FOODS 37 



Compositioii. — Frankland gives the following table, contrasting ths 

 milk of woman, ass, and cow : — 



Cow 



I Per cent. Per cent. Per coit. 



Proteids (chiefly caseinogen) . ! 2-7 I'T i 4*2^ I 



Butter (fat) . . . . i 3-5 1-3 I 3-8 



Lactose '5-0 4-5 { 3-8 ! 



j Salts 0-2 0-5 1 0-7 



Hence, in feeding infants on cow's milk, it will be necessary to 

 dilute it, and add sugar to make it approximately equal to natural 

 human milk. 



The Proteids of Milk.— The principal proteid in milk is called 

 caseinogen ; this is the one which is coagulated by rennet to form 

 casein. Cheese consists of casein with the entangled fat. The other 

 proteid in milk is an albumin. It is present in small quantities only ; 

 it differs in some of its properties (specific rotation, coagulation 

 temperature, and solubilities) from serum-albumin ; it is called lact- 

 alhumin. 



The Coagulation of Milk. — Rennet is the agent usually employed 

 for this purpose : it is a ferment secreted by the stomach, especially 

 by sucking animals, and is generally obtained from the calf. 



The cxird consists of the casein and entangled fat : the hquid 

 residue called whey contains the sugar, salts, and albumