45 45.

21 21.

Coconut oil 86 86.

6 6.

2 2.

Palm kernel oil 81 81.

11 11.

2 2.

Palm oil 49 49.

37 37.

9 9.

Cocoa b.u.t.ter 60 60.

35 35.

2 2.

Vegetable shortening 31 31.

51 51.

14 14.

Cottonseed oil 26 26.

18 18.

50 50.

Stick margarine 19 19.

59 59.

18 18.

Tub margarine 17 17.

47 47.

31 31.

Peanut oil 17 17.

46 46.

32 32.

Soybean oil 14 14.

23 23.

58 58.

Olive oil 13 13.

74 74.

8 8.

Corn oil 13 13.

24 24.

59 59.

Sunflower seed oil 13 13.

24 24.

59 59.

Grapeseed oil 11 11.

16 16.

68 68.

Canola oil 7 7.

55 55.

33 33.

Safflower oil 9 9.

12 12.

75 75.

Walnut oil 9 9.

16 16.

70 70.

Hydrogenation: Altering Fat Saturation For more than a century now, manufacturers have been making solid, fat-like shortenings and margarines from liquid seed oils to obtain both the desired texture and improved keeping qualities. There are several ways to do this, the simplest and most common being to saturate the unsaturated fatty acids artificially. This process is called For more than a century now, manufacturers have been making solid, fat-like shortenings and margarines from liquid seed oils to obtain both the desired texture and improved keeping qualities. There are several ways to do this, the simplest and most common being to saturate the unsaturated fatty acids artificially. This process is called hydrogenation, hydrogenation, because it adds hydrogen atoms to the unsaturated chains. A small amount of nickel is added to the oil as a catalyst, and the mixture is then exposed to hydrogen gas at high temperature and pressure. After the fat has absorbed the desired amount of hydrogen, the nickel is filtered out. because it adds hydrogen atoms to the unsaturated chains. A small amount of nickel is added to the oil as a catalyst, and the mixture is then exposed to hydrogen gas at high temperature and pressure. After the fat has absorbed the desired amount of hydrogen, the nickel is filtered out.

Trans Fatty Acids It turns out that the hydrogenation process straightens a certain proportion of the kinks in unsaturated fatty acids not by adding hydrogen atoms to them, but by rearranging the double bond, twisting it so that its bend is less extreme. These molecules remain chemically unsaturated - the double bond between two carbons remains - but they have been transformed from an acutely irregular It turns out that the hydrogenation process straightens a certain proportion of the kinks in unsaturated fatty acids not by adding hydrogen atoms to them, but by rearranging the double bond, twisting it so that its bend is less extreme. These molecules remain chemically unsaturated - the double bond between two carbons remains - but they have been transformed from an acutely irregular cis cis geometry to a more regular geometry to a more regular trans trans structure (see ill.u.s.tration, p. 799). structure (see ill.u.s.tration, p. 799). Cis Cis is Latin for "on this side of," and is Latin for "on this side of," and trans trans for "across from"; the terms describe the positions of neighboring hydrogen atoms on the double bond between carbon atoms. Because the trans fatty acids are less kinked, more like a saturated fat chain in structure, they make it easier for the fat to crystallize and so make it firmer. They also make the fatty acid less p.r.o.ne to attack by oxygen, so it's more stable. Unfortunately, trans fatty acids also resemble saturated fats in raising blood cholesterol levels, which can contribute to the development of heart disease (p. 38). Manufacturers will soon be required to list the trans fatty acid content of their foods, and they're beginning to implement other processing techniques that harden fat consistency without creating trans fatty acids. for "across from"; the terms describe the positions of neighboring hydrogen atoms on the double bond between carbon atoms. Because the trans fatty acids are less kinked, more like a saturated fat chain in structure, they make it easier for the fat to crystallize and so make it firmer. They also make the fatty acid less p.r.o.ne to attack by oxygen, so it's more stable. Unfortunately, trans fatty acids also resemble saturated fats in raising blood cholesterol levels, which can contribute to the development of heart disease (p. 38). Manufacturers will soon be required to list the trans fatty acid content of their foods, and they're beginning to implement other processing techniques that harden fat consistency without creating trans fatty acids.

