MICltOBKS, OK BACTERIA. Ill 



mented, and obtained from the milk of cows, sheep, 

 or goats, skimmed or unskimmed, according to the 

 kind of cheese desired. 



Sweet-milk cheese do not differ in their composi- 

 tion from those of curdled milk. They consist of casein, 

 albuminoid matter which encloses particles of butter : 

 the liquid residue is the serum or whey, which con- 

 tains lactic acid and mineral salts. 



Cheese, strictly so called, such as Gruyere and 

 Roquefort, only differ from the foregoing because they 

 have been exposed, for a shorter or longer time to the 

 action of the air, and of the microbes suspended in it. 

 Cheese is first oxidized under the influence of the 

 uxygen of the air ; butyric and even alcoholic fermen- 

 tation soon follows lactic fermentation, together with 

 the disengagement of hydrogen and of putrid pro- 

 ducts, when the action of the ferments which effect 

 these transformations has gone on too long. 



In order to obtain the different kinds of cheese 

 which come into the market, they are exposed to the 

 weather, generally in holes which have been excavated 

 in the rock for this purpose, on a bed of straw, t)r 

 sometimes partially covered with it, until the cheese 

 is ripe and has attained the desired quality. 



Butyric and ammoniacal fermentations lead us 

 directly to the study of putrefaction; that is, the fer- 

 mentation of dead organic matter. 



