516 BACTERIA IN BUTTER AND CHEESE-MAKING 



Bacteria and Other Microorganisms in the Ripening of Cheese. 

 Proteids in Milk. Cow's milk contains about 3 to 4 per cent, of 

 nitrogenous organic compounds or proteids, and these are, as shown 

 by Hammarsten, not of one kind, but three chemically different bodies 

 known as casein, lactalbumin, and globulin. The casein is equal to 

 about 80 per cent, of the entire amount of proteids, and it is present 

 in milk as a calcium compound. It is not in true solution, but in a 

 swollen, finely divided condition known as the colloidal state. When 

 milk is acidulated beyond a certain degree, either by the addition of 

 acid from without or by the growth and development of lactic-acid 

 bacteria, the casein is precipitated as a more or less finely flocculent 

 mass. It is more finely flocculent in human milk, more coarsely 

 flocculent in cow's milk. When this change has occurred the milk 

 is said to have coagulated. 



Coagulation of Milk. Coagulation of milk can be brought about 

 by another procedure aside from acidulation, namely, by the addition 

 of an enzyme called labferment, or rennet. This enzyme is furnished 

 by the mucous membrane of the stomach of animals, and also by a 

 number of bacteria, as first shown by Ducleaux, who demonstrated 

 that certain bacteria growing in milk coagulate it by the aid of this 

 ferment. Conn succeeded in separating the rennet enzyme from 

 the bacteria which had produced it. While very similar in action the 

 bacterial rennet is not absolutely identical with the rennet obtained 

 from calves' stomachs or the stomachs of other mammals. The 

 bacterial rennet can also coagulate sterilized milk, the latter only 

 raw milk. There are quite a number of bacteria which can coagulate 

 milk by the aid of their rennet enzyme even without acidulating it. 

 Among such bacteria are particularly the potato bacilli (Bacillus 

 mesentericus vulgatus and other bacteria of this group). The rennet 

 prepared from calves' stomachs cannot coagulate boiled milk because it 

 acts only in the presence of soluble lime salts, and these are precipitated 

 when milk is boiled; it can likewise not act in an alkaline solution, 

 but acts best in a slightly acid medium at a temperature of 37 C. 

 At 25 C. the action is slow; at 45 C. it does not take place, and at 

 70 C. the enzyme is destroyed. 



The Formation of the Curd. In order to prepare cheese, milk may 

 be coagulated by the addition of the lab- or rennet enzyme 1 from 

 calves' stomachs, by permitting it to become sour spontaneously, or 

 by adding milk already soured or a starter. According to the method 

 used in coagulation, cheeses are distinguished as rennet milk cheeses 

 and sour milk cheeses. The soft, spongy mass, full of fluid, which is 

 formed when milk has been coagulated by either one of the two 

 methods, is called a curd. Most of the whey may be pressed out of 

 the curd or a considerable portion may be left in it. In the former 

 case the comparatively dry raw material will form the hard, in the latter 



1 An exceedingly small amount of rennet will coagulate a very large amount of milk. 



