Substitute Cornstarch for Flour in Roux: The Right Way

You can use cornstarch instead of flour to thicken a sauce, but you cannot cook it into a traditional roux. A roux is flour cooked in fat; for a cornstarch version, make a smooth cold-water slurry and whisk it into a warm, buttery sauce for a glossy finish.

This recipe makes about 2 cups (475 ml) of mild, creamy sauce using broth, milk, butter, and cornstarch. It works for gravy-style sauces and casseroles, though its texture and flavor differ from a flour roux.

Ingredients

2 tablespoons unsalted butter (28 g)
1½ cups low-sodium chicken or vegetable broth (355 ml)
½ cup whole milk (120 ml)
1 tablespoon cornstarch (8 g)
2 tablespoons cold water (30 ml)
¼ teaspoon fine salt (1.5 g), or to taste
⅛ teaspoon ground black pepper (about 0.3 g)

Instructions

Measure and mix the slurry. Before heating the sauce, whisk the cornstarch with the cold water in a small bowl until no dry lumps remain. Set it beside the stove. Stir it again just before using, because cornstarch can settle at the bottom while it stands.
Melt the butter. Set a small saucepan over medium-low heat. Add the butter and let it melt for about 1 minute. Swirl the pan so the butter coats the bottom, but do not let it brown or sizzle aggressively. Unlike flour, cornstarch should not be added to this fat to make a roux.
Warm the liquids. Slowly whisk in the broth and milk. Bring the mixture to a gentle simmer over medium heat, whisking now and then, for about 4 minutes. Keep it below a rolling boil; the goal is to heat the liquid evenly before adding the starch.
Thicken with the slurry. Whisk the slurry once more, then pour it into the simmering liquid in a thin stream while whisking constantly. Continue whisking for 1 to 2 minutes, until the sauce looks glossy and lightly coats the back of a spoon. A few small bubbles are fine; avoid a hard boil or extended cooking, which can weaken cornstarch’s thickening power.
Season and adjust. Take the pan off the heat and stir in the salt and pepper. Dip in a spoon to check the consistency: the sauce should cling lightly, not set like pudding. If it is too thick, whisk in broth or milk a tablespoon (15 ml) at a time. If it is too thin, mix another ½ teaspoon cornstarch (about 1.3 g) with 1 teaspoon cold water (5 ml), add a little at a time, and simmer briefly until thickened.

Tips

Use about half as much cornstarch as flour, as a starting point. Flour and cornstarch do not thicken identically, and the best amount depends on the liquid and the texture you want. For a sauce made with 2 cups (475 ml) of liquid, 1 tablespoon (8 g) cornstarch gives a moderately light coating. A flour-thickened sauce often needs more flour, but do not swap spoon for spoon without adjusting.
Keep the starch away from hot fat until it is diluted. Stirring dry cornstarch directly into melted butter can make stubborn lumps, and it will not develop the browned, nutty flavor of a cooked flour roux. A cold-water slurry disperses the starch first so it can thicken evenly when heated.
Whisk as you pour, then give it a short simmer. Adding the slurry all at once or skipping the whisking can leave pockets of gel. Cornstarch thickens quickly once hot, so wait a minute before deciding whether to add more; the sauce will also firm slightly as it cools.
Choose another thickener if you need roux’s specific character. For a dark, toasted gumbo-style base or a sauce that must withstand long simmering, cornstarch is not a direct stand-in. Use flour for a true roux, or consider a suitable alternative such as arrowroot for a glossy, clear finish; arrowroot can become stringy with prolonged high heat.

Serving Suggestions

Spoon the sauce over roast chicken, turkey, pork chops, mashed potatoes, or steamed vegetables. It also works as a quick creamy base for cooked mushrooms, peas, or sliced onions: sauté those ingredients separately until tender, then stir them into the finished sauce. For a casserole, make the sauce slightly looser than your desired final texture because it may thicken further as the dish bakes or cools.

This is a useful option when a recipe calls for a light white sauce or gravy and flour is unavailable. It is not identical to béchamel, which is traditionally thickened with a flour-and-butter roux, and it will not have the same soft, opaque body. If serving a group, bring the sauce to the table promptly and offer black pepper or chopped herbs separately so people can season their portions.

