1 84 



BACTERIA, YEASTS, AND MOLDS 



milk belong to a type known as lactic bacteria (Fig. 67). 

 These produce a change in the milk sugar, converting it 

 into lactic acid, which causes the milk to taste sour and 

 curdle. Curdling and souring will never occur if bacteria 

 can be kept out of the milk. Although the souring is a 

 o o n nuisance, it does not injure 



>g 9 qrf>_ the wholesomeness of the 

 milk, and sour milk could 

 be used freely were it not 

 for its unpleasant taste. 

 Indeed, souring is, under 

 some circumstances, desir- 

 able, since milk properly 

 soured is protected from a 

 variety of other changes 

 far less agreeable. If the 

 lactic bacteria do not cause 

 the milk to sour, it is 

 almost sure to putrefy, and 



i , the most common lactic bacterium, B. lactis putrefaction is far more 



acidi; 2, a less common lactic bacterium unp l e asant and Unwhole- 

 n. lactis aerogenes ; 3, common cocci found 



in milk; 4, a bacillus producing cheese SOmC than Ordinary SOlir- 



flavors ; s, a common bacillus with no action . .-p., . r .,, 



on milk, *. *,7; 6, a bacillus causing "lg. The SOUring of milk, 



slimy milk, B. lactis viscosus ; 7 and 8, com- therefore, is SL natural phe- 

 mon organisms with no action on milk; 9, 



bacillus causing swelling of cheese; 10, a n O m 6 n O n, and OttC that 



bacillus causing milk to become putrid. should be expected and 



desired in milk after it has become a day or two old. 

 Milk which will not sour is suspicious, unless it has been 

 kept at a very low temperature for preservation. 



Sometimes milk a day or two old becomes slimy or 

 slippery to the touch, rather sweetish to the taste, and is 



9 ^ fo 



FIG. 67. Group of milk bacteria. 



