of properties over a wide frequency domain would be necessary before any relationship between perceived texture and measurement is possible. However, most foods display distinct nonlinear properties, making the problem much greater. To complicate matters further, as we chew the food in the mouth it interacts with saliva and changes its properties, leading to changes with time that are more or less impossible to simulate in the laboratory. Worse still, everyone will chew their food differently, so that the perceptions they have will differ. Consider, for example, a food that is distinctly nonlinear, one that has a very low modulus and shear strength at low frequency and small amplitude but which at higher frequencies and amplitudes becomes a rigid solid with a high shear strength. Such a food might seem soft and smooth to a person who chews slowly but hard and brittle to somebody in a rush to eat their food. Accordingly, successful attempts to relate perceived texture to measurable physical properties are few and generally limited to specific types of food and textural descriptions.

The best documented cases of direct relationships between measurable physical properties and perceived texture lie in the area of “semi-solid” foods such as yogurts and custards. The perceived thickness, T, of such foods can be modeled in terms of the shear stress, σ, “felt” on the tongue as the food is consumed. Several authors (e.g., Kokini^202,203) have reported a relationship of the form T = a·σ^b, where a and b are constants that depend both on the food being consumed and on the method by which perceived thickness is evaluated by the taste panels. The shear stress felt by the tongue has itself to be calculated from a knowledge of the viscosity of the food; again, there are different models to do this (see, for example, the fluid mechanical calculations of DeMartine and Cussler(204)). Terpstra et al.(205) provided a detailed discussion of these models together with some comments on their limitations.

Although it is difficult to provide clear predictive relationships between measured physical properties and perceived texture in the mouth, textural measurements remain useful for food scientists developing new products, if only to permit elimination of products that are likely to prove to have unpleasant textures or as quality control techniques. In both cases, it is possible to create specific measurements that can be used to select products that may need further testing.

The range of texture-measuring methods that have been devised over the years reflects on the ingenuity of those working in the field. All manner of devices have been constructed to simulate what happens as we eat our food, from the simplest of measurements of stiffness and failure stress using standard testing machines to a specially designed and built apparatus that simulates the movement of jaws and teeth as we eat.^206–209

The texture of (solid-like) foods varies from incredibly light and soft foams to very hard and brittle boiled sugars. The range of possibilities is so vast that it is not possible to list them all. In this section we first outline the wide range of possible microstructures that occur in the world of foods and catalog them as foams, emulsions, and colloids or more complex multiphasic materials. At the same time we try to show how the different microstructures relate to the physics properties of these foods.

Very few foods and food ingredients can be characterized as being a single phase. Examples include water and simple solutions. Aqueous solutions include naturally occurring ingredients such as egg whites (which can be thought of as a 10% aqueous solution of the proteins ovalbumin, ovotransferring, ovomomucoid, globulins, lysozyme, ovomucin, and avidin as well as small amounts of salts and carbohydrates) and red and white wines where the major solute is ethanol, although it is the other minor solutes such as tartaric acid, flavanoids, tannins, and aldehydes that provide the wine’s flavor. The various oils used in cooking are nominally mixtures of various liquid triglycerides. Olive oil, for example, is mainly composed of a mixture of four triglycerides composed of oleic acid, O, lauric acid, L, and palmitic acid, P; these may be referred to as OOO (three oleic acids joined together), LOO (one lauric acid together with two oleic acids), POO, and SOO. These triglycerides further act as a solvating medium for various minor components: short-chain alcohols, chlorophylls, fat-soluble vitamins, etc. Sugar and salt are the most common single-phase solid ingredients used in the kitchen. Sugar can, as we shall see later, be heated to a liquid phase and then spun and cooled quickly to form a glass: examples of spun glass foods include candy floss. Glassy sugars can also act as solvents in the case of hard candy. Such products, in their simplest versions, are single-phase materials where colorants and flavor molecules are dissolved in amorphous sugar.

While it is possible to find a number of examples of single-phase foods, the vast majority of the materials in the world of foods are of multiphasic nature. Simply moving from a still white wine to a sparkling wine such as champagne immediately doubles the number of phases. Champagne, when in the bottle, may be considered as a pressurized, supersaturated solution of carbon dioxide. As soon as it is served it is better considered as an unstable dispersion of rapidly nucleating carbon dioxide bubbles. These bubbles constitute the second phase of the material. Table 4 shows examples for the development of the complexity of food materials.

Most foods can be described in terms of a number of dispersed phases, surrounded by a continuous phase or matrix. Such systems are, in general, termed colloids, and depending on the properties of the continuous and dispersed phases they can be classified into the categories solid sols, solid foams, sols, emulsions, foams, solid aerosols, and liquid aerosols.

Not all of these colloidal states are equally important in foods and cookery. A liquid dispersed into a gas is called a liquid aerosol. We do not know of any examples of dishes or foods with this type of structure. Nevertheless, if one sprays a liquid on some food, an intermediate mist-like material is produced, which indeed is a liquid phase that is dispersed into a gas phase. The smoke used for smoking foods belongs to the category of solid aerosols: solid particles dispersed in a gas phase.

Moving to combinations that are found commonly in foods, we begin with the case of a solid dispersed into a liquid. Such systems are termed sols; examples include melted chocolate, chocolate sauce, and cold cream (where the fat particles are cool enough to be solid). Sols are mostly used in cooking as sauces and some soups; the important properties for the chef are the “thickness” and “creaminess”. In general, a more viscous liquid matrix and a higher concentration of solid particles will increase the “thickness” of a sol-based sauce, while higher proportions of small particles will tend to increase its creaminess.

A solid phase dispersed into another solid phase constitutes a solid sol. Dark chocolate is an example of a solid sol as both the two solids, sugar and cocoa powder, are dispersed into the continuous solid fat phase. In the case of solid foods chefs tend to be concerned about whether they are tough, brittle, hard, or soft. Generally, in systems where one or more solid phases are dispersed in a single matrix, the properties of the matrix dominate, so if the matrix has a high tensile modulus the product will be perceived as hard if it is ductile, with a low yield stress, as soft, and so on. However, the size and concentration of the solid inclusions also affect the perception of the overall properties in the mouth, especially if the inclusions are macroscopic so they are felt by the teeth (or tongue) as the food is eaten.

A gas dispersed into a solid matrix is called a solid foam. The range of properties that can be achieved with solid foams is vast; the properties of the foam relate mainly to the properties of the solid matrix. At one extreme are the brittle foams that melt in the mouth; these can be light and delicate (such as dry meringues) or hard and brittle (the foamed chocolate “Aero” bars). At the other extreme are the ductile foams that produce, for example, sponge cakes. Stiff matrices with a high-yield stress provide hard or tough foams (e.g., stale and toasted breads), while low-modulus matrices with low-yield stress tend to provide softer weaker foams (e.g., light sponge cakes); brittle matrices produce foams that burst open when bitten into (e.g., dry meringues), while ductile matrices produce foams that melt in the mouth as they are chewed. While the matrix properties dominate the general character of the product (hard, soft, brittle, etc.), the size and proportion of the dispersed bubbles determine how “light” the product is perceived, more small bubbles tending to produce a “lighter” result.

When a gas phase is dispersed into a liquid phase it produces foam. Foams are very common in cookery, both as an intermediate step to a product (bread dough may be considered a foam prior to cooking), as a part of a dish (the whipped cream on top of the strawberries), or even a complete dish in its own right (soufflés). The key properties of foams, from the point of view of the cook, are similar to those of sols. Foams are interesting low-viscosity fluids, air is dispersed, often in high volume fraction into a fluid of higher viscosity. The material formed by the two phases; the foam can behave as a solid (a bowl containing beaten egg whites can often be turned upside down). However, if we consider its nonlinear rheological behavior then foams are better described as plastic materials that can be permanently deformed beyond the yield stress limit. The yield stress as well as the static shear modulus depends on interfacial tension, the average bubble radius, and the volume fraction of air; a theoretical expression yield in foams has been derived by Princen and Kiss.^210,211 Emulsions are formed when a liquid phase is dispersed into another liquid phase. Emulsions constitute perhaps the largest group of food colloids. In gastronomy most sauces and dressings fall in this category.

The key properties of the sauces for the chef are “thickness”, “creaminess”, and “stability”. The first two generally follow those of the matrix, but in some cases (e.g., mayonnaises) the properties of the product can be very different from those of either component. The third, stability, is far more complex and deserves a section on its own.

Hervé This(8) recently attempted to provide a schema to classify the wide variety of possible structures in foods. To do this he considers four basic “phases”: gaseous phases (G), normally simply air but including any gaseous phase, for example, steam or alcohol vapor; liquid aqueous phases (W), pure water or any aqueous solution irrespective of the type or amount of solutes; liquid oil-based phases (O), any oil, mixture of oils, or solutions of molecules of any type in oils (an oil is taken to be any lipophilic liquid); and any solid phases (S) no matter what their chemical composition or internal structure, including solid fats, ice, carbohydrates, etc.

Having thus defined four basic continuous phases from which all foods can be made, This suggests that we may consider the structure of foods in terms of how these are arranged and provides a formalism to permit a concise description of the internal structure of a foodstuff. For example, cream (a mixture of solid and oily liquid particles in a continuous aqueous phase) would be described as (O + S)/W, the “/” symbol indicating the oil (O) and solid (S) inclusions are contained within a continuous aqueous phase (W). Whipped cream occurs when gas bubbles are incorporated into the cream, so it would have structural code G/[(O + S)/W]. This further suggests that such symbolism can be used to describe the transformations that occur in kitchen operations, so the production of whipped cream could be represented by the equation Further, with the use of superscripts and subscripts it is possible to include information about the sizes (or range of sizes) of the various phases and denote complex cases where already multiphasic inclusions are contained within one another, building up complex hierarchical structures. We show in Table 5 a few examples of how particular foods can be classified in the This schema.

The utility of such a classification is that it can be used to see the generality of the various types of products used in the kitchen. For example, This carefully analyzed all the classical French sauces according to this schema and found that just 23 categories have been used.(8) More interestingly, he found that some simple types have not been used by traditional French chefs. The simplest sauce not used in traditional French cuisine is G/W (bubbles in a liquid). Many gastronomic restaurants now use such foams in a wide range of sweet and savory dishes. While such developments were not directly inspired by the This schema, it is possible that future generations of chefs may find novel dishes and variations on existing dishes by examining such classifications.

4.3.2.1. Stability of Food Colloids. From a thermodynamic point of view, more or less all emulsions and food colloids are unstable. That is, the free energy of food is higher in the emulsion or colloidal state than it would be if the food were to separate fully into two (or more) macroscopic regions. For example, a simple vinaigrette dressing will, given time, separate to form two completely separate regions of oil and vinegar; the dressing is only temporarily mixed into an emulsion when the oil and water are shaken together and demixes spontaneously once the agitation stops.

