Liver Liver is a flavorful thickener, but has the disadvantage of requiring disintegration before it can be used. The coagulable proteins are concentrated inside its cells, so the cook must break the cells open by pounding the tissue, and then strain away the particles of connective tissue that hold the cells together. Liver is a flavorful thickener, but has the disadvantage of requiring disintegration before it can be used. The coagulable proteins are concentrated inside its cells, so the cook must break the cells open by pounding the tissue, and then strain away the particles of connective tissue that hold the cells together.

Blood Blood is the traditional thickening agent in Blood is the traditional thickening agent in coq au vin, coq au vin, the French rooster in wine sauce, and in braises of game animals (civets). It's about 80% water and 17% protein, and consists of two phases: the various cells, including the red cells colored by hemoglobin, and the fluid plasma in which the cells float. The plasma makes up about two thirds of cattle and pig blood and contains dispersed proteins, about 7% by weight. Alb.u.min is the protein that causes blood to thicken when heated above 167F/75C . the French rooster in wine sauce, and in braises of game animals (civets). It's about 80% water and 17% protein, and consists of two phases: the various cells, including the red cells colored by hemoglobin, and the fluid plasma in which the cells float. The plasma makes up about two thirds of cattle and pig blood and contains dispersed proteins, about 7% by weight. Alb.u.min is the protein that causes blood to thicken when heated above 167F/75C .

Sh.e.l.lfish Organs The liver and eggs of crustaceans, and the s.e.xual tissues of sea urchins have the same advantages and disadvantages as liver, and thicken and coagulate at much lower temperatures. They should be added carefully to a sauce that has first been allowed to cool well below the boil. The liver and eggs of crustaceans, and the s.e.xual tissues of sea urchins have the same advantages and disadvantages as liver, and thicken and coagulate at much lower temperatures. They should be added carefully to a sauce that has first been allowed to cool well below the boil.

Cheese and Yogurt These cultured milk products differ from the other protein thickeners in that their casein proteins have already been coagulated by enzyme activity and/or acidity. They're thus unable to develop a new thickness by being heated with a sauce. Instead, they lend their own thickness as they're mixed into the sauce. They're best subjected to only moderate heat, since temperatures approaching the boil can cause curdling. Yogurt is a more effective thickener if it has been drained of its watery whey. The best cheeses for thickening have a creamy consistency themselves, an indication that the protein network has been broken down into small, easily dispersed pieces; more intact casein fibers can form stringy aggregates (p. 65). Most cheeses are a concentrated source of fat, emulsified droplets of which also contribute body. These cultured milk products differ from the other protein thickeners in that their casein proteins have already been coagulated by enzyme activity and/or acidity. They're thus unable to develop a new thickness by being heated with a sauce. Instead, they lend their own thickness as they're mixed into the sauce. They're best subjected to only moderate heat, since temperatures approaching the boil can cause curdling. Yogurt is a more effective thickener if it has been drained of its watery whey. The best cheeses for thickening have a creamy consistency themselves, an indication that the protein network has been broken down into small, easily dispersed pieces; more intact casein fibers can form stringy aggregates (p. 65). Most cheeses are a concentrated source of fat, emulsified droplets of which also contribute body.

Almond Milk This water extract of soaked ground almonds contains a significant amount of protein that causes the liquid to thicken when heated or acidified(p. 504). This water extract of soaked ground almonds contains a significant amount of protein that causes the liquid to thicken when heated or acidified(p. 504).

Solid Sauces: Gelatin Jellies and Carbohydrate Jellies When a meat or fish stock is allowed to cool to room temperature, it may set into a fragile solid, or gel. This behavior can be undesirable, for example when it causes some sauce to congeal on the plate. But cooks also exploit it to make delightful jellies, a sort of solid sauce. A gel forms when the gelatin concentration is sufficiently high, around 1% or more of the stock's total weight. At these concentrations, there are enough gelatin molecules in the stock that their long chains can overlap with each other to form a continuous network throughout the stock. As the hot stock cools down to the melting temperature of gelatin, around 100F/40C, the extended gelatin chains begin to a.s.sume the coiled shape that they had in the original triple helix of the collagen fibers (p. 597). And when coils on different molecules approach each other, they nest closely alongside each other and bond to form new double and triple helixes. These rea.s.sembled collagen junctions give some rigidity to the network of gelatin molecules, and they and the water molecules they surround can no longer flow freely: so the liquid turns into a solid. A 1% gelatin gel is fragile and quivery and breaks easily when handled; the more familiar and robust dessert jellies made with commercial gelatin are usually 3% gelatin or more. The higher the proportion of gelatin, the more firm and rubbery the gel is.

Jellies are remarkable in two ways. At their best they are translucent, glistening, beautiful on their own or as settings for the foods embedded in them. And the temperature at which the gelatin junctions are shaken apart is right around body temperature: so gelatin gels melt effortlessly in the mouth to a full-bodied fluid. They bathe the mouth in sauce. No other thickener has this quality.

Protein-Thickened Sauces and HealthSauces thickened with proteins are very nutritious, and microbes can multiply rapidly in them. They're best held either above 140F/60C or below 40F/5C to prevent the growth of bacteria that can cause food poisoning. When cooling large quant.i.ties of meat stock, the cook should divide the stock into small portions so that their temperature will fall rapidly through the potentially dangerous temperature range.Like well-browned meats themselves, meat stocks and sauces whose flavor comes from browned pan juices or from long reduction carry small quant.i.ties of chemicals called heterocyclic amines. HCAs are known to damage DNA and therefore may contribute to the development of cancer (p. 124). We don't yet know whether the levels found in meats and sauces pose a significant risk. Vegetables in the cabbage family contain chemicals that prevent HCAs from damaging DNA, so it may be that other foods in a well-balanced diet protect us from the toxic effects of HCAs.

Jelly Consistency The firmness or strength of a gelatin gel, and therefore its tolerance of handling and its texture in the mouth, depends on several factors: the gelatin molecules themselves, the presence of other ingredients, and the way in which the mixture is cooled.

