There are two general classes of ferments: (1) living organisms, or organized ferments; (2) chemical, or unorganized ferments. Organized ferments are living microorganisms, capable, as a result of their growth, of causing the changes. Unorganized ferments are chemical substances or ferments without life, capable of causing marked changes in many complex organic compounds, while the enzymes themselves undergo little or no change. These unorganized ferments are such as rennin, pepsin, trypsin, ptyalin. The rennet and pepsin must, therefore, be very thoroughly mixed into the milk to insure complete and uniform results, because they act by contact, and theoretically, if they could be recovered, might be used over and over again. Practically, the amount used is so small a percentage that recovery would be impractical even if possible. +45. Nature of rennet.+--Two enzymes or ferments are found in rennet extract, rennin and pepsin. They are prepared from the secreting areas of living membranes of the stomachs of mammalian young. For rennet-making, these stomachs are most valuable if taken before the young have received any other feed than milk. Rennin at this stage appears to predominate over pepsin which is already secreted to some extent. With the inclusion of other feed, the secretion of pepsin comes to predominate. Rennin has never been separated entirely from pepsin. Both of these enzymes are secreted by digestive glands in the same area, perhaps even by the same glands. They are so closely related that many workers have regarded them as identical. In practical work the effectiveness of rennet preparations has been greatest when stomachs which have digested feed other than milk are excluded. The differences, therefore, however difficult to define, appear to be important in the commercial preparation of rennet. It was the practice until a few years ago for each cheese-maker to prepare his own rennet extract. Each patron was supposed to supply so many rennets. Now commercial rennet extract and pepsin are on the market; however, some Swiss cheese-makers prefer to make their own rennet extract. For sheep's and goat's milk cheese, some makers hold that rennet made from kid or lamb stomachs is best for handling the milk of the respective species. The objection to the cheese-maker preparing his own rennet extract is that it varies in strength from batch to batch and is liable to spoil quickly. Taints and bad odors and flavors develop in it and so taint the cheese. +46. Preparation of rennet extract.+--This extract may be manufactured commercially from digestive stomachs of calves, pigs or sheep. An animal is given a full meal just before slaughtering; this stimulates a large flow of the digestive juices, containing the desired enzymes.