CHAPTER VII. 

 RELATION OF BACTERIA TO CHEESE. 



Cheese is an important item in the dietary of European 

 people. It has a high food value because it represents 

 a concentration of three-fourths of the solids of the milk 

 to about one-tenth of their original volume. It is also 

 largely used as a condiment. In many European coun- 

 tries the food supply of a large part of the people is 

 coarse and relatively tasteless. A small amount of high- 

 ly flavored cheese renders such food more appetizing. 



Manufacture of cheese. Cheese is made from the fats 

 of the milk, the casein, certain ash constituents, and a 

 part of the milk serum. The casein is curdled by allow- 

 ing the milk to sour or by the addition of rennet. The 

 curd holds the fat globules of the milk, and when it 

 shrinks under the influence of heat and acid, whey is 

 expressed, leaving a mixture rich in fat and casein, and 

 containing from 30 to 50 per cent of milk serum or whey, 

 depending on the kind of cheese. 



Types of cheese. Cheese may be grouped into two 

 classes: (1) Those in which the curd is obtained by 

 allowing the milk to sour ; (2) those in which the curd 

 is obtained by the use of rennet. 



The only important kind of the first class in this coun- 

 try is the cottage or Dutch cheese. Bacteria function in 

 the manufacture of this variety as the curdling of the 

 casein is due to the souring of the milk. Heating the 



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