22.2 Thickeners: roux, slurries, starches and others

How heat swells starch and thickens a liquid is explained with the science of heat. see chapter 7: Starch gelatinization

A sauce is set apart from a broth or soup by being both thicker and stronger in flavor. Today's diners tend to prefer sauces that look light, so many modern sauces are only lightly thickened, or thickened by reduction alone.

How thickeners work

Thickeners work in three main ways. Solids in a liquid: starch and vegetable purée spread through the liquid and get in the way of the water, so it flows less freely. Starch molecules unfold in the liquid and hold on to water. The scientific word for this kind of mixture is a sol. Liquid in a liquid: an emulsion, such as hollandaise or mayonnaise, holds tiny drops of fat in water. Air in a liquid: a foam, such as sabayon. A good béarnaise is both an emulsion and a foam at once.

Vegetable purées thicken too. Some hold enough starch to act like flour; most give a slightly rough, matt sauce, but they add flavor, which plain starch does not. Tomato and green vegetable purées hold very little starch. A sauce thickened by such a purée alone will separate on standing unless another thickener joins it.

Roux

The basic roux is equal parts flour and fat by weight. Heat or melt the fat, add all the flour, stir until they combine, and cook until the flour no longer tastes raw and the color is right. Use a heavy pan so it does not scorch, and stir often.

The flour matters. The thickening power of a flour comes from its starch. Cake or pastry flour, low in protein and high in starch, thickens better than bread flour; high-gluten flour has too little starch to use.

Roux is cooked to a range of colors, from nearly white to nearly black:

• White roux is cooked only briefly, just until it looks frothy and bubbly. It goes into white sauces such as béchamel, where little or no color is wanted.
  ▶ Watch
• Blond roux cooks a little longer and starts to take on color. It suits ivory sauces such as velouté.
  ▶ Watch
• Brown roux cooks until it is darker and smells and tastes nutty. It goes into brown sauces.
  ▶ Watch
• Dark brown roux, the Cajun dark roux, goes past a French brown roux. It is hard to make and gives dark gumbos their color.
  ▶ Watch

How long to cook it depends on the heat and the size of the batch. White roux needs only a few minutes. Brown roux takes much longer; in classic French kitchens the flour either cooks in clarified butter for 15 to 20 minutes or is browned in the oven first. Brown roux is rarely made in restaurants today. A finished roux should be stiff, not runny, with a slight shine and no taste or feel of raw flour.

Escoffier's three roux

Escoffier gave the same amounts for every roux and changed only the cooking. White roux cooks just a few minutes, long enough to remove the unpleasant raw flour taste. Pale (blond) roux comes off the heat the moment its color starts to change, before it colors at all. For about a pound of brown roux he used 8 oz of clarified butter measured by volume and 8 oz of flour by weight.

Brown roux, in his method, cooks in a very heavy pan, either in a moderate oven or pushed to the edge of the stove, stirred often so the heat spreads evenly. It is done when it is a light brown and smells of hazelnut, like baked flour. Do not rush it. Too much heat at the start burns the starch, so it can no longer bind; you then need two or three times as much roux, which clogs the sauce without thickening it and leaves a bitterness that nothing can remove.

Choosing the fat

Any fat you can eat will make roux. How fats behave under heat is covered with fats and oils. see chapter 19: Smoke point and flash point

• Butter is the usual French choice. Whole butter burns at the heat a dark roux needs, so use clarified butter or oil for dark roux.
  ▶ Watch
• Oil that smokes only at a high heat, soybean or canola for example, lets a dark roux cook hotter and faster. Each fat gives the roux a slightly different taste.
• Meat drippings suit meat gravies. Rendered bacon fat is common in regional American cooking.
  ▶ Watch
• Cajun roux uses bacon fat or oil, cooked to medium or dark brown: rich in flavor, but it gives a thinner result.
  ▶ Watch
• Central European cooks use rendered lard or, more recently, vegetable oil. In German the roux is called Mehlschwitze.
• Japanese curry is thickened with flour fried with butter or oil and yellow curry powder. In Ottoman and Turkish cooking, roux (meyane) goes back at least to the 1400s.
  ▶ Watch

Getting roux into a liquid without lumps

The trick is a difference in temperature between the roux and the liquid:

• Whisk warm or hot stock into hot roux, whisking hard.
• Or whisk room-temperature roux into hot stock.
  ▶ Watch

A stricter rule is to always add the liquid to the roux, a little at a time while stirring, never the roux to the liquid. Once the roux has loosened with enough liquid, it can safely go into more.

