CHEESE. 227 



made from fresh milk, cheese consists of caseine and 

 fatty matter; when made from skimmed milk, it con- 

 tains little or no fat: in either case, during the press- 

 ing and curing of the cheese, it undergoes a peculiar 

 kind of fermentation, which gives rise to several 

 complicated chemical changes in its composition. 

 Cheese is generally made from milk, by the action of 

 rennet, the lining membrane of the stomach of the 

 calf, which has the property of causing the coagula- 

 tion of the caseine. Cheese which contains butter, 

 becomes soft and viscid when toasted ; whilst that 

 which contains none, becomes hard and shrivels up 

 almost like horn. 



583. The curd of milk may be separated in a 

 great variety of ways ; any substance capable of 

 entering into fermentation, and the greater number 

 of acids, organic as well as inorganic, cause its 

 coagulation. As the caseine of milk is held in solu- 

 tion by a small quantity of alkali, which causes fresh 

 milk always to give a slightly alkaline reaction to 

 colored vegetable test-papers; so the addition of an 

 acid, or of any substance which by inducing fer- 

 mentation can generate lactic acid, neutralizes this 

 alkali, and consequently precipitates the caseine. 



584. The most powerful of all the ferments which 

 are thus able to cause this separation of the caseine, 

 is rennet, which is in fact a membrane in a state of 

 slow putrefaction. When a piece of this substance 

 is put into milk, the temperature of which is then 

 slowly raised to 120°, or thereabouts, a slight degree 