Fats and Heat Most fats do not have sharply defined melting points. Instead, they soften gradually over a broad temperature range. As the temperature rises, the different kinds of fat molecules melt at different points and slowly weaken the whole structure. (An interesting exception to this rule is cocoa b.u.t.ter, p. 705). This behavior is especially important in making pastries and cakes, and it's what makes b.u.t.ter spreadable at room temperature. b.u.t.ter, p. 705). This behavior is especially important in making pastries and cakes, and it's what makes b.u.t.ter spreadable at room temperature.

Melted fats do eventually change from a liquid to a gas: but only at very high temperatures, from 500 to 750F/260 400C. This high boiling point, far above water's, is the indirect result of the fats' large molecular size. While they can't form hydrogen bonds, the carbon chains of fats do form weaker bonds with each other(p. 814). Because fat molecules are capable of forming so many bonds along their lengthy hydrocarbon chains, the individually weak interactions have a large net effect: it takes a lot of heat energy to knock the molecules apart from each other.

Omega-3 fatty acids. Omega-3 fatty acids are unsaturated fatty acids whose first double bond begins at the third carbon atom from the end. (The most common unsaturated fatty acids are omega-6 fatty acids.) They are essential in our diet for, among other things, the proper function of the immune and cardiovascular systems. Linolenic acid has 3 double bonds among its 18 carbon atoms, and is found in green leaves and in some seed oils. Eicosapentaenoic acid has 20 carbons and 5 double bonds, and is found almost exclusively in seafood (p. 183).

The Smoke Point Most fats begin to decompose at temperatures well below their boiling points, and may even spontaneously ignite on the stovetop if their fumes come into contact with the gas flame. These facts limit the maximum useful temperature of cooking fats. The characteristic temperature at which a fat breaks down into visible gaseous products is called the Most fats begin to decompose at temperatures well below their boiling points, and may even spontaneously ignite on the stovetop if their fumes come into contact with the gas flame. These facts limit the maximum useful temperature of cooking fats. The characteristic temperature at which a fat breaks down into visible gaseous products is called the smoke point. smoke point. Not only are the smoky fumes obnoxious, but the other materials that remain in the liquid, including chemically active free fatty acids, tend to ruin the flavor of the food being cooked. Not only are the smoky fumes obnoxious, but the other materials that remain in the liquid, including chemically active free fatty acids, tend to ruin the flavor of the food being cooked.

The smoke point depends on the initial free fatty acid content of the fat: the lower the free fatty acid content, the more stable the fat, and the higher the smoke point. Free fatty acid levels are generally lower in vegetable oils than in animal fats, lower in refined oils than unrefined ones, and lower in fresh fats and oils than in old ones. Fresh refined vegetable oils begin to smoke around 450F/230C, animal fats around 375F/190C. Fats that contain other substances, such as emulsifiers, preservatives, and in the case of b.u.t.ter, proteins and carbohydrates, will smoke at lower temperatures than pure fats. Fat breakdown during deep frying can be slowed by using a tall, narrow pan and so reducing the area of contact between fat and atmosphere. The smoke point of a deep-frying fat is lowered every time it's used, since some breakdown is inevitable even at moderate temperatures, and trouble-making particles of food are always left behind.