Avoid extremes. Very cold stock hits hot roux so hard that the mixture can seize, spit steam or burn, and roux colder than room temperature sets in hard bits that are difficult to whisk out. Roux that is extremely hot is a hazard: it can spit when the liquid hits it. Depending on the dish, the sauce's other ingredients may go in before the roux has finished cooking.

A roux does not start to thicken until the sauce is close to boiling, and it keeps thickening for several minutes as the sauce simmers. So do not add more too soon. A long simmer thickens the sauce further by reduction. Once it boils, simmer the sauce to cook out any flour taste; most chefs allow at least 20 minutes, and many recipes 30, skimming off froth and impurities as it goes. Flour carries proteins and other flavor compounds, so a roux sauce needs that skimming. Use a heavy pot so the long simmer does not scorch, and not an aluminum one: the whisk scrapes the metal, turns a pale sauce gray and gives it a metallic taste.

Skim the fat off the stock before the flour goes in. Flour holds fat in suspension so it cannot be skimmed later, and the sauce turns greasy and muddy. One good method is to use half the flour a classic recipe calls for, then reduce the sauce slowly to the thickness you want. That gives more time to skim and softens any taste of flour.

How much roux

Roux to liquid for a light, medium or heavy sauce.

On a home scale, white sauce takes 1, 2, 3 or 4 tablespoons of flour per cup of milk, for a thin, medium, thick or very thick sauce. To make it:

• Melt the fat.
  ▶ Watch
• Blend in the flour and salt.
  ▶ Watch
• Take the pan off the heat and stir in the milk.
  ▶ Watch
• Return it to the heat and stir gently while bringing it quickly to a full boil.
  ▶ Watch
• Boil for 1 minute if it will not be cooked further.
  ▶ Watch

Each thickness has its uses:

• Thin: cream soups and vegetable sauces.
  ▶ Watch
• Medium: a base for casseroles and cheese sauce.
  ▶ Watch
• Thick: soufflé.
  ▶ Watch
• Very thick: croquettes.
  ▶ Watch

Two variations start from the medium sauce. Tomato juice in place of milk, with no salt, gives a tomato sauce for soup or gravy. Cooking the flour and butter to light brown and using water, bouillon or vegetable juice gives a brown sauce for gravy and meat. A good cream soup made on this base is smooth, never slick or lumpy, with no skin, no fat on top and no raw starch taste.

Other ways to use flour

Beurre manié (kneaded butter) is equal weights of flour and soft whole butter worked into a smooth paste with a fork. Unlike roux, it is not cooked, and it goes in at the end, most often to a stew whose liquid is too thin. Roll it into pea-sized balls and whisk them into the simmering liquid a few at a time. You cannot judge the result until the liquid simmers again, so wait each time. Stop cooking once it is thick enough. The butter adds shine and flavor as it melts.

Flour has an odd habit. After about 2 minutes of cooking it tastes strongly of flour, and that taste fades only as cooking goes on, usually by 30 minutes. That is why a sauce finished with beurre manié should not keep cooking.

In country cooking, stew meat is often dusted with flour before browning (in French, singer). The flour browns fully, so it loses its starchy taste, and it helps the meat form a crust. Only a little flour goes in, so you can still add beurre manié or reduce the liquid at the end. Some cooks instead cook flour with the vegetables after the meat comes out; if so, pour off any burnt fat first and use fresh butter or olive oil. Browning and stewing are covered with slow cooking. see chapter 10: Cut and coat

A white wash, or "cowboy roux", is cold water and wheat flour stirred into a dish. The boiling liquid frees the starch, so it thickens, but it never gets hot enough to remove the raw flour taste, and restaurants seldom use it. For a lower-fat gumbo, some Creole chefs toast the flour in a hot dry pan with no oil.

Pure starches and the slurry

Cornstarch, arrowroot and potato starch are nearly pure starch. They give a shiny sauce, while the protein in flour leaves a slightly matt one. They thicken about twice as much liquid as the same amount of flour, and since less goes in, cornstarch leaves less of a floury taste. They cost little and add no flavor, but they only thicken. Unlike roux, they bring nothing to the taste of a dish.

A pure starch must be stirred into a little cool liquid first, to a thin paste called a slurry. Dropped straight into hot liquid it forms lumps that have to be strained out. Stir the slurry into hot liquid without stopping. Unlike roux, cornstarch starts thickening almost at once in a hot liquid. Then cook it gently, usually about 5 minutes, until the raw starch taste is gone. see chapter 7: Hydrocolloids

• Cornstarch is the most common thickener in the industry and popular in Asian cooking. Its gloss may or may not be wanted. Long heating can break it down so it loses its thickness, so a cornstarch sauce is less stable than a roux sauce and should not be reheated.
  ▶ Watch
• Arrowroot, from the roots of tropical plants, is used exactly like cornstarch. It holds up longer under heat and is a little clearer, but costs much more. It suits delicate soups, sauces, puddings and custards.
• Potato starch came early to French cooking but never caught on for sauces in the United States. Like cornstarch, it breaks down with long heating.

