230 THE CHEMISTRY OP THE FARM 



yeasts find admission. The ordinary souring of milk 

 is produced by various species of bacteria, which 

 during their growth convert the sugar of milk into 

 lactic acid. This acidification of the milk induces the 

 coagulation of the casein. The higher is the tem- 

 perature the smaller is the proportion of acid which 

 will curdle milk. Milk is also curdled by other species 

 of bacteria, which produce no, or very little, acidity, 

 but apparently act by the formation of a rennet-like 

 enzyme. Other ferments (enzymes), altering the con- 

 dition of the albuminoids in milk, are produced by 

 other species of bacteria. The presence of these 

 enzyme-forming bacteria sometimes occasions mucTi 

 difficulty in dairy work, and is the cause of many of the 

 so-called " diseases " of milk. The development of 

 these mischievous bacteria may be checked by cooling 

 the milk while the cream is rising. The speedy work 

 done by the centrifugal machine also enables the 

 required products to be obtained before change has 

 occurred. All micro-organisms may be destroyed by 

 a boiling heat, and many by prolonged exposure to a 

 temperature of 140° Fahr. Milk that has been heated 

 for some time to the temperature of boiling water in a 

 closed vessel is said to be sterilised. Milk that has been 

 heated for twenty minutes to 158° Fahr. is said to be 

 pasteurised. In pasteurised milk all organisms, save 

 spores, have been destroyed, while the character of the 

 milk is little altered. 



Cream. — The fat of milk occurs in the form of 



globules ; the largest are about "0005 inch in diameter, 

 the smallest may be one-tenth this diameter. The 

 average size of the globules is different with different 



