Fermentations of Milk. 83 



tions ' \ of milk, each being most commonly named from 

 the most important by-product formed. 



Acid fermentation of milk. Fresh milk has a sweet 

 taste and little or no odor, but if it is allowed to stand 

 at ordinary temperatures, it sours ; the taste is no longer 

 sweet because the sweetness of the sugar of the milk is 

 masked by the acid produced from the decomposition 

 of a portion of the sugar by the bacteria. The change 

 in odor and taste of milk is apparent long before the 

 appearance is altered and increases in intensity as the 

 acid-fermentation progresses. The first alteration in 

 appearance is most usually one of consistency; the 

 liquid milk is transformed into a semi-solid mass. The 

 terms ' ' curdling ' ' and ' ' sour ' ' are usually synonymous. 

 Milk is, how r ever, often said to be sour as soon as the 

 acid fermentation has progressed to a point where it 

 is evident to taste or smell. This process of souring, or 

 the acid fermentation is so common a change that raw 

 milk which does not show this type of fermentation is 

 looked upon- with suspicion, and, usually, justly so. 

 The process in the past was thought to be something 

 inherent in the milk, a natural and inevitable change. 

 It is now known that this is not so, but that it is due 

 to certain kinds of bacteria, and that if these are pre- 

 vented from getting into milk, it will not sour, but 

 will undergo some other less desirable type of decom- 

 position. 



The acid-forming bacteria comprise but a very small 

 part of the total number of organisms that find their 

 way into the milk during its production on the farm, 

 yet in sour milk scarcely any other kinds of bacteria 

 can be found. At ordinary air temperatures, the acid- 

 forming bacteria grow more rapidly in milk than do 



