22 PRACTICAL DAIRY BACTERIOLOGY 



less common, occurring in milk under certain conditions but 

 which can generally, by proper means, be prevented from getting 

 into the milk in quantities large enough to produce any effect. 



NORMAL MILK BACTERIA 



The Souring of Milk. The most common change which oc- 

 curs in milk is its souring. This has always been recognized 

 and it has been supposed to be a natural change in the milk, the 

 supposition being that milk would sour, just as blood will 

 clot. The studies of bacteriologists have shown, however, that 

 this is a mistake. The souring of milk is not a characteristic 

 of the milk itself, for it will not occur in perfectly pure milk. 

 It is always brought about by the growth of bacteria. Never- 

 theless, it is of almost universal occurrence even in good milk. 

 Indeed, if one obtains a lot of milk which will not sour when 

 kept for a day or longer in a warm temperature, it at once 

 arouses suspicion that some means have been adopted for keep- 

 ing the milk sweet, either by the use of preservatives, or by 

 pasteurization. This is not necessarily the case, however, for, 

 under some conditions, pure milk may be obtained which will 

 not undergo the ordinary souring; but the phenomenon is so 

 common that it is to be expected in all samples of normal milk. 

 The souring is the result of the production of lactic acid from 

 milk sugar. Milk contains about 4 per cent, milk sugar and, under 

 the action of certain bacteria that grow in milk, this milk sugar 

 is changed into lactic acid. The change is frequently ex- 

 pressed by the following formula: 



Milk sugar Lactic acid 

 CH, 2 O 6 2C 3 HO 8 



There is no question, however, that this does not represent the 

 change that actually takes place. 



A molecule of milk sugar contains a certain multiple of the 

 formula above given, but the multiple is not known, and the 

 change into lactic acid is a far more complicated one than 



