84 Dairy Bacteriology. 



any other forms, and the acid produced by them ren- 

 ders the milk an unfavorable medium for the growth 

 of other bacteria. This is the reason why milk practi- 

 cally always undergoes the acid fermentation, although 

 it is contaminated with a host of other kinds of bac- 

 teria. If a mixture of seeds is sown on low wet ground, 

 certain kinds will grow best; if the same mixture is 

 sown on drier land, other types will find most favorable 

 conditions for growth, and the plants which appeared 

 on the low land will not appear. The same condition is 

 found in milk where the environment is most favorable 

 for the acid-forming bacteria. 



Amount of acid formed in milk. In this country the 

 acidity of milk is expressed as so many per cent of lac- 

 tic acid. A milk that shows an acidity of one per cent 

 should, theoretically, contain one pound of lactic acid 

 in each one hundred pounds of milk. The acid deter- 

 mined does not actually represent lactic acid, as there 

 are other substances in milk which act as acids, with 

 the reagents used in the present methods of determin- 

 ing the acidity of milk. For instance, perfectly fresh 

 milk has an apparent acidity of 0.13 to 0.18 per cent, 

 although no fermentation has occurred. Other acids 

 than lactic are formed in the acid fermentation, but 

 the entire acid content is referred to as lactic when 

 speaking of the acidity of milk. When the developing 

 acidity of milk reaches 0.25 to 0.3 per cent, a sour taste 

 becomes evident and the milk will curdle on heating. 

 When the acidity increases to 0.6 to 0.7 per cent, the 

 milk curdles at ordinary temperatures. The acidity 

 continues, however, to increase until it reaches about 

 1 per cent, which is the maximum amount that will be 



