RELATION OF MICROORGANISMS TO CHEESE. 351 



In order to insure proper rennet action the maker of Cheddar cheese 

 desires the milk to have an acidity of about 0.2 per cent. He thus wishes 

 milk that has passed through the period of incubation and in which the 

 acidity has begun to increase. If the milk shows the desired acidity when 

 it reaches the factory, the making process is immediately begun. If the 

 milk is too sweet, the period of incubation is shortened by warming the 

 milk to temperatures most favorable for the lactic bacteria, 30 to 32, and 

 by the addition of pure lactic cultures, which are identical in nature and 

 in method of propagation with those used in butter making. The devel- 

 opment of a slight acidity is known as the "ripening" of milk. 



CURDLING OF MILK. Under the influence of a favorable temperature 

 and the slight acidity, the milk is quickly changed by the rennin* to a 

 firm, jelly-like mass that is cut, with appropriate knives, into small cubes. 

 The curd encloses 95 to 99 per cent of the bacteria in the milk. The same 

 factors that favor the curdling of the milk favor the shrinking of the curd 

 and the expulsion of the whey from the cubes. The development of acid 

 within the curd is rapid, due to the concentration of large numbers of 

 bacteria in a small volume and to the favorable environment. During the 

 six to eight hours that elapse between the curdling of the milk and the pres- 

 sing of the curd, the increase of acidity is over o. i per cent per hour. The 

 following table gives the acidity of milk and the whey expressed from 

 the curd at various stages in the making of a typical Cheddar cheese. 



Acidity of milk before adding rennet 0.20-0.21 per cent. 



Acidity of whey immediately after cutting curd. o. 14-0. 145 per cent. 



Acidity of whey when removed from the curd. o. 16 o. 18 per cent. 



Acidity of whey when curd is packed o. 24-0. 30 per cent. 



Acidity of whey when curd is milled o. 65-0. 75 per cent. 



Acidity of whey when curd is salted o. 90-1 . 10 per cent. 



MANIPULATION OF THE CuRD.f The curd particles at first show little 

 tendency to cohere; but, as the acidity increases, the nature of the curd 

 changes, and, when the whey is removed, the pieces of curd soon cohere 

 and ultimately form a single mass in which the original cubes of curd 

 cannot be detected. The fusion of the curd particles is known as "mat- 

 ting" and is an important step in the Cheddar process. The lack of acid 



* The rennet used in cheese-making is obtained by extracting the abomasum the true diges- 

 tive stomach of the calf with a solution of sodium chloride. The extract contains two enzymes 

 a clotting or curdling enzyme, rennin, and a proteolytic enzyme, pepsin. 



f Cheddar cheese. 



