v] CHANGES WHICH MILK UNDERGOES 57 



increased before thunder, and that this promotes a 

 more active development of bacteria in the milk. 

 That it is not due to electricity is proved by the fact 

 that sterilised milk will not sour, no matter how 

 violent the thunderstorm is. 



Amphoteric Reaction. And here it may be well 

 to say a word or two on rather a curious property of 

 milk. This is the amphoteric reaction it possesses 

 a property which we have already referred to. By 

 warming the milk, it is found that the alkaline reac- 

 tion becomes more pronounced. Warming, however, 

 has no influence on the acid reaction. By the 

 gradual formation of free lactic acid in the process 

 of fermentation, milk loses its amphoteric reaction, 

 the alkaline reaction disappears, and the acid re- 

 action alone remains and gradually increases in 

 strength. This takes place, after a time, to such an 

 extent that, although the milk remains liquid at 

 ordinary temperatures, a slight increase of tem- 

 perature, or the addition of carbonic acid, causes 

 immediate coagulation. Finally, the casein, even at 

 the ordinary temperature, becomes coagulated. 



Coagulation of Milk. The coagulation of the 

 casein is thus due to the development of acidity in 

 the rnilk. The higher the milk is heated up to a 

 certain point the more quickly will coagulation 

 take place. The coagulation of the casein may be 

 effected by the addition of various precipitating 



