IO2 GROWTH OF BACTERIA IN MILK. 



without a typical souring and curdling. It is sometimes pos- 

 sible to keep the milk for five or six weeks without curdling 

 if it be retained at 50, simply because, at this temperature, 

 the common lactic bacteria do not grow. Such milk contains 

 in the end great numbers of bacteria, but they do not belong 

 to the lactic type. At 70 Bact. lactis acidi develops readily, 

 and from this temperature up to 90 or over, this and other 

 lactic species multiply rapidly. At 86 and above, however, 

 some other varieties of lactic bacteria are more especially 

 stimulated, and if milk is kept at the higher temperature it 

 is likely to be soured by some of the other species of lactic 

 organisms. Some milk bacteria, as we have seen, require a 

 temperature of 140-! 50 for their proper growth. 



From these facts it is evident that the temperature will 

 greatly modify the types of bacteria which grow in milk, 

 and that the number is not the only factor affected by 

 cooling. By cooling, milk may be kept for a long time with- 

 out souring or curdling, but this does not mean that the milk 

 will be fit to use, for it will be almost sure to contain great 

 quantities of other species of bacteria .which do not make 

 themselves evident by any visible action on the milk, but 

 are probably more deleterious to the persons using the 

 milk than the more common lactic bacteria which develop at 

 the higher temperatures. Milk which is old cannot be 

 regarded as reliable simply because it has not soured or 

 shown any other visible change. 



Comparative Growth of Bacteria in Normal Milk. For 

 three or four days there is a great increase in the numbers of 

 bacteria in normal milk. But this does not mean that all of 

 the species which are found in fresh milk increase equally 

 and that an increase from 2,000 to 1,000,000,000 is a uni- 

 form one of all species. On the contrary, some of the species 

 grow very rapidly, some grow moderately, some grow slowly, 

 and some fail to grow at all and even seem to disappear. The 

 analysis of the kinds of bacteria in milk of various ages 



