Thickening with cornstarch

Cornstarch in cold water is insoluble and granular, and it will settle out if left standing, so cooks first mix it into a cool liquid. This mixture is called a slurry. College baking texts describe adding the slurry to a simmering liquid while whisking until it dissolves and the liquid thickens.

The thickening comes from gelatinization: when heated, the granules absorb water, swell, and rupture. It begins at about 140 degrees F (60 degrees C) and is complete at the boiling point. A guideline in one text is about 4 ounces (100 g) of starch for a liter of liquid.

Sugar slows gelatinization, so some cooks hold back part of the sugar until the starch has cooked. For acidic fruit fillings, the starch paste is cooked first and the fruit is added afterward. Cornstarch thickens pie fillings, puddings, and pastry creams.