228 PRACTICAL DAIRY BACTERIOLOGY 



been curdled by the use of rennet, the whey is extracted as 

 completely as possible, first by cutting the curd and draining, 

 and then by pressing under a high pressure until all of the whey 

 which can be squeezed out is removed. This makes a hard, 

 tough mass, which is put aside for ripening. The details of 

 manufacture differ, of course, in different types of cheeses. In 

 the Swiss cheese the milk is sweet when it is curdled, while 

 in the American or Cheddar cheese the milk is allowed to be- 

 come sour. Each of the other types of hard cheeses has its 

 own special method of manufacture, with slight differences in 

 the product. 



The ripening of these cheeses involves both digestive and 

 chemical changes, and the production of flavors. To bring about 

 these changes there are, at least, two different agents, either or 

 both of which may play a part in the process. The cheeses 

 contain both enzymes and bacteria. 



Enzymes. In all cheeses there is present a certain quantity 

 of enzymes, capable of digesting casein, derived from, at least, 

 two sources: i. The rennet used in cheese curdling is derived 

 from the stomachs of sucking mammals, and because of its 

 method of preparation rennet will always contain considerable 

 quantities of pepsin,, the juice that carries on the digestive func- 

 tions in the mammal's stomach. It is impracticable to prepare 

 rennet from a stomach without its containing some pepsin. 2. 

 It has also been shown by Babcock and Russell that milk itself, 1 

 when secreted, contains a ferment, which they have called 

 galactase, and which has also the power of digesting casein. 

 This is either secreted by the cow or by bacteria growing in the 

 milk ducts of the udder. Both pepsin and galactase are capable 

 of converting casein into peptones or proteoses, and during the 

 ripening period they are both acting slowly but constantly. By 

 some it is believed that they are, to a large extent, responsible 



1 Babcock and Russell. Ann. Rep. Wis. Exper. Sta., 1900. 