Variations

Make it dairy-free: Replace the butter with 2 tablespoons (28 g) of a neutral plant-based butter or olive oil, and use unsweetened, unflavored oat or soy milk instead of dairy milk. The sauce may taste less rich, and some plant milks have a noticeable flavor, so choose one that suits the dish. Check labels if you are cooking for someone with an allergy.

Use all broth: Replace the milk with another ½ cup (120 ml) broth for a more savory, less creamy sauce. This version pairs well with vegetables and roasted meat. A splash of milk or cream stirred in at the end can soften the flavor, but avoid boiling a cream-enriched sauce hard.

Add aromatics or herbs: Before adding the liquids, gently cook 1 tablespoon (about 15 g) finely minced shallot in the butter for 2 to 3 minutes, without browning it. You can also finish the sauce with 1 teaspoon (5 ml) Dijon mustard, or 1 tablespoon (about 4 g) chopped parsley or chives. Add acidic ingredients in small amounts and taste as you go, since they can make a dairy-based sauce taste sharper.

Make it gluten-free: Cornstarch itself is naturally gluten-free, but packaged ingredients can be processed or blended in different facilities. Check the labels on the broth and starch if gluten cross-contact is a concern. This recipe does not need flour at any stage.

Storage and Reheating

Transfer leftover sauce to a shallow, covered container and refrigerate it within 2 hours of cooking. Keep it refrigerated for 3 to 4 days. Cornstarch-thickened sauces can become quite firm or slightly watery after chilling; that separation is a texture issue, not a reason to leave the sauce at room temperature.

Reheat gently in a small saucepan over low to medium-low heat, whisking often. Add broth or milk a tablespoon (15 ml) at a time until it loosens, and heat it until steaming throughout. If the sauce contains cooked meat or poultry, USDA FSIS guidance is to reheat leftovers to 165°F (74°C), checked with a food thermometer. For a sauce without meat, heat until hot and steaming, but avoid a prolonged rolling boil because that can thin a cornstarch sauce.

Freezing is possible, but thawed cornstarch sauces may separate and feel grainy. If you freeze it, use a freezer-safe container, leave room for expansion, and use it within about 2 months for better quality. Thaw overnight in the refrigerator, then whisk while reheating; if the texture will not smooth out, use it in a casserole or soup where a perfectly silky finish is less important.

Frequently Asked Questions

Can I put cornstarch directly into melted butter like flour?

No. Flour can be stirred into fat and cooked to form a roux, but cornstarch needs to be dispersed in liquid before heating. If you stir dry starch into butter, it is likely to clump and will not brown into a roux. Mix it with cold water first, then whisk that slurry into the warm sauce.

How much cornstarch replaces 1 tablespoon of flour?

There is no exact one-to-one conversion because flour and cornstarch produce different textures. As a general starting point, use about half as much cornstarch as flour for similar thickening, then adjust cautiously. For a sauce, make a slurry and add it gradually; the recipe’s liquid, simmer time, and desired thickness all affect how much you need.

Will cornstarch sauce taste like a flour-roux sauce?

No. A flour roux brings a mild cooked flavor and an opaque, fuller texture, and browning it adds toasted notes. Cornstarch has little flavor and tends to make a smoother, glossier sauce. It is handy for a quick thickener, but it cannot recreate the flavor of a blonde or dark roux.

Why did my cornstarch sauce turn lumpy or thin?

Lumps usually form when the starch is not mixed with cold liquid first, or when the slurry is added without steady whisking. Thinness can mean the sauce did not get hot enough to activate the starch, or that it boiled hard for too long and lost thickness. Whisk in a fresh, small slurry and bring the sauce to a gentle simmer to correct it.

Can I use this method for gumbo or a long-simmered stew?

You can thicken a finished stew with a cornstarch slurry, but it will not replace the roux used for gumbo’s flavor and color. Add the slurry near the end of cooking and simmer only briefly. For a dish that depends on a dark, cooked-flour base, use flour or another recipe specifically designed for a different thickener.

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