Emulsifiers: Phospholipids, Lecithin, Monoglycerides Some very useful chemical relatives of the true fats, the triglycerides, are the diglycerides and monoglycerides. These molecules act as emulsifiers emulsifiers to make fine, cream-like mixtures of fat and water - such sauces as mayonnaise and hollandaise - even though fat and water don't normally mix with each other. The most prominent natural emulsifiers are the diglyceride to make fine, cream-like mixtures of fat and water - such sauces as mayonnaise and hollandaise - even though fat and water don't normally mix with each other. The most prominent natural emulsifiers are the diglyceride phospholipids phospholipids in egg yolks, the most abundant of which is in egg yolks, the most abundant of which is lecithin lecithin (it makes up about a third of the yolk lipids). Diglycerides have only two fatty-acid chains attached to the glycerol frame, and monoglycerides just one, with the remaining positions on the frame being occupied by small polar groups of atoms. These molecules are thus water-soluble at the head, and fat-soluble at the tail. In cell membranes, the phospholipids a.s.semble themselves in two layers, with one set of polar heads facing the watery interior, the other set the watery exterior, and the tails of both sets mingling in between. When the cook whisks some fat into a water-based liquid that contains emulsifiers - oil into egg yolks, for example - the fat forms tiny droplets that would normally coalesce and separate again. But the emulsifier tails become dissolved in the droplets, and the electrically charged heads project from the droplets and shield the droplets from each other. The emulsion of fat droplets is now stable. (it makes up about a third of the yolk lipids). Diglycerides have only two fatty-acid chains attached to the glycerol frame, and monoglycerides just one, with the remaining positions on the frame being occupied by small polar groups of atoms. These molecules are thus water-soluble at the head, and fat-soluble at the tail. In cell membranes, the phospholipids a.s.semble themselves in two layers, with one set of polar heads facing the watery interior, the other set the watery exterior, and the tails of both sets mingling in between. When the cook whisks some fat into a water-based liquid that contains emulsifiers - oil into egg yolks, for example - the fat forms tiny droplets that would normally coalesce and separate again. But the emulsifier tails become dissolved in the droplets, and the electrically charged heads project from the droplets and shield the droplets from each other. The emulsion of fat droplets is now stable.

These "surface-active" molecules have many other applications as well. For example, monoglycerides have been used for decades in the baking business because they help r.e.t.a.r.d staling, apparently by complexing with amylose and blocking starch retrogradation.

Carbohydrates The name for this large group of molecules comes from the early idea that they were made up of carbon and water. They are indeed made up of carbon, hydrogen, and oxygen atoms, though the oxygen and hydrogen are not found as intact water complexes within the molecules. Carbohydrates are produced by all plants and animals for the purpose of storing chemical energy, and by plants to make a supporting skeleton for its cells. Simple sugars and starch are energy stores, while pectins, cellulose, and other cell-wall carbohydrates are the plant's structural materials.

SUGARS.

Sugars are the simplest carbohydrates. There are many different kinds of sugar molecules, each distinguished by the number of carbon atoms it contains, and then by the particular arrangement it a.s.sumes. Five-carbon sugars are especially important to all life because two of them, ribose and deoxyribose, form the backbones of ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), the carriers of the genetic code. And the 6-carbon sugar glucose is the molecule from which most living things obtain the energy to run the biochemical machinery of their cells. Sugars are such an important nutrient that we have a special sense designed specifically to detect them. Sugars taste sweet, and sweetness is a nearly universal source of pleasure. It's the essence of the dishes we serve at the end of the meal, as well as of candies and confections. Sugars and their properties are described in detail in chapter 12.

Oligosaccharides The oligosaccharides ("several-unit sugars") raffinose, stachyose, and verbascose are 3-, 4-, and 5-ring sugars, respectively, all too large to trigger our sweet detectors, so they're tasteless. They're commonly found in the seeds and other organs of plants, where they make up part of the energy supply. These sugars all affect our digestive system, thanks to the fact that we don't have digestive enzymes capable of breaking them down into single sugars that can be absorbed by the intestine. As a result, the oligosaccharides are not digested and pa.s.s intact into the colon, where various bacteria do digest them, producing large quant.i.ties of carbon dioxide and other gases in the process (p. 486).

Phospholipid emulsifiers. Phospholipids are diglycerides, and are excellent emulsifiers, molecules that make possible a stable mixture of oil and water. Unlike the triglycerides of fat and oil, they have a polar, water-compatible head. Such emulsifiers bury their fatty-acid tails in oil droplets, while their water-compatible, electrically charged heads project from the surface and block the droplets from contacting each other and coalescing.

