The Influence of Other Ingredients on Starch Sauces Flavorings: Salt, Sugar, Acid Starch and water are the basis for a sauce's structure, and most other ingredients have only secondary effects on that structure. Salt, acid, and sugar are frequently added for their contributions to flavor. Salt slightly lowers the gelation temperature of starch, while sugars increase it. Acids in the form of wine or vinegar encourage the fragmentation of starch chains into much shorter lengths, so that starch granules gelate and disintegrate at lower temperatures, and the final product is less viscous for a given amount of starch. Root starches are noticeably affected by moderate acidity (a pH lower than 5), while grain starches can withstand the acidities typical of yogurt and many fruits (pH 4). Gentle and brief cooking will minimize acid breakdown. Starch and water are the basis for a sauce's structure, and most other ingredients have only secondary effects on that structure. Salt, acid, and sugar are frequently added for their contributions to flavor. Salt slightly lowers the gelation temperature of starch, while sugars increase it. Acids in the form of wine or vinegar encourage the fragmentation of starch chains into much shorter lengths, so that starch granules gelate and disintegrate at lower temperatures, and the final product is less viscous for a given amount of starch. Root starches are noticeably affected by moderate acidity (a pH lower than 5), while grain starches can withstand the acidities typical of yogurt and many fruits (pH 4). Gentle and brief cooking will minimize acid breakdown.

Proteins and Fats Two other materials are commonly found in sauces and have some influence on their texture. Flour is about 10% protein by weight, and much of this fraction is insoluble gluten. Gluten aggregations probably get caught in the starch network and so slightly increase the viscosity of the solution, though the pure starches are generally more powerful thickeners overall. Sauces based on concentrated meat stocks also contain a good deal of gelatin, but gelatin and starch seem not to affect each other's behavior. Two other materials are commonly found in sauces and have some influence on their texture. Flour is about 10% protein by weight, and much of this fraction is insoluble gluten. Gluten aggregations probably get caught in the starch network and so slightly increase the viscosity of the solution, though the pure starches are generally more powerful thickeners overall. Sauces based on concentrated meat stocks also contain a good deal of gelatin, but gelatin and starch seem not to affect each other's behavior.

Finally, fats are usually present in the form of b.u.t.ter, oil, or the drippings from a roast. They do not mix with water or water-soluble compounds, but they do slow the penetration of water into starch granules. Fat does contribute the sensations of smoothness and moistness to a sauce, and when used to precook the flour in a roux, it coats the flour particles, prevents them from clumping together in the water, and so safeguards against lumps.

Incorporating Starch into Sauces In order to thicken a sauce with starch, the cook must get the starch into the sauce. Very basic, but not so simple! If you add flour or starch directly to a hot sauce, it lumps up and never disperses evenly: the moment they hit the hot liquid, the clumps of starch granules develop a partly gelated, sticky surface that seals the dry granules inside and prevents them from dispersing.

Slurries, Beurre Manie, Floured Meat Cooks use four methods for incorporating starch into a sauce. The first is to mix the starch with some cold water, so that the granules are wetted and separated before they encounter gelation temperatures. The starch-water slurry can then be added directly to the sauce. A second method is to separate the starch or flour particles not with water, but with fat. Cooks use four methods for incorporating starch into a sauce. The first is to mix the starch with some cold water, so that the granules are wetted and separated before they encounter gelation temperatures. The starch-water slurry can then be added directly to the sauce. A second method is to separate the starch or flour particles not with water, but with fat. Beurre manie, Beurre manie, or "kneaded b.u.t.ter," is flour worked into a paste with its weight in b.u.t.ter. When a piece of the paste is added to a hot sauce in need of last-minute thickening, the b.u.t.ter melts and gradually releases greased starch particles into the liquid, where their swelling and gelation are slowed by the water-repelling surface layer. or "kneaded b.u.t.ter," is flour worked into a paste with its weight in b.u.t.ter. When a piece of the paste is added to a hot sauce in need of last-minute thickening, the b.u.t.ter melts and gradually releases greased starch particles into the liquid, where their swelling and gelation are slowed by the water-repelling surface layer.

A third method for getting starch into a sauce is to introduce it early in the cooking, not late. Many stews and frica.s.sees are made by dusting pieces of meat with flour, then sauteing the pieces, and only then adding a cooking liquid that will become the sauce. In this way the starch has already been dispersed over the large surface area of the meat pieces, and it has been precoated with the sauteing fat, which prevents clumping when the liquid is added.

