Bacteria in the Cheese Industry. 149 



In precipitating the casein, the fat is caught in the 

 curd particles, and is thus incorporated in the cheese. 

 This curdling of the casein is brought about in two ways. 



1. Addition of rennet, a chemical ferment extracted 

 from calves' stomachs 1 , which has the specific property 

 of rendering the casein of milk insoluble, e. g., cheddar 

 and Swiss varieties of cheese. 



2. Development of acid until the casein is precipitated, 

 e. </., sour milk, cottage cheese. 



By far the larger amount of cheese is made in the first 

 way. The various kinds manufactured may be divided 

 into two general classes, soft and hard cheese, depending 

 upon the character of the finished product, which is pro- 

 duced by the kind of treatment that is given the milk in 

 the manufacture of the cheese and its subsequent curing. 

 By far the larger amount of cheese made in this country 

 belongs to hard, firm type of cheese. 



156. Summary of manufacturing' details. In ma- 

 king cheese, it is necessary to concentrate the casein and 

 adherent fat into a compact mass, containing only a lim- 

 ited amount of water. The addition of the rennet pre- 

 cipitates the casein and causes it to shrink in volume. In 

 order to facilitate this process, the coagulated casein is 

 cut into small pieces, thereby giving a better opportunity 

 for the continued shrinking action, and so expel the milk- 

 serum or whey. This change is greatly affected by the 

 presence of acid and heat. In order to make the process 

 uniform from day to day, the milk is held or ''ripened" 

 until a certain degree of acidity is reached from the de- 

 velopment of the bacteria that are invariably present in 

 the milk. The curds are then heated to a varying tem- 

 perature, the action of the heat being to further expel 



1 In Italy this ferment is sometimes secured from artichokes and 

 is used in cheese manufacture. 