Gelatin Quality and Concentration The most important influence on the texture of a jelly is the concentration and quality of its gelatin. Gelatin is a highly variable material. Even manufactured gelatin (below) is only 6070% intact, full-length gelatin molecules; the remainder consists of smaller pieces that are less efficient thickeners. The gelatin in a stock is especially unpredictable, since meat and bones vary in their collagen content, and long cooking causes progressive breakdown of the gelatin chains. The best way to a.s.sess gel strength is to cool a spoonful of the liquid in a bowl resting in ice water, see if the liquid sets, and how firm the gel is. A liquid lacking in firmness can be reduced further to concentrate the gelatin, or it can be supplemented with a small amount of pure gelatin. The most important influence on the texture of a jelly is the concentration and quality of its gelatin. Gelatin is a highly variable material. Even manufactured gelatin (below) is only 6070% intact, full-length gelatin molecules; the remainder consists of smaller pieces that are less efficient thickeners. The gelatin in a stock is especially unpredictable, since meat and bones vary in their collagen content, and long cooking causes progressive breakdown of the gelatin chains. The best way to a.s.sess gel strength is to cool a spoonful of the liquid in a bowl resting in ice water, see if the liquid sets, and how firm the gel is. A liquid lacking in firmness can be reduced further to concentrate the gelatin, or it can be supplemented with a small amount of pure gelatin.

How gelatin turns a liquid into a solid. When the gelatin solution is hot (left), (left), the water and protein molecules are in constant, forceful movement. As the solution cools and the molecules move more gently, the proteins naturally begin to form little regions of collagen-like helical a.s.sociation the water and protein molecules are in constant, forceful movement. As the solution cools and the molecules move more gently, the proteins naturally begin to form little regions of collagen-like helical a.s.sociation (right). (right). These "junctions" gradually form a continuous meshwork of gelatin molecules that traps the liquid in its interstices, preventing any noticeable flow. The solution has become a solid gel. These "junctions" gradually form a continuous meshwork of gelatin molecules that traps the liquid in its interstices, preventing any noticeable flow. The solution has become a solid gel.

Additional Ingredients Other common ingredients have various effects on gel strength when included in a jelly. Other common ingredients have various effects on gel strength when included in a jelly.

Salt lowers gel strength by interfering with gelatin bonding.

Sugars (except for fructose) increase gel strength by attracting water molecules away from the gelatin molecules.

Milk increases gel strength.

Alcohol increases gel strength until it becomes 30 to 50% of the gel, when it will cause the gelatin to precipitate into solid particles.

Acids - vinegar, fruit juices, wine - with a pH below 4 produce a weaker jelly by increasing repulsive electrical charges on the gelatin molecules.

The gel-weakening effects of salt and acids can be compensated for by increasing the gelatin concentration.

Both strongly acid ingredients and the tannins in tea or red wine can cloud a jelly, the acids by precipitating proteins in a meat or fish stock into tiny particles, and the tannins by binding to and precipitating the gelatin molecules themselves. These ingredients are best cooked briefly with the gelatin solution so that it can be strained or clarified before setting.

A number of fruits - papaya, pineapple, melons, and kiwi among them - contain protein-digesting enzymes that break gelatin chains into short pieces and thus prevent them from setting into a gel at all. They and their juices can be made into a jelly only after a brief cooking to inactivate the enzymes.

Cooling Temperatures The temperature at which the gel forms and ages affects its texture. When "snap-chilled" in the refrigerator, the gelatin molecules are immobilized in place and bond to each other quickly and randomly, so the bonds and the structure of the network are relatively weak. When allowed to set slowly at room temperature, the gelatin molecules have time to move around and form more regular helix junctions, so once it forms, the network is more firm and stable. In practice, jellies should be set in the refrigerator to minimize the growth of bacteria. Gelatin bonds continue to form slowly in the solid jelly, so snap-chilled jellies become as firm as slow-chilled jellies after a few days. The temperature at which the gel forms and ages affects its texture. When "snap-chilled" in the refrigerator, the gelatin molecules are immobilized in place and bond to each other quickly and randomly, so the bonds and the structure of the network are relatively weak. When allowed to set slowly at room temperature, the gelatin molecules have time to move around and form more regular helix junctions, so once it forms, the network is more firm and stable. In practice, jellies should be set in the refrigerator to minimize the growth of bacteria. Gelatin bonds continue to form slowly in the solid jelly, so snap-chilled jellies become as firm as slow-chilled jellies after a few days.

Jellies from Meat and Fish: Aspics Meat and fish jellies go back to the Middle Ages (p. 584), and are still delightful showpieces. They're made much as consomme is, ideally from a flavorful meat stock - often cooked with a veal foot to provide enough gelatin - or from a double fish stock. The stocks are clarified with egg whites and chopped meat or fish, then filled and flavored just before they set. Aspics should be firm enough to be cut as necessary, but quivery and tender in the mouth, not rubbery. When made to coat a terrine or whole portion of meat, or to bind chopped meat together, they must be firmer, around 1015% gelatin, so that they don't run off the food or crumble. Fish jellies and aspics are especially delicate due to the low melting temperature of fish gelatin; they and their plates should be kept distinctly cold to prevent premature melting. A homely version of the meat aspic is boeuf a la mode, boeuf a la mode, a pot roast braised in stock and wine along with a veal foot, then sliced and embedded in the strained jelly made by the cooking liquid. a pot roast braised in stock and wine along with a veal foot, then sliced and embedded in the strained jelly made by the cooking liquid. Chauds-froids Chauds-froids are meat or fish jellies that include cream. are meat or fish jellies that include cream.

Food Words: Gel, Gelatin, Jelly Gel, Gelatin, JellyGel and and jelly, jelly, words for a fragile solid that is largely water, and words for a fragile solid that is largely water, and gelatin, gelatin, the name of the protein that can gel water into a solid, all come from an Indo-European root meaning "cold" or "to freeze." The jelly maker freezes liquids with molecules instead of icy temperatures. the name of the protein that can gel water into a solid, all come from an Indo-European root meaning "cold" or "to freeze." The jelly maker freezes liquids with molecules instead of icy temperatures.

