Cornstarch is the fine white starch obtained from the endosperm of maize kernels, and it is commonly mixed with cold water to make a slurry for thickening sauces and gravies. Adding it to cold liquid first helps disperse the powder and prevents stubborn lumps.
When the slurry is stirred into a hot liquid, the starch granules absorb water and swell. Heat causes the starch molecules to unravel and form a structure that thickens the sauce, a process known as gelatinization. Cornstarch often creates a smoother, more translucent finish than wheat flour.
Cornstarch is also used in soups, custards, pie fillings, and some fried coatings. It should not be confused with semolina, which is a coarse wheat product; gelatin, a protein-based setting agent; or baking soda, an alkaline leavener. Prolonged vigorous boiling can weaken a cornstarch-thickened mixture and make it thinner again.