The excess free energy of an emulsion is created by the internal interface area of the system. The excess Gibbs free energy of creating a surface of area, dA, can be written as dG = γ(dA), where γ is the surface free energy density or the surface tension γ = ((∂G)/(∂A))_T,p. The surface tension is always positive (a hypothetical negative surface tension would cause an unstable and spontaneously growing surface). Since colloids are systems of excess surface area, as compared to the macroscopically phase-separated systems, the dispersed situation is always going to be thermodynamically unstable. However, it is possible to construct colloids so that they lie at local (rather than global) minima of the free energy and so are at least metastable.

While preparing an emulsion, foam, or other colloidal dispersion, a surface has to be formed. The work needed to create the surfaces in emulsions and foams is w = γΔA, where ΔA is the excess area created. While a chef might not be fully aware of the thermodynamics of emulsions and foams, he or she will fully acknowledge the work needed for stirring, whipping, and beating the food materials. Often powerful kitchen machines are used to obtain the right degree of dispersion.

A simple example serves to illustrate the power needed to achieve an oil−water emulsion. For a hypothetical oil in water emulsion with a volume fraction of 0.1 and droplet size of 0.30 mm the surface area is 2 × 10³ m²/L. A typical value of the triacylglycerol water surface tension is 30 mN m⁻¹, so that the resulting excess free energy (or work needed) is 60 J/L. Thus, to make a liter of such an emulsion at least 60 J of work needs to be done on the mixture. However, since the emulsion is inherently unstable, the work needs to be applied in a short time, less than the “relaxation time” of the droplets, which will typically be significantly less than a second. Thus, any mixing device used to create emulsions needs to have a power in the region of hundreds of Watts and to run at high speeds to induce the high shear strain necessary both to deform the phase to be dispersed and to break it into droplets. Examples of such industrial devices include rotor stator types with narrow slits and high-pressure homogenizers using large pressure drops and those that force the liquid through a narrow slit at high velocity. Neither of these will, on their own, create an emulsion; they simply disperse an existing emulsion into finer droplets. As yet we are not aware of any purpose built equipment for the domestic kitchen designed to create emulsions. Rather, chefs and cooks tend to use inefficient methods such as manual whisking, where most of the energy is “wasted” going into viscous heating of the liquids rather than into the creation of (and reduction in size of existing) droplets.

4.3.2.2. Kinetics/Dynamics of Degradation of Structure. The stability of a structure can be characterized by a lifetime before severe degradation (in this case before macroscopic phase separation becomes apparent to the diner). This lifetime should be compared with other relevant time scales for the product, such as those for chemical degradation, microbiological contamination, and consumption.

Knowledge and empirical understanding of stability have become built into traditional recipes and procedures; indeed, recipes for many sauces have evolved so that the sauce remains stable long enough to be taken from the kitchen to the table and consumed over a prolonged period. However, when chefs develop novel dishes they have to address the various questions of colloidal stability to ensure the product remains stable from the time it is completed in the kitchen to the time it is finally consumed in the dining room. To this end they must select the appropriate ingredients and stabilizing additives. Further, they need at least an empirical understanding of the conditions under which the structure will remain stable and how long it will remain sufficiently stable. In some cases longer time scales become important. Mayonnaise, if well prepared, has a lifetime of much longer than the time between manufacture and serving, so a more relevant time scale to consider is that of chemical or microbiological degradation.

4.3.2.3. Mechanisms/Processes That Result in Phase Separation or Severe Degradation of Structure. The field of liquid−liquid phase separation is well studied and understood in terms of the underlying thermodynamic drivers. Accordingly, the stability of sols and emulsions is perhaps best viewed in terms of the various mechanisms of phase separation.

4.3.2.3.1. Sedimentation/Creaming. In the food industry the process of “creaming” in emulsions is well understood Essentially, provided the droplets are large enough that Brownian motion is not sufficient on its own to keep them suspended, the droplets will slowly diffuse either upwards or downwards due to the density difference between the two phases. This is what happens to whole milk when left to stand—the cream rises slowly to the top.

The simplest approach is to find the steady-state drift speed of dispersed droplets assuming spherical droplets and frictional forces obeying Stoke’s equation, where d is the droplet diameter, ρ_D and ρ_C are the densities of the dispersed and continuous phases, and η_C is the viscosity of the continuous phase.(212) Stoke’s equation is rarely used quantitatively as several additional effects lead to deviations from this simple estimate of drift velocity; nevertheless, it can provide some guidance and insights for the improvement of the stability of colloids and emulsions in the kitchen.

If the drift velocity can be reduced, the stability will be increased. The simplest approach is to reduce the particle size; reducing the particle size by a factor of 10 will lead to a 100-fold reduction in drift velocity and correspondingly increased stability. The simple rule of thumb for chefs is that smaller droplets (generally made by doing more work) will produce more stable sauces.

One method to reduce droplet size is that used commercially in the homogenization process of milk.(212) Fat droplets in raw milk have a fairly broad size distribution, ranging approximately from 1 to 7 µm.(213) After a typical homogenization process at 20 MPa, the distribution is quite narrow and typically peaks at 0.25 µm. Gastronomic and domestic kitchens might benefit from similar machinery for efficient homogenization.

Alternatively, the stability of such systems can be manipulated by controlling the viscosity of the continuous phase perhaps by adding thickening agents (such as the starches used to thicken some sauces; or the use of sodium carboxymethyl cellulose, locust bean gum, etc. in some commercial products). Using such additives, the viscosity and thereby the colloidal stability can, in many cases, be improved a few orders of magnitude.

4.3.2.3.2. Aggregation and Coalescence. As well as droplets separating under gravity, they can aggregate into usually undesirable structures of larger size, which then, due to their increased size, may be subject to enhanced rates of sedimentation/creaming.

On the most basic level, aggregation can be described through Smoluchowski theory,^214,215 which emphasizes the rate of the process as being diffusion limited. However, the theory does not include the rather important effect of repulsive forces between particles and thus underestimates the stability of food dispersions.

Coalescence is a small step further and results in the complete fusion of two or more particles. For example, in the case of a vinegar/oil dressing, coalescence typically takes place once the droplets become closely packed together, having gained a high concentration (either at the top or bottom of the container) through the sedimentation/creaming process. In beer foams, a large quantity of the liquid phase drains back into the beer under gravity and the air bubbles coalesce in to a coarser structure, which eventually collapses.

Partial coalescence is an intermediate between complete fusion and aggregation. In whipped cream the fat globules reach a partial coalesced state and act to form a solid-like network that gives the whipped cream its physical stability.^216,217

There is little the chef can do to prevent aggregation and coalescence except to be aware that they will happen and will generally happen more rapidly where the droplets are closer together and where the surrounding medium is less viscous. But it is worth noting the importance of electronic repulsion between like charged droplets—this is often the main stabilizing effect where the droplets become charged—increasing the charge on the droplets (usually by more and harder beating) can significantly affect stability—as every chef knows from experience making mayonnaise.

4.3.2.3.3. Ostwald Ripening. Ostwald ripening is due to diffusion of individual molecules through the continuous phase from smaller, less stable droplets to larger droplets. The process is driven by the destabilizing surface contribution to the free energy that makes the chemical potential larger for migrants present in the small particles as compared to larger particles. This effect creates concentration gradients of the migrant in the continuous phase and thus a net diffusion. Ostwald ripening is well understood and has been observed in a wide range of phase-separating systems.

In particular, Ostwald ripening provides for fast growth of the dispersed phase if it is highly soluble in the continuous phase. The process, therefore, is slow for dressings such as water-in-oil emulsions. However, it can be very fast in the case of beer foam since carbon dioxide is highly soluble in the aqueous phase. The mechanism also becomes relevant for products that are stored for long times, such as commercially produced frozen desserts. In such cases, the phenomenon can result in large ice crystals and unpleasant textures.(218)

In systems where one phase is partially soluble in the other, small fluctuations in temperature (or pressure) will cause changes in the amount of the dispersed phase as a new equilibrium partition is reached. When the external factors return to the original state and the net amount of dispersed phase is restored, the size distribution of particle will change toward larger sizes. The phenomenon can be seen as an enhancement of the Ostwald ripening process. This sort of process is particularly important for storage of frozen desserts as temperature fluctuation in freezers can lead to the irreversible growth of large ice crystals.

To conclude, while chefs have some tools available to combat phase-separation processes that inevitably destabilize the product they make, it is never possible to eliminate the tendency of small droplets of dispersed phases from growing larger over time. All that we can do is to be aware of the time scales involved and make sure that products remain sufficiently stable over a long enough period that the diner can enjoy them to the fullest extent possible. It is perhaps for this reason that it can be particularly challenging to transfer the gastronomic dishes prepared in restaurants to the wider food industry. In a restaurant environment, it is possible to produce and serve dishes that have a lifetime of minutes, while the commercial retail industry demands products with lifetimes in the region of days or even months.

To provide such long lifetimes it is often necessary to introduce additional “stabilizing” compounds. These stabilizers in turn introduce their own tastes and textures, which often change the character of the original dishes, often making them less attractive. However, as we shall see in the gels section, some chefs are now starting to use some of these “stabilizers” and use them to enhance the range of gastronomic dishes they prepare. So there is some hope that in the future with superb creative chefs regularly using the types of food additives that were seen as only fit for “processed” foods a few years ago, we may soon see gastronomic quality foods appearing on the supermarket shelves.

4.3.2.4. Ingredients and Compounds That Enhance Colloidal Stability. There is a wide range of food molecules which can be used to increase the stability of foams, sols, and emulsions. Generally, these molecules are grouped into two categories: emulsifiers and surfactants as well as stabilizers. They are widely present in food ingredients. Typical examples include polar lipids (monoglycerides, diglycerides, phospholipids, glycollipids, etc.) and globular proteins (such as beta-lacto-globulin); some common food examples are given in Table 6. These molecules tend to reduce the surface tension between the phases. The reduced excess surface energy decreases the driving force for phase separation. This affects both the ease of formation of emulsions and the stability of the final preparation.

The presence of an emulsifier decreases the work associated with deforming and breaking up larger droplets into smaller ones. This usually results in a size distribution shifted to smaller sizes, and thus, the rate of the creaming/sedimentation process is lowered. The kinetics of the phase separation process into bulk phases is also modified by the presence of surface active components. The lowering of the surface tension also lowers the concentration/pressure differences responsible for the Ostwald ripening.

The accumulation of surfactant at the internal surfaces of the food colloidal will also in most cases modify the forces acting between the dispersed particles. Most prominent is the accumulation of charged surfactant, which gives rise to long-range electrostatic repulsion between dispersed particles and thus a kinetic stabilization against aggregation phenomena. In Table 7 we listed some examples of prepared foods which are stabilized using such molecules together with the stabilizers used.

