Since cornstarch is used in many popular recipes to thicken custards, sauces, and stews, it’s essential to know exactly how to mix it with water.
Ordinary cornstarch doesn’t dissolve in cold or room temperature water. Its solid particles spread through the water to form a suspension, or slurry. When you heat it with enough water, the granules swell and release some starch into the liquid. This process, called gelatinization, is what thickens a sauce.
Cornstarch is a fine, white powder made from the starchy endosperm inside a corn kernel. The outer covering is called the pericarp. Also known as cornflour in some countries, cornstarch is useful for thickening food without adding much flavor.
What Happens When You Mix Cornstarch And Water?
Cornstarch is a carbohydrate composed of two starches: amylose and amylopectin.
These starch molecules are packed into tiny granules, which keep their structure in cold water rather than dissolving.
When you stir cornstarch into cold water, the granules spread through the water. This suspension is what cooks call a slurry.
A concentrated mixture of uncooked cornstarch and water can also behave like oobleck. It flows when handled gently but can become much harder to move when squeezed or stirred quickly. This behavior is called shear thickening.
As Columbia University explains, the effect comes from the way the suspended particles move:
- With slow movement, the water lets the particles slide past one another, so the mixture flows.
- With sudden force, the particles can press against one another and temporarily jam, making the mixture act more like a solid.
- The effect depends on how concentrated the mixture is and how forcefully it is moved. A thin cooking slurry does not necessarily behave this way.
Does Cornstarch Dissolve In Cold Water?
Ordinary, uncooked cornstarch doesn’t dissolve in cold water.
As the starch granules are too small to see with the naked eye, they may seem to have dissolved but are actually suspended in the water and remain solid.
A cold slurry still needs to be cooked to thicken the liquid in a sauce.
Stirring disperses the particles throughout the water, but they remain suspended rather than dissolved.
This mixture forms a runny paste that’s used in cooking as a thickener, known as slurry.
If you leave the slurry standing, the particles can settle to the bottom. Stir it again just before you use it.
This explanation applies to ordinary cook-up cornstarch. Some instant or pregelatinized starch products can thicken cold liquids. For example, Ingredion makes a cold-water-swelling corn starch. Follow the directions for your product rather than assuming every starch works the same way.
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Does Cornstarch Dissolve In Hot Water?
Adding dry cornstarch directly to hot water can create lumps, because the outside of a clump thickens before the powder inside has dispersed.
When you first mix cornstarch with cool water and then heat the mixture, the starch granules take up water and swell.
During starch gelatinization, the structure of the granules changes and some starch molecules move into the surrounding water. The mixture thickens as the granules swell and starch is released. This is more involved than sugar simply dissolving in water.
The temperature and amount of swelling depend on the starch and the mixture. Use the cooking method in your recipe or on the package rather than treating one temperature range as the point when every sauce is ready.
The mixture can become firmer as it cools, depending on the recipe and how much starch it contains.
This feature makes cornstarch a stellar thickening agent for sauce, gravy, custard, and pies.
Long cooking or vigorous stirring can eventually break down the thickened mixture and make it thinner. That doesn’t mean you should stop at the first sign of thickening; allow the recommended cooking time first.
How To Make And Use A Cornstarch Slurry?
A simple mixture of cornstarch and cool water creates a quick, fuss-free, and highly effective thickening agent known as a slurry.
If you add dry cornstarch directly to an already hot dish, it can thicken on the outside before it has mixed through the liquid, leaving lumps.
For this method, first mix the cornstarch with enough cool water to make a smooth, pourable slurry. Recipes use different proportions, so equal parts cornstarch and water is one option rather than a fixed rule.
A cornstarch slurry can be used to thicken the following dishes:
- Soup,
- Stew,
- Curry,
- Sauces and gravies,
- Cooked sauces made from reserved marinade,
- Stir-fry sauces, and
- Pie fillings.
If you plan to use a marinade as a sauce, reserve a portion before adding raw meat or poultry, as the USDA recommends. Thickening a marinade is not a substitute for handling it safely.
Cornstarch Slurry Recipe
The amount below is a starting point for about 1 cup of liquid, rather than a guarantee of the same thickness in every dish. For example, Argo’s Classic White Sauce uses 1 tablespoon of cornstarch with 1 cup of milk, along with butter and seasonings.
This slurry uses equal volumes of cornstarch and cool water. If your recipe calls for a different proportion, follow that recipe. The amount of liquid, sugar, fat, and acid in a dish can all affect how it thickens.
Ingredients
- 1 tablespoon cornstarch, and
- 1 tablespoon cool or room temperature water.
Equipment
- Small mixing bowl, and
- Small whisk or fork for mixing.
Instructions
- Place cornstarch in a small mixing bowl and slowly add the water.
- Mix with a small whisk or fork until the slurry is smooth. If it is too stiff to pour, stir in a little more cool water.
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How To Use A Cornstarch Slurry?
Now that the cornstarch is evenly combined in the cool water, it can be used as a slurry to thicken liquids.
The starch is suspended rather than dissolved. It still needs to cook in the dish to thicken the surrounding liquid.
For a basic stovetop sauce or gravy, follow these steps. Use a dedicated recipe for custards, pie fillings, and other mixtures that need their own cooking method.
- Re-stir the slurry. The particles can settle while it stands, so mix it again just before adding it to the dish.
- Add the slurry toward the end of cooking. For a dish that needs a long simmer, leave enough time at the end for the starch to cook after you add it.
- Stir the slurry into the hot liquid.
- Slowly pour in a little slurry while stirring gently, so it spreads through the sauce rather than collecting in one spot.
- For a basic stovetop sauce or gravy, bring the mixture to a full boil over medium heat and boil for 1 minute while stirring. This follows Argo’s basic cooking instructions.
- Let the starch cook before deciding whether the sauce needs more slurry. If you add more, cook that addition through as well.
- Finish cooking before removing the pan from the heat.
- Once the sauce has completed its cooking time and thickened, remove it from the heat. Avoid prolonged boiling or rough stirring, which can make it thin out.
- Allow for some further thickening as the sauce cools, rather than assuming its hot and cool textures will be identical.
A separate slurry isn’t necessary for every recipe.
If a recipe starts by mixing cornstarch into cold milk or another cold liquid before heating, you can follow that method. Argo’s white sauce uses this approach. What you want to avoid is sprinkling dry cornstarch straight into an already hot sauce and expecting it to mix smoothly.
