SPONTANEOUS CHANGES IN MILK 123 



If the question be asked : What is the sanitary signifi- 

 cance of the existence of this mass of bacteria ? we must 

 consider the changes which bacteria in pure culture may 

 cause in milk, and which were mentioned above, for it 

 is evident that these same changes occur, although often 

 very much less clearly marked, in market milk rich in 

 bacteria. When milk is kept a long time it may become 

 altered in one of several ways, of which the following 

 are the most common : 



1. The well known coagulation (curdling), following 

 the formation of acid, is the most frequent. 



2. Less often, a more or less disagreeable (putrid) 

 odor and taste occur without much acid formation and 

 without coagulation. 



3. Still less frequently, the milk becomes slimy or 

 i i stringy, ' ' either without souring, or accompanying 

 the process of souring. 



Sour milk. It is known that sour milk has no harmful 

 effect on healthy people. But it is different with those 

 suffering with catarrh of the stomach, and with small 

 children. In these cases a degree of acidity not be- 

 trayed by coagulation or appreciable sour taste appar- 

 ently may cause vomiting and indigestion, possibly be- 

 cause the numerous lactic acid bacteria rapidly increase 

 with the warming of the milk in the stomach, and cause 

 coagulation before the gastric juice can check their 

 growth. Milk in which the lactic acid fermentation is so 

 far advanced that it will not stand boiling, or the taste 

 of which is slightly sour, must therefore be regarded as 

 unfit for food for small children. The number of bac- 

 teria in milk does not give us a safe criterion for judging 

 it in this connection, but the degree of acidity furnishes 

 a reliable guide. (See below.) 



Putrid milk. Even though the taste is not yet decid- 

 edly foul, but is only bitter (formation of peptone) 



