5 6 MILK [v 



influenced in the rate of their development by tem- 

 perature. This, indeed, is the reason why the 

 influence of heat is so enormous in regulating the 

 changes which take place in inilk. Eapid develop- 

 ment of these classes of bacteria may be said only to 

 take place at temperatures above 15 C. (60 Fahr.). 

 It is for this reason that milk ought to be cooled down, 

 immediately after milking, below this temperature, 

 if it is desired to keep it for any time. Similarly, 

 the temperature ought not subsequently to be allowed 

 to rise above this point. But, just as a low tempera- 

 ture is unfavourable for the development of bacteria, 

 so also is a high temperature. Thus, heating the 

 milk to 50 C. (122 Fahr.) checks this kind of fer- 

 mentation. The effect of the formation of this lactic 

 acid, when it reaches certain proportions in the milk, 

 is to cause the coagulation of another important con- 

 stituent of milk, viz. the caseous matter. 



And here reference may be made to the widely 

 spread belief that electricity in the air has the effect 

 of souring milk. There can be no doubt that in 

 " thundery " weather milk or cream sours more 

 quickly than under ordinary conditions ; but this 

 cannot be traced to the effect of electricity. In fact, 

 experiments have shown that electricity, instead of 

 accelerating the souring of milk, actually retards it. 

 The true explanation is probably to be found in the 

 fact that the temperature of the air is generally 



