l6o BACTERIA IN MILK. 



Fresh milk contained 6,525 bacteria per c.c. 



After 25 hours at 50 the same milk contained, 6,425 bacteria 

 After 25 hours at 70 the same milk contained, 6,275,000 bacteria " 



In this example it is seen that for twenty-five hours the bacteria 

 in milk kept at 50 did not multiply at all, while in that kept at 70 

 they multiplied one thousand fold. It is not common to find such 

 a striking difference, but in all cases there is a very marked contrast. 



PROTECTIVE ACTION OF LACTIC ACID 

 BACTERIA. 



If ordinary proteids, like eggs or meat, are left undisturbed to 

 the action of bacteria, they will putrefy. Milk also contains a 

 proteid, casein, which is just as liable to putrefaction as other 

 proteids. But under ordinary conditions it does not undergo this 

 unpleasant change. Milk sours, but rarely putrefies. The reason 

 for this is found in the power of the lactic acid bacteria to restrain 

 the growth of other species. Almost from the start, the lactic acid 

 bacteria in milk grow more rapidly than the other types, and as 

 they become more abundant, they prevent the other kinds from 

 growing; they thus effectually restrain the growth of the putrefactive 

 bacteria, so that milk that has begun to sour will not putrefy. This 

 is really a very useful function, for, whereas soured.milk is wholesome, 

 putrefied milk is not wholesome, and the lactic acid bacteria thus 

 protect the milk from a decomposition which would be far worse 

 than souring. It has also come to be a recognized fact that many 

 of the troublesome faults in milk may be remedied by using a culture 

 of lactic acid bacteria. In cases of bitter milk, of premature curd- 

 ling, and of other miscellaneous troubles, due to undesired bacteria, 

 a remedy is found by putting into the milk a culture of a vigorous 

 lactic acid bacterium that will grow rapidly and prevent the 

 undesirable bacteria from developing sufficiently to cause trouble. 

 The use of this principle in butter- and cheese-making has become 

 very widely extended. 



It is this restraining action of the lactic acid bacteria that explains 

 the generally recognized fact that sour milk, or butter-milk, is not 

 only a wholesome, but a very useful beverage. It seems a little 



