Can you use flour instead of cornstarch?

What's the flour to cornstarch ratio?

2 tablespoons of all-purpose flour for each tablespoon of cornstarch. Bob's Red Mill says cornstarch has twice the thickening power of flour, and University of Illinois Extension gives the same amounts per cup of liquid: 1 tablespoon of cornstarch, or 2 of flour. Cornstarch is pure starch. Flour is ground wheat, so part of every spoonful is protein, gluten included, and less of it is starch.

A cup holds 16 tablespoons, and anything in between, or in milliliters, is a job for Hedgehog Chef's free cooking converter. Use the 2 to 1 ratio for gravy, pan sauces, soups and stews. Fruit pie works differently, and has its own numbers below.

How to thicken a sauce with flour instead of cornstarch

Mix the flour with cold liquid first, or cook it in fat, and then give it time on the heat. Starch dropped straight into hot liquid clumps before it can spread out, which is why the Culinary Institute of America starts every slurry in cold liquid.

1. Shake the flour with cold water or cold stock in a jar until no dry bits are left. The CIA's starting point for a slurry is 1 part starch to 2 parts cold liquid.
2. Pour it into the simmering sauce a little at a time, stirring the whole time.
3. Let it simmer, still stirring, for several minutes. Mississippi State University Extension's turkey gravy cooks for 5 to 10 minutes over low heat after the broth goes in.

If the recipe starts with butter or oil in the pan, use that instead. Stir the flour into the fat until it bubbles, then whisk in the liquid. Illinois Extension says flour works best in sauces made with fat, which is how a roux gravy or a cheese sauce is built.

Expect a different look. Illinois Extension describes flour sauces as cloudy and opaque, and cornstarch sauces as transparent and glossy. In brown gravy nobody will notice. In a lemon sauce or a sweet-and-sour glaze, they will.

Does flour work for gravy, pie filling and stir-fry?

For gravy, yes, without a second thought. Flour is the traditional gravy thickener, and our posts on fixing lumpy or thin gravy and gravy without drippings both use it.

For fruit pie, yes, but the 2 to 1 rule doesn't carry over. King Arthur Baking's pie thickener chart uses only a little more flour than cornstarch for juicy fruit, and about 3½ times as much for apples, which need the least thickener of all. In King Arthur's side-by-side test of five thickeners in strawberry-rhubarb pies, the flour filling worked but was cloudier, with a faintly starchy feel in the mouth.

King Arthur suggests about 25% more thickener for particularly juicy fresh fruit. With cornstarch, bake until the filling bubbles up through the crust, which King Arthur gives as the sign it has set.

For stir-fry, flour is the weakest swap. A cornstarch slurry thickens the sauce after a minute's boil and leaves it clear and shiny. Flour needs several minutes on the heat, which the vegetables don't have, and turns the sauce opaque. If you have arrowroot or potato starch, use either of those instead. If flour is all there is, simmer the sauce and its flour slurry in a small pan for a few minutes, then pour it over the stir-fry at the end.

Other substitutes for cornstarch

Arrowroot and potato starch both stay clear like cornstarch, and the CIA lists both for slurries made the same way. Bob's Red Mill sells arrowroot as a direct substitute for cornstarch, with no flavor of its own, but it's a little weaker: Cook's Country found that almost 1½ teaspoons of arrowroot thicken as much liquid as 1 teaspoon of cornstarch, and that it turned a butterscotch pudding slimy, so keep it out of milk and cream sauces. Potato starch goes the other way. America's Test Kitchen uses 2 teaspoons of potato starch for every tablespoon of cornstarch, and it thickens before the sauce reaches a full simmer. Bob's Red Mill also says potato starch tolerates higher temperatures than cornstarch.

If you're cooking for someone who avoids gluten, flour is out. Illinois Extension describes cornstarch as pure starch made from corn, while flour is made from wheat and contains gluten. Arrowroot and potato starch are the swaps to reach for. Check each package for a gluten-free label.

The other fix is not running out. In Hedgehog Chef's pantry, mark cornstarch as a staple, and when it's running low it goes on the shopping list in one tap.

Why do cornstarch sauces go thin?

Usually because the sauce never got hot enough, or because it was stirred hard after it set. The Exploratorium's Science of Cooking says cornstarch has to reach 203°F (95°C) before it starts to thicken, and that once the starch network forms, agitation breaks it and lets the liquid back out. Bob's Red Mill's instructions are to bring it to a boil, stirring constantly, and boil for 1 minute. After that, stir gently or leave it alone.

• Liquid seeping out as the sauce stands is syneresis, which the Exploratorium says is more noticeable when the gel holds eggs or a lot of sugar.
• Egg yolks carry an enzyme, amylase, that breaks down starch. Cook's Illustrated brings pastry cream almost to a boil, about 200°F (93°C) with a few bubbles breaking the surface, to destroy it. Undercooked, the cream won't set as it cools. Cream pie fillings and puddings made with yolks need the same treatment, whether they're thickened with flour or cornstarch.
• Freezing is hard on cornstarch. Ohio State University Extension's freezing fact sheet says flour-thickened sauces and gravies may look curdled after thawing but usually come back together when stirred, and that cornstarch may be less successful. For sauces headed for the freezer it recommends Clear Jel, a modified starch.

Once you've found a swap that works in a recipe, write it down where you'll see it next time.