Slurry
A slurry is a mixture of starch and cold liquid, usually cornstarch and water, that's stirred into a hot sauce, soup or filling to thicken it. Pre-mixing the starch with cold liquid separates the granules so they swell evenly in the heat instead of clumping into lumps.
How a slurry thickens
Starch granules are tightly packed bundles of two molecules, amylose and amylopectin. In cold water they barely change. Heated in liquid, they absorb water and swell many times their size, then leak starch chains that tangle together. That's starch gelatinization.
Cornstarch thickens most strongly once the liquid reaches about 95 °C / 203 °F, so bring the sauce to a simmer and let it bubble for about 30–60 seconds after adding the slurry. It will look cloudy at first, then turn clear and glossy as it thickens.
Slurry ratios
| Starch | Per 250 ml (1 cup) liquid, medium sauce | Notes |
|---|---|---|
| Cornstarch | 1 tbsp (8 g) | Glossy, translucent; brief simmer |
| Potato starch | 2–3 tsp | Thickens fast at lower heat; don't boil long |
| Arrowroot | 1 tbsp | Clear, freezes well; avoid with dairy (turns slimy) |
| Tapioca starch | 1 tbsp | Glossy, slightly stretchy; good in pie fillings |
| All-purpose flour | 2 tbsp (16 g) | Opaque; simmer 5+ minutes |
Mix the starch with about twice its volume of cold water, stock, wine or juice.
How to use one
- Stir starch into cold liquid until completely smooth, with no dry bits.
- Re-stir right before adding; starch settles quickly.
- Pour slowly into the simmering liquid while whisking.
- Simmer briefly until clear and thickened.
- Add more slurry a little at a time if needed.
When slurries fail
- Too much stirring or boiling: breaks down swollen granules, thinning the sauce. Cornstarch only needs a short simmer.
- Strong acids: lemon juice, vinegar and tomatoes weaken starch. Add acid near the end, or use a little extra starch.
- Freezing: cornstarch-thickened sauces often turn spongy and weep when thawed, as starch molecules recrystallize. Arrowroot and tapioca freeze better.
- Sugar: large amounts delay thickening, so fruit fillings need more starch.
Slurry vs roux
A roux cooks flour in fat first and suits creamy, opaque sauces like béchamel and gravy. A slurry is fat-free, fast and gives the glossy sheen of stir-fry sauces, pie fillings and sweet-and-sour sauce. For a last-minute butter-rich fix, see beurre manié.
Questions
What is the ratio for a cornstarch slurry?
Mix 1 part cornstarch with about 2 parts cold water, such as 1 tbsp cornstarch in 2 tbsp water. That amount thickens roughly 250 ml (1 cup) of liquid to a medium sauce.
Why do you mix cornstarch with cold water first?
Dry starch added to hot liquid gelatinizes on the outside instantly and forms lumps with dry powder inside. Cold liquid separates the granules so each one swells evenly.
Why did my cornstarch sauce get thin again?
It was boiled or stirred too long, it contains a lot of acid, or it was frozen. All of these break down the swollen starch network.
Can you use flour instead of cornstarch in a slurry?
Yes, but use about twice as much (2 tbsp flour per 250 ml liquid) and simmer at least 5 minutes to cook out the raw flour taste. The sauce will be opaque rather than glossy.