Roux The fourth method for getting starch into a sauce, and one that has been developed into a minor art of its own, is to preheat the starch separately in fat to make what the French call a The fourth method for getting starch into a sauce, and one that has been developed into a minor art of its own, is to preheat the starch separately in fat to make what the French call a roux, roux, from the word for "red." The basic principle works with any form of starch and any fat or oil. In the traditional French system, the cook carefully heats equal weights of flour and b.u.t.ter in a pan to one of three consecutive endpoints: the mixture has had the moisture cooked out of it, but the flour remains white; the flour develops a light yellow color; or the flour develops a distinctly brown color. from the word for "red." The basic principle works with any form of starch and any fat or oil. In the traditional French system, the cook carefully heats equal weights of flour and b.u.t.ter in a pan to one of three consecutive endpoints: the mixture has had the moisture cooked out of it, but the flour remains white; the flour develops a light yellow color; or the flour develops a distinctly brown color.

Improvements in Flavor, Color, and Dispersability In addition to coating the flour particles with fat and making them easier to disperse in a hot liquid, roux making has three other useful effects on the flour. First, it cooks out the raw cereal flavor and develops a rounded, toasty flavor that becomes more p.r.o.nounced and intense as the color darkens. Second, the color itself - the product of the same browning reactions between carbohydrates and proteins that produce the toasty flavor - can lend some depth to the color of the sauce. In addition to coating the flour particles with fat and making them easier to disperse in a hot liquid, roux making has three other useful effects on the flour. First, it cooks out the raw cereal flavor and develops a rounded, toasty flavor that becomes more p.r.o.nounced and intense as the color darkens. Second, the color itself - the product of the same browning reactions between carbohydrates and proteins that produce the toasty flavor - can lend some depth to the color of the sauce.

Finally, the heat causes some of the starch chains to split, and then to form new bonds with each other. This generally means that long chains and branches are broken down into smaller pieces that then form short branches on other molecules. The short, branched molecules are less efficient at thickening liquids than the long chains, but they're also slower to bond to each other and form a continuous network as the liquid cools. The sauce is therefore less p.r.o.ne to congeal on the plate. The darker the roux, the more starch chains are modified in this way, and so the more roux is required to create a given thickness. It takes more of a dark brown roux than a light one to thicken a given amount of liquid. (The industrial version of roux making to make a starch more dispersable and stable to cooling is called dextrinization, dextrinization, and involves heating dry starch together with some dilute acid or alkali to 375F/190C.) and involves heating dry starch together with some dilute acid or alkali to 375F/190C.) Outside of France, roux are especially prominent in the cooking of New Orleans, where flour is cooked to a number of different stages from pale to chocolate-brown, and where cooks may use several roux in a single gumbo or stew to lend their distinct layers of flavor.

Starch in Cla.s.sic French Sauces In the code formalized by Auguste Escoffier in 1902, there are three leading mother sauces that are thickened in part with flour: the stock-based brown and white sauces, or espagnole and veloute; and the milk-based bechamel. Each of these relies on a distinctive combination of roux and liquid. Brown sauce consists of a stock made from browned vegetables, meat, and bones, then reduced after thickening with a roux that is cooked until the flour browns as well. White sauce uses a stock made from un un browned meat, vegetables, and bones, and is bound with a pale yellow roux. Bechamel combines milk with a roux that is not allowed to change color at all. From these three parent sauces, the cook can produce scores of offspring simply by finishing the sauce with different seasonings and enrichments. browned meat, vegetables, and bones, and is bound with a pale yellow roux. Bechamel combines milk with a roux that is not allowed to change color at all. From these three parent sauces, the cook can produce scores of offspring simply by finishing the sauce with different seasonings and enrichments.

Once the roux has been added to the stock, the mixture is allowed to simmer for quite a while - two hours for veloute, and up to ten in the case of brown sauce. During this time, the flavor is concentrated as water evaporates, and the starch granules dissolve and disperse among the gelatin molecules, with a very smooth texture the result. Brown sauce is cooked for the longest time because it's meant to be quite clear to the eye, and this requires that the gluten proteins coagulate and be carried to the surface, where they and the tomato solids can be skimmed off.

Escoffier said that a sauce should have three characteristics: a "decided" taste, a texture that's smooth and light without being runny, and a glossy appearance. The taste is a matter of making fine stocks and being judicious in seasoning, while the consistency and appearance depend on how the thickening is accomplished. Generally, long and patient simmering is necessary, so that there will be little or no vestige of granular structure left to the starch, and the insoluble gluten proteins will be caught up in the surface sc.u.m and so removed from the sauce. Gelatin contributes some body to the stock-based sauces, but the starch is what gives them most of their viscosity. After reduction, the concentration of starch in these sauces is around 5%, the gelatin concentration probably about half that.