Other Jellies and Gelees; Manufactured Gelatins The first jellies were meat and fish dishes, but cooks soon began to use animal gelatins to set other ingredients into pleasing solids, especially creams and fruit juices, and prepared gelatin became a standard ingredient for the pastry cook, who also uses it to give a melting firmness to some mousses, whipped creams, and pastry creams. The most familiar jellies in the United States today, both made from manufactured gelatin powders, are sweet, fruit-flavored, fluorescently colored desserts, and "shooters" fortified with vodka and other spirits. More refined preparations, often named by the French gelee, gelee, take advantage of the fact that other ingredients can be added at the last minute when the mix is barely warm and about to set, so fresh and delicate flavors can be preserved in the jelly: such things as champagne or the "water" from a seeded tomato. take advantage of the fact that other ingredients can be added at the last minute when the mix is barely warm and about to set, so fresh and delicate flavors can be preserved in the jelly: such things as champagne or the "water" from a seeded tomato.

Gelatin Production Most manufactured gelatin in the United States and Europe comes from pigskin, though some is also made from cattle skins and from bones. Industrial extraction is far more efficient and gentler on the gelatin chains than kitchen extractions. The pigskins are soaked in dilute acid for 1824 hours to break the collagen's cross-linking bonds, and then are extracted in several changes in water, beginning at just 130F/55C, and ending around 195F/90C. The low-temperature extracts contain the most intact gelatin molecules, produce the strongest gels, and are the lightest in color; higher temperatures damage more gelatin chains and cause a yellow discoloration. The extracts are then filtered, purified, their pH adjusted to 5.5, evaporated, sterilized, and dried into sheets or granules that are 8590% gelatin, 815% water, 12% salts, and 1% glucose. Gelatin quality is sometimes indicated by a "Bloom" number (named for Oscar Bloom, inventor of the measuring device), with high numbers (250) indicating high gelling power. Most manufactured gelatin in the United States and Europe comes from pigskin, though some is also made from cattle skins and from bones. Industrial extraction is far more efficient and gentler on the gelatin chains than kitchen extractions. The pigskins are soaked in dilute acid for 1824 hours to break the collagen's cross-linking bonds, and then are extracted in several changes in water, beginning at just 130F/55C, and ending around 195F/90C. The low-temperature extracts contain the most intact gelatin molecules, produce the strongest gels, and are the lightest in color; higher temperatures damage more gelatin chains and cause a yellow discoloration. The extracts are then filtered, purified, their pH adjusted to 5.5, evaporated, sterilized, and dried into sheets or granules that are 8590% gelatin, 815% water, 12% salts, and 1% glucose. Gelatin quality is sometimes indicated by a "Bloom" number (named for Oscar Bloom, inventor of the measuring device), with high numbers (250) indicating high gelling power.

Types of Gelatin Gelatin is sold in several different forms. Granulated gelatin and sheet gelatin are given an initial soaking in cold water so that the solid gelatin network can absorb moisture and dissolve readily when warm liquid is added. If added to the warm liquid directly, the outer layers of the solid granules can become gluey and stick neighboring granules together, though even these cl.u.s.ters eventually disperse. Sheets with their small surface area introduce less air into the liquid, which can be an advantage when the cook wants great clarity in the jelly. There is also an "instant" gelatin that is manufactured by drying the extract rapidly before the gelatin chains can form junctions, so it disperses directly in warm liquid. And hydrolyzed gelatins have been intentionally broken into chains too short to form a gel; they're used in food manufacturing as an emulsifying agent (p. 627). Gelatin is sold in several different forms. Granulated gelatin and sheet gelatin are given an initial soaking in cold water so that the solid gelatin network can absorb moisture and dissolve readily when warm liquid is added. If added to the warm liquid directly, the outer layers of the solid granules can become gluey and stick neighboring granules together, though even these cl.u.s.ters eventually disperse. Sheets with their small surface area introduce less air into the liquid, which can be an advantage when the cook wants great clarity in the jelly. There is also an "instant" gelatin that is manufactured by drying the extract rapidly before the gelatin chains can form junctions, so it disperses directly in warm liquid. And hydrolyzed gelatins have been intentionally broken into chains too short to form a gel; they're used in food manufacturing as an emulsifying agent (p. 627).

The standard proportion advised by dessert gelatin packages is one 7-gm package per cup/240 ml, or about a 3% solution; 2% and 1% solutions are progressively more tender.

Gelatin Doesn't Strengthen Nails or HairThough it's widely believed that gelatin supplements strengthen both nails and hair, there is no good evidence that this is true. Nails and hair are made of a very different protein called keratin, and gelatin has no advantage over other protein sources in supplying the building blocks for keratin production.

Carbohydrate Gelling agents: Agar, Carrageenan, Alginates Gelatin isn't the only ingredient that cooks have at their disposal for turning a flavorful liquid into an intriguing solid. Starch gels give us various pie fillings and the candy called Turkish Delight, and pectin gels the many fruit jellies and jams (p. 296). Along the seacoasts of the world, cooks found long ago that various seaweeds release a viscous substance into hot water that forms a gel when the water cools. These substances are not proteins like gelatin, but unusual carbohydrates with some unusual and useful properties. Food manufacturers use them to make gels and to stabilize emulsions (cream and ice cream, for example).

Agar Agar, a shortened version of the Malay Agar, a shortened version of the Malay agar agar, agar agar, is a mixture of several different carbohydrates and other materials that has long been extracted from several genera of red algae (p. 341). It's now manufactured by boiling the seaweeds, filtering the liquid, and freeze-drying it in the form of sticks or strands, which are readily available in Asian groceries. The solid pieces of agar can be eaten uncooked as a chewy ingredient in cold salads, soaked and cut into bite-sized pieces. In China agar is made into an unflavored gel that's sliced and served in a complex sauce; it's also used to gel flavorful mixtures of fruit juice and sugar, and stews of meats, fish, or vegetables. In j.a.pan agar is made into jellied sweets. is a mixture of several different carbohydrates and other materials that has long been extracted from several genera of red algae (p. 341). It's now manufactured by boiling the seaweeds, filtering the liquid, and freeze-drying it in the form of sticks or strands, which are readily available in Asian groceries. The solid pieces of agar can be eaten uncooked as a chewy ingredient in cold salads, soaked and cut into bite-sized pieces. In China agar is made into an unflavored gel that's sliced and served in a complex sauce; it's also used to gel flavorful mixtures of fruit juice and sugar, and stews of meats, fish, or vegetables. In j.a.pan agar is made into jellied sweets.

