THE DISTRIBUTION OF MICRO-ORGANISMS IQI 



The development of these various microbes in the milk 

 depends very much upon the temperature at which it is kept. 

 At o to 10 C. the acid-forming bacteria grow very slowly or 

 not at all, and the milk may remain practically unchanged for 

 many days or even weeks. Eventually some of- the liquefying 

 bacilli or the slime-producing types may gain the upper hand 

 and change the consistency and flavor. Between 10 and 21 

 the Bad. acidi lactici is almost certain to gain the dominance 

 and rapidly to suppress the other types, and it produces the 

 normal souring of milk. Between 21 and 35 C. the organisms 

 of the B. coli and B. lactis aero genes groups are likely to pre- 

 dominate and at temperatures from 37 C. to 40 C. the B. bul- 

 garicus is likely to gain the ascendency, after a few days at 

 any rate. These may be regarded as the normal fermentations 

 of unheated milk of very good quality. The other microbes in 

 the milk are not destroyed by these fermentations but their 

 development is usually held in check somewhat. 



Shortly after the coagulation of the milk, which occurs when 

 the lactic acid reaches a concentration of about 0.45 per cent, 

 the living bacteria begin to diminish in number, and gradually 

 Oidium lactis and other molds become prominent, although acid- 

 resisting forms such as B. bulgaricus still continue to grow. 

 Organisms of these kinds seem to be specially concerned in 

 the ripening of acid curd in cheese making. Finally the acidity 

 may disappear as a result of the activity of molds, and putre- 

 factive bacteria find the opportunity to develop. 



If the milk be pasteurized, the bacteria which form lactic 

 acid are killed, and when fermentation occurs it is likely to be 

 different from the normal souring. At a high temperature, 

 the stormy butyric-acid fermentation due to B. welchii may be 

 observed. At a lower temperature, a slow putrefaction due to 

 spore-forming putrefactive anaerobes in conjunction with other 

 bacteria may occur. These fermentations are ordinarily inhib- 

 ited by the lactic acid produced in the normal souring of milk. 



Alcoholic fermentation of milk occurs as a rule only when 