The First Printed Recipe for Roux RouxIt was long thought that the first recipes for roux appeared in late 17th-century French cookbooks, but here is one of two German recipes from 150 years before La Varenne. They suggest that this version of the starch thickener was developed in late medieval times.How to Cook a Wild Boar's Head, Also How to Prepare a Sauce for ItA wild boar's head should be boiled well in water and, when it is done, laid on a grate and basted with wine, then it will be thought to have been cooked in wine. Afterwards make a black or yellow sauce with it. First, when you would make a black sauce, you should heat up a little fat and brown a small spoonful of wheat flour in the fat and after that put good wine into it and good cherry syrup, so that it becomes black, and sugar, ginger, pepper, cloves and cinnamon, grapes, raisins and finely chopped almonds. And taste it, however it seems good to you, make it so.- Das Kochbuch der Sabina Welserin, Das Kochbuch der Sabina Welserin, 1533, transl. Valoise Armstrong 1533, transl. Valoise Armstrong Milk-Based Sauces: Bechamel and Boiled Sauces based on milk rather than stock are of course much easier to make, and more forgiving; because they're already milky, the cook doesn't have to worry about long simmering to clarify them. The cla.s.sic starch-thickened milk sauce is bechamel, whose only other ingredients are seasonings and the b.u.t.ter in which the starch is precooked for a couple of minutes. Once the milk has been added to the roux, the sauce is simmered for 3060 minutes with occasional skimming of the skin of milk and flour proteins that forms at the surface. Starch is more effective at thickening milk than it is meat stocks, apparently because it bonds both to the milk proteins and the fat globules and so recruits these weighty ingredients into its flow-slowing network. Thanks to its pleasant but neutral flavor, bechamel is a versatile sauce that can be imbued with many flavors and served with many mainingredients. It's also made in several thicknesses for a variety of purposes. Thick preparations (6% flour by weight) serve as the base for souffles, somewhat thinner ones as a moistening and enrichment for gratins. Sauces based on milk rather than stock are of course much easier to make, and more forgiving; because they're already milky, the cook doesn't have to worry about long simmering to clarify them. The cla.s.sic starch-thickened milk sauce is bechamel, whose only other ingredients are seasonings and the b.u.t.ter in which the starch is precooked for a couple of minutes. Once the milk has been added to the roux, the sauce is simmered for 3060 minutes with occasional skimming of the skin of milk and flour proteins that forms at the surface. Starch is more effective at thickening milk than it is meat stocks, apparently because it bonds both to the milk proteins and the fat globules and so recruits these weighty ingredients into its flow-slowing network. Thanks to its pleasant but neutral flavor, bechamel is a versatile sauce that can be imbued with many flavors and served with many mainingredients. It's also made in several thicknesses for a variety of purposes. Thick preparations (6% flour by weight) serve as the base for souffles, somewhat thinner ones as a moistening and enrichment for gratins.

In the "boiled dressing" often used in the United States to moisten coleslaw and other robust salads, flour not only thickens the milk and/or cream, but also helps prevent the vinegar from curdling the milk and egg-yolk proteins into coa.r.s.e particles.

Gravy We come now to the homely Anglo-American cousin of French sauces, the starch-thickened gravy typically made to accompany a roast. This is a last-minute sauce that's put together just before serving, and consists of the roast's juices, extended with additional liquid, and thickened with flour. The drippings from the roast, both fat and browned solids, give the gravy its flavor and color. First the fat is poured off and reserved, and the pan is "deglazed": the browned solids are lifted from the roasting pan with a small amount of water, wine, beer, or stock. The liquid dissolves the browning-reaction products that have stuck to the pan and so takes up their especially rich flavors. The deglazing liquid is poured off and reserved separately. Now some of the fat is returned to the pan with an equal volume of flour, and the flour cooked until it has lost its raw aroma. The deglazing liquid is added, around a cup/250 ml for every 12 tablespoons/1020 gm flour. The mixture is cooked until it thickens, a matter of a few minutes.

Escoffier on Future RouxThough he was a traditionalist in many ways, Escoffier openly looked forward to the day when pure starch would replace flour as the thickener in stock-based sauces.Indeed, if [starch] is absolutely necessary to give the mellowness and velvetiness to the sauce, it is much simpler to give it pure, which permits one to bring it to the point in as little time as possible, and to avoid a too prolonged sojourn on the fire. It is therefore infinitely probable that before long starch, fecula, or arrowroot obtained in a state of absolute purity will replace flour in the roux.Today's proponents of the cla.s.sic sauces, however, generally remain loyal to flour.

