82 PRACTICAL DAIRY BACTERIOLOGY 



a fairly regular increase of nearly all species, although the 

 common lactic bacterium grows more rapidly than any other. 

 The length of time of this period of increase depends upon the 

 temperature of the milk. If kept cold, there may be a day, or 

 even two days, during which time there will be shown a general 

 increase in the numbers of nearly all kinds of bacteria. 



Second Phase. After the numbers of bacteria have become 

 fairly large, 10,000,000 to 20,000,000 per c.c., it is found that 

 the Bact. lactis acidi has gained the upper hand of the others. 1 

 This species, present in small numbers at the outset, has in- 

 creased in proportion until at the close of the first period there 

 may be from 20 to 40 per cent, of them. During the second 

 period, which now follows, they rapidly increase until they 

 finally come to be nearly 100 per cent. This type of organism 

 has, in other words, almost or completely replaced the numerous 

 species found at the outset. This second phase ends with the 

 souring and the curdling of the milk. 



Third Phase. After souring and curdling, the bacteria in the 

 milk cease growing, the amount of acidity being so high that 

 they cannot thrive. Many of the bacteria die, and there is 

 a fairly rapid reduction in numbers until, after a few days, they 

 are once more very few. The milk may now remain for a long 

 time in this condition without further bacteria growth. Later, 

 however, there are sure to develop some conditions which re- 

 duce the acidity. This third phase, however, does not affect 

 the milk handler, but chiefly the cheese producer; for it is 

 only in milk products that have been kept many days, or several 

 weeks, that the later changes occur. These later changes are 

 variable, but, in general, they consist of a growth of molds 

 over the surface of the milk or cheese, which reduces the 

 acidity to a neutral, or slightly alkaline, condition. After the 

 acid has disappeared, bacteria will again begin to grow. The 

 bacteria which develop in the third phase are not lactic germs, 



1 Bienstock. Archv f. Hyg., xxxix., p. 390, 1901. 