Polysaccharides: Starch, Pectins, Gums Polysaccharides, which include starch and cellulose, are sugar polymers, polymers, or molecules composed of numerous individual sugar units, as many as several thousand. Usually only one or a very few kinds of sugars are found in a given polysaccharide. Polysaccharides are cla.s.sified according to the overall characteristics of the large molecules: a general size range, an average composition, and a common set of properties. Like the sugars of which they're composed, polysaccharides contain many exposed oxygen and hydrogen atoms, so they can form hydrogen bonds and absorb water. However, they may or may not dissolve in water, depending on the attractive forces among the polymers themselves. or molecules composed of numerous individual sugar units, as many as several thousand. Usually only one or a very few kinds of sugars are found in a given polysaccharide. Polysaccharides are cla.s.sified according to the overall characteristics of the large molecules: a general size range, an average composition, and a common set of properties. Like the sugars of which they're composed, polysaccharides contain many exposed oxygen and hydrogen atoms, so they can form hydrogen bonds and absorb water. However, they may or may not dissolve in water, depending on the attractive forces among the polymers themselves.

Starch By far the most important polysaccharide for the cook is By far the most important polysaccharide for the cook is starch, starch, the compact, unreactive polymer in which plants store their supply of sugar. Starch is simply a chain of glucose sugars. Plants produce starch in two different configurations: a completely linear chain called the compact, unreactive polymer in which plants store their supply of sugar. Starch is simply a chain of glucose sugars. Plants produce starch in two different configurations: a completely linear chain called amylose, amylose, and a highly branched form called and a highly branched form called amylopectin, amylopectin, each of which may contain thousands of glucose units. Starch molecules are deposited together in a series of concentric layers to form solid microscopic granules. When starchy plant tissue is cooked in water, the granules absorb water, swell, and release starch molecules; when cooled again, the starch molecules rebond to each other and can form a moist but solid gel. Various aspects of starch - the way it determines the texture of cooked rice, its formation into pure starch noodles, its role in breads, pastries, and sauces - are described in detail in chapters 911. each of which may contain thousands of glucose units. Starch molecules are deposited together in a series of concentric layers to form solid microscopic granules. When starchy plant tissue is cooked in water, the granules absorb water, swell, and release starch molecules; when cooled again, the starch molecules rebond to each other and can form a moist but solid gel. Various aspects of starch - the way it determines the texture of cooked rice, its formation into pure starch noodles, its role in breads, pastries, and sauces - are described in detail in chapters 911.

Glycogen Glycogen, or "animal starch," is an animal carbohydrate similar to amylopectin, though more highly branched. It's a fairly minor component of animal tissue and so of meats, although its concentration at the time of slaughter will affect the ultimate pH of the meat, and thereby its texture (p. 142). Glycogen, or "animal starch," is an animal carbohydrate similar to amylopectin, though more highly branched. It's a fairly minor component of animal tissue and so of meats, although its concentration at the time of slaughter will affect the ultimate pH of the meat, and thereby its texture (p. 142).

A sugar, glucose, and a polysaccharide, starch, which is a chain of glucose molecules. Plants produce two broadly different forms of starch: simple long chains called amylose, and highly branched chains called amylopectin.

Cellulose Cellulose is, like amylose, a linear plant polysaccharide made up solely of glucose sugars. Yet thanks to a minor difference in the way the sugars are linked to each other, the two compounds have very different properties: cooking dissolves starch granules but leaves cellulose fibers intact; most animals can digest starch, but not cellulose. Cellulose is a structural support that's laid down in cell walls in the form of tiny fibers a.n.a.logous to steel reinforcing bars, and it's made to be durable. Few animals can digest cellulose, and hay-eating cattle and wood-eating termites can do so only because their guts are populated by cellulose-digesting bacteria. To other animals, including ourselves, cellulose is indigestible fiber (which has its own value; see p. 258). Cellulose is, like amylose, a linear plant polysaccharide made up solely of glucose sugars. Yet thanks to a minor difference in the way the sugars are linked to each other, the two compounds have very different properties: cooking dissolves starch granules but leaves cellulose fibers intact; most animals can digest starch, but not cellulose. Cellulose is a structural support that's laid down in cell walls in the form of tiny fibers a.n.a.logous to steel reinforcing bars, and it's made to be durable. Few animals can digest cellulose, and hay-eating cattle and wood-eating termites can do so only because their guts are populated by cellulose-digesting bacteria. To other animals, including ourselves, cellulose is indigestible fiber (which has its own value; see p. 258).