Because they're made at the last minute, gravies are not cooked long enough to cause the disintegration of the starch granules, and therefore generally have a slightly coa.r.s.e texture, even when lump-free. This gives gravies a character very different from that of the suave sauce: hearty, and when they are extremely thick, almost bready. The cook can obtain a smoother consistency by making an initial preparation from the flour and a fraction of the deglazing liquid, heating the mixture until the starch granules gelate and crowd up against each other to form a thick paste, and whisking the paste vigorously to smash the granules into each other and break them up into finer pieces. This paste is then mixed with the rest of the deglazing liquid and simmered until it's evenly dispersed and the liquid reaches the desired consistency.

Sauces Thickened with Plant Particles: Purees Some of the most delicious sauces we eat, including tomato sauces and applesauce, are made simply by crushing fruits and vegetables. Crushing, or pureeing, frees the juices from the cells of the fruit or vegetable, and breaks the cell walls into fragments that become suspended in the juices and block their flow, so giving them some thickness. Crushed nuts and spices have no juices of their own, but they thicken a liquid to which they're added by absorbing some of its water and providing dry cell particles that obstruct the liquid's flow.

Until recently, most purees of plant tissue would have been made by cooking the tissue to soften it, and then either grinding it in a mortar or forcing it through a fine sieve. Raw purees could only be made from fruits softened by ripening, or from brittle nuts. Today's cooks can use powerful machines - blenders, food processors - to puree any fruit or vegetable or seed with ease, whether they're raw or cooked.

Plant Particles: Coa.r.s.e and Inefficient Thickeners Compared to the other ways of thickening, simple pureeing tends to produce a coa.r.s.e sauce that more readily separates into solid particles and thin fluid. The solid fragments of plant cell walls are clumps of many thousands of carbohydrate and protein molecules. If those molecules were dispersed separately and finely throughout the fluid - as gelatin or starch molecules are in other sauces - they would bind many more water molecules, get tangled up in each other, and be far too small for the tongue to detect as particles. But plant-cell fragments range from 0.01 to 1 millimeter across; they give a grainy impression on the tongue and they're far less efficient than individual molecules at binding water or interfering with fluid flow. And because the fragments are usually denser than the cell fluids, they end up sinking and separating from the fluids. Heating without stirring tends to speed this separation, because the free water is able to flow and rise from the bottom of the pot through the thicker particle phase, and acc.u.mulate above it.

Some sauces and related preparations aren't meant to be suave and smooth; instead the cook leaves some pieces of tissue intact to highlight the texture of the fruit or vegetable itself. Mexican tomato and tomatillo salsas, unstrained cranberry sauce, and applesauce are familiar examples.

Food Words: Puree PureeThe word puree, puree, meaning thoroughly crushed fruits, vegetables, or animal tissue, comes ultimately from the Latin meaning thoroughly crushed fruits, vegetables, or animal tissue, comes ultimately from the Latin purus, purus, meaning "pure." England borrowed a form of the French descendent, the verb meaning "pure." England borrowed a form of the French descendent, the verb purer, purer, which had both a general meaning, "to purify," and a specific one: to drain excess water from beans and peas left to soak. The beans and peas would go on to be cooked into a mush, and the consistency of that mush appears to be the prototype of other purees. which had both a general meaning, "to purify," and a specific one: to drain excess water from beans and peas left to soak. The beans and peas would go on to be cooked into a mush, and the consistency of that mush appears to be the prototype of other purees.

Refining the Texture of Purees Cooks can refine the basic coa.r.s.eness of purees by modifying either the solid plant particles or the fluid that surrounds them. Cooks can refine the basic coa.r.s.eness of purees by modifying either the solid plant particles or the fluid that surrounds them.

Making the Plant Particles Smaller There are several ways to make the plant particles as fine as possible. There are several ways to make the plant particles as fine as possible.

The pureeing process itself is a physical crushing or shearing that breaks the plant tissue into pieces and liberates thickening molecules from them. Blenders and mortars are the most effective tools for this; food processors slice rather than crush. Producing a fine puree can take some time even in a blender, several minutes or more.

Straining through a sieve or cheesecloth removes the large particles, and forcing the puree through a fine mesh breaks large particles into smaller ones.

