Cornstarch is, obviously, a starch thickener which means it thickens much the same way wheat flour does: with tangles of long-chain sugars that slow down the flow of the water around them. However there are several important differences between wheat flour and corn starch. One of those is particle size. Cornstarch is milled much finer than wheat flour, which means when those particles come in contact with hot water, they begin to shed starch molecules much faster. So cornstarch thickening happens faster than wheat flour thickening, but then also “un-thickens” that much faster.
Still the advantages of the very small particles are legion. Small as they are they don’t register on the tongue in liquid so cornstarch-thickened liquids have no cereal texture or aftertaste, even when cooked for only a short period of time. The smaller particles also allow more light rays to pass through the mixture, which gives cornstarch-thickened liquids a glossy and only faintly cloudy — or even totally clear — appearance.
Cornstarch is especially good at thickening dairy, though the higher the acidity of the liquid you’re trying to thicken, the weaker a cornstarch gel will become. Like wheat flour gels, cornstarch gels don’t hold up well after they’re frozen.
The best way to apply cornstarch to a mixture is to make a paste out of it first, using a roughly equivalent amount of cold water (which prevents any unwanted pre-gelling). When it boils it’s as thick as it’s going to get.
Because cornstarch is a pure starch, with no protein or other substances in it, it has double the thickening power of wheat flour.