152 BACTERIA IN MILK. 



fresh milk, they very rarely get an opportunity to have any consider- 

 able effect upon the milk. The lactic bacteria grow so very much 

 more rapidly that they soon entirely outnumber the enzyme class, 

 and, indeed, in most cases stop their growth. As a result, whereas 

 the latter may be comparatively numerous in fresh milk, they become 

 less rather than more abundant as the lactic bacteria grow, and 

 finally disappear. Under such conditions their significance in the 

 milk is probably nothing. Occasionally, however, it may happen 

 that a sample of milk does not chance to have any lactic organisms 

 in it, or that they are so few as to fail to get the upper hand of the 

 others. If this occurs, the other species of bacteria may find the 

 conditions favorable to their growth, as in cases of sweet curdling. 

 This class of bacteria plays an important part in the changes which 

 may take place in so-called sterilized milk, which has been heated 

 to a temperature of boiling water. Such milk still contains a 

 considerable number of spore-bearing bacteria that resist this 

 temperature. The milk does not sour, inasmuch as all lactic acid 

 bacteria are killed, since they never produce spores. The class of 

 enzyme-forming bacteria, however, are very commonly spore 

 bearers, and resist the temperature of boiling water. Milk which 

 has been boiled, therefore, not infrequently undergoes changes 

 which affect its taste and its chemical nature, due to the class of 

 bacteria here considered. Occasionally they are of significance in 

 cheese-making. During the long ripening of cheese they have a 

 better chance to grow than in milk. Whether they have much 

 influence upon hard cheeses seems doubtful, but in the ripening of 

 soft cheeses they sometimes produce very bad results, causing much 

 loss to the cheese-makers. While, therefore, they are of little impor- 

 tance to the one who handles milk, they play a considerable part 

 in the making of cheese. 



This class of liquefying bacteria usually produces no acid; 

 but there is a small group of the same class that differs from the 

 others in producing both a digesting enzyme and an acid. They 

 are sometimes called acid liquefiers. It has been thought that they 

 play a part in the ripening of cheese, but this is by no means 

 certain and in general they are of little significance. 