In restaurant and domestic cooking food needs only to be stable long enough to go from the kitchen to the table and be consumed. Thus, it is possible to prepare and use foods that are in a metastable and even an unstable state. An interesting example, from the Fat Duck, is that of a green tea foam palate cleanser (for a recipe and photograph see page 133 of The Big Fat Duck Cookbook(62)). The basic concept arose once it was noted that a mixture of green tea and vodka in an egg white foam acts as an excellent palate cleanser. Being a foam which disappears quickly it can be served right at the start of a meal with no risk of spoiling the appetite as might a larger more dense version of the same ingredients. However, there is a problem of stability: the foam very quickly collapses, leaving some liquid at the bottom of the glass in which it is served. Thus, it has to be prepared right at the table and consumed immediately. However, not everyone is cooperative; some people will leave it to stand (perhaps to savor it), but the stability is insufficient. In the end a solution was found by freezing the foam in liquid nitrogen to make a small “poached” hard-shelled meringue-like sphere that can be put in the mouth where it instantly disappears, cleaning the palate.

Many of the ingredients we use in our kitchens are fully or partly crystalline materials (examples include sugars, fats, many carbohydrates, and salt). During the preparation of the food these materials will often pass through other liquid or amorphous states due to action of heat or solvents such as water. Eventually the original ingredients may, through solvent removal during baking and cooking and by cooling after heat treatment, be turned into new materials which in many cases also involve (new) crystalline structures. Often multiple competing crystalline forms can coexist. The recrystallization from one metastable form into a more thermodynamically stable form is part of the (usually undesired) aging of food. Overall, an understanding of these phase transitions of food ingredients can enable the chef to obtain a wide range of textures using the right combination of ingredients, heating, and drying.

4.3.3.1. Water. Of all the food molecules that occur in crystalline form, water is the most common. Ice is used in many desserts such as ice creams and sorbets. Ice is also present in all frozen foods, and so the growth of ice crystals is important in considering their long-term storage.

In sorbets and ice creams it is important that there is a liquid phase present to provide enough fluidity to make the dish soft and easy to eat. With no liquid phase the ice would remain hard and unpleasant. This is achieved through the addition of sucrose, other sugars, or other solutes such as ethanol to create a sufficient freezing point depression to leave a substantial portion of unfrozen aqueous solution at the typical serving temperature from −13 to −6 °C and even at conventional storage temperatures from −15 to −20 °C. In many ice creams the sugar content is as high as 15%, somewhat higher than the typical sugar content of other desserts. However, at the low temperatures we become less sensitive to the sweet taste.

Some frozen desserts such as granites are made with only limited amounts of sugar or other solutes, and the final snowy texture of such dishes relies on mechanical treatment of the initially rather solid and hard ice. Recently, novel blenders for use with frozen foods (such as the Paco Jet, see later, section .5) have been exploited in restaurants to make finely dispersed ice particles and expanded the range of possibilities within frozen desserts.

When considering the storage of frozen foods we need to note first that the initial freezing creates ice crystals that can damage the texture of the food being frozen, so that it is usually best to freeze as quickly as possible to create the smallest possible crystals. We discuss how crystallization temperatures and rates affect crystal sizes later. Second, it is important to realize that these small ice crystals are not thermodynamically stable. Larger crystals will grow larger at the expense of smaller crystals that will disappear in a process comparable to that of Ostwald ripening described previously. Such recrystallization is also promoted by the presence of a liquid phase. In most foods there are some salts and sugars present in the aqueous phase. On freezing, as ice forms, so the concentration of the sugars and salts increases, but since the eutectic temperatures of most salt mixtures are below those found in most domestic freezers, some aqueous solution usually remains in the frozen foods.

4.3.3.2. Carbohydrates. Simple sugars are generally refined into crystalline powders and granules of high chemical purity. These are essentially single-component materials so that, in principle, they possess sharp, well-defined thermodynamic melting transitions. However, most sugars are not chemically stable; they undergo dehydration reactions upon heating, which affect the melting behavior. These caramelization reactions have been discussed previously. There are a number of dishes (such as the créme brulees) where the flavors and color of the caramelization reactions are desirable. However, it is also often the case that chefs may wish to produce sugar products that do not have the caramelized taste or color but which nonetheless need to be heated; examples include spun sugars and candy flosses.

Two solutions are possible. First, it is possible to select sugars that are more stable; there are some sugars, in particular, the sugar alcohols, xylitol, sorbitol, and maltitol, which are stable against dehydration reactions and can be melted without any significant browning. Second, mixtures of different sugars can be used so as to reduce the melting temperature range.

For example, boiled sweets are based on a combination of sucrose and glucose (in some countries glucose syrups). The preparation involves boiling the solution at an increasing temperature (the temperature rises as water boils off due to pronounced boiling point elevation of the highly concentrated solutions). The process is stopped at a temperature of about 146−154 °C, indicating a very low water content. However, due to the presence of two or more sugars (or glucose oligomers) as well removal of most of the water, crystallization of sucrose and glucose on subsequent cooling is kinetically suppressed,(222) so that the process directly provides a supercooled (glassy) sugar melt without entering the high-temperature regions used to melt sugars that accelerate the dehydration reaction of sugar. The effect of glucose, glucose oligo- or polymers on the crystallization of sucrose has been systematically studied by differential scanning calorimetry.(223)

As we shall see later, sugars can also be quenched to form glassy materials. Crystalline sugars are desirable in many dishes and products (including chocolate, fondant, fudge, dry meringues), while glassy sugars are required in other dishes (such as hard boiled candy). The presence of crystalline sugars not only contributes to a sweet taste but also changes the texture of the product depending on the amount and size distribution of the crystals. Some sugars, mainly xylitol and to some extent sorbitol, dissolve quickly in water through a strongly endothermic process and can be used to create a cold sensation in the mouth.

As we shall see in detail in the gels section starches are present in many basic ingredients such as flour and potatoes, etc., in the form of small, partly crystalline, micrometer-sized starch granules. The crystalline parts consist mainly of the macromolecules amylopectin and amylose. Due to the action of a combination of water and heat the crystalline parts of the granules will melt and the granule will take up water and swell; eventually the whole granule can disintegrate. The process is called starch gelatinization. The melting of the crystalline regions occurs at increasingly high temperatures with decreasing water content.(224) The melting temperature reaches its lowest level (about 58 °C in the case of potato starch) independent of water content if excess water is present.(225)

4.3.3.3. Fats and Oils: Triglycerides. Fats and oils are multicomponent materials with a multitude of crystal phases that can all coexist, making their crystallization and melting behavior significantly more complex than simple systems such as the sugars. Mixtures of triglycerides in a liquid state are normally called oils, whereas purely or mainly crystalline triglycerides mixtures are called fats. Triglycerides show a richness in polymorphic forms, which can be grouped into three main types, α, β′, and β. Thermodynamically these polymorphs are distinguished by increasing stability (decreasing free energy) and increasing melting point in the order α, β′, and β. On a structural level, they differ in the methylene group packing arrangements as seen by wide angle X-ray diffraction experiments.(226)

The sizes of individual crystallites are typically a few micrometers. In fats with a content of liquid triglycerides, the crystals of fat can form a network that penetrates the materials, making a gel-like system with plastic deformability, giving rise to the well-known texture and spreadability of ingredients such as butter, margarine, and pork fat. For most fat types metastable crystals of β′ type give rise to the most optimal crystal size distribution and crystal network and are therefore desired in ingredients such as butter, margarine, and some forms of chocolate. A culturally based exception to this is the middle eastern ingredient, butter ghee (a butter fat without water and proteins), where a grainy texture based on β crystals is preferred.

Phase diagrams of some triglyceride mixtures have been determined.^227,228 However, in general fats can be considered to be multicomponent mixtures as well as multiphase materials. For such complex systems the derivation of complete composition temperature phase diagrams is almost impossible. Instead, for a specific fat or combination of triglycerides the SFI (solid fraction index), the fraction of crystalline material present, can be derived as a function of temperature using techniques such as DSC or low-field nuclear magnetic resonance spectroscopy (NMR).(229) In such studies the detailed connection between the underlying phase behavior and the SFI-melting profile remains unclear. However, it is clear that the incompatibility between crystals of different triglycerides can lead to eutectic-like phenomena. The number of phases present is comparable to the number of different triglycerides, which for most fats can be large (>10), making the detailed materials science of fats extremely complex. In general, all that can be said is that they usually have very broad melting ranges.

However, in some systems with fewer triglycerides and where they have compositions that lie close to the eutectic-like proportions a comparatively sharp melting temperature can be seen. Cocoa butter, a chemically simple fat, has three major components: POS, POP, and SOS (using the notation defined in section ). When cocoa butter is in the so-called state V, a β′ polymorph (the desired polymorph for chocolate), it has a melting temperature just below body temperature. This form of cocoa butter lies close to the eutectic composition for the tempered system of the three triglycerides POS, POP, and SOS, explaining the sharpness of the melting process of dark chocolate. The much broader melting interval of butter and pork fat can be understood in this context in terms of a triglyceride composition further way from the eutectic composition of the relevant triglycerides.

Finally, we should mention that the presence of metastable polymorphs further complicates the measurement and observation of phase behavior since the melting behavior becomes strongly dependent on the thermal history. For example, it is often found that parts of a material will crystallize and release heat upon heating. Unstable polymorphs melt at relatively low temperatures on heating (for example, in a differential scanning calorimeter (DSC)) to form supercooled melts which subsequently crystallize into more stable polymorphs and finally melt again.(230)

4.3.3.4. Nucleation, Growth, and Other Dynamic Aspects of Crystalline Phases in Gastronomy. One of the most important issues with crystalline food systems is the size of the individual crystals. The overall crystallization rate and distribution of crystal sizes are determined by the rates of both nucleation of new crystals and growth of existing crystals. Both rates increase with the thermodynamic driving force (which depends, inter alia, on the supercooling or supersaturation), and both decrease as the mobility of the molecules decreases. The result is usually a bell-shaped curve for the rates as a function of, i.e., temperature, with rates at first increasing and then passing through a maximum before decreasing as the temperature is lowered. However, the relative rate of nucleation to that of growth tends to increase as the thermodynamic driving force increases, leading to significantly smaller crystals being formed at the highest supercoolings (and for crystallization from solution the highest supersaturations).

Implicit qualitative knowledge is often inherent to classical gastronomic procedures. In the making of fondants and chocolate, often the hot sugar melt/solution or parts of the melted chocolate are poured on to and kneaded on top of a marble block, which ensures fast cooling and with it formation of very small crystals. When making ice cream the ice cream mix is usually stirred vigorously while being cooled in order to maximize the supercooling and break up any large crystals that may grow to produce a very fine dispersion of ice crystals, which is very important to the perceived texture of this frozen dessert. Dry meringues are baked for a long time at sufficiently low temperature (≤105 °C, well below the melting point, ca. 185 °C, of sucrose) in order to ensure that sucrose will crystallize as the water is removed.

In cases where prevention of crystallization is desired, as in the making of hard-boiled sweets, we exploit the fact that crystallization can be quenched by sufficiently fast cooling through the temperature interval of overall fast crystallization and into a more kinetically stable temperature region. However, it should be noted that crystallization of such impure material must involve accumulation of impurities and diffusion over increasingly longer distances as crystallization progresses (rather than forming a supersaturated/supercooled melt that eventually will be glassy).

