Cornstarch is a common thickening agent in the culinary arts, but if you add it directly to the liquid you want to thicken, it will clump up. To thicken a sauce or soup with cornstarch, you first need to make a slurry, which is a mixture of equal parts cornstarch and liquid (usually water, stock or wine).
It's important to make the slurry with cold liquid, and then add the slurry to the simmering sauce.
How it works is that the cornstarch molecules are like little sponges: they soak up water and expand as they do so. The same thing happens with any starch: think of the way rice or oatmeal or polenta thicken and expand in volume when simmered.
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Cornstarch imparts a glossy sheen to the liquids it thickens, so it tends to be used more in sweet sauces and pie fillings than in savory sauces and gravies. Still, it works really well, and it's easy to use:
- For each cup of liquid, you want to thicken, start with 1 tablespoon of cornstarch in a small bowl. Add an equal amount of cold liquid and stir until smooth paste forms. This is your slurry.
- Whisk the slurry into the hot, simmering liquid that you want to thicken. Bring to a boil and simmer until any starchy taste has been cooked away. Don't cook longer, though, as the starch may break down and the liquid will thin out again.
Something to remember when you're using cornstarch: If your sauce is quite acidic (like maybe it's tomato-based), the acid will cause cornstarch to lose some of its effectiveness as a thickener. In that case, you can substitute arrowroot or tapioca starch.
Tapioca or arrowroot are also better options if what you're making is something you're planning to freeze because cornstarch can take on a spongy texture when frozen.
Conversely, don't use arrowroot to thicken a cream or milk-based sauce as arrowroot combined with milk can be a bit slimy.
Cornstarch's thickening properties can help you out in other ways, too. Suppose you're making a stir-fry, and it's become watery. That often happens when your wok or pan isn't hot enough: all the liquid from the veggies and meat leaks out, causing the food to steam rather than fry. You could let it reduce, but you'll just overcook your veggies. Instead, add some cornstarch (again, make sure to make a slurry) and in a moment or two, all that extra liquid will thicken into a flavorful sauce.