208 ELEMENTARY AGRICULTURAL CHEMISTRY 



35° C. (95° F.). Some of the better qualities of hard cheese 

 are made from enriched milk — i.e,, from a mixture of milk and 

 cream— others from whole milk, others from a mixture of whole 

 and skimmed milk, while some poor, horny cheeses are made 

 from skimmed milk. 



It would be beyond the scope of the present volume to 

 attempt to describe the varieties of cheese and their methods 

 of manufacture. 



The practice usually followed is to " ripen " the milk — i.e., 

 to impart the necessary acidity, often corresponding to about 

 0'2 per cent, lactic acid — by adding to it a " starter " consisting 

 of sour milk or a pure culture of lactic organisms, then to 

 add the necessary amount of rennet, the milk being previously 

 warmed to the proper temperature. When coagulation occurs, 

 which should be in from twenty to forty minutes, the tempera- 

 ture is raised to the optimum temperature, about 37" or 

 38** C, and kept at that for some time, usually one or two 

 hours. The whey is then run off, the curd stirred and cut, 

 and lastly broken in a mill, salted and pressed into moulds. 

 The cheeses are then ripened at a temperature of 15° 

 to 20° C. 



^ During ripening many changes of complex character occur ; 

 the sugar is converted into lactic acid, water evaporates and 

 the casein is converted into more digestible nitrogenous 

 bodies of the nature of albumoses and peptones. These 

 changes, according to one view, are produced by the lactic 

 organisms, while another theory ascribes them to enzymic 

 action, the enzyme being probably mainly galactase^ which is 

 said to be present in all milk, and possesses the power of 

 peptonising casein. 



Whatever may be the cause of the change, there can be 

 no doubt that in well-ripened cheese a considerable portion 

 of the casein is converted into albumoses, peptones, amides 

 and even ammonia. In most analyses, however, the 

 whole of the nitrogen present is expressed as being pre- 

 _sent as casein, though in fully ripened cheese perhaps not 