Agar forms gels at even lower concentrations than gelatin does, less than 1% by weight. An agar jelly is somewhat opaque, and has a more crumbly texture than a gelatin jelly. To make an agar jelly, the dried agar is soaked in cold water, then heated to the boil to fully dissolve the carbohydrate chains, mixed with the other ingredients, and the mixture strained and cooled until it sets, at around 110F/38C. But where a gelatin gel sets and remelts at around the same temperature, an agar gel only melts again when its temperature reaches 185F/85C. So an agar gel won't melt in the mouth; it must be chewed into particles. On the other hand, it will remain solid on hot days, and can even be served hot. Modern cooks have used this property to disperse small agar-gelled morsels of contrasting flavor into a hot dish.

Gelatinous Delicacies: Tendons, Fins, and NestsThe Chinese are great admirers of gelatinous textures, the semi-solid stickiness of long-cooked gelatin-rich connective tissue, and make soups from several ingredients that in the West are hardly considered to be edible. Beef tendons are one example; they are essentially pure connective tissue, and when simmered for hours develop a texture that is simultaneously gelatinous and crunchy. Shark fins are a delicacy that are dried after being taken from that cartilaginous fish, then rehydrated, simmered in several changes of water to remove off-flavors, and then simmered in broth.Most unusual of all are the nests of cave-dwelling birds in the swallow family, swiftlets of the genus Collocalia, Collocalia, which are found throughout Southeast and South Asia. The males build their nests up from strands of their saliva, which stick to the cave walls and dry to form a small but strong cup. The harvested nests are soaked in cold water to rinse out impurities and to let them absorb water and swell. They're then simmered in broth, and enjoyed for their semisolid, gelatinous consistency, which is due not to gelatin itself, but to salivary proteins called mucins, which are related to the mucins in egg white (p. 77). which are found throughout Southeast and South Asia. The males build their nests up from strands of their saliva, which stick to the cave walls and dry to form a small but strong cup. The harvested nests are soaked in cold water to rinse out impurities and to let them absorb water and swell. They're then simmered in broth, and enjoyed for their semisolid, gelatinous consistency, which is due not to gelatin itself, but to salivary proteins called mucins, which are related to the mucins in egg white (p. 77).

Carrageenan, Alginates, Gellan Experimentally minded cooks are exploring a number of other unusual carbohydrate gelling agents, some traditional and some not. Experimentally minded cooks are exploring a number of other unusual carbohydrate gelling agents, some traditional and some not. Carrageenan, Carrageenan, from certain red algae(p. 341), has long been used in China to gel stews and flavored liquids, and in Ireland to make a kind of milk pudding. Purified fractions of crude carrageenan produce gels with a range of textures, from brittle to elastic. from certain red algae(p. 341), has long been used in China to gel stews and flavored liquids, and in Ireland to make a kind of milk pudding. Purified fractions of crude carrageenan produce gels with a range of textures, from brittle to elastic. Alginates Alginates come from a number of brown seaweeds, and form gels only in the presence of calcium (in milk and cream, for example). Inventive cooks have taken advantage of this to make small flavored spheres and threads: they prepare a calcium-free alginate solution of the desired flavor and color, and then drip or inject it into a calcium solution, where it immediately gels. come from a number of brown seaweeds, and form gels only in the presence of calcium (in milk and cream, for example). Inventive cooks have taken advantage of this to make small flavored spheres and threads: they prepare a calcium-free alginate solution of the desired flavor and color, and then drip or inject it into a calcium solution, where it immediately gels. Gellan, Gellan, an industrial discovery, is a carbohydrate secreted by a bacterium, and in the presence of salts or acid forms very clear gels that release their flavor well. an industrial discovery, is a carbohydrate secreted by a bacterium, and in the presence of salts or acid forms very clear gels that release their flavor well.

Sauces Thickened with Flour and Starch Many sauces, from long-simmered cla.s.sic French brown sauces to last-minute gravies, owe at least part of their consistency to the substance called starch. starch. Unlike the other thickening agents, starch is a major component of our daily diet. It's the molecule in which most plants store the energy they generate from photosynthesis, and provides about three-quarters of the calories for the earth's human population, mainly in the form of grains and root vegetables. It's the least expensive and most versatile thickener the cook has to work with, a worthy adjunct to gelatin and fat. The cook can choose among several different kinds of starch, each with its own qualities. Unlike the other thickening agents, starch is a major component of our daily diet. It's the molecule in which most plants store the energy they generate from photosynthesis, and provides about three-quarters of the calories for the earth's human population, mainly in the form of grains and root vegetables. It's the least expensive and most versatile thickener the cook has to work with, a worthy adjunct to gelatin and fat. The cook can choose among several different kinds of starch, each with its own qualities.

Agar: From Pudding to Petri DishSolid gels made from agar have long been a standard tool in the study of microbes. Scientists make them up to include various nutrients, and then grow colonies of microbes on their surface. Agar gels have several important advantages over the original growth medium, which was gelatin. Very few bacteria can digest the unusual agar carbohydrates, so agar gels remain intact and the bacterial colonies separate, while many bacteria digest proteins and can quickly liquefy a gelatin gel into a useless soup. And agar gels remain solid at the ideal temperatures for bacterial growth, often around 100F/38C, a temperature at which gelatin begins to melt.How did microbiologists come to use agar? In the late 19th century, Lina Hesse, the American wife of a German scientist, recalled the advice of family friends who had lived in Asia, and made agar jellies and puddings that stayed solid in the summer heat of Dresden. Her husband relayed his wife's suggestion to his boss, the pioneering microbiologist Robert Koch, who then used agar to isolate the bacterium that causes tuberculosis.