Some of the macrocomponents of food, such as sugars, larger carbohydrates, and to some extent proteins, form glasses when melts or solutions of these food components are cooled or dried sufficiently fast to avoid the crystallization processes. Glass formation is important for the description and understanding of hard-boiled candy, candy floss, cookies, crusts, and crackers, etc. In fact, more or less all crunchy or crisp foods are in a glassy state.

Briefly, a glass is, from a structural point of view, a molecularly disordered material that behaves from a simple mechanical point of view as a solid, characterized by a high shear modulus of the order of 10¹² Pa. However, food glasses are often more like very viscoelastic liquids which show very slow relaxation processes (with relaxation times, τ, of many hours) and can thus also formally be assigned a viscosity >10¹² Pa s. Glasses are also brittle, which is very important for the perceived crunchy textures they produce.

The rates of the relaxation processes as well the viscosity of food glasses are heavily dependent on temperature with a strongly non-Arrhenius temperature dependency. When cooling amorphous food materials, the very steep dependency on temperature gives rise to a moderately well-defined transition from liquid behavior to solid behavior at the so-called glass transition temperature, T_g. Differential scanning calorimetry using standardized scanning rates of 5−10 deg/min is the preferred technique for the determination of glass transition temperatures in food systems, although other methods can be employed.(231)

The strong temperature dependence of the viscosity near the glass transition is most commonly described by the Williams−Landell−Ferry expression (the WLF equation): where η is the viscosity, T₀ is some reference temperature, and C₁ and C₂ are constants pertinent to the combination of the glass-forming material and reference temperature. If no specific knowledge is available of the material under investigation a combination of a reference temperature of T_g and the so-called universal values of C₁ (17.44K) and C₂ (51.6 K) can be employed.

Although, accurate numerical prediction within gastronomy is not relevant in most cases, the WLF equation gives an impression of the changes of the material properties as food materials are cooled during gastronomic processing. A set of constants, close to the so-called universal values, predict that the viscosity will change by 10 orders of magnitude over a temperature interval of about 20 °C.

Table 8 shows that the (calorimetrically measured) glass transition temperature depends strongly on the nature of the food components. For sugars the calorimetric glass transition ranges between −29 °C for xylitol and 110 °C for trehalose. The glass transition temperature of dry starch and dry starch components is high from the viewpoint of cookery (>200 °C). The glass transition temperature of water has been heavily debated(232) and reported in a temperature range which can be considered irrelevant from the viewpoint of gastronomy. The diversity of glass transition temperatures gives the creative chef possibilities of manipulating textures of foods by interchanging components and changing the composition of ingredients.

Unlike the pure food components of Table 8, food is usually made out of mixtures. The glass transition temperature of such mixtures can be calculated as a weighted average of the glass transition temperatures of the individual components. Various expressions have been reported for the calculation of the glass transition temperature of mixtures.(231) Most importantly, components with low glass transition temperatures strongly depress the glass transition temperature of mixtures; the effect is known as plasticization. The ever present solvent water is the most prominent and efficient plasticizer in food systems. The uptake of moisture is responsible for the loss of crispiness in many foods, for example, crackers, which can be treated as starch-based composites with glassy regions.

In the kitchen many sweet dishes involve boiling sugar syrup to various “stages”. Essentially, as water is driven off, so the solution concentration increases with a corresponding increase in boiling point (and decrease in the plasticization). The progress of the process is usually monitored by measurement of the boiling temperature and terminated at a specific temperature to obtain the required consistency (or “stage”). Of course, the glass transition temperature and viscosity of both the hot melt/solution and the cooled product all depend on the water content. The various stages of syrups are usually called thread, soft ball, firm ball, hard ball, soft crack, and hard crack. The glass transition temperatures and viscosities of the various stages is given in Table 9.

4.3.5.1. Introduction to Gels. The original jellies are the aspics, meat-based jellies that arise naturally from the juices of boiled meats and bones. They get their name from the Latin gelare (to freeze), presumably as the hot clarified meat juices eventually set in a transparent solid on cooling. Thus, when the underlying structure was found to be that a solution of long molecules formed a three-dimensional network on cooling it is not surprising that the molecules identified as being responsible for the formation of the jelly were named gelatin.

Today we understand a gel to be a system where a large volume of liquid is stabilized in a solid-like form by a network of partially dissolved long-chain polymer molecules. Provided these long molecules form a complete three-dimensional network throughout the system it will have a solid-like behavior and becomes a gel. The properties of the gel then depend largely on the properties of this polymeric network. Since this network is normally very dilute and made up from molecules that are, more or less, random (although swollen by the presence of the surrounding liquid) it is reasonable in many cases to treat the network as a rubber-like system.

Most gels start off as a solution of polymeric molecules in a fluid (in food gels nearly always water, although alcohol- and oil-based gels are also possible). Some process whereby the molecules become cross-linked occurs; this may be chemical or physical. As more and more cross-links are formed, so the effective molecular weight of the dissolved polymer molecules increases and the viscosity increases correspondingly. As molecules form links to one another it soon becomes possible to build a full three-dimensional network throughout the solution, at which point it forms a gel. The cross-link density at which this full 3-D network occurs is sometimes referred to as the percolation limit.(234)

In a rubber the stiffness and extensibility are determined by the molecular weight between the cross-links and the equivalent segment length of the polymers. The simplest theories assume the molecules between the cross-links adopt random configurations with an equivalent segment length (l) being the length of a segment of a hypothetical molecule of the same total length (L) as the real molecule, which would ensure its end to end distance (r) is that predicted by that of a simple random walk. The stiffness, G, of the rubber is then given by G ≈ (3kT)/(2Ll) and the maximum extensibility λ_max by λ_max = n^1/2, where n is the number of segments in the molecule. Thus, a gel made from a more rigid molecule will have a longer equivalent segment length and, thus, for the same cross-link density will be correspondingly stiffer and less extensible. Similarly, for the same system if the cross-link density is lower the gel will be softer and more extensible. A further complication is that not all cross-links are stable. In some gels they are labile, so that the gel may flow to some extent and if broken has the chance to reform.

4.3.5.2. Methods of Gelation. There is a very wide range of possible gelation mechanisms depending on the type of junctions or cross-links and their relative stability. We can divide the junctions into two broad categories: chemical and physical. Chemical junctions are irreversible, while physical junctions can usually be undone as easily as they are made.

Examples of gels with chemical junctions in food are well illustrated in the cooking of eggs. Both the egg white and the yolks can form gels as they are heated. In both cases the covalent cross-links are created between the proteins once they have denatured. Both the denaturation and the cross-linking are thermally activated processes with different activation temperatures for different proteins. Egg whites will form a gel due to the denaturation and cross-linking of the albumin proteins at temperatures above ca. 52 °C, while the proteins in the egg yolks require a higher temperature (>ca. 58 °C). Conveniently, this provides a method of preparing perfect soft boiled eggs: simply place the eggs in a temperature-controlled water bath for a long time, enough for them to reach thermal equilibrium (∼30 min) at a temperature above that at which the albumin will cross-link but below that at which the yolk proteins do so.

However, most of the gels we encounter in the kitchen are physical rather than chemical in nature. The most common is probably the gelatin gel. Despite the common nature of gelatin gels the details of the mechanisms by which the individual gelatin molecules become associated is still at best poorly understood.^235,236 It seems that most if not all junctions involve several molecules and may be semiordered. It is probable that there are actually a range of different types of junctions, simple molecular entanglements, regions where pairs of molecules interact via perhaps mutual intertwining, and zones where several molecules come together to form semiordered structures. Further, not all gelatins are equal; they come from different sources and have widely differing molecular weights, so the range of possible junction types may differ significantly between products. Overall, this leaves a complex situation for the chef. The minimum concentration of gelatin required to make a system gel depends on the ability to form a three-dimensional cross-linked network; so, shorter (lower molecular weight) molecules need higher concentrations. Thus, the gelatin gels can form and melt over quite large temperature ranges. For example, the concentration of gelatin appears to affect the melting temperature, more concentrated gels tending to have higher melting ranges. However, other properties of the solution (e.g., pH) can also affect the propensity for junctions to form as can the temperature. Some junctions only form at comparatively low temperatures, and others form very slowly, so a gel can change its properties on storage, usually by increasing the cross-link density, hence both making the gel stiffer and less extensible, in food terms providing a harder tougher gel. The only hard and fast rules are that the more gelatin used the stiffer the resulting gel will be.

Other gel-forming food molecules use a range of mechanisms to create the junctions. These include simple entanglements, electrostatic forces, e.g., the use of counterions to bind specific sites (alginates), local precipitation caused by pH changes, and crystallization.^237–248

In recent years a wide variety of gelling agents have found new uses in gastronomic restaurants. Here we give just a couple of examples. First, work at El Bulli by Ferran Adria using the fact that alginates can be made to gel simple by changing the counterion environment led to the process that has become known in gastronomic circles as spherification.(200) The process can be used to make, for example, small spheres with a tough outer skin and a liquid center that look like and have a texture similar to caviar photographs of some examples, and the process can be found in A day at el Bulli,(249) but which have any chosen flavor. An alginate solution with the desired flavor is prepared and then dropped into a water bath with a suitable solution of a calcium salt, as the solution falls so it forms into small spheres, as these fall into the calcium solution the outer layer gels quickly to create the “caviar”. One example of the use of this technique in the restaurant is the spherical green “olives” served on a spoon at El Bulli.(201)

Another application of the properties of gelling agents is to exploit the fact that they can be quite temperature resistant; unlike gelatin gels which typically melt around 30−40 °C, some gels such as agar can have a melting temperature up to almost 100 °C. Such gels have been used as flavored layers in hot dishes to keep different foods apart. However, perhaps the most spectacular use is the flaming sorbet invented at the Fat Duck.(62) Here a sorbet is doped with a suitable gelling agent (pectin) so that it will keep its shape even as the ice melts. Such a sorbet can then be flambéed at the table, providing a sorbet that is truly hot on the outside and completely frozen in the middle.

4.3.5.3. Gel Properties. Gels may be characterized by their main properties, e.g., hard, elastic, brittle, fluid, etc. Most of these properties are inherent to a particular system; within a particular system the degree of cross-linking can sometimes be controlled (up to a limit), and the concentration of the gelling agent can be varied. These two variables usually provide control over the stiffness of the gel but not over whether it is “brittle” or “elastic”; such properties are determined by the rigidity of the gelling agents themselves.

There is a wealth of gelling agents now available to the cook. Thus, it is possible to make gels that retain strength even when very hot (using, for example, agar as a gelling agent) as well as gels that form “crusts” on small liquid drops (using, for example, alginates that gel under changes in counterion content). The possibilities are almost endless. A list of readily available gelling agents that are approved for food use is provided in Table 10 together with some notes on the type of gels they produce and the conditions under which they form gels.

It is unfortunate that the precise conditions under which any gelling agent will form a gel always depends on the molecular weight of the specific material, but manufacturers rarely supply this vital information, so that the chef is usually reduced to carrying out some trial and error experiments to establish the best conditions for each specific application.

