18 BUTTER-MAKING AND COMPOSITION OF MILK 



heating milk in a glass flask to a high temperature, and letting it 

 stand for a time, it will be found that a mineral precipitate has 

 settled to the bottom. This precipitate is believed to be a lime 

 phosphate, which, previous to heating, was combined with the 

 casein of the milk. By adding calcium chloride (CaCy to milk 

 which has been heated, its normal condition towards the action of 

 rennet is again restored. 



Albumen. If the casein is removed from the milk by pre- 

 cipitation, and then filtered off, the filtrate will contain a sub- 

 stance which will precipitate when boiled. This is albumen, 

 and is similar in character to albumen from the white of an 

 egg. It differs from casein in that it is not precipitated by 

 rennet or acids, but precipitates on heating. It does not contain 

 any phosphates, but contains a comparatively large amount of 

 sulphur. 



As the albumen is soluble in rennet and dilute acids, it can 

 readily be seen that it is retained in the whey obtained in cheese- 

 making. When albumen is present in small quantities, as it is in 

 normal milk, heating does not completely precipitate it, unless 

 the casein or curd is first removed. If, on the other hand, 

 albumen is present in excess, as is the case in colostrum, the major 

 portion of the albumen is precipitated when heat is applied with- 

 out first removing the casein. 



Sugar. Milk-sugar occurs in milk to the extent of about 5 

 per cent. It varies very little in quantity, seldom falling below 

 3^ per cent and seldom rising above 5^ per cent. It occurs in 

 solution, and is not found elsewhere in nature. 



Milk-sugar is the most unstable component of milk; its 

 decomposition is brought about quickly and easily by the action 

 of micro-organisms. If these could be entirely excluded from the 

 milk, it would keep for an almost indefinite length of time. As it 

 is impossible under practical conditions to entirely exclude 

 organisms from the milk, the only way to retard and prevent the 

 growth of germs and thereby prevent the changing of the sugar 

 into other products, is to cool the milk to a low temperature 

 (50 F.), or to heat the milk to a sufficiently high temperature 

 (180 F.) to destroy most of the germs. According to Van Slyke 



