IO6 GROWTH OF BACTERIA IN MILK. 



is filled .with bacteria in enormous quantities (1,000,000,000 

 or more per c.c. being frequently found), but most of them 

 are of the lactic type and Bact. lactis acidi is usually by far 

 the most abundant. 



It will be clear that the chemical changes which occur in 

 milk must be different during these two periods. In the first 

 period the general growth of the bacteria will produce a 

 variety of chemical changes characteristic of the different 

 species of bacteria. The nature of these changes is little 

 known, but the development of lactic acid is not one of them. 

 During this first period the acidity of the milk does not 

 increase, remaining, as shown by careful tests, exactly what 

 it was at the start. Of the other changes which occur we can 

 say little or nothing, except that the increase of the many 

 species from a few thousands to a few millions per c.c. must 

 result in some chemical changes in the milk. During the 

 second period it is probable that, for a time, these miscel- 

 laneous changes continue, at least so long as the miscellaneous 

 bacteria continue to grow. But the distinctive characteristic 

 of the second period is the development of lactic acid. This 

 begins to increase very rapidly, more rapidly, in comparison, 

 than the bacteria growth. The amount of acid increases until 

 it is sufficient to curdle the milk, and even then does not stop, 

 but may go on accumulating for some time longer. Even- 

 tually, however, the abundance of lactic acid is sufficient to 

 stop the growth of bacteria so that they begin to die, and 

 finally, some days after curdling, they almost disappear. The 

 milk may now remain without further change for a long time, 

 although eventually moulds will grow upon it and, finally, a 

 further putrefaction may occur. But with these later changes 

 we are not concerned, since they have nothing to do with 

 ordinary dairy phenomena. 



It will be manifest that the most striking feature of this 

 bacteria growth in milk concerns the relation of the lactic 

 bacteria. The lactic bacteria are the milk bacteria par excel- 