Heating softens cell walls so that they'll break into smaller pieces, and shakes loose long-chain carbohydrates from the cell walls and gets them into the watery phase, where they can act as separate starch and gelatin molecules do.

Freezing a puree and then thawing it causes ice crystals to damage cell walls, which can help liberate more pectin and hemicellulose molecules into the liquid.

A fruit or vegetable puree. Grinding plant tissue turns it inside out, freeing the cell fluids and breaking the cell walls and other structures into small particles. A puree is a mixture of plant particles and molecules floating in water (left). (left). If left to stand, most purees will separate, with the larger particles settling to the bottom If left to stand, most purees will separate, with the larger particles settling to the bottom (center). (center). This separation can be prevented, and the puree consistency thickened, by cooking the puree down and evaporating the excess water This separation can be prevented, and the puree consistency thickened, by cooking the puree down and evaporating the excess water (right). (right).

Preventing Separation The consistency of a puree is also improved by reducing the amount of water in the continuous phase. The simplest way to do this is to cook the whole puree down, simmering gently, until the separate thin phase disappears. Another way that better preserves the puree's fresh flavor is to drain the thin fluid off the solids and either discard it, or cook it down separately and then add it back. Or the cook can remove some of the fruit's or vegetable's water before crushing it, for example by partly drying halved tomatoes in the oven. The consistency of a puree is also improved by reducing the amount of water in the continuous phase. The simplest way to do this is to cook the whole puree down, simmering gently, until the separate thin phase disappears. Another way that better preserves the puree's fresh flavor is to drain the thin fluid off the solids and either discard it, or cook it down separately and then add it back. Or the cook can remove some of the fruit's or vegetable's water before crushing it, for example by partly drying halved tomatoes in the oven.

The binding power of the puree particles themselves can be supplemented by adding some other thickener, dry spices or nuts for example, or flour or starch.

Fruit and Vegetable Purees Any fruit or vegetable can be turned into a sauce by crushing it. Here are brief observations about some of the more commonly pureed foods.

Raw Purees: Fruits Raw purees are generally made from fruits, whose ripening enzymes often break down their cell walls from within, and thus allow their intact flesh to turn into a puree in the mouth. Raspberries, strawberries, melons, mangoes, and bananas are examples of such naturally soft fruits. The flavor of a raw puree is usually accentuated by the addition of sugar, lemon juice, and aromatic herbs or spices. But that flavor is fragile and changeable. Pureeing mixes the cell contents with each other and with oxygen in the air, so enzyme action and oxidation begin immediately (see below for the effects in cooked purees of tomato, a botanical fruit). The best way to minimize this change is to chill the puree, which slows all chemical reactions. Raw purees are generally made from fruits, whose ripening enzymes often break down their cell walls from within, and thus allow their intact flesh to turn into a puree in the mouth. Raspberries, strawberries, melons, mangoes, and bananas are examples of such naturally soft fruits. The flavor of a raw puree is usually accentuated by the addition of sugar, lemon juice, and aromatic herbs or spices. But that flavor is fragile and changeable. Pureeing mixes the cell contents with each other and with oxygen in the air, so enzyme action and oxidation begin immediately (see below for the effects in cooked purees of tomato, a botanical fruit). The best way to minimize this change is to chill the puree, which slows all chemical reactions.

Raw Purees: Pesto The Italian puree of basil leaves, The Italian puree of basil leaves, pesto genovese, pesto genovese, also contains olive oil and so is partly an emulsion as well. Pesto takes its name from the same root that gives us also contains olive oil and so is partly an emulsion as well. Pesto takes its name from the same root that gives us pestle, pestle, and the basil leaves and garlic were traditionally ground with a pestle and mortar. Because this takes some time and effort, modern cooks usually prepare pesto in a blender or food processor. The choice of appliance and how it's used influence both consistency and flavor. The crushing and shearing action of the pestle, the shearing action of the blender, and the slicing action of the processor all produce different proportions of intact and broken cells. The more thoroughly the cells are broken, the more their contents are exposed to each other and to the air, and the more their flavor evolves. A coa.r.s.e pesto will have a flavor most like the flavor of fresh basil leaves. and the basil leaves and garlic were traditionally ground with a pestle and mortar. Because this takes some time and effort, modern cooks usually prepare pesto in a blender or food processor. The choice of appliance and how it's used influence both consistency and flavor. The crushing and shearing action of the pestle, the shearing action of the blender, and the slicing action of the processor all produce different proportions of intact and broken cells. The more thoroughly the cells are broken, the more their contents are exposed to each other and to the air, and the more their flavor evolves. A coa.r.s.e pesto will have a flavor most like the flavor of fresh basil leaves.

