EFFECT OF TEMPERATURE. 99 



property of milk, but better to speak of it as a period of 

 preliminary adaptation of bacteria to the new conditions. 

 Practically it makes no difference to what we attribute the 

 phenomenon. The fact is that, for a period ranging from two 

 to forty hours, according to conditions, there is no appre- 

 ciable growth of bacteria but, on the contrary, frequently an 

 actual decline. This phenomenon is probably universal, 

 although it varies with the milk of different cows and has 

 not always been observed. In all recent work where a care- 

 ful study has been made this initial period of bacteria decline 

 is very apparent. 



EFFECT OF TEMPERATURE UPON BACTERIA GROWTH. 



Effect upon Numbers. After the initial period has passed 

 the bacteria begin to increase rapidly. In general terms, the 

 longer the milk stands the larger the number of bacteria 

 which it contains. But this general statement is true only 

 within limits. As already noticed they do not grow at all for 

 a few hours, and at the other end also we find that, after they 

 become very numerous, they cease to grow. At about the 

 time of the curdling of the milk, or a few hours after, the 

 bacteria reach their maximum, and from this time decline 

 in numbers. Old milk contains fewer bacteria than 

 milk of less age, and after several days they are greatly 

 reduced in numbers. Such old milk plays no part in dairy 

 processes except cheese-making, and for the period covered 

 by the time in which milk is handled by the dairyman as milk 

 it is quite true that the older the milk, other things being 

 equal, the greater the number of bacteria. 



But the number is most extraordinarily modified by tem- 

 perature. Life processes in general are more rapid at high 

 temperatures, but in regard to the growth of milk bacteria 

 the influence of temperature is surprising. The following 

 figures show this influence of temperature. The figures at 

 70 and 50 were obtained from the same sample of milk, 



