How to Thicken a Sauce

Sauces thicken when water leaves them. Simmering uncovered over medium heat lets steam escape, which concentrates everything left in the pan: the flavor gets stronger and the body gets heavier at the same time. This is the right first move whenever the sauce tastes good but looks watery, and you have a few minutes to spare.

Keep the heat at a steady simmer, not a hard boil, and stir every minute or two so the bottom does not scorch. A wide pan works faster than a tall pot because more surface is exposed to the air. Taste as you go: a sauce reduced by a third can taste noticeably saltier, so hold off on extra salt until it reaches the thickness you want.

Reduction does not suit every sauce. Cream sauces can split if they boil hard, and sauces with fresh herbs lose brightness with long cooking. If you cannot afford to lose volume, skip ahead to the starch methods.

A cornstarch slurry is the quickest way to thicken a sauce that needs to stay put right now. Mix 1 tablespoon of cornstarch with 2 tablespoons of cold water in a small bowl until smooth, then pour it into the simmering sauce while stirring. The sauce will turn glossy and thicken within 1 to 2 minutes of simmering. Add it gradually: you can always stir in more, but a sauce thickened too far turns sticky and dull.

Use cold water for the slurry, never hot. Cornstarch clumps the moment it hits hot liquid, and those clumps will not dissolve no matter how hard you whisk. The sauce must also simmer after the slurry goes in; cornstarch only reaches full thickness near a boil, so a warm-but-not-simmering sauce will stay thin.

Cornstarch gives a clear, glossy finish, which suits stir-fry sauces and fruit sauces. It is less suited to gravies, where flour looks and tastes more natural. Cornstarch-thickened sauces also thin out when reheated or frozen, so serve them right away.

Flour thickens more slowly than cornstarch but gives an opaque, hearty body that suits gravies, stews, and cream sauces. The cleanest way to use it is a roux: cook equal parts flour and butter or oil in the pan over medium heat for 2 to 3 minutes, stirring, until it smells nutty and turns a shade darker. Then pour in your liquid a little at a time, whisking well after each addition, and simmer 5 to 10 minutes until the raw flour taste is gone.

If the sauce is already made, use a beurre manie instead: knead equal parts soft butter and flour into a smooth paste, then whisk in small lumps, one at a time, into the simmering sauce. Each lump should melt and disappear before you add the next. The sauce needs 2 to 3 minutes of simmering after the last lump to cook out the flour taste and reach full thickness.

Use about 1 tablespoon each of flour and butter per cup of sauce for a light coating, or double that for a thick gravy. Flour-thickened sauces reheat better than cornstarch ones, so flour is the better choice for gravy served later. Remember that flour contains gluten.

Some sauces carry their own thickener already. Starchy pasta water is the best-known example: the starch that cooked out of the pasta binds oil and water together, so a splash tossed with pasta and sauce in the pan gives a silky coating without any added starch. Save a cup before draining, and add it a few tablespoons at a time.

Beans and potatoes do the same job in soups and stews. Mashing a spoonful of the beans or a few chunks of potato against the side of the pot and stirring them back in thickens the liquid with no extra ingredients.

Fat and eggs thicken by coating and binding rather than by absorbing water, and they add richness at the same time. A splash of heavy cream simmered into a sauce reduces and thickens gently; it suits tomato sauces and pan sauces that can take a little richness.

Cold butter whisked into a hot sauce a piece at a time, off the boil, melts into a glossy, velvety finish. This works best as a final step, right before serving, because a boiled butter sauce can split into oil and liquid. Egg yolk is the strongest of the three: whisk a yolk with a spoonful of the hot sauce to warm it, then stir the mixture back into the pan over low heat. Never let an egg-thickened sauce boil, or the yolk will scramble into grainy bits.

A sauce that went too thick is easy to fix: whisk in warm water, stock, or milk a tablespoon at a time over low heat until it loosens. Taste again after thinning, since the extra liquid can dilute the seasoning. A sauce that went lumpy needs more work: whisk it hard to break up small lumps, or pour it through a fine sieve into a clean pan.

A sauce that split, with oil pooling on top, usually got too hot. Take it off the heat, whisk in a tablespoon of cold water or an ice cube, and keep whisking until it comes back together. If it will not come back, start a fresh spoonful of the base in a clean pan and whisk the split sauce into it a little at a time.

Start with the sauce simmering gently over medium heat in a wide pan. Taste it first: if the flavor is right but the body is thin, simmer uncovered for 3 to 5 minutes, stirring every minute, until it coats the back of a spoon. Run your finger across the spoon; the line should hold for a second before the sauce closes over it.

If the sauce is still thin, mix 1 tablespoon of cornstarch with 2 tablespoons of cold water and stir half of it into the pan. Simmer 1 to 2 minutes, check the thickness, and add the rest only if you need it. Taste again for salt, since the starch itself adds no flavor, and take the pan off the heat as soon as the sauce coats the spoon. Serve it right away; a sauce left standing keeps thickening as it cools.