Cooked Purees: Vegetables, Applesauce Most vegetable purees are made by first cooking the vegetable to soften its tissues, break apart the cells, and free their thickening molecules. Some that develop an especially suave smoothness have cell walls rich in soluble pectin, which escapes from the softened wall fragments during pureeing. These vegetables include carrots, cauliflower, and capsic.u.m peppers; more than 75% of the cell-wall solids in capsic.u.m puree is pectin. Many root and tuber vegetables (though not carrots) contain starch granules, which when cooked absorb much of the water in the vegetable and make it less watery. However, such vegetables are best crushed gently, without breaking open the cells. Thorough pureeing that liberates the gelated starch turns the vegetable into a super-thick potato gravy, gluey and stringy. Most vegetable purees are made by first cooking the vegetable to soften its tissues, break apart the cells, and free their thickening molecules. Some that develop an especially suave smoothness have cell walls rich in soluble pectin, which escapes from the softened wall fragments during pureeing. These vegetables include carrots, cauliflower, and capsic.u.m peppers; more than 75% of the cell-wall solids in capsic.u.m puree is pectin. Many root and tuber vegetables (though not carrots) contain starch granules, which when cooked absorb much of the water in the vegetable and make it less watery. However, such vegetables are best crushed gently, without breaking open the cells. Thorough pureeing that liberates the gelated starch turns the vegetable into a super-thick potato gravy, gluey and stringy.

Even though fruits are presoftened by ripening, cooks often heat them to improve their texture, flavor, and storage life. One of the most popular cooked fruit sauces is applesauce, which is meant to have a certain coa.r.s.eness and yet not seem grainy. The cells of different varieties have different tendencies to adhere to each other, and that tendency can change with time in storage. Most of the soft varieties used to make sauce produce finer-grained purees with time, while the Macintosh produces coa.r.s.er ones.

Tomato Sauce: The Importance of Enzymes and Temperatures The most familiar vegetable puree in the West, and perhaps in the world, is tomato sauce and paste. The solids in tomatoes are about two-thirds flavorful sugars and organic acids, and 20% cell-wall carbohydrates that have some thickening power (10% cellulose, and 5% each pectin and hemicelluloses). In the United States, commercial tomato purees may include all the water in the original tomatoes, or just a third. Tomato paste is tomato puree cooked down so that it contains less than a fifth of the water of the raw vegetable. Tomato paste is thus a concentrated source of flavor, color, and thickening power. (It's also an effective emulsion stabilizer; see p. 628.) The most familiar vegetable puree in the West, and perhaps in the world, is tomato sauce and paste. The solids in tomatoes are about two-thirds flavorful sugars and organic acids, and 20% cell-wall carbohydrates that have some thickening power (10% cellulose, and 5% each pectin and hemicelluloses). In the United States, commercial tomato purees may include all the water in the original tomatoes, or just a third. Tomato paste is tomato puree cooked down so that it contains less than a fifth of the water of the raw vegetable. Tomato paste is thus a concentrated source of flavor, color, and thickening power. (It's also an effective emulsion stabilizer; see p. 628.) There are several variables in the preparation of purees that can affect their final texture and flavor. Food scientists have shown this most clearly for ma.s.s-produced tomato puree. The general lessons are also relevant to the preparation of purees from other fruits and vegetables.

Tomato Enzymes and Consistency The final consistency of a tomato puree depends not just on how much water has been removed, but also on how long the puree spends at either moderate or high temperatures. Ripe tomatoes have very active enzymes whose job is to break down pectin and cellulose molecules in the fruit cell walls, and so give the fruit its soft, fragile texture. When the tomatoes are firstcrushed, the enzymes and their target molecules are thoroughly mixed together, and the enzymes start breaking down the cell-wall structures. If the raw puree is held at room temperature for a while, or heated to a temperature below the denaturation temperature of the pectin enzymes, around 180F/80C, then the enzymes will break down a lot of the cell-wall reinforcements, and these liberated molecules will give a noticeably thicker consistency to the puree. The final consistency of a tomato puree depends not just on how much water has been removed, but also on how long the puree spends at either moderate or high temperatures. Ripe tomatoes have very active enzymes whose job is to break down pectin and cellulose molecules in the fruit cell walls, and so give the fruit its soft, fragile texture. When the tomatoes are firstcrushed, the enzymes and their target molecules are thoroughly mixed together, and the enzymes start breaking down the cell-wall structures. If the raw puree is held at room temperature for a while, or heated to a temperature below the denaturation temperature of the pectin enzymes, around 180F/80C, then the enzymes will break down a lot of the cell-wall reinforcements, and these liberated molecules will give a noticeably thicker consistency to the puree.

