FOOD AND DIET. 



375 



By a comparison of these figures, it will be seen that cow's milk 

 contains too large a proportion of protein and fat, and too small a 

 proportion of lactose, for the human infant. It is therefore necessary, 

 if an infant is fed on cow's milk, to dilute the latter with water and to 

 add lactose in order to obtain the correct proportions. 



Fresh milk has a specific gravity of 1028 to 1034, and is neutral to 

 litmus. Microscopically, it consists of small fat globules floating in an 

 almost colourless fluid, that is, it is a permanent fine emulsion. The 

 globules appear to be prevented from running together by the proteins 

 of the milk forming a fine pellicle on the surface of each globule. 



The proteins of milk are caseinogen and lactalbumin. Caseinogen 

 is a phosphoprotein, and is insoluble in water, but soluble in dilute 

 alkalies. It exists in milk as a compound with calcium, and is pre- 

 cipitated by the addition of acetic acid, the precipitate being soluble in 

 excess of the acid. The precipitate of caseinogen obtained from milk 

 by the addition of acetic acid carries down the fats entangled with it, 

 and may be purified from these by washing with ether. When purified, 

 it is a white powder. Lactalbumin remains in the filtrate when the 

 caseinogen and fats have been filtered off. If the excess of acid in the 

 filtrate be almost neutralised so that only a trace of acidity remains, 

 the lactalbumin may be coagulated by heating the fluid. 



The fats of milk consist mainly of tripalmitin, tristearin, and triolein. 

 There are, in addition, small quantities of fatty acids lower in the 

 scale myristic, caproic, caprylic, capric, and lauric. Lactose is the 

 carbohydrate present in milk. It is a disaccharide, C 12 H 22 O n , and 

 reduces an alkaline solution of copper sulphate on boiling. It is 

 not fermented by ordinary yeast, and in this way it can be distinguished 

 from dextrose. It can be obtained from the filtrate from milk, after re- 

 moval of the proteins and fats, by slow evaporation, when it crystallises 

 out. The enzyme lactase, by which lactose is converted during digestion 

 into dextrose and galactose, is especially abundant in young animals. 



The salts of milk consist chiefly of phosphates and chlorides of 

 potassium, sodium, calcium, magnesium, and iron, calcium phosphate 



