igS GENERAL BIOLOGY OF MICRO-ORGANISMS 



The development of these various microbes in the milk de- 

 pends very much upon the. temperature at which it is kept. At 

 o"* to lo" 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 lo" and 21" the Bad. acidi 

 lactici is almost certain to gain the dominance and rapidly to 

 suppress the other tj^es, and it produces the normal souring of 

 milk. Between 21" and 35" C. the organisms of the B. coli and 

 B. lactis aerogenes groups are likely to predominate and at tempera- 

 tures from 37° C. to 40° C. the B. bulgaricus is likely to gain the 

 ascendency, after a few days at any rate. These may be regarded 

 as the normal fermentations of unhealed 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 putrefactive 

 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 



