56 Dairy Bacteriology. 



about. Of such a nature is the lactic acid fermentation, 

 by far the most common change that occurs in milk. 



57. Souring 1 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 forma- 

 tion of lactic acid, which is produced by the decomposition 

 of the milk-sugar. While this change is wellnigh uni- 

 versal, it does not occur without a pre-existing cause, and 

 that is the presence of certain living bacterial forms. 

 These organisms develop in milk with great rapidity, and 

 the decomposition 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-pro- 

 ducts as other organic acids (acetic, formic, and succinic) , 

 different alcohols, and gaseous products, as CO 2 , H, N, 

 and methane (CH 4 ) are produced in small amounts. 



In the souring of milk, the formation of acid does not 

 continue until the sugar is all exhausted. When the 

 acidity reaches about 0.4% , milk begins to taste sour. In- 

 cipient curdling takes place at about 0.6% , and soon after 

 this, bacterial development is checked altogether. The 

 acid formation goes on, however, until from 0.8 to 1.0% 

 is reached. The amount of acid formed varies consider- 

 ably in different cases. 1 



If the acid produced in any case is removed by neu- 

 tralizing it with a carbonate, its development will begin 

 anew, showing that it is suspended by the inability of 

 the organisms to grow in such a medium. 



Cream never contains as much acid as milk for the 

 reason that a considerable proportion of its volume is oc- 

 cupied by the butter-fat which is not subject to this de- 

 composition. 



1 Warrington, Journ. Chem. Soc., 53: 727, 1888. 



