BACTERIA IN MILK. 1 95 



almost clear, watery solution, which may have meantime de- 

 veloped certain colors. The essential phenomenon is, however, 

 the solution of the casein, usually called digestion. 



More commonly the action of these bacteria is somewhat 

 different because of the second enzyme. This is similar in its 

 nature to rennet and it produces a curdling of the milk. Under 

 the action of these bacteria milk becomes in a few days a soft 

 gelatinous curd, which is alkaline in reaction and has a sweet 

 taste, a phenomenon sometimes called "sweet curdling." It 

 generally happens that a few hours after the milk is thus 

 curdled the curd begins to become softened and dissolved. 

 The trypsin ferment (casease), produced by the same bacteria, 

 digests the curdled casein, and, as fast as it is digested, it is dis- 

 solved. As the digestion goes on the whole curd may in time 

 become dissolved and the milk become a transparent, watery 

 fluid, of a brownish, yellowish or greenish color, or with no 

 color at all. The final result is the same whether or not the 

 milk first curdles. Milk that has curdled from the formation 

 of lactic acid never becomes digested in this manner, inasmuch 

 as the acid prevents the growth of these enzyme-forming 

 bacteria. 



The number of bacteria producing these enzymes is very 

 large. The bacteriologist in his classification of bacteria 

 divides them into two great divisions according to their power 

 of liquefying gelatin. The dairy bacteriologist finds that the 

 bacteria which liquefy gelatin produce enzymes as above de- 

 scribed. All of them appear to give rise to the digestive fer- 

 ment, for the power of liquefying gelatin and the power of 

 digesting casein appear to be the same. Not all of the lique- 

 fiers, however, curdle milk for some of them digest the casein 

 without curdling. Even these do, in some cases at least, pro- 

 duce rennet, but the amount is too small, or it is produced too 

 slowly, to curdle the milk. 



The study of the bacteria of cow's excrement and of water 