A particularly interesting class of gels are those formed by starches; these not only can be made in the kitchen but more often occur inside foods while they are being cooked. In the following section we will look at the structure of starch granules and how they form small gel particles when they are swollen and heated.

4.3.5.4. Swelling Starch, Including Potato, Rice, and Flour Cookery. Starch is formed by many plants in small granules; a typical granule may be a few micrometers across. Within a granule the plant lays down successive rings, each with a higher or lower proportion of amylopectin.(224) In rings with a lower amylopectin content the molecules are packed close together in a well-ordered form, making these parts of the granules more resistant to attack from enzymes; such layers are often referred to as “crystalline”. The granules are not purely amylopectin and amylose; the plants also incorporate some proteins as they make the granules. Importantly, different plants (and different varieties of the same plant) incorporate widely differing amounts of protein in their starch granules.

The amount of protein and where it is located in the starch granules is crucial when cooking with starchy foods. Cold water added to starch granules will be absorbed by the proteins but hardly penetrate the amylose and amylopectin. Accordingly, high-protein granules absorb significant amounts of moisture at room temperature compared to low-protein starches.

Water absorption can be important as it affects greatly how the starch granules are used. If there are sufficient proteins around the outside of a starch granule and they absorb enough water they can bind granules together. Once a large group of granules become bound, those near the center are unlikely to be further swollen by any additional water; this can be the origin of “lump” in sauces, etc.

While cold water will not greatly affect the amylose or amylopectin in a starch granule, hot water certainly will. When starch granules are heated the ordered “crystalline” layers start to melt as the temperature exceeds 60 °C. The actual melting temperature depends on the relative amounts of amylopectin and amylose and on how well the amylose molecules pack together to form small crystals inside the granules. This disordering and opening up of the structure of the granules allows water to penetrate. The linear amylose molecules are quite soluble in the water and the branched amylopectin less easily dissolved. As the molecules overlap with one another to a significant degree they do not fully dissolve in the water but rather form a soft gel. Starch granules can absorb an enormous amount of water without losing their integrity; this is one of the reasons why they are such good thickening agents. For example, potato starch granules can swell up to 100 times their original volume. This swelling provides the thickening effect of starches used in the kitchen.

The thickening of a sauce by ingredients such as flour or corn starch is an example of the gelatinization of starch granules into an optimal swollen state. Overcooking the sauce can result in a complete disintegration of granules, thereby releasing amylopectin and amylose into solution with the consequence of an undesired thinning of the sauce. Cooling a suspension of gelatinized starch results in gels where crystalline regions serve to link the amylopectin and amylose into an overall nonliquid structure. Further storage of such a gel can lead to the, usually undesired, so-called retrogradation, which is caused by recrystallization of amylopectin into a thermodynamically more stable form. The gel expels water and becomes denser and more heat stable during this process.

The baking of breads involves gelatinization of starch granules; during the cooling and initial storage of the fresh bread the amylose partly crystallizes and transforms the doughy texture of the crumb of very fresh or still warm bread into the more desired texture of a fresh bread. Further storage will lead to crystallization of amylopectin and formation of stale bread, which has a dry and hard sensation despite the fact that staling does not necessarily imply the loss of water.

Potatoes provide an excellent system to see what can be done with a starch gelling system. One can simply cook the potatoes and then mash them (mechanically break the major structures) so as to allow the starch molecules to absorb whatever liquid is present; typically potatoes can easily hold more than three times their own weight of additional liquid while still remaining firm enough to be eaten with a fork. Thus, it is possible to create a wide range of flavored potato dishes; traditionally fats such as butter or milk are often added to provide a creamy texture and flavorants such a garlic to provide a suitable savory note. However, in practice it is possible to use more or less any liquid to make mashed potatoes, so the water from red cabbages provides a distinctly flavored pink mash or the use of a dark beer provides a slightly bitter and malty tasting brown mash.

However, if the water content is low enough then, as we have seen previously, the starchy component can become glassy; then we have a crisp texture, as in chips (crisps in the United Kingdom). A further refinement is to create a glassy texture on the outside, leaving a smooth creamy gel in the interior. This can be done in heavily processed dishes such a croquette potatoes, where mashed potato is rolled into suitable shapes and then fried to remove water from the outer layer, or in pieces of potato where no additional liquid has been added, such as French fries (chips in the United Kingdom). The key to crisp fries is to prevent water from the interior gelled starch migrating to the outer glassy layer and so reducing its glass transition temperature, rendering it soggy. A remarkable solution, pioneered at the Fat Duck, is to begin by cooking the fries in water until they are just about to break up; they will have absorbed some water during this stage and will form firm gels on cooling. Next, the cooled fries are dried by placing them in a vacuum desiccator; during this stage the outer layers become quite dry to the touch but of course still retain significant amounts of water. These fries are then put in oil at a temperature of around 110 °C to boil off the water in the outer layers and turn them glassy. Then, in a third stage the fries are put into hotter oil at ca. 190 °C; in this stage the glassy outer layer is temporarily softened and as steam is generated from the gel within it pushes much of the outer now softened glassy layer away from the interior, leaving a distinct puffed up nature to the fries. On subsequent cooling and serving the small air gap between the interior and exterior slows down diffusion of water, ensuring the crisp texture remains for at least as long as it takes the diner to eat the fries.

A similar interesting phenomenon is the popping of cereal grains; we are all familiar with popcorn. On heating, the outer layer of the corn, which is initially glassy in nature, softens to become rubbery. At the same time, pressure builds up inside the grains as some water in the slightly swollen starch turns to steam. If the balance is right, then there is sufficient pressure inside the grains to make them explode just as the glassy outer layer is soft enough that it can no longer withstand the pressure. The key to the process is to ensure there is enough water in the interior of the grains to generate the pressure and little enough water in the outer layers that they remain glassy until the temperature is high enough to convert enough water to steam to allow the grains to “pop”. More or less any grains can be popped if they are initially cooked in water to increase the water content at the center and then dried in a cool oven (at around 50 °C) to reduce the water content of the outer layers while leaving the interior well hydrated.

4.3.6.1. Denaturation of Protein and the Associated Textural Changes. At the start of cooking meat has a flaccid feel. During the cooking process the most obvious changes are the shrinkage in muscle volume with a consequent loss of fluid and development of a rigidity absent in raw meat. The texture changes in the meat are related to all stages in the denaturation of the fiber and connective tissue proteins. The myosin begins to coagulate at about 50 °C, which gives the meat some firmness. The myosin squeezes out some of the water molecules which are further squeezed out of the cell by the sheath of connective tissue. In steaks and chops the water also escapes out of the cut ends of the fibers. At this stage meat is firm and juicy. At around 60 °C more of the proteins inside the cell coagulate and the cells become more segregated into a solid core of coagulated protein and a surrounding liquid. In this temperature range the meat progressively gets firmer and more juicy. At 60−65 °C the denaturation of the collagen in the connective tissue happens. The collagen shrinks and forces the liquid out of the cells. At this stage the meat releases lots of juice, shrinks, and becomes more chewy and dry. A continued cooking leads to dryer and continuously more compact meat. However, at around 70 °C the connective tissue collagen begins to dissolve into gelatin and the muscle fibers are more easily pushed apart. At this stage the meat seems more tender, although the fibers are still stiff and dry because the fibers do not any longer form a substantial mass and because the gelatin provides a juiciness on its own.(253)

4.3.6.2. Meat Tenderness and Appropriate Cooking Methods. The ideal cooking of meat would minimize moisture loss and toughening of the fibers while also maximizing the conversion of collagen to gelatin. In ordinary kitchen practice this is difficult, and hence, the cooking method is usually adjusted to the meat’s tenderness.(253) The main difference between tender and less tender cuts of beef is the relative quantity of connective tissue. Tender meat has a small and the less tender a large proportion of the connective tissue. When cooking meat with a high content of connective tissue the point is to bring about hydrolysis of collagen and leave the fibers free to fall apart, in which case the meat appears to be tender. This can be accomplished by cooking the meat in the presence of water. If a little acid is added, the hydrolytic process is accelerated. Steam is even more efficient than water, and if it is under pressure so that the temperature is above that of boiling, hydrolysis is brought about rapidly. The methods using water or steam include braising and stewing. The dry-heat methods of broiling and roasting are used for the tender cuts since these presumably have so little connective tissue that none of it needs to be removed to make the meat tender.(254)

Cooking of tender meat can be a challenge since the desired temperature range is very narrow and it is difficult to obtain a uniform temperature in a piece of meat. When frying or grilling meat at a high temperature, a temperature gradient from the outside to the center will be present, which means the meat will dry out on the outside before reaching the desired temperature at the center. By using a long cooking time at a lower temperature this problem can be eliminated; however, a high temperature is needed for the desired browning reactions on the outside. This issue leads to the common kitchen procedure of heating the meat at a high temperature for a short time to obtain the browning (Maillard) reactions and finish the cooking at a lower temperature. The cook can also remove the meat from the heat before it is fully cooked and rely on the afterheat to finish the cooking more gradually. The ideal cooking time is affected by a number of factors such as the meat’s starting temperature, the cooking temperature, flipping of the meat, fat content of the meat, and the extent of bones in the meat. Hence, there is no way of making a precise prediction of the ideal cooking time.(253)

4.3.6.3. Effect of Cooking Conditions on the Texture of Meat. In order to determine the optimum conditions for cooking of meat scientifically it is necessary to understand the heat-induced processes like the denaturation of the various proteins and the textural consequences, as described above. Several studies have investigated the effect of time and temperature on meat tenderness and the textural changes.^255–257 Differential scanning calorimetry (DSC) is being used to investigate the denaturation of proteins by measuring the energy input when heating up the meat sample. The Warner Bratzler method, which measures the shear force when cutting meat, has often been used to determine the tenderness objectively. However, the results do not often correlate to a sensory evaluation.(258) Nevertheless, the shear force is a common method for meat quality assessment.

4.3.6.3.1. Detailed Description of Processes during Heating. An increase in shear force with increasing temperature occurs in two distinct phases. The first occurs at 45−50 °C and has been associated with denaturation of myofibrillar proteins (actin and myosin). Denaturation of actinomyosin leads to a release of tension and fluid is forced out of the space between the endomysium and the denatured myofibrils, accounting for the observed loss of fluid at these temperatures. The higher temperature (65−70 °C) increase in shear force is attributed to shrinkage of the perimysial collagen. The fibers are denatured and may be seen to change from an opaque inelastic fiber with characteristic banding pattern to a swollen fiber with elastic properties. The extent of shrinkage and loss of fluid depends on the nature of the intermolecular cross-links, which stabilize the perimysial collagen fibers. For this reason the extent of shrinkage is greater in older animals. Heating for prolonged periods at temperatures above 70 °C eventually causes a reduction in shear value, probably due to cleavage of peptide bonds in the molecule. The residual strength of fibers binding the muscle together contributes to toughness of the meat, in addition to the tension generated by the thermal shrinkage of perimysial collagen. Overall, the mass of the meat depends on the amount of denatured myofibrillar protein, while the texture is mainly determined by the collagen fibers of the perimysium. Two effects are involved: compression of the muscle bundles during collagen shrinkage and binding of the muscle bundles due to the residual strength of the denatured collagen fibers. In each case the effects are determined by the nature and extent of collagen cross-linking.(259)

4.3.6.3.2. Texture Changes Due to Denaturation (Time and Temperature). Martens et al.(257) investigated the texture changes for beef during cooking. Table 11 shows the results interpreted in terms of thermal denaturation of the three major proteins.

