BACTERIA IN MILK. 189 



LACTIC ACID BACTERIA. 



By far the most universal type of bacteria in milk is the class 

 which produces lactic acid and causes the milk to sour and 

 curdle. This lactic acid is produced from the milk sugar by 

 a chemical change the details of which are not known and 

 which, indeed, may not be the same under different conditions, 

 or when produced by different bacteria. The essential nature 

 of the change is expressed by the formula : 



C 6 H 12 6 =2C 3 H 6 3 



Milk sugar Lactic acid 



While this represents the essential nature of the change it 

 is certain that it is a more complicated phenomenon than here 

 indicated, and involves a number of other products, acetic acid 

 and alcohol being frequently formed. For ordinary purposes, 

 however, the essential effect is the production of lactic acid out 

 of milk sugar. 



The number of types of bacteria which can produce the 

 lactic fermentation is large. Indeed, the property of produc- 

 ing lactic acid from milk sugar is characteristic of a large 

 number of bacteria which are not dairy organisms. Several 

 scores of bacteria with this power have been described by 

 different bacteriologists, all apparently more or less distinct. 

 But the more carefully the dairy bacteria have been studied, 

 the more evident has it become that the long list must be con- 

 siderably shortened, and to-day there is a tendency toward the 

 belief that lactic bacteria which are concerned in the normal 

 souring of milk are very few. The most important of these 

 are two. 



I. Bacillus acidi lactid. This name was first used by 

 Hueppe and has been subsequently applied to quite a large 

 number of more or less different types. It is still in use and 

 applied now to a group of related bacteria, slightly different 

 from the original described by Hueppe, but a group which 