The Nature of Starch Starch molecules are long chains of thousands of glucose sugar molecules linked up together. There are two kinds of starch molecules: long straight chains called amylose, and short, branched, bushy chains called amylopectin. Plants deposit starch molecules in microscopic solid granules. The size, shape, amylose and amylopectin contents, and cooking qualities of the starch granules vary from species to species.

Linear Amylose and Bushy Amylopectin The shapes of amylose and amylopectin molecules have a direct effect on their ability to thicken a sauce. The straight amylose chains coil up into long helical structures when dissolved in water, but they retain their basically linear shape. Their elongation makes it very likely that one chain will knock into another or into a granule: each sweeps through a relatively large volume of liquid. By contrast, the branched shape of amylopectin makes for a compact target and therefore a molecule less likely to collide with others; and even if it does collide, it's less likely to get tangled up and slow the motion of other molecules and granules in the vicinity. A small number of very long amylose molecules, then, will do the job of more but shorter amylose molecules, and of many more bushy amylopectins. For this reason, the cook can obtain the same degree of thickening from a smaller amount of long-amylose potato starch than from moderate-amylose wheat and corn starches. The shapes of amylose and amylopectin molecules have a direct effect on their ability to thicken a sauce. The straight amylose chains coil up into long helical structures when dissolved in water, but they retain their basically linear shape. Their elongation makes it very likely that one chain will knock into another or into a granule: each sweeps through a relatively large volume of liquid. By contrast, the branched shape of amylopectin makes for a compact target and therefore a molecule less likely to collide with others; and even if it does collide, it's less likely to get tangled up and slow the motion of other molecules and granules in the vicinity. A small number of very long amylose molecules, then, will do the job of more but shorter amylose molecules, and of many more bushy amylopectins. For this reason, the cook can obtain the same degree of thickening from a smaller amount of long-amylose potato starch than from moderate-amylose wheat and corn starches.

Two kinds of starch. Starch molecules are chains made up of hundreds or thousands of glucose molecules bonded together. They take two forms: straight chains of amylose (left), (left), and branched chains of amylopectin and branched chains of amylopectin (right). (right). A long amylose chain moves around in a larger volume than the more compact amylopectin containing the same number of glucose molecules, and is more likely to tangle with other chains. Amylose is therefore a more effective thickener than amylopectin. A long amylose chain moves around in a larger volume than the more compact amylopectin containing the same number of glucose molecules, and is more likely to tangle with other chains. Amylose is therefore a more effective thickener than amylopectin.

Pure StarchStarch has been separated from the proteins and other materials in grains since ancient times. The Romans called it amylum, amylum, which meant "not ground at the mill." They made it by grinding wheat in a mortar and then soaking the flour for days, during which bacteria grew and digested the grain's cell walls and gluten proteins while leaving the dense, solid starch grains intact. They reground the dregs, and then pressed them through fine linen, which retained the small grains. The starch grains were dried in the sun, and then either cooked in milk or used to thicken sauces (p. 583). which meant "not ground at the mill." They made it by grinding wheat in a mortar and then soaking the flour for days, during which bacteria grew and digested the grain's cell walls and gluten proteins while leaving the dense, solid starch grains intact. They reground the dregs, and then pressed them through fine linen, which retained the small grains. The starch grains were dried in the sun, and then either cooked in milk or used to thicken sauces (p. 583).

Swelling and Gelation What makes starch so useful is its behavior in hot water. Mix some flour or cornstarch into cold water, and nothing much happens. The starch granules slowly absorb a limited amount of water, about 30% of their own weight, and they simply sink to the bottom of the pot and sit there. But when the water gets hot enough, the energy of its molecules is sufficient to disrupt the weaker regions of the granule. The granules then absorb more water and swell up, thereby putting greater and greater stress on the more organized, stronger granule regions. Within a certain range of temperatures characteristic of each starch source but usually beginning around 120140F/5060C, the granules suddenly lose their organized structure, absorb a great deal of water, and become amorphous networks of starch and water intermingled. This temperature is called the What makes starch so useful is its behavior in hot water. Mix some flour or cornstarch into cold water, and nothing much happens. The starch granules slowly absorb a limited amount of water, about 30% of their own weight, and they simply sink to the bottom of the pot and sit there. But when the water gets hot enough, the energy of its molecules is sufficient to disrupt the weaker regions of the granule. The granules then absorb more water and swell up, thereby putting greater and greater stress on the more organized, stronger granule regions. Within a certain range of temperatures characteristic of each starch source but usually beginning around 120140F/5060C, the granules suddenly lose their organized structure, absorb a great deal of water, and become amorphous networks of starch and water intermingled. This temperature is called the gelation range, gelation range, because the granules become individual gels, or water-containing meshworks of long molecules. This range can be recognized by the fact that the initially cloudy suspension of granules suddenly becomes more translucent. The individual starch molecules become less closely packed together and don't deflect as many light rays, and so the mixture becomes clearer. because the granules become individual gels, or water-containing meshworks of long molecules. This range can be recognized by the fact that the initially cloudy suspension of granules suddenly becomes more translucent. The individual starch molecules become less closely packed together and don't deflect as many light rays, and so the mixture becomes clearer.

Thickening: The Granules Leak Starch Depending on how concentrated the starch granules are to begin with, the starch-water mixture may noticeably thicken at various points during their swelling and gelation. Most sauces are rather dilute (less than 5% starch by weight) and thicken during gelation, when the mixture begins to become translucent. They reach their greatest thickness after the gelated granules begin to leak amylose and amylopectin molecules into the surrounding liquid. The long amylose molecules form something like a three-dimensional fishnet that not only entraps pockets of water, but blocks the movements of the whale-like, water-swollen starch granules. Depending on how concentrated the starch granules are to begin with, the starch-water mixture may noticeably thicken at various points during their swelling and gelation. Most sauces are rather dilute (less than 5% starch by weight) and thicken during gelation, when the mixture begins to become translucent. They reach their greatest thickness after the gelated granules begin to leak amylose and amylopectin molecules into the surrounding liquid. The long amylose molecules form something like a three-dimensional fishnet that not only entraps pockets of water, but blocks the movements of the whale-like, water-swollen starch granules.