From Table 11 it can be seen that both myosin and actin denaturation results in increased firmness, bite-off force, and total chewing work (usually referred to as toughness). Actin denaturation also results in increased amounts of residual bolus and has a negative effect on juiciness and total texture impression (the meat becomes drier). Denaturation of collagen leads to reduced fiber cohesivity, bite-off force, residual bolus, juiciness, and total chewing work and increased total texture impression (in everyday terms, tenderness). This means that to achieve the optimal eating experience the meat should be heated to a temperature where collagen is denatured but actin is still native, i.e., between 62 and 65 °C.(257) The rate of actin denaturation is dependent on time as well as temperature; at 66 °C denaturation of 10% of the actin is accomplished in approximately 10 min, 50% denaturation in approximately 40 min, and 90% denaturation in approximately 100 min.(257)

Tornberg(260) suggests that the denaturation of sarcoplasmic proteins has a big impact on the texture of the meat, as the denaturation below 50 °C causes increased toughness of the meat but in the temperature range 50−65 °C makes the meat appear more tender as these proteins form a gel in the space surrounding the fibers and fiber bundles. At temperatures above 65 °C this gel is much firmer and leads thus to a less tender texture.(260) This is in accordance with the results found by Martens et al.(257)

4.3.6.3.3. Pressure. Ma and Ledward(261) studied the effect of pressure (0−800 MPa) on the texture of beef muscle. They found that a pressure of 200 MPa caused a large significant decrease in hardness, chewiness, and gumminess (evaluated by texture profile analysis: Stable Micro System Type) in the temperature range of 60−70 °C. By using DSC the texture changes were further analyzed. Collagen was, as expected, inert to pressure, whereas myosin was unfolded by both pressure and temperature. From the results obtained it was concluded that the structurally induced changes are unlikely to be a major cause of the significant loss of hardness observed when beef is treated at 60−70 °C and 200 MPa. Instead, accelerated proteolysis under these conditions is suggested.

4.3.6.3.4. Heat Transfer. The heat-induced processes, as described above, all depend on the heat transfer in a piece of meat when cooking. Some studies have worked on the modeling of heat transfer in meat. Most studies are mainly concerned with the safety issue (i.e., when does the meat reach 75 °C in the center^262–266), but a few studies have been conducted on the impact of heat transfer on the textural properties.^267–270

Modeling heat transfer in meat is an immensely difficult task. Meat is seldom uniform in size or shape; it consists of several fractions of protein each with their own thermal properties, and a large number of processes occur during heating. These processes will cause changes in the thermal properties, e.g., changes in heat capacity, thermal conductivity, etc., and changes in dimensions, water-holding capacity, etc. Furthermore, water is transported in the meat during heating, which leads to transport of heat and, if the temperature is high enough, evaporation of water at the surface. To complicate things further the fiber direction also plays a role in the heat transfer. However, understanding the basics of heat transfer in meat (and using a thermometer when cooking) can lead to improved gastronomic quality of cooked meat, as this will reduce the risk of overcooking.^271–273

Califano et al. simulated the heat transfer during the cooking process of beef and the related textural changes. They propose a model for the cooking process as a tool for analyzing the effect of cooking operations on the texture of meat. The effect of heat transfer coefficient, temperature of the cooking medium, and size of the meat piece on the hardness (by the Warner Bratzler method) of the cooked meat was studied. If the cooking medium was above 85 °C the average hardness was high regardless of processing time. Conversely, a low temperature gives a more tender product.(267)

4.3.6.3.5. Marinating Meat: Softening. Marinating, immersing meat in a fluid medium, has been used for many years to flavor meat as well as tenderize it in the domestic kitchen. Marinating of meat can be considered as a chemical method to help tenderize meat, which also changes the flavor of the meat. The literature on this topic is however driven mostly by the demand in the food industry to tenderize meat. Marinade solutions can improve the perceived juiciness and tenderness of the meat as well as increase the weight of the product. However, there is a particular problem with marinating meat in that the marinade penetrates the meat very slowly and hence only works on the outer layers. This problem is sometimes overcome by injecting the marinade into the meat.

Several studies have investigated the properties of tenderizing agents, focusing on obtaining the optimal juiciness and tenderness without causing any undesirable effect on color or flavor. The mechanisms for increased tenderness and juiciness are generally connected with higher water holding capacity (WHC) and swelling of myofibrils.

Acids, like vinegar, lemon juice, or wine, are very common ingredients in a marinade. Sour marinating of meat has been found to improve tenderness and juiciness and increase the weight of the product due to retention of water. However, sour marinating is also found to affect the flavor, giving an unpleasant sour taste.^274,275 The mechanism behind the tenderizing action of acidic marinades is shown to involve increased proteolysis and increased conversion of collagen to gelatin.(276) The pH value is important to the swelling capacity and hence the WHC of meat. Both low and high muscle pH after marinating have positive effects on texture and give an increase in water binding capacity and swelling of myofibrillar protein.^277,278 As the pH moves further from the isoelectric point, the water-holding capacity increases due to an increase in the amount of negative charges on the meat proteins that can bind water.^279,280

Since an increase in WHC on either side of the isoelectric point is seen on the myofibrillar protein, alkaline solutions also have tenderizing properties. Alkaline marination is a common method of tenderization in Chinese cookery,(281) and it is likewise commonly used as a marinade in household cooking in India.(282) A study by Hsieh et al. in 1980 showed that that bicarbonate treatment caused swelling and fusion of the myofibrils and obscured the structures within the sacomer. Although alkaline marinating generally has been overlooked in the West, recent work has focused on using bicarbonate to minimize the problem of pale, soft, and exudative (PSE) pork meat.

Wynveen(283) found that postmortem injection of sodium bicarbonate and sodium phosphate improved WHC and color in pork. Likewise, Yang(284) found that postmortem injection of sodium bicarbonate in meat gave an increase in WHC and solubility of myofibrillar protein as well as decrease in drip loss, weight loss during cooking, and shear force. Sheard(285) compared the effect of sodium bicarbonate with that of salt and phosphate, which are also common tenderization agents in the food industry. All solutions gave a significantly higher yield and a decrease in shear force. The effect of sodium bicarbonate on shear force and cooking loss was as great as salt and phosphate. However, the bicarbonate-treated samples contained air-filled pockets between fibers, giving an unusual appearance that might not be appreciated by consumers. This problem might be caused by generation of carbon dioxide produced during cooking, which is likely also to be a contributing factor for the texture change. A recent study by Anna et al.(286) investigated the synergistic effect of sodium bicarbonate and blade tenderization on tenderness of buffalo rumen meat. They found that sodium bicarbonate gave improved tenderness based on analytical and sensory analysis.

The alkaline marinating techniques used in some places in Asia can be compared to the traditional Swedish/Norwegian way of preparing dried fish (lutefisk).(166) Drying was a common way of preserving fish (it can keep for years in Arctic climate), and preparation of lutefisk was a primitive way of reconstituting the moistureless tough fish. Lutefisk is made by soaking the dried cod in fresh water for a week, then submerging it in a alkaline solution for several more days, and resoaking it in fresh water for another 2 days.(287) Originally the ashes from a wood fire (rich in carbonate and minerals) or lime stone (calcium carbonate) and later lye (sodium hydroxide) was used.(166) The fish becomes very soft after cooking, which can be explained by the fish proteins accumulating a positive charge in the alkaline solution. This causes the proteins to repel each other, and only weak bonds are formed between the muscle fibers, giving a soft texture.(166)

Another chemical way of tenderizing meat is by using enzymes. For many years the Mexican Indians have known that the latex from the papaya leaves has a tenderizing effect on meat during cooking. The major enzyme, papain (a cysteinyl-proteinase), has been investigated. It has little effect at room temperature, so the main action occurs while cooking. Above 50 °C the collagen structure is loosened and vulnerable to attack from the proteinase;(288) unless the enzyme is deactivated it will continue to attack the collagen until none is left and the meat has fallen apart. Ashie et al. in 2002(289) compared the use of papain and an aspartic protease (expressed by Aspergillus oryzae) for their tenderizing effect on meat.(290) They found that aspartic protease has an advantage over papain by having a limited specificity on meat proteins and being readily inactivated by cooking. Additionally, the use of proteases from kiwi fruit,(291) bromelain from pineapple, and ficin from figs(292) has been reported.

4.4.1.1. Use of Heat. Many cooking methods or gastronomic unit operations involve heating the food material to induce chemical and/or physical changes that favor the development of pleasant flavors and textures and create microbiologically safe food. However, we also often heat food simply so as to serve a hot meal. Gastronomic foods are often served above room temperature (for example, warm salads) even when there are no specific physical, chemical, or microbiological reasons for the heating. However, heating the food enhances the release of aroma compounds, changes the perception of the food, and can create the direct sensation of heat.

During heating the elevated temperatures will shift the thermodynamic stability of food components and make chemical changes and especially phase transitions possible (e.g., melting, evaporation, gelatinization, denaturation of proteins). Through such transitions the food itself can be transformed: plant cell walls can be broken, making vegetables soft, meat protein can be denatured, rendering it tougher, etc. For the gastronomic cook it is important to be aware of the range of possible transformations and their impact on the flavor and texture of the food being heated.

Elevated temperatures change the rates of chemical reactions; chemical compounds that are otherwise metastable may become unstable, and severe changes may take place within the time scale of the cooking process (minutes to days). For example, proteins may be hydrolyzed into peptides and amino acids during the cooking of stocks, Maillard reactions will take place during the frying process at elevated temperatures, etc.

The phase transitions that occur in foodstuffs are mostly strongly endothermic. The demand for significant amounts of heat can in turn influence the cooking process by perturbing the dynamics of transport of heat and moisture. Since most foods have rather high water content, the evaporation of water is a particularly important phenomenon for understanding most cooking methods.

Water has a particularly high specific heat (heat capacity), 4.19 kJ/K. Accordingly, the energy needed to heat 100 mL of water from room temperature to (normal) boiling point is about 31 kJ. However, a much more important issue is the high latent heat of evaporation of water (2.26 MJ/kg). Thus, the energy needed to evaporate 100 mL of water is about 222 kJ; about 7 times more heat is needed in order to evaporate water as compared to heating it from room temperature to boiling temperature. This large heat requirement is one of the more important limiting factors in the kitchen, especially when scaling up recipes.

