Slurry

A slurry is a thin mix of starch and cold liquid, usually cornstarch (cornflour in the UK) and water. You stir it into a simmering sauce, soup or gravy, and the liquid thickens within a minute or two. It is the usual thickener for stir-fry sauces, fruit pie fillings and gluten-free gravies.

How to do it

1. Mix 1 tablespoon of cornstarch with 2 tablespoons of cold water until smooth.
2. Bring the liquid you want to thicken to a simmer.
3. Stir the slurry again, since starch settles fast, then pour it in while stirring.
4. Simmer, stirring, for 1 to 2 minutes until the sauce turns glossy and thick.
5. Add more slurry a little at a time if you need it.

As a guide, 1 tablespoon of cornstarch thickens about 250 to 375 ml (1 to 1 1/2 cups) of liquid to a medium sauce.

Why it works

Starch grains do not dissolve in cold water, so they spread out without clumping. In hot liquid they absorb water and swell, which thickens the sauce. This is called gelatinization. Cornstarch needs to come close to a boil to fully thicken.

Substitutes

Starch	Instead of 1 tablespoon cornstarch	Notes
Potato starch	1 tablespoon	Thickens at a lower heat; do not boil long
Arrowroot	1 tablespoon	Clear and glossy; works with acidic fruit; turns slimy with dairy
Tapioca starch	1 to 1 1/2 tablespoons	Good for pies; slightly stretchy
All-purpose flour	2 tablespoons	Cloudier; cook 3 to 5 minutes

Common mistakes

• Adding dry starch straight to hot liquid, which makes lumps.
• Not bringing the sauce back to a simmer, so it stays thin and tastes chalky.
• Boiling for a long time or stirring hard after it thickens, which can break the starch down and thin the sauce again.
• Adding too much at once. Sauces with starch set thicker as they cool.

In recipes

"Thicken with a cornstarch slurry" or "mix cornstarch with water and add" both mean this method. Compared with a roux, a slurry is quicker and gluten-free, but less stable for reheating.