Hemicelluloses and Pectic Substances These polysaccharides (made from a variety of sugars, including galactose, xylose, arabinose) are found together with cellulose in the plant cell walls. If the cellulose fibrils are the reinforcing bars in the cell walls, the amorphous hemicelluloses and pectic substances are a sort of jelly-like cement in which the bars are embedded. Their significance for the cook is that, unlike cellulose, they are partly soluble in water, and therefore contribute to the softening of cooked vegetables and fruits. Pectin is abundant enough to be extracted from citrus fruits and apples and used to thicken fruit syrups into jams and jellies. These carbohydrates are described in detail in chapter 5. These polysaccharides (made from a variety of sugars, including galactose, xylose, arabinose) are found together with cellulose in the plant cell walls. If the cellulose fibrils are the reinforcing bars in the cell walls, the amorphous hemicelluloses and pectic substances are a sort of jelly-like cement in which the bars are embedded. Their significance for the cook is that, unlike cellulose, they are partly soluble in water, and therefore contribute to the softening of cooked vegetables and fruits. Pectin is abundant enough to be extracted from citrus fruits and apples and used to thicken fruit syrups into jams and jellies. These carbohydrates are described in detail in chapter 5.

Inulin Inulin is a polymer of fructose sugars, from a handful to hundreds per molecule. Inulin is a form of energy storage and a source of antifreeze (sugars lower the freezing point of a water solution) in members of the onion and lettuce families, notably garlic and the sunchoke. Like the oligosaccharides, inulin is not digestible, and so feeds bacteria in our large intestine and generates gas. Inulin is a polymer of fructose sugars, from a handful to hundreds per molecule. Inulin is a form of energy storage and a source of antifreeze (sugars lower the freezing point of a water solution) in members of the onion and lettuce families, notably garlic and the sunchoke. Like the oligosaccharides, inulin is not digestible, and so feeds bacteria in our large intestine and generates gas.

Plant Gums There are a number of other plant carbohydrates that cooks and manufacturers have found useful for thickening and gelling liquid foods, helping to stabilize emulsions, and producing smoother consistencies in frozen goods and candies. Like the cell-wall cements, they're generally complex polymers of several different sugars or related carbohydrates. They include: There are a number of other plant carbohydrates that cooks and manufacturers have found useful for thickening and gelling liquid foods, helping to stabilize emulsions, and producing smoother consistencies in frozen goods and candies. Like the cell-wall cements, they're generally complex polymers of several different sugars or related carbohydrates. They include: Agarose, alginates, and carrageenans, cell-wall polymers from various seaweeds Gum arabic, which exudes from cuts in various species of Acacia Acacia trees trees Gum tragacanth, an exudate from various species of Astralagus Astralagus shrubs shrubs Guar gum, from seeds of a shrub in the bean family (Cyamopsis tetragon.o.bola) Locust-bean gum, from seeds of the carob tree, Ceratonia siliqua Ceratonia siliqua Xanthan gum and gellan, polysaccharides produced by certain bacteria in industrial fermentation Proteins Of all the major food molecules, proteins are the most challenging and mercurial. The others, water and fats and carbohydrates, are pretty stable and staid. But expose proteins to a little heat, or acid, or salt, or air, and their behavior changes drastically. This changeability reflects their biological mission. Carbohydrates and fats are mainly pa.s.sive forms of stored energy, or structural materials. But proteins are the active machinery of life. They a.s.semble all the molecules that make a cell, themselves included, and tear them down as well; they move molecules from one place in the cell to another; in the form of muscle fibers, they move whole animals. They're at the heart of all organic activity, growth, and movement. So it's the nature of proteins to be active and sensitive. When we cook foods that contain them, we take advantage of their dynamic nature to make new structures and consistencies.

