With the exception of some fresh cheeses, the cheesemaker curdles nearly all cheeses with a combination of starter-bacteria acid and rennet. Acid and rennet form very different kinds of curd structures—acid a fine, fragile gel, rennet a coarse but robust, rubbery one—so their relative contributions, and how quickly they act, help determine the ultimate texture of the cheese. In a predominantly acid coagulation, the curd forms over the course of many hours, is relatively soft and weak, and has to be handled gently, so it retains much of its moisture. This is how fresh cheeses and small, surface-ripened goat cheeses begin. In a predominantly rennet coagulation, the curd forms in less than an hour, is quite firm, and can be cut into pieces the size of a wheat grain to extract large amounts of whey. This is how large semihard and hard cheeses begin, from Cheddar and Gouda to Emmental and Parmesan. Cheeses of moderate size and moisture content are curdled with a moderate amount of rennet.