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 theie 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. 