Amino Acids and Peptides Like starch and cellulose, proteins are large polymers of smaller molecular units. The smaller units are called amino acids. amino acids. They consist of between 10 and 40 atoms, mainly carbon, hydrogen, and oxygen, with at least one nitrogen atom in an They consist of between 10 and 40 atoms, mainly carbon, hydrogen, and oxygen, with at least one nitrogen atom in an amine amine group - NH group - NH>2- that gives the amino acids their family name. A couple of amino acids include sulfur atoms. There are about 20 different kinds of amino acids that occur in significant quant.i.ties in food. Particular protein molecules are dozens to hundreds of amino acids long, and often contain many of the 20 different kinds. Short chains of amino acids are called peptides. peptides.

Amino Acids and Peptides Contribute Flavor Three aspects of amino acids are especially important to the cook. First, amino acids partic.i.p.ate in the browning reactions that generate flavor at high cooking temperatures (p. 778). Second, many single amino acids and short peptides have tastes of their own, and in foods where proteins have been partly broken down - aged cheeses, cured hams, soy sauce - these tastes can contribute to the overall flavor. Most tasty amino acids are either sweet or bitter to some degree, and a number of peptides are also bitter. But glutamic acid, better known in its concentrated commercial form MSG (monosodium glutamate), and some peptides have a unique taste that is designated by such words as Three aspects of amino acids are especially important to the cook. First, amino acids partic.i.p.ate in the browning reactions that generate flavor at high cooking temperatures (p. 778). Second, many single amino acids and short peptides have tastes of their own, and in foods where proteins have been partly broken down - aged cheeses, cured hams, soy sauce - these tastes can contribute to the overall flavor. Most tasty amino acids are either sweet or bitter to some degree, and a number of peptides are also bitter. But glutamic acid, better known in its concentrated commercial form MSG (monosodium glutamate), and some peptides have a unique taste that is designated by such words as savory, brothy, savory, brothy, and and umami umami (j.a.panese for "delicious"). They lend an added dimension of flavor to foods that are rich in them, including tomatoes and certain seaweeds as well as salt-cured and fermented products. When heated, sulfur-containing amino acids break down and contribute eggy, meaty aroma notes. (j.a.panese for "delicious"). They lend an added dimension of flavor to foods that are rich in them, including tomatoes and certain seaweeds as well as salt-cured and fermented products. When heated, sulfur-containing amino acids break down and contribute eggy, meaty aroma notes.

Amino Acids Influence Protein Behavior The third important characteristic of amino acids is that they have a variety of chemical natures, and these influence the structure and behavior of the protein they're a part of. Some amino acids have portions resembling water and can form hydrogen bonds with other molecules, including water. Some have short carbon chains or carbon rings that resemble fats, and can form van der Waals bonds with other similar molecules. And some, especially those that include a sulfur atom, are especially reactive, and can form strong covalent bonds with other molecules, including other sulfur-containing amino acids. This means that a single protein has many different chemical environments along its chain: parts that attract water molecules, parts that avoid water molecules, and parts that are ready to form strong bonds with similar parts on other proteins, or on other parts of the same protein. The third important characteristic of amino acids is that they have a variety of chemical natures, and these influence the structure and behavior of the protein they're a part of. Some amino acids have portions resembling water and can form hydrogen bonds with other molecules, including water. Some have short carbon chains or carbon rings that resemble fats, and can form van der Waals bonds with other similar molecules. And some, especially those that include a sulfur atom, are especially reactive, and can form strong covalent bonds with other molecules, including other sulfur-containing amino acids. This means that a single protein has many different chemical environments along its chain: parts that attract water molecules, parts that avoid water molecules, and parts that are ready to form strong bonds with similar parts on other proteins, or on other parts of the same protein.

