56 Agricultural Bacteriology. 



teria produce in milk is perfectly homogeneous, showing 

 no holes due to gas or no whey expressed from the curd. 

 The odor is agreeable, the taste sour, but not bitter or of- 

 fensive in any way. 



The lactic bacteria do not form spores and hence are 

 easily killed, if milk is heated. If milk is pasteurized 

 and subsequently kept free from lactic bacteria, it will 

 not sour, but will putrefy due to the development of the 

 spores not killed by the heating. Often the first sign of 

 spoiling in pasteurized milk is the appearance of a bitter 

 or other undesirable taste. Frequently it does not cur- 

 dle for a long time. One of the dangers in the use of 

 pasteurized milk is the fact that the consumer has no way 

 of telling how old it is. It may appear normal in every 

 way and yet be harmful to the health. 



The lactic bacteria grow best at temperatures from 

 70-95 F. If the milk is cooled below 50, growth goes 

 on very slowly, at still lower temperatures, 32-35, the 

 growth of lactic bacteria is wholly prevented, but other 

 types develop which may cause the milk to become harm- 

 ful. 



It is a widespread belief that thunder storms cause the 

 milk to sour. Electricity has no effect on milk, the ap- 

 parent effect is due to the high temperatures that always 

 accompany such storms, causing the bacteria to grow 

 more rapidly. 



Undesirable lactic bacteria. The desirable types of 

 lactic bacteria produce other by-products than the lactic 

 acid and to these is due the odor of sour milk. The un- 

 desirable lactic germs form some lactic acid but larger 

 amounts of other acids such as acetic acid. They also 

 form gaseous by-products that give to the milk an ob- 

 jectionable odor and taste, injuring it especially for but- 