However, when the puree is then heated to remove water and concentrate it, the high temperatures break the already enzyme-damaged molecules into smaller pieces that are less efficient thickeners, and the paste requires that much greater reduction to obtain the desired thickness. If instead the raw puree is cooked quickly close to the boil, the result is a thicker sauce that requires less subsequent reduction. The pectin and cellulose enzymes are denatured and become inactive, while at the same time the cell walls are disrupted by the heat. The cell-wall pectins that escape into the fluid phase during the concentration heating are longer molecules and more efficient thickeners.

The Thickening Components of Tomatoes

Total Solids, % by weight Pectin and Hemicellulose Content, % by weight Pectin and Hemicellulose Content, % by weight

Raw tomatoes 510 510 0.51.0 0.51.0

Canned tomato puree 824 824 0.82.4 0.82.4

Canned tomato paste 40 40.

4 4.

Tomato Enzymes and Flavor In addition to tomato enzymes that affect texture, there are enzymes that affect flavor: and in the case of flavor, some initial enzyme activity can be desirable. The fresh, "green"-smelling molecules (hexa.n.a.l and hexanol, p. 274) that are an important element in ripe tomato flavor are generated by the action of enzymes on fatty acids when the fruit tissue is crushed, either in the mouth or in the pot. Rapid cooking to the boil minimizes this fresh flavor element, while allowing the raw puree to sit at room temperature - in a Mexican salsa, for example - or only slowly heating it, will cause the acc.u.mulation of these flavor molecules in the puree. A method that home cooks sometimes use is to halve or quarter tomatoes, then bake them in a slow oven to remove water, and finally cook them relatively quickly into a sauce. This technique minimizes the mixing of enzymes and targets, so cells stay relatively intact, and relatively little green aroma develops. In addition to tomato enzymes that affect texture, there are enzymes that affect flavor: and in the case of flavor, some initial enzyme activity can be desirable. The fresh, "green"-smelling molecules (hexa.n.a.l and hexanol, p. 274) that are an important element in ripe tomato flavor are generated by the action of enzymes on fatty acids when the fruit tissue is crushed, either in the mouth or in the pot. Rapid cooking to the boil minimizes this fresh flavor element, while allowing the raw puree to sit at room temperature - in a Mexican salsa, for example - or only slowly heating it, will cause the acc.u.mulation of these flavor molecules in the puree. A method that home cooks sometimes use is to halve or quarter tomatoes, then bake them in a slow oven to remove water, and finally cook them relatively quickly into a sauce. This technique minimizes the mixing of enzymes and targets, so cells stay relatively intact, and relatively little green aroma develops.

Then there's the traditional Italian preparation called estratto, estratto, which begins with fresh tomatoes cooked down to some extent, and then mixes them with some olive oil and spreads the paste on boards to dry down further in the sun. This is often described as a relatively "gentle" process compared to cooking, and it probably does spare some damage to the pectin molecules. But in fact it subjects a number of sensitive molecules - including the antioxidant tomato pigment lycopene and unsaturated fatty acids in the olive oil - to powerful and damaging ultraviolet light, which gives which begins with fresh tomatoes cooked down to some extent, and then mixes them with some olive oil and spreads the paste on boards to dry down further in the sun. This is often described as a relatively "gentle" process compared to cooking, and it probably does spare some damage to the pectin molecules. But in fact it subjects a number of sensitive molecules - including the antioxidant tomato pigment lycopene and unsaturated fatty acids in the olive oil - to powerful and damaging ultraviolet light, which gives estratto estratto a distinctively strong, cooked flavor. a distinctively strong, cooked flavor.

Nuts and Spices as Thickeners Among seeds and other dry plant materials, only oily nuts can be made into a sauce base on their own. When such nuts are ground into "b.u.t.ters," the oil provides the fluid continuous phase that lubricates the particles of cell walls and proteins. But most of the time, nuts are mixed with other ingredients, including liquids, so they become part of a complex suspension and help thicken both with their dry particles and with their oil, which becomes emulsified into tiny droplets. Almonds have long served this purpose in the Middle East and Mediterranean in such sauces as romesco (with red peppers, tomatoes, and olive oil) and picada (garlic, parsley, oil), and the coconut in tropical Asia, where it's pounded along with spices and herbs to become part of the sauce for cooked meats, fish, and vegetables.