Thinning: The Granules Break Once it reaches its thickest consistency, the starch-water mixture will slowly thin out again. There are three different things that the cook may do that encourage thinning: heating for a long period of time after thickening occurs, heating all the way to the boil, and vigorous stirring. All of these have the same effect: they shatter the swollen and fragile granules into very small fragments. While this does mean that even more amylose is released into the water, it also means that there are many fewer large bodies to get caught in the amylose tangle. In other words, the amount of netting increases, the mesh grows finer, but at the same time the big whales become small minnows. This thinning effect is especially striking in the case of very thick pastes, less obvious in normal sauces. If the granules are few and far between to begin with, their disintegration is less noticeable. This thinning is accompanied by a greater refinement of texture, as the starch particles disappear and only indetectably small molecules remain. Once it reaches its thickest consistency, the starch-water mixture will slowly thin out again. There are three different things that the cook may do that encourage thinning: heating for a long period of time after thickening occurs, heating all the way to the boil, and vigorous stirring. All of these have the same effect: they shatter the swollen and fragile granules into very small fragments. While this does mean that even more amylose is released into the water, it also means that there are many fewer large bodies to get caught in the amylose tangle. In other words, the amount of netting increases, the mesh grows finer, but at the same time the big whales become small minnows. This thinning effect is especially striking in the case of very thick pastes, less obvious in normal sauces. If the granules are few and far between to begin with, their disintegration is less noticeable. This thinning is accompanied by a greater refinement of texture, as the starch particles disappear and only indetectably small molecules remain.

Some of the thinning of long-simmered starch-based sauces is caused by the gradual breakdown of the starch molecules themselves into smaller fragments. Acidity accelerates this breakdown.

Thickening a sauce with starch. Uncooked starch granules offer little obstruction to the flow of the surrounding liquid (left). (left). As the sauce heats up and the temperature reaches the gelation range, the granules absorb water and swell, and the sauce consistency begins to thicken As the sauce heats up and the temperature reaches the gelation range, the granules absorb water and swell, and the sauce consistency begins to thicken (center). (center). As cooking continues and the temperature approaches the boil, the granules swell even more and leak starch chains into the liquid As cooking continues and the temperature approaches the boil, the granules swell even more and leak starch chains into the liquid (right). (right). It's at this stage that the sauce reaches its maximum thickness. It's at this stage that the sauce reaches its maximum thickness.

Cooling, Further Thickening, and Congealing Once the starch in a sauce has gelated, its amylose has leaked out, and the cook judges the sauce to be properly cooked, he stops the cooking, and the temperature of the sauce begins to fall. As the mixture cools down, the water and starch molecules move with less and less energy, and at a certain point the force of the temporary bonds among them begins to hold the molecules together longer than they are kept apart by random collisions. Gradually, the longer amylose molecules form stable bonds among themselves, the kind of bonds that held them together in the granule initially. Water molecules settle in the pockets between starch chains. As a result, the liquid mixture gets progressively thicker. If the amylose molecules are concentrated enough, and the temperature falls far enough, the liquid mixture congeals into a solid gel, just as a gelatin solution settles into a jelly. (Bushy amylopectin molecules take much longer to bond to each other, so low-amylose starches are slow to congeal.) This is the way in which pie fillings, puddings, and similar solid but moist starch concoctions are made. Once the starch in a sauce has gelated, its amylose has leaked out, and the cook judges the sauce to be properly cooked, he stops the cooking, and the temperature of the sauce begins to fall. As the mixture cools down, the water and starch molecules move with less and less energy, and at a certain point the force of the temporary bonds among them begins to hold the molecules together longer than they are kept apart by random collisions. Gradually, the longer amylose molecules form stable bonds among themselves, the kind of bonds that held them together in the granule initially. Water molecules settle in the pockets between starch chains. As a result, the liquid mixture gets progressively thicker. If the amylose molecules are concentrated enough, and the temperature falls far enough, the liquid mixture congeals into a solid gel, just as a gelatin solution settles into a jelly. (Bushy amylopectin molecules take much longer to bond to each other, so low-amylose starches are slow to congeal.) This is the way in which pie fillings, puddings, and similar solid but moist starch concoctions are made.

Judge Sauce Consistency at Serving Temperatures It's important for the cook to antic.i.p.ate this cooling and thickening. We create and evaluate most sauces on the stove at high temperatures, around 200F/93C, but when they're poured in a thin layer onto food and served, they immediately begin to cool and thicken. However thick a sauce is in the pan, it's going to be thicker when the diner actually tastes it, and it may even congeal on the plate. So sauces should be thinner at the stove than they're meant to be at the table. (Minimizing the amount of thickener will also reduce the extent to which the sauce's flavor is muted.) The best way to predict the final texture of a sauce is to pour a spoonful into a cool dish and then sample it. It's important for the cook to antic.i.p.ate this cooling and thickening. We create and evaluate most sauces on the stove at high temperatures, around 200F/93C, but when they're poured in a thin layer onto food and served, they immediately begin to cool and thicken. However thick a sauce is in the pan, it's going to be thicker when the diner actually tastes it, and it may even congeal on the plate. So sauces should be thinner at the stove than they're meant to be at the table. (Minimizing the amount of thickener will also reduce the extent to which the sauce's flavor is muted.) The best way to predict the final texture of a sauce is to pour a spoonful into a cool dish and then sample it.

Starch in sauce making. A swollen granule of potato starch caught in a meshwork of molecules freed from it and other granules (left). (left). A starch-thickened sauce is thickest at this stage, when both starch granules and molecules block the movement of water. A granule of wheat starch that has lost nearly all of its starch molecules to the surrounding liquid A starch-thickened sauce is thickest at this stage, when both starch granules and molecules block the movement of water. A granule of wheat starch that has lost nearly all of its starch molecules to the surrounding liquid (right). (right). As the granules in a starch-thickened sauce disintegrate, they no longer get caught in the mesh of free starch, and the sauce thins out. As the granules in a starch-thickened sauce disintegrate, they no longer get caught in the mesh of free starch, and the sauce thins out.

