Fermentations in Milk. 63 



tyric changes, a decomposition process in which butyric 

 acid is the chief product formed, but this may be associ- 

 ated with an alkaline condition of the milk and the pro- 

 duction of a bitter substance in the same. Thus, the sub- 

 division followed here will , of necessity be imperfect, and 

 occasional instances will be noted where some changes in 

 milk might well be described under several heads. It is 

 possible that milk may acquire an abnormal odor or taint, 

 such as is due to direct absorption, without having under- 

 gone any fermentative change, but the introduction of 

 various forms of bacteria is so common that fermentative 

 changes due to living ferments are constantly at work. 



Souring Of milk. Milk naturally undergoes a change 

 known as souring, if allowed to stand for several days at 

 ordinary temperature. This is due to the formation of 

 lactic acid, which is produced by the decomposition of the 

 milk-sugar. While this change is wellnigh universal, it 

 does not occur without a pre-existing cause, and that is the 

 presence of certain living bacterial forms. These organ- 

 isms develop in milk with great rapidity, and the decom- 

 position changes that are noted in souring are due to the 

 by-products of their development. 



The milk-sugar undergoes fermentation, the chief pro- 

 duct being lactic acid, although various other by-products, 

 .as other organic acids (acetic, formic and succinic), dif- 

 ferent alcohols and gaseous products, as C0 2 , H, N and 

 methane (CH 4 ) are produced in small amounts. 



In this fermentation, the acidity begins to be evident to 

 the taste when it reaches about 0.3 per cent, calculated as 

 lactic acid. As the formation of acid goes on, the casein 

 is precipitated and incipient curdling or lobbering of the 

 milk occurs. This begins to be apparent when the acidity 