The temperature distribution in food during preparation can be strongly influenced by evaporation phenomena. Since many food ingredients are very moist their thermal behavior can be modeled, to a first approximation, by the behavior of pure water. In cooking operations where the surface temperature can exceed 100 °C (e.g., frying) a dry crust has to be developed in order for the local temperature to be above that of boiling water; the loss of water enables high temperature and accompanying acceleration of chemical reactions such as the Maillard reactions specific to the frying process.

The presence and evaporation of water affect the transport of heat through foods as they are being cooked. In particular, they tend to reduce the rate at which the temperature at the center of foods being cooked increases and thus also increase temperature gradients within the food.

4.4.1.2. Heating Methods.4.4.1.2.1. Boiling. Boiling (heating in boiling water) is perhaps the simplest of all kitchen techniques. For most vegetables we can assume that the temperature is close to 100 °C (of course, addition of salt or changes in atmospheric pressure will affect the boiling temperature, but such changes are generally small). We can then treat the cooking process as a relatively straightforward heat transfer problem with the boundary conditions of a uniform constant temperature (100 °C) at the food surface. Thus, cooking times will be reproducible.

The time required to cook different foods will vary greatly; the actual temperature required will differ between foods. In many cases, food is cooked for a short time in boiling water, creating a significant temperature gradient across the food. Consider, for example, green beans cooked for 3 or 4 min. The surface temperature will be 100 °C, while the center will have a temperature of only around 30 °C. As the food is left on a plate heat continues to flow to reduce (and eventually remove) this temperature gradient.

In some foods we wish to ensure that some physical or chemical process proceeds. In such cases the time scale of the cooking process may be much longer than that of heat transport, leading to fewer temperature gradients within the food during the cooking process. Examples where longer cooking times are used include the need to gelatinize starch in cooking of rice (and to a lesser extent potatoes) and hydrolysis of proteins, which is required to provide the flavor of stocks.

Some complete dishes are prepared in boiling water. We can consider stews and some sauces simply as a dilute solution with solutes of low molecular weight (salts, amino acids, etc). The boiling takes place close to the normal boiling point of water (any boiling point elevation due to the dissolved salts is of no practical importance). The temperature will remain uniform as long as there is no accumulation of solid material at the bottom of the pan near the heat source, in which case it can start to overheat and “burn”; stirring is required to prevent this from happening.

In the kitchen there are many different ways to describe boiling water. The main differences are simply in the rate of heat input. The minimum heat input is that which will just maintain the temperature at 100 °C with the minimal amount of evaporation of steam. As the heat input is increased, so the rate of creation of steam and the rapidity of bubbles rising to the surface increases; at some heat input the flow caused by the bubbles becomes rather chaotic, leading to violent movement and mixing of food in the pan.

Gentle boiling with minimum heat input, just enough for ensuring the boiling of water, is usually termed “simmering”. Some chefs claim that simmering causes less toughening of meat than more “rapid” boiling, although to our knowledge there is no actual evidence for such an effect. However, it may be that the heat input in these cases is not sufficient to maintain the water at boiling point and the actual temperature was significantly lower, in which case a more tender product could be quite possible.

At the opposite extreme with a high heat input chefs often say water is at a “rolling boil”, indicating the large-scale movements in the water. If food is added to a pan at a “rolling boil” the heat input may be sufficient to avoid a significant temperature drop as the cold food has to be heated, thus reducing the cooking time, and the violent movement of the water may ensure the food is kept moving and prevented from sticking to itself. However, there is no literature where any clear distinctions are drawn, and personal experience suggests these are not important effects.

4.4.1.2.2. Steaming. In the steaming process the food (such as vegetables, fish, or bread dough) is in contact with steam above boiling water. Heat is transferred to the food as the gaseous water condenses at the food surface and releases its latent heat. The steam is thus a very effective medium for transferring heat as opposed to hot air at the same temperature (as seen in the baking process). One could consider steaming as the opposite of baking, condensing rather than evaporating water at the food surface. No crust is formed, and the food is not dehydrated, which creates very different results. In steaming the surface temperature is 100 °C due to the equilibrium between the two states of water.

Steaming thus resembles boiling and simmering in many respects, although some differences may be important. The food is not immersed in water, so that probably fewer soluble compounds are lost from the food. Steaming is generally believed to create textures with a better bite and is a more gentle form of heating compared to boiling.

4.4.1.2.3. Frying and Deep Frying. In frying a much higher temperature is used than when cooking in water. In deep fat frying the oil temperature is usually around 160−180 °C. The surface temperature of food is thus well above the boiling point of water, and rapid boiling occurs. This boiling requires a large amount of heat, so the temperature of the cooking oil normally falls rapidly, leading to a reduction in this surface boiling. In practice, the amount of food put in a deep fat fryer will greatly affect the way in which it cooks. If too much food is put in at one time the temperature is reduced below 100 °C and the food will not become “crisp”.

Shallow (or dry) frying uses a high (surface) temperature (typically 200 to 240 °C) and, usually, some frying oil to ensure a good heat conduction. The surface temperature is (unlike boiling) controlled by a balance between the heat needed to evaporate water and the supplied heat from the stove. Unlike the boiling of water, there is no phase transition to control the temperature of frying oil or the pan surface. The temperature remains nonuniform throughout the frying process due to evaporation of water.

Two main transport processes take place: heat conduction from the hot surface toward the center and transport of water toward the surface where water is evaporated (of course, it can evaporate from any surface, but most evaporation will come from the hot surface in contact with the pan). A dry, hot, and relatively well-defined frying crust is built up. Just below the crust the temperature is expected to be close to 100 °C due to evaporation of water. Thus, a large temperature gradient is created over the frying crust. The temperature of the crust surface is comparable to the frying oil. If the temperature in the crust is high enough, browning reactions can take place at considerable rates.

The temperature at the center of the food item will however only slowly increase with time as heat is conducted inward. For example, in the case of steak frying, the process is terminated when the temperature has reached 40−60 °C, giving the center an appropriate red color. This can take (for a typical 3 cm thick steak) between 5 and 12 min.

During heating the food (especially meat) releases considerable amounts of water (due to changes of meat at temperatures above the denaturation temperature of muscle proteins). The energy input must match this release in order to quickly evaporate all the water so as to keep the surface temperature of the food well above 100 °C. Too little power will result in a cooking/broiling process where the surface temperature will be close to 100 °C, giving rise to less browning and a different flavor formation. It is often forgotten by cooks that they should not attempt to cook too many steaks (or brown too much meat) at a time. The amount that can be cooked is limited by the power of the burner used on their stove. The power of the burner must be significantly greater than that required to boil off the water that is being released from the cooking meat. This problem is particularly important when scaling up recipes. A recipe that works well when cooking for four may completely fail when doubled for a dinner party of eight all because the pan temperature cannot get above 100 °C with the increased amount of meat.

The heating power of the burners on modern domestic stoves is typically up to 2.5 kW, while restaurant stoves can often provide up to 8 kW. The power limits the amount of meat that can be browned at a time; for a domestic stove the limit is around 800 g, and for a commercial stove it is nearer 2.5 kg.

In a wok or when using any continuously stirred process the surfaces in contact with the pan will heat up briefly. The surfaces facing away from the pan will quickly be cooled down due to evaporation of water. The average surface temperature during stir frying is thus quite low (below 100 °C). Heat conduction into the vegetable is thus quite slow and gentle despite the high surface temperature of the wok. The center temperature of the end product will be low compared to a boiled vegetable; this will ensure less transformation of the cell wall carbohydrates, etc. However, the vegetables are subjected to severe dehydration. The combination of gentle heating and dehydration gives the product its texture or bite. In meats, the Maillard reactions and pyrolysis at the surface ensure development of taste and aroma compounds.

4.4.1.2.4. Baking. In a baking oven the air temperature is kept fixed in the range 150−250 °C by a thermostat system. Air circulates the oven due to convection or forced circulation. Confusingly, some manufacturers (particularly in the United States) refer to ovens where the air is circulated using a fan as “convection” ovens.

Oven surfaces and heating elements may have much higher temperatures so that some radiant heating can also occur.

(a) Leavening of Cakes, Breads, and Soufflés. The expansion of various baked goods during the heating can be created by several kinds of endothermic processes where the thermodynamic equilibrium is shifted by the increasing temperature. For example, in the endothermic decomposition of baking powder into gaseous compounds carbon dioxide is formed from sodium bicarbonate (baking soda) or carbon dioxide and ammonia are created from ammonium bicarbonate. Dissolved gases can be released and water can evaporate. At elevated temperatures the equilibrium of these processes will be shifted toward the formation of more gaseous compounds, so that the baked goods rise due to an increase in the gas content.

Industrially, but not so much in home cooking, leavening acids are used. These leavening acids (such as sodium aluminum sulfate, dicalcium phosphate dihydrate, potassium acid tartrate, or δ-gluconolactone) enable the decomposition of bicarbonate at lower temperature and prevent the pH elevation and soapy flavors due to formation of carbonate ions. The action of leavening acids is kinetically determined and can be classified into slow, medium, and fast reacting.

(b) Role of Water Evaporation for the Expansion of Bread Cakes and Soufflés. The dough or soufflé base is very moist with water activity (relative humidity) close to 1. Therefore, in many respects they can be treated as if they behave like water. The doughs belong to the colloidal category of foams containing many small air bubbles. The air is trapped within a soft material and is thus approximately subjected to pressure very close to the external pressure, which usually is close to 1 atm. The entrapped dry air can be a result of mechanical treatment of the dough, leavening using yeast, or leavening using baking powders. In the case of soufflés since no yeast or baking powder is used, evaporation of water is particularly important and the main cause of the rising of the soufflé.

During the baking process the macroscopic transport of heat and moisture over distances comparable to the cake/bread/soufflé size are rate limiting and responsible for the overall duration of the baking process. On a much shorter time scale we can expect water to evaporate into the air bubbles of the dough in order to attain a local equilibrium between gaseous and liquid water. Some water will be lost from the surface of the soufflé, but the inside will remain a very moist structure. It is reasonable to assume the pressure of water will attain its equilibrium-saturated value inside the bubbles.

At elevated temperature the saturated water pressure will increase and ultimately approach the external pressure (1 atm at 100 °C). Likewise, the molar and volume fraction of water in the gas phase increase (ultimately to 1). Due to the dilution effect of the gaseous components already present (N₂, O₂, Ar, etc.) a large amount of water has to evaporate, making the overall gas phase expand. Under these somewhat simplified assumptions the gas volume will in principle diverge when the equilibrium partial pressure of water approaches the external pressure (at the boiling point temperature of water) where V_g(T₀) is the volume of dry gases at low temperature (T₀), p_w^o(T) is the temperature-dependent saturated vapour pressure of water, p_ex is the external pressure, and T is the absolute temperature.

