vi] THE BACTERIA OF MILK 91 



which are characterised by their great powers of 

 resistance, explains why it is that the bitter taste is 

 most frequently found in boiled milk which has been 

 allowed to stand for some time. In such cases, by 

 boiling the milk, the bacteria effecting lactic ferment- 

 ation are killed, and the spores of the bitter-milk 

 bacteria only are allowed to remain. Such treat- 

 ment is favourable to their development, since the 

 presence of lactic acid in unboiled milk is inimical 

 to their growth. Among the micro-organisms which 

 have been identified as causing this fault may be 

 mentioned Weigmann's litter -milk bacillus, which 

 possesses a length of 1 6 * Q th to 18 \ Q O th of an inch 

 long, and from 27> o 00 th to ^^th of an inch 

 broad ; and Conn's bitter-milk micrococcus. 



There are a number of other faults to which milk 

 is liable, and which are due to micro-organic life, 

 such as premature curdling, and the development of a 

 salt taste, regarding which not much is as yet known. 



Having discussed the different " faults " to which 

 milk is liable, we may pass on to describe the cause 

 of the more common changes which milk undergoes 

 in keeping. The bacteria causing these changes may 

 be most conveniently classified according to their 

 products/ 1 We have two principal groups, known 

 as Lactic ferments and Casein ferments. 



Lactic Fermentation of Milk. The most com- 

 mon fermentative change in milk is its souring, 



