There are three stages in the transformation of milk into cheese. In the first stage, lactic acid bacteria convert milk sugar into lactic acid. In the second stage, while the acidifying bacteria are still at work, the cheese-maker adds the rennet, curdles the casein proteins, and drains the watery whey from the concentrated curds. In the last stage, ripening, a host of enzymes work together to create the unique texture and flavor of each cheese. These are mainly protein- and fat-digesting enzymes, and they come from the milk, from bacteria originally present in the milk, the acidifying bacteria, the rennet, and any bacteria or molds enlisted especially for the ripening process.