The total gas volume is proportional to the initial volume of dry gases. The expansion mechanism due to water will only work if other gaseous components are present. In other words, evaporation of water will boost other leavening mechanisms. In a soufflé, the chef beats in air in the egg white before cooking; during baking this increased air volume is further boosted by evaporation of water. The normalized gas volume as a function of temperature for an idealized model air/water soufflé is shown in Figure 17. The trivial temperature expansion of the dry air contributes in itself with an almost negligible expansion, whereas evaporation of water contributes dramatically when approaching the boiling temperature.

All the various mechanisms of heat-assisted rising of bakery products, do not work independently but mutually boost one another other. For example, the expansion due to evaporation of water will influence the equilibrium between dissolved and chemically bound carbon dioxide and gaseous carbon dioxide. The expansion due to evaporation lowers the partial pressure of carbon dioxide and thus favors release of more gaseous carbon dioxide in order to restore equilibrium. At the same time, the released carbon dioxide boosts the expansion due to evaporation of water.

4.4.2.1. Microwaves. It is the dipolar nature of water molecules that permits them to be heated by microwaves. The alternating field (typically 2.45 GHz in a domestic microwave oven) causes the dipoles to rotate; the inability of the dipoles to keep up with the field leads to the heating effect. Any dipolar material will be heated in a microwave oven, but water having the strongest dipoles in common food stuffs displays the greatest heating effect. In general, any material with hydroxyl groups will display dipolar heating in a microwave field; thus, oils, sugars, proteins, and carbohydrates can all be heated to a greater or lesser extent with a microwave oven. However, materials where the dipoles are not able to rotate (such as ice) are much less susceptible to microwave heating.

It is worth realizing that the microwave energy will always be absorbed from the outside inward (in general, the higher the dielectric constant the greater the absorption). In wet foods the typical penetration depth of microwaves is a few centimeters. The common myth that microwaves heat from the inside out probably arises from reheating foods such as jam doughnuts where the outer regions have significantly lower water content than the center so that the center can become hotter than the outside.

An interesting example of the use of a microwave oven to produce a novel food was described by Nicholas Kurti in his highly influential 1969 Royal Institution lecture(9) which perhaps can be traced as the origin of the modern science of Molecular Gastronomy. In the lecture he demonstrated the concept of an “inside out” Baked Alaska, by freezing ice cream at low temperatures, leaving very little liquid water, and placing some very high sugar content jam which retains a significant liquid content at the center; he was able to use a microwave oven so that the microwaves were hardly absorbed by the ice and passed through heating the jam center, thus producing a novel dessert with a cold outside and a hot center.

However, despite this early potential novel use of a microwave the device has not found many real gastronomic applications, perhaps due to the nonuniformity of the heating (there are always nodes and antinodes separated by a few centimeters in a microwave oven) or maybe to the fact that it is very difficult to achieve temperatures above 100 °C so that browning and Maillard reactions do not usually occur. Of course, a variety of methods have been introduced to permit browning in microwave ovens: the use of containers that themselves heat up or the use of combination ovens which have conventional as well as microwave heating.

It is instructive to note that boiling water in a microwave oven is (in certain cases) very different than boiling water in a kettle. In a conventional kettle the element (or for a kettle on a stove the base) is heated to a temperature well above 100 °C and the water starts to boil readily at this hot surface. However, in a microwave oven the water is heated directly, so that it can superheat if no bubbles are nucleated. In practice, tap water has a good deal of dissolved air which starts to come out of solution as the water is heated (the solubility of the gas being lower at elevated temperatures), and this leads to the nucleation of bubbles and allows the water to start to boil as soon as the temperature reaches 100 °C. However, in previously boiled water in which there is no dissolved air, it is quite possible for the water to superheat significantly. If this happens and you remove say a cup of reheated coffee from the microwave oven that has superheated and not yet started to boil it can at the least disturbance (for example, adding a little sugar) boil over in an explosive fashion.

4.4.2.2. High Pressure. Some useful processes in cooking, such as extruding, pressure cooking, and homogenization, use moderately elevated pressures. In these cases it is the increase in the boiling temperature of water with pressure that provides the useful effects. In this section we are concerned with the effects of cooking at much higher pressures where more interesting effects can be found.

The use of high-pressure techniques in the pressure range from 2000 to 10 000 atm, where pressure effects become significant, has been limited for technical reasons; the exploration of the technique is not new. Hite(216) used pressure as a means of microbial control in milk as early as 1899. Pressure treatment normally has no effect on covalent bonds but denatures proteins, as denatured proteins have smaller partial molar volume than native proteins owing to differences in solvation. High-pressure processed foods commercially available today included juices with noncooked flavor but with long shelf life, dried cured hams, where residual microbial contamination following slicing has been removed without heat treatment, and products such as guacamole where browning enzymes are deactivated by pressure. Equipment for high-pressure processing is becoming less costly and will eventually find its way to the restaurant kitchen, opening up possibilities for new types of dishes. Meats can be decontaminated by pressure treatment prior to shorter heat treatment such as in a wok or heat treatment at lower temperatures. Eggs can be hardened by pressure treatment instead of boiling but with preservation of the fresh egg flavor. Milk can be solidified by pressure without requiring acidification and addition of sugars, creating new pH-neutral types of deserts. The effects of pressure on fat crystallization deserve further attention for optimization of spreadability and smoothness. The drawbacks seem few except for the cost of equipment. One potential drawback is that lipids in poultry show increasing oxidation following pressure treatment and subsequent cooking.(293) It should, however, be possible to define pressure−temperature−time windows, where microbial decontamination is acceptable without pressure-induced lipid oxidation.

Pressure effects on ice are unique as pressure lowers the melting point of normal ice down to almost −20 °C around 2000 atm. Pressure-assisted freezing involves pressurizing up to 2000 atm followed by cooling to below zero. Upon pressure release, water in the subzero liquid food item solidifies all at once, not just from the outside, to form smaller ice crystals than normal methods. Pressure-assisted thawing, which has already been used to prepare of raw fish dishes from frozen fish without lengthy thawing at room temperature or thawing at high temperature, depends on an initial pressurization of the frozen fish during which the ice melts at the low temperature followed by a controlled pressure release with simultaneous temperature compensation to reach ambient pressure and temperature. The technique produces a fish which is thawed uniformly and without high-temperature domains.(294)

High-pressure techniques should find their way to the kitchen for creation of new dishes, where freshness and uncooked flavor can be combined with changed texture.

4.4.2.3. Improved Temperature Control. One area where commercial kitchens have already learned from the science laboratory is in the use of accurately controlled temperature baths. Until quite recently a kitchen “bain marie” was simply a warm water bath; the temperature might have been anything from 40 to 80 °C. Temperature control (if any) was via a crude bimetallic strip type of thermostat, and temperatures were set using a simple variable resistor, usually with no calibration at all. However, in the past decade or so many restaurant kitchens have started using recirculating water baths with modern, accurate temperature controllers (the same as those found in any good laboratory). The results are immediately noticeable. Precise temperature control permits all sorts of cooking that is not otherwise possible. For example, an egg can be cooked in water at 52 °C, a temperature where the white will set but the yolk will remain fluid; thus, perfect poached or soft-boiled eggs can be served to order with complete confidence that the product on the plate will be quite perfect.

Another use for such baths is in sous vide cooking of meats. Meat is placed in a plastic bag and sealed under vacuum to exclude any air and provide a barrier between it and the surrounding water (or other heat transfer medium). Of course, care is taken to ensure any bacteria are killed prior to the low-temperature cooking, either by quick pasteurization in a hot (85 °C) bath for a couple of minutes or by passing the flame of a blow torch over the surface of the meat before putting it in the bag. The meat is then cooked at a low temperature, where the myosin and actin proteins are scarcely denatured, but for a sufficiently long time that the collagen is slowly softened. The required temperatures and times vary according to both type and cut of meat. Thus, for example, a lamb fillet might require 90 min at 56 °C, while belly pork might take 6 h at 60 °C. The best results are found by trial and error, but chefs who use the technique find significant differences in the texture and flavor of meats cooked at temperatures that differ by as little as 1°C.

4.4.2.4. Low Temperatures. Another area where modern laboratory equipment can be of real use in the kitchen is in cooling. Freezers used in the kitchen typically can only reach temperatures down to −20 °C. However, many processes demand lower temperatures. For example, to kill some parasites, etc., in some fish it is necessary to cool them below −30 °C. Similar low temperatures are needed to prepare ices with high alcohol content and foods for freeze drying. In this context, freeze dryers are also potentially useful kitchen appliances (and are already in use at some restaurants).

Liquid nitrogen can be particularly useful in the kitchen; it not only provides quick and easy access to low temperatures but also permits rapid cooling of all sorts of foods and so prevents the growth of large ice crystals that so often damage frozen foods. Two particular uses for liquid nitrogen are to permit the easy grinding of herbs; simply mixing the herbs with some liquid nitrogen in a mortar quickly freezes them into brittle solids, and grinding with the pestle turns these into a useful powder that can be used to provide an instant hit of fresh flavor in any dish. However, perhaps the best use of liquid nitrogen was pioneered in 1901 soon after nitrogen was first liquefied by Wroblewski and Olszewscki in Warsaw in 1887(295) by Mrs. Agnes Marshall.^296,297 Mrs. Marshall was the celebrity chef of her day, giving large-scale demonstrations and selling her recipes, books, and equipment along the way. Unfortunately for us on her death all the rights to her work were bought up by Mrs. Beeton’s publishers and never again saw the light of day. However, she describes in some detail how to use liquid gases at the table to prepare ice cream. The principle is extremely simple: you just add liquid nitrogen to the ice cream mixture and stir while it freezes; it is possible to freeze a liter of ice cream in under 20 s in this way.^298,299 The advantage is that the speed is so great and the temperature so low that only very small ice crystals can form, making a wonderfully smooth ice cream.(300)

4.4.2.5. High-Power Mixing and Cutting Machines. A further group of items of modern equipment that is finding increased use in the kitchen are powerful mixing, grinding, and cutting tools. One of the most interesting of these is one that was indeed designed for the kitchen, the Paco Jet. The Paco Jet consists of a very sharp knife that rotates at around 2000 rpm and is driven slowly into a solid frozen block of food, shaving layers of thickness (ca. 1 μm) in each revolution.(301) The machine is most commonly used to produce ice creams and sorbets. It can produce particularly smooth ices by ensuring the crystals are kept very small. To ensure all the ice crystals are cut into pieces that are small enough that we can hardly detect them it is important that the block is completely solid (i.e., frozen to a temperature below the eutectic point for the solutes, sugars and proteins, present), which for most ices would mean a temperature below about −18 °C. The machine has a suitable headspace above the solid block to permit aeration of the resulting mixture, while it is still fully frozen so as to produce light ices.

Another useful high-power technique can be to use ultrasonic agitation to induce emulsification. A simple ultrasonic probe placed in a small container of oils and aqueous liquids will very quickly emulsify the mixture; provided suitable emulsifiers are present, the resulting emulsions can be very stable. This is a particularly easy way to prepare small emulsions; typical probes have diameters of 1 cm or less, so volumes of 1 or 2 mL can be prepared to order.