272 MILK, BUTTER, AND CHEESE [chap. 



Protein. — Of the proteins of milk the chief is a body 

 called casein, which is distinguished by its property of 

 forming a firm curd when treated with the enzyme of 

 rennet, or a flocculent curd when treated with acids. 

 The casein is a compound of carbon containing 15-7 per 

 cent, of nitrogen and about -8 per cent, of sulphur and 

 phosphorus respectively. It is present in milk 

 probably combined with calcium ; the resulting salt is 

 not soluble in water, but exists in normal milk in a 

 finely divided state diffused through the serum, forming 

 what is known as a colloidal suspension. Rennet, 

 which consists of a solution of the enzyme which is 

 present in the fourth stomach of a calf, causes the 

 casein to coagulate and form a firm curd, in which are 

 also enclosed the fat globules of the milk. The rapidity 

 of the action of the rennet and the firmness of the 

 resulting curd are increased at higher temperatures, but 

 the presence of a soluble lime salt is necessary before 

 the curd will form. For this reason, perfectly fresh 

 milk will not curdle with rennet, a certain amount of 

 acid must first have been developed, which formation of 

 acid takes place naturally when the milk stands, by the 

 action of certain bacteria, which always find their way 

 into the milk and convert the milk sugar into lactic 

 acid, the substance characterising sour milk. It will be 

 seen later that the development of acidity and the 

 establishment of a suitable temperature are very 

 essential to obtain the curd of proper consistency for 

 cheese-making. After the casein has been precipitated 

 from milk by the action of rennet or acids, the milk still 

 contains about o-6 per cent, of other proteins, chiefly of 

 albumen, which begins to coagulate when the whey is 

 heated to temperatures above 70°. 



Milk Sugar. — The solid body which is present in 

 largest amount in milk is a particular sugar known as 