Different Starches and Their Qualities Cooks have several different forms of starch to choose among for thickening sauces, each with its own particular qualities. They fall into two families: starches from grains, including flour and cornstarch, and starches from tubers and roots, including potato starch and arrowroot. Less commonly seen except on the ingredient labels of processed foods is sago starch, from the stem pith of a Pacific palm (Metroxylon sagu).

Grain Starches Starches from grains tend to share several characteristics. Their starch granules are medium-sized, and contain small but significant amounts of lipids (fats, fatty acids, phospholipids) and protein. These impurities somehow give the starch granules some structural stability, which means that it takes a higher temperature to gelate them; and they lend a cloudiness and distinct "cereal" flavor to starch-water mixtures. Light that pa.s.ses right through a gelated mesh of pure starch and water is scattered by tiny starch-lipid or starch-protein complexes, producing a milky, impenetrable appearance. Grain starches contain a high proportion of moderately long amylose molecules that readily form a network with each other, and so make sauces that quickly thicken and congeal when cooled. Starches from grains tend to share several characteristics. Their starch granules are medium-sized, and contain small but significant amounts of lipids (fats, fatty acids, phospholipids) and protein. These impurities somehow give the starch granules some structural stability, which means that it takes a higher temperature to gelate them; and they lend a cloudiness and distinct "cereal" flavor to starch-water mixtures. Light that pa.s.ses right through a gelated mesh of pure starch and water is scattered by tiny starch-lipid or starch-protein complexes, producing a milky, impenetrable appearance. Grain starches contain a high proportion of moderately long amylose molecules that readily form a network with each other, and so make sauces that quickly thicken and congeal when cooled.

Wheat Flour Wheat flour is made by grinding wheat grains and sieving the bran and germ from the starch-rich endosperm(p. 528). Wheat flour is only about 75% starch, and includes about 10% by weight of protein, mainly the insoluble gluten proteins. It's therefore a less efficient thickener than pure cornstarch or potato starch; it takes more flour to obtain the same consistency. A common rule of thumb is to use 1.5 times as much flour as starch. Flour has a distinct wheat flavor that cooks often transform by precooking the flour before adding it to a sauce (p. 617). The suspended particles of gluten protein make flour-based sauces especially opaque and give their surface a matte appearance, unless the sauce is cooked for hours and skimmed to remove the gluten. Wheat flour is made by grinding wheat grains and sieving the bran and germ from the starch-rich endosperm(p. 528). Wheat flour is only about 75% starch, and includes about 10% by weight of protein, mainly the insoluble gluten proteins. It's therefore a less efficient thickener than pure cornstarch or potato starch; it takes more flour to obtain the same consistency. A common rule of thumb is to use 1.5 times as much flour as starch. Flour has a distinct wheat flavor that cooks often transform by precooking the flour before adding it to a sauce (p. 617). The suspended particles of gluten protein make flour-based sauces especially opaque and give their surface a matte appearance, unless the sauce is cooked for hours and skimmed to remove the gluten.

Cornstarch Cornstarch is practically pure starch and so a more efficient thickener than flour. Cornstarch is manufactured by soaking the whole maize grain, milling it coa.r.s.ely to remove the germ and hull, and grinding, sieving, and centrifuging the remainder to separate the seed proteins. The resulting starch is washed, dried, and reground into a fine powder consisting of single granules or small aggregates. During this wet processing, the starch granules absorb odors and develop their own when their traces of lipids are oxidized, so cornstarch has a distinctive flavor unlike that of wheat flour, which is milled dry. Cornstarch is practically pure starch and so a more efficient thickener than flour. Cornstarch is manufactured by soaking the whole maize grain, milling it coa.r.s.ely to remove the germ and hull, and grinding, sieving, and centrifuging the remainder to separate the seed proteins. The resulting starch is washed, dried, and reground into a fine powder consisting of single granules or small aggregates. During this wet processing, the starch granules absorb odors and develop their own when their traces of lipids are oxidized, so cornstarch has a distinctive flavor unlike that of wheat flour, which is milled dry.

Rice Starch Rice starch is seldom seen in Western markets. Its granules have the smallest average size of the starches, and produce an especially fine texture in the early stages of thickening. Rice starch is seldom seen in Western markets. Its granules have the smallest average size of the starches, and produce an especially fine texture in the early stages of thickening.

Tuber and Root Starches Compared to the starches from dry grains, the starches from moist underground storage organs come in the form of larger granules that retain more water molecules, cook faster, and release starch at lower temperatures. They contain less amylose, but their amylose chains are up to four times longer than cereal amyloses. Root and tuber starches contain a fraction of the lipids and proteins that are a.s.sociated with cereal starches, which makes them more readily gelated - lipids delay gelation by stabilizing granule structure - and gives them less p.r.o.nounced flavors. These starches leave their sauces with a translucent, glossy appearance. The properties of root starches suit them for last-minute corrections of sauce consistency: less of them is required to lend a given thickness, they thicken quickly, and don't need precooking to improve their flavor. Compared to the starches from dry grains, the starches from moist underground storage organs come in the form of larger granules that retain more water molecules, cook faster, and release starch at lower temperatures. They contain less amylose, but their amylose chains are up to four times longer than cereal amyloses. Root and tuber starches contain a fraction of the lipids and proteins that are a.s.sociated with cereal starches, which makes them more readily gelated - lipids delay gelation by stabilizing granule structure - and gives them less p.r.o.nounced flavors. These starches leave their sauces with a translucent, glossy appearance. The properties of root starches suit them for last-minute corrections of sauce consistency: less of them is required to lend a given thickness, they thicken quickly, and don't need precooking to improve their flavor.

