BACTERIA IN MILK 



20S 



"niC^^ 



milk belong to a type known as lactic-acid 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 



nuisance, if does not injure 

 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 

 g ^=' \ lo' *^^ milk to sour, it is 



Fig. 67. Group of milk bacteria. almost sure to putrefy, and 



, the most common lactic bacterium, B. lactis pUtrCfaCtion is far mOrC 

 acidi; ., . less common lactic bacterium UUpleaSant and UHwholC- 

 £>. aeroge7ies ; 3, common cocci found ■■ 



in mUk; 4, a bacillus producing cheese SOmC than Ordinary SOUr- 

 ilavors ; 5, a common bacillus with no action 

 on milk, B. subtilU; 6, a bacillus causing 

 slimy milk, B, laciis viscosus ; 7 and 8, com- 

 mon organisms with no action on milk ; 9, 

 bacillus causing swelling of cheese ; 10, a 

 bacillus causing milk to become putrid. 





ing. The souring of milk, 

 therefore, is a natural phe- 

 nomenon, and one that 

 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 



