Many sauces, from long-simmered classic French brown sauces to last-minute gravies, owe at least part of their consistency to the substance called starch. Unlike the other thickening agents, starch is a major component of our daily diet. It’s the molecule in which most plants store the energy they generate from photosynthesis, and provides about three-quarters of the calories for the earth’s human population, mainly in the form of grains and root vegetables. It’s the least expensive and most versatile thickener the cook has to work with, a worthy adjunct to gelatin and fat. The cook can choose among several different kinds of starch, each with its own qualities.