CURDLING. 145 



The Jersey breed gives the richest milk, the fat fre- 

 quently amounting to 5 6 per cent. 



The influence of diet on the production of milk and 

 butter has been already considered (p. 130). 



Curdling. 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 acidi- 

 fication of the milk induces the coagulation of the 

 casein. The higher is the temperature, 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 ferment. Other fer- 

 ments, altering the condition of the albuminoids in milk, 

 are produced by other species of bacteria. The presence 

 of these ferment-forming bacteria sometimes occasions 

 much 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 is also most valuable 

 for this purpose. All bacteria are destroyed at a boiling 

 heat. Milk that is free from micro-organisms is un- 

 changed by keeping. 



Cream. The fat of milk occurs in the form of globules; 

 the largest are about '0005 to '0006 inch in diameter, 

 the smallest may be one-tenth this diameter, or even less. 

 The average size of the globules is different with different 

 breeds of cattle ; thus they are larger in the milk of the 

 Jersey than in the milk of the Ayrshire or Holstein breed. 



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