Cornstarch starts to gelatinize at about 60°C (140°F) and finishes at the boil. Several ingredients hold the swelling back:

• sugar in large amounts
• egg
• fat
• dry milk solids
• very acidic fruit

In a sweet filling, hold back some sugar or the acidic fruit until the starch has set. About 100 g (4 oz) of cornstarch thickens 1 L of water or juice. Ordinary cornstarch turns cloudy; waxy maize starch stays clearer and runnier. Pre-gelatinized starch is already cooked, so it thickens with no heat when mixed first with sugar; put straight into water, it lumps at once. see chapter 7: Thickeners and gelling agents compared

Escoffier, writing at the start of the twentieth century, expected pure starches to replace roux, because they need no skimming. He was right that flour sauces would decline, but wrong about what would take their place. Western diners have never quite warmed to the glassy shine of a starch sauce. A little starch in a stock that is already well reduced will hardly show, though. Some kitchens reduce and degrease a brown stock, then thicken all of it lightly with arrowroot as a base for brown sauces made to order. Go easy: a dark, glossy sauce with no flavor is the worst result of all.

Egg yolks and the liaison

In sauce making, a liaison usually means one part egg yolk to three parts heavy cream. It adds richness and smoothness and only a little thickness; it does not work by swelling starch. Mixing the yolks into cream raises the heat they can take before they set, from 65–70°C (149–158°F) for plain yolks to about 82–85°C (180–185°F). Above 85°C (185°F) they curdle.

To add it, temper the liaison: whisk a little of the hot sauce into it to warm it, then stir the warmed liaison back into the rest of the sauce. Added straight to hot liquid, the yolks set into lumps.

Egg yolks hold several emulsifiers, among them lecithin and cholesterol. That is why they can bind fat and water into mayonnaise and hollandaise, trap air in a sabayon, and enrich crème anglaise. They are seldom used alone to thicken a sauce. Do not let a yolk sauce boil unless it also holds flour, which steadies it: sauce allemande and pastry cream contain flour and are boiled after the yolks go in. Sugar and acids such as lemon or vinegar steady yolks a little, but not enough to boil them. Never cook them in aluminum, which turns the sauce gray.

A white veal stew, blanquette de veau, shows the classic use. The poaching liquid becomes a velouté with about 100 g (3½ oz) of roux to 1 L of liquid. Each yolk is mixed with 45–60 ml (3–4 tbsp) of heavy cream, and three or four yolks' worth goes into each liter. The sauce is stirred gently until it coats the back of a spoon, and most recipes warn against boiling it.

Holding a liaison sauce. Hot food must be held at 60°C (140°F) or hotter, and the range from 4°C to 60°C (40°F to 140°F) is the danger zone, where most harmful bacteria grow. The ceiling for this sauce is 85°C (185°F), to stop curdling. How eggs set under heat is covered with eggs. see chapter 15: Heat and the egg

Gelatin and reduction

Meat and fish release gelatin, a protein that dissolves in water, as they cook. Concentrated enough, it gives a cooking liquid a natural, lightly syrupy body. The costly way to get it is to reuse stock to cook more meat, again and again, until the stock is thick on its own. Pork skin, veal feet and knuckles, or chicken and turkey wing tips added to stocks and stews give extra gelatin. Restaurants usually add it at the last minute as a glace: glace de viande for meat, glace de poisson for fish. A glace gives body and a deep flavor that balances stronger finishes such as wine or herbs.

Reduction alone has limits. A sauce reduced too far feels gluey, sets fast on a hot plate and can taste flat and overcooked. So gelatin is rarely the only thickener, and butter is often added at the end to take away the stickiness.

Cream

Heavy cream softens and smooths a sauce, but it thickens only once it is reduced. You can reduce it alone and keep it for sauces made to order, or reduce it together with the sauce base, which suits sauces made ahead.

• Use a pan three or four times the volume of the cream; boiling cream foams over in seconds.
• Whisk it at least every 2 minutes. Left alone, even on a low flame, it turns grainy and may break.
• Use a pan wide enough for the flame. A small pan lets flame lick up the sides and brown the cream there.
• Never cover it. Steam drips back from the lid and makes the cream grainy.
• Reduce wine fully before cream goes in. Raw wine's acid can break the cream and leaves a harsh taste.
  ▶ Watch

Cream is reduced by between a third and two thirds, depending on its fat and the thickness you want. For example, 375 ml (1½ cups) of cream added to 125 ml (½ cup) of flavor base may be reduced to 125 ml for a very thick sauce or to 250 ml for a lighter one. On its own, reduced cream is very rich and can feel chalky, so it usually works with butter, yolks or flour. Some chefs start the base with roux, or finish with beurre manié, so the sauce needs less reduction, which makes it less rich and cheaper.

