THE LACTIC FERMENTATION. 63 



are rare or uncommon in others. A few species, however, 

 appear to be very widely distributed and these we shall 

 notice particularly. The effects produced upon milk by dif- 

 ferent bacteria are very numerous, but they may be grouped 



into a comparatively few types as follows: 



i ' 



THE LACTIC FERMENTATION. 



By lactic bacteria are meant species which give rise to a 

 considerable amount of lactic acid, the production of this 

 acid being, therefore, a primary characteristic of their growth 

 so far as concerns their relation to milk. Along with the 

 lactic acid, however, they all produce other products and 

 they have numerous secondary characters. For example, 

 there is frequently produced carbon dioxide, hydrogen, traces 

 of alcohol, acetic acid and salicylic acid, and other more ob- 

 scure chemical products. The development of these second- 

 ary products is somewhat variable, and is in a measure 

 dependent upon temperature. At moderately high tempera- 

 tures the secondary products are developed in larger amount 

 than they are at low temperatures. But the chief product 

 of this group of bacteria is lactic acid, which is produced 

 from the milk sugar, and since this is a prominent charac- 

 teristic of a considerable group of bacteria we recognize 

 them as lactic bacteria. The lactic acid produced is not al- 

 ways the same. At 70 F. it is the type of lactic acid that 

 turns the plane of polarized light to the right. At 98 it is 

 likely to be both the right-handed and the inactive type (50). 

 In old cultures at high temperatures the left-handed variety is 

 more apt to be present (50). 



The Souring of Milk. The souring of milk is a well-nigh 

 universal phenomenon. It is true that occasionally normal 

 milk does not sour, due to factors to be noticed later, but 

 these cases are exceptional and, in the vast majority of cases, 

 milk becomes acid after a few hours and eventually the acid 

 precipitates the casein and the milk curdles. Curdling may 