Protein Structure Proteins are formed by linking the amine nitrogen of one amino acid with a carbon atom on another amino acid, and then repeating this "peptide bond" to make a chain dozens or hundreds of amino acids long. The carbon-nitrogen backbone of the protein molecule forms a sort of zigzag pattern, with the "side groups" - the other atoms on each amino acid - sticking out to the sides.

The Protein Helix One effect of the peptide bond is a certain kind of regularity that causes the molecule as a whole to twist and form a spiral, or helix. Very few proteins exist as a simple regular helix, but those that do tend to join together in strong fibers. These include connective-tissue collagen in meat, an important factor in its tenderness, and the source of gelatin (pp. 130, 597). One effect of the peptide bond is a certain kind of regularity that causes the molecule as a whole to twist and form a spiral, or helix. Very few proteins exist as a simple regular helix, but those that do tend to join together in strong fibers. These include connective-tissue collagen in meat, an important factor in its tenderness, and the source of gelatin (pp. 130, 597).

Protein Folds The other influence on protein structure is the side groups of its amino acids. Because the protein chain is so long, it can bend back on itself and bring together amino acids that are some distance along the chain from each other. Amino acids with similar side groups can then bond to each other in various ways, including via hydrogen bonds, van der Waals bonds, ionic bonds (p. 813), and strong covalent bonds (especially between sulfur atoms). This bonding is what gives a particular protein molecule the characteristic shape that allows it to carry out its particular job. The weak, temporary nature of the hydrogen and hydrophobic bonds allows it to change its shape as it works. The overall shape of a protein can range from a long, extended, mostly helical molecule with a few kinks or loops, to compact, elaborately folded molecules that are called "globular" proteins. Collagen is an example of a helical protein, and the various proteins in eggs are mainly globular. The other influence on protein structure is the side groups of its amino acids. Because the protein chain is so long, it can bend back on itself and bring together amino acids that are some distance along the chain from each other. Amino acids with similar side groups can then bond to each other in various ways, including via hydrogen bonds, van der Waals bonds, ionic bonds (p. 813), and strong covalent bonds (especially between sulfur atoms). This bonding is what gives a particular protein molecule the characteristic shape that allows it to carry out its particular job. The weak, temporary nature of the hydrogen and hydrophobic bonds allows it to change its shape as it works. The overall shape of a protein can range from a long, extended, mostly helical molecule with a few kinks or loops, to compact, elaborately folded molecules that are called "globular" proteins. Collagen is an example of a helical protein, and the various proteins in eggs are mainly globular.

Amino acids and proteins, denaturation and coagulation. Top: Top: Three of the 20-odd amino acids important in food. Each amino acid has a common end including an amino (NH Three of the 20-odd amino acids important in food. Each amino acid has a common end including an amino (NH2) group, by which amino acids bond to each other into long chains called proteins, and a variable end or "side group" that can form different kinds of bonds with other amino acids. Center: Center: A chain of amino acids shown schematically, with some of the side groups projecting from the chain. The amino acid chain can fold back on itself, and some of the side groups form bonds with each other to hold the chain in a folded shape. A chain of amino acids shown schematically, with some of the side groups projecting from the chain. The amino acid chain can fold back on itself, and some of the side groups form bonds with each other to hold the chain in a folded shape. Bottom: Bottom: Heating and other cooking processes can break the fold-stabilizing bonds and cause the long chains to unfold, or Heating and other cooking processes can break the fold-stabilizing bonds and cause the long chains to unfold, or denature (left, center) denature (left, center) . Eventually the exposed side groups form new bonds between different protein chains, and the proteins . Eventually the exposed side groups form new bonds between different protein chains, and the proteins coagulate coagulate , or form a permanently bonded solid ma.s.s , or form a permanently bonded solid ma.s.s (right) (right) . .