BACTERIA IN MILK. 197 



bacteria plays an important part in the changes which may 

 take place in so-called sterilized milk. Sterilized milk has 

 been heated to a temperature of boiling water, or even higher, 

 a temperature supposed to destroy all bacteria. Careful test- 

 ing has shown, however, that, in a considerable number of 

 samples, certain spore-bearing bacteria resist these high 

 temperatures. Such milk will never sour, inasmuch as all 

 lactic bacteria are killed, since they never produce spores. 

 This class of enzyme-forming bacteria, however, are very 

 commonly spore-bearers, and resist the temperature of boil- 

 ing water. Milk which has been sterilized, therefore, not in- 

 frequently undergoes changes which affect its taste and its 

 chemical nature, due to the class of bacteria here considered. 

 This class of bacteria has been supposed to have important 

 functions in cheese-ripening. 



BUTYRIC ACID FERMENTATION. 



A third type of bacteria, which may always be expected 

 in milk, are bacteria which produce a butyric acid. Butyric 

 acid is produced by a large number of bacteria and is in no 

 case to be regarded as a chief product, but rather as a by- 

 product of general decomposition produced by a number of 

 species. As a rule it is produced by bacteria which grow 

 only in the absence of oxygen, and is developed quite slowly. 

 In ordinary dairying this type of fermentation has, therefore, 

 no special significance in the handling of milk, since the milk 

 is consumed before the butyric acid forms. In butter which 

 is kept for some time butyric acid is almost sure to be pro- 

 duced. The chemical action is usually given as follows : 

 C 6 H 12 6 = c 4 H 8 o. 2 + 2 co 2 + 211, 



Milk sugar Butyric acid 



This certainly does not express the complete reaction. 

 A long list of bacteria has been described which produce 

 butyric acid. Most of them are anaerobic though some grow 



