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 