Double cream has extra butterfat and needs less reduction. Where it is not sold, hang crème fraîche in a wet cloth or cheesecloth in the refrigerator overnight. The whey drips out and the fat in what is left rises. If the cream loses half its volume as whey, its fat roughly doubles from 35 percent to 70 percent.

Butter

Butter whisked into a hot liquid forms an emulsion, much as egg yolk does. The milk solids and proteins in the butter keep tiny drops of fat suspended and give butter sauces their gloss and body. Because those solids do the work, clarified butter cannot thicken a sauce. Cold butter works better than warm butter that has started to turn oily. A sauce bound only by butter is less stable than one bound by cream, yolks or flour, so many butter sauces are made to order.

Finishing a hot sauce at the last moment by swirling in pieces of cold butter is called monter au beurre. It adds shine, flavor and richness. A compound (flavored) butter can take the place of plain butter to add a particular flavor.

The amount has to be right. Too little butter floats to the top unless the sauce already holds cream, yolks or flour. Too much drowns the flavor, makes the sauce thick and waxy, and may break it. As a rule, a butter sauce made to order is about one third butter: 125 g (½ cup) of butter for 250 ml (1 cup) of base. Reduce the base a little too far rather than not enough, because a butter sauce is easy to thin but boiling it to thicken it will break it. Tomato or mushroom purée can start the thickening of a brown sauce, so less butter is needed.

Gums and modern thickeners

Modern kitchens also use thickeners that need no heat. Modified starch or xanthan gum is simply blended into a cold liquid, which keeps a fresh, uncooked taste. Many chefs now reach for hydrocolloids: flavorless, they thicken in very small amounts, and some cooks prefer the texture. Gelatin, agar and pectin are covered with the science of heat. see chapter 7: Gelatin, agar and pectin

• Xanthan gum is made by fermenting glucose. A small dose gives a thick liquid that pours easily and holds particles in suspension without masking flavor. It dissolves whether the liquid is hot or cold, acidic or alcoholic, but once set it can lose some strength when heated.
  ▶ Watch
• Guar gum, from the seed of the guar plant, thickens and steadies sauces and dressings in the food industry. Dissolve it in cold liquid; the more you use, the thicker the liquid.
  ▶ Watch
• Carrageenan, from red seaweed, works well with dairy and does not change flavor. Sprinkle it into cold liquid and blend. Algin (sodium alginate) dissolves in cold water and is used in uncooked icings, but does not work well with acidic fruit.
  ▶ Watch
• Locust bean gum (carob bean gum) has a neutral taste and a creamy feel, and dissolves in hot or cold liquid. Maltodextrin thickens and steadies sauces and dressings and can turn fat into a powder.
  ▶ Watch
• Soy lecithin, from soybeans, is an emulsifier that can be whisked straight into a liquid. It holds oil and water together and makes long-lasting foams.
  ▶ Watch

Some modern sauces are served as foams. Gelatin, lecithin or another stabilizer goes into the liquid, which is whipped or pushed through a siphon charged with gas. A foam made in a siphon is called an espuma, Spanish for froth. A fluid gel sits between a sauce, a gel and a purée: it holds still on the plate but flows when stirred, can be served hot or cold, and feels rich and creamy.

Finishing a sauce

• Reduction. Simmering drives off water as steam, so what is left thickens and tastes stronger, with nothing added.
  ▶ Watch
• Straining. A fine mesh chinois, or a china cap (a cone-shaped strainer) with a few layers of cheesecloth inside, removes the flavorings and any lumps of thickener.
  ▶ Watch
• Nappe. This French word names the thickness at which a sauce coats food evenly, neither too thick nor too thin. Dip a spoon in the sauce and turn it over. Draw a finger down the back. If the line stays clean, the sauce is ready. Keep stirring with a metal whisk and hold the heat steady.
  ▶ Watch

The sauce families

The five leading or mother sauces are béchamel, velouté, espagnole (brown sauce), tomato and hollandaise. What sets them apart is mainly the liquid and the thickener. Each is the head of a family of small or compound sauces, made by adding seasonings and garnishes. A small sauce may be named for what is in it, where it came from or who made it.

The liquid and thickener for each mother sauce.