30 THE BOOK OF CHEESE 



The important change taking place is the formation of 

 lactic acid from the milk-sugar. The change is brought 

 about by certain living organisms, namely, the lactic acid- 

 forming bacteria. Another familiar case of fermentation 

 is the coagulation of milk by rennet extract or pepsin. 

 In this case, the change is produced by a chemical sub- 

 stance, not a living organism. Fermentation may be 

 defined as a chemical change of an organic compound 

 through the action of living organisms or of chemical 

 agents. 



There are two general classes of ferments : (1) living 

 organisms, or organized ferments; (2) chemical, or un- 

 organized ferments. Organized ferments are living micro- 

 organisms, capable, as a result of their growth, of causing 

 the changes. Unorganized ferments are chemical sub- 

 stances 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, there- 

 fore, be very thoroughly mixed into the milk to insure 

 complete and uniform results, because they act by con- 

 tact, 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. Ren- 

 nin at this stage appears to predominate over pepsin 



