The majority of the starches we use in cooking come from either grains or from roots and tubers:
Wheat flour and cornstarch are the two most common forms of grain starches we use in our cooking. Because it is almost pure starch, cornstarch is a more efficient thickener than wheat flour. Both are medium-sized starch granules that gelatinize at a higher temperature than root starches. However, once that temperature is reached, thickening happens very quickly!
Grain starches also contain a relatively high percentage of fats and proteins, which can make sauces thickened with these starches look opaque and matte-like. These starches also tend to have a distinctive cereal taste once cooked.
Root and Tuber Starches
Potato starch, tapioca (made from manioc root), and arrowroot are larger-grained starches that gelatinize at relatively lower temperatures. Sauces thickened with these starches are more translucent and glossy, and they have a silkier mouthfeel. Root starches also have less forward flavors once cooked.
These root starches don't stand up as well as grain starches to longer cooking and so they're best used to thicken sauces toward the very end of cooking.
Choosing Which Starch to Use
If you need to thicken at the beginning of cooking, as for macaroni and cheese or a classic beef stew, go for a grain starch. If you need to quickly thicken a sauce just before it comes off the stove, use a root starch.
We also prefer using root starches in baking for custards, puddings, and pie fillings. We find that the flavor is more neutral and our results are more consistent.
Any other bits of advice to share regarding starches and thickeners? Are there specific times you use one kind over the other?