Potato Starch Potato starch was the first commercially important refined starch and is still an important food starch in Europe. It is unusual for several characteristics. Its granules are very large, up to a tenth of a millimeter across, and its amylose molecules are very long. This combination gives potato starch an initial thickening power far greater than that of any other starch. The long amylose chains tangle with each other and with the giant granules to block easy movement of the sauce fluid. This entanglement also creates long aggregates of amylose and granules that can give the impression of stringiness. And the large swollen granules give a noticeable initial graininess to sauces. However the granules are fragile, and readily fragment into finer particles; so having reached its thickest and grainiest, the consistency of a potato-starch sauce rapidly gets both finer and thinner. Potato starch is also unusual for having a large number of attached phosphate groups, which carry a weak electric charge and cause the starch chains to repel each other. This repulsion helps keep the starch chains evenly dispersed in a sauce, and contributes to the thickness and clarity of the dispersion and its low tendency to congeal into a gel on cooling. Potato starch was the first commercially important refined starch and is still an important food starch in Europe. It is unusual for several characteristics. Its granules are very large, up to a tenth of a millimeter across, and its amylose molecules are very long. This combination gives potato starch an initial thickening power far greater than that of any other starch. The long amylose chains tangle with each other and with the giant granules to block easy movement of the sauce fluid. This entanglement also creates long aggregates of amylose and granules that can give the impression of stringiness. And the large swollen granules give a noticeable initial graininess to sauces. However the granules are fragile, and readily fragment into finer particles; so having reached its thickest and grainiest, the consistency of a potato-starch sauce rapidly gets both finer and thinner. Potato starch is also unusual for having a large number of attached phosphate groups, which carry a weak electric charge and cause the starch chains to repel each other. This repulsion helps keep the starch chains evenly dispersed in a sauce, and contributes to the thickness and clarity of the dispersion and its low tendency to congeal into a gel on cooling.

Tapioca Tapioca, derived from the root of a tropical plant known as manioc or ca.s.sava ( Tapioca, derived from the root of a tropical plant known as manioc or ca.s.sava (Manihot esculenta, p. 305), is a root starch used mostly in puddings. It tends to form unpleasantly stringy a.s.sociations in water and so is usually made into large pregelatinized pearls (p. 578), which are then cooked only long enough to be softened. Because tapioca keeps well in the ground and is processed into starch within days of harvest, it doesn't develop the strong aromas found in wheat and corn starches or in potato starch, which is typically extracted from long-stored, second-quality tubers. Tapioca starch is especially prized for its neutral flavor. p. 305), is a root starch used mostly in puddings. It tends to form unpleasantly stringy a.s.sociations in water and so is usually made into large pregelatinized pearls (p. 578), which are then cooked only long enough to be softened. Because tapioca keeps well in the ground and is processed into starch within days of harvest, it doesn't develop the strong aromas found in wheat and corn starches or in potato starch, which is typically extracted from long-stored, second-quality tubers. Tapioca starch is especially prized for its neutral flavor.

Properties of Some Common Thickening Starches Cooked in Water

Gelation Temperature Maximum Thickness Maximum Thickness

Wheat 126185F 5285C 126185F 5285C + +.

Corn 144180F 6280C 144180F 6280C ++ ++.

Potato 136150F 5865C 136150F 5865C +++++ +++++.

Tapioca 126150F 5265C 126150F 5265C +++ +++.

Arrowroot 140187F 6086C 140187F 6086C +++ +++.

Consistency Stability to Prolonged Cooking Stability to Prolonged Cooking

Wheat Short Short Good Good

Corn Short Short Moderate Moderate

Potato Stringy Stringy Poor Poor

Tapioca Stringy Stringy Poor Poor

Arrowroot Stringy Stringy Good Good

Appearance Flavor Flavor

Wheat Opaque Opaque Strong Strong

Corn Opaque Opaque Strong Strong

Potato Clear Clear Moderate Moderate

Tapioca Clear Clear Neutral Neutral

Arrowroot Clear Clear Neutral Neutral

Arrowroot Arrowroot starch as it's known in the West is refined from the roots of a West Indian plant ( Arrowroot starch as it's known in the West is refined from the roots of a West Indian plant (Maranta arundinacea). Arrowroot starch has smaller granules than potato or tapioca starches, produces a less stringy consistency, and doesn't thin out as much on prolonged cooking. Its gelation temperature is higher than the other root starches, more like the range for cornstarch. A number of other plants and their starches are also called arrowroot in Asia and Australia (species of Tacca, Hutchenia, Canna Tacca, Hutchenia, Canna).

Root Starches in China In China, starch was originally extracted from millet and water chestnuts. Nowadays most Chinese sauces are thickened with corn, potato, or sweet potato starch - all plants from the New World. Other Asian sources of starch are yams, ginger, lotus, and the tuber of the kudzu vine ( In China, starch was originally extracted from millet and water chestnuts. Nowadays most Chinese sauces are thickened with corn, potato, or sweet potato starch - all plants from the New World. Other Asian sources of starch are yams, ginger, lotus, and the tuber of the kudzu vine (Pueraria).

Modified Starches Food manufacturers have not been content with the starches available in nature, mainly because the consistency they create isn't stable throughout the cycle of production, distribution, storage, and use by the consumer. They've therefore engineered a variety of starches that are more stable. Plant breeders have developed so-called "waxy" varieties of corn whose seeds contain little or no amylose and are nearly all amylopectin, which doesn't form networks as readily as amylose. Waxy starches therefore make sauces and gels that resist congealing and separation into a firm solid phase and watery residue, a problem to which high-amylose starches are p.r.o.ne. Food manufacturers have not been content with the starches available in nature, mainly because the consistency they create isn't stable throughout the cycle of production, distribution, storage, and use by the consumer. They've therefore engineered a variety of starches that are more stable. Plant breeders have developed so-called "waxy" varieties of corn whose seeds contain little or no amylose and are nearly all amylopectin, which doesn't form networks as readily as amylose. Waxy starches therefore make sauces and gels that resist congealing and separation into a firm solid phase and watery residue, a problem to which high-amylose starches are p.r.o.ne.

Ingredient manufacturers also use physical and chemical treatments to modify the starch molecules from standard plant varieties. They precook and dry starches in various ways to produce powders or granules that readily absorb cold water or disperse in and thicken liquids without requiring cooking. And they alter them with chemicals - cross-linking chains to each other, or oxidizing them, or subst.i.tuting fat-soluble side groups along the chain - to make them less p.r.o.ne to breakdown during cooking, to make them more effective emulsion stabilizers, and to give them other qualities that "native" starches don't normally have. Such starches are listed on ingredient labels as "modified starch."