The next step, almost invariably, is to set the curd. Usually rennet is used; vegetarian cheeses and some minor varieties (see caciotta; pecorino) may be curdled with vegetable extracts, and there are many cheeses, usually fresh, that are set with some sort of acid (vinegar or lemon juice for instance). For a general description of this process, see curdling. The casein fraction of the proteins coagulates and shrinks, trapping the fat globules and forcing out the whey. The milk is kept at a temperature near blood heat while it separates.