Nuts and other finely ground seeds and spices help thicken liquid sauces thanks to their very dryness, which allows their particles to absorb water from the sauce and thus reduce the amount of liquid that needs to be filled with particles. At the same time, the particles themselves swell and become larger obstacles to the liquid's flow. Dry spices such as turmeric, c.u.min, and cinnamon are both flavorings and thickeners in Indian sauces, and coriander is especially effective thanks to its fibrous, absorbant seed coat. Dried chilli peppers, ground nuts, and spices thicken Mexican mole sauces. Powdered versions of dried chilli pepper are prominent in Spanish and Hungarian sauces (pimenton, paprika); mustard is also widely used. Some spices also release efficient thickening molecules into the liquid. Fenugreek seeds exude a gum that gives a gelatinous consistency to the Yemeni sauce hilbeh; and the dried leaves of the sa.s.safras tree, ground to make file powder, release carbohydrates that give Louisiana gumbos a slightly stringy viscosity.

Complex Mixtures: Indian Curries, Mexican Moles The most complex and sophisticated puree sauces are made in Asia and Mexico. The sauce or "gravy" for many Indian and Thai dishes begins with finely ground plant tissues - onions, ginger, garlic in northern India, coconut in southern India and in Thailand - and a number of different spices and herbs. These ingredients are then fried in hot oil until much of the moisture has boiled off, and the plant solids are sufficiently concentrated that the sauce clings to itself and the oil separates. The frying also cooks the sauce, eliminating raw flavors and developing new ones. The sauce is then slightly thinned with some water, and the main ingredient cooked in it. Mexican mole sauces are prepared in much the same way, except that the foundation ingredient is usually rehydrated dried chillis; pumpkin and other seeds are another major element. Thanks to the high pectin content of the chillis, moles have a more suave, finer consistency than the Asian purees. But both are marvels of mouth-filling pleasure.

Sauces Thickened with Droplets of Oil or Water: Emulsions The sauces we've examined so far are liquids thickened with a fine dispersion of solid materials: protein molecules, starch granules and molecules, particles of plant tissue and cell-wall molecules. A very different thickening method is to fill the water-based liquid with droplets of oil, which are much more ma.s.sive and slow-moving than individual molecules of water, impede their motion, and so create a thick and creamy consistency in the mixture as a whole. Such a dispersion of one liquid in another is called an emulsion. emulsion. The word comes from the Latin for "to milk out," and referred originally to the milky fluids that can be pressed from nuts and other plant tissues. Milk, cream, and egg yolks are natural emulsions, while sauce emulsions include mayonnaise, hollandaise sauce, beurre blanc, and oil-and-vinegar salad dressings. Modern chefs have applied the basic idea to the thickening of all kinds of liquids, and often actually describe the result on the menu as an The word comes from the Latin for "to milk out," and referred originally to the milky fluids that can be pressed from nuts and other plant tissues. Milk, cream, and egg yolks are natural emulsions, while sauce emulsions include mayonnaise, hollandaise sauce, beurre blanc, and oil-and-vinegar salad dressings. Modern chefs have applied the basic idea to the thickening of all kinds of liquids, and often actually describe the result on the menu as an emulsion, emulsion, a word that lingers on the tongue longer than a word that lingers on the tongue longer than sauce sauce does. does.

Emulsified sauces offer a special challenge to the cook: unlike sauces thickened with solids, emulsions are basically unstable. Whisk oil and a little vinegar together in a bowl, and the vinegar forms droplets in the oil: but they soon sink and coalesce, and in a few minutes the two liquids have separated again. Cooks not only have to form the emulsion, they also have to prevent the emulsion from being undone by the basic incompatibility of the two liquids.

The Nature of Emulsions An emulsion can only be made from two liquids that don't dissolve in each other, and therefore retain their distinct ident.i.ties even when mixed. The molecules of water and alcohol, for example, mix freely and so can't form an emulsion. In addition to sauces, cosmetic creams, floor and furniture waxes, some paints, asphalt, and crude oil are all emulsions of water and oil.

The Relative Proportions of Fat and Water in Common Food Emulsions

Food

Parts Fat to 100 Parts Water Parts Fat to 100 Parts Water

Fat-in-Water Emulsions

Whole milk

5 5.

Half-and-half

15 15.

Light cream

25 25.

Heavy cream

70 70.

Heavy cream reduced by a third

160 160.

Egg yolk

65 65.

Mayonnaise

400 400.

Water-in-Fat Emulsions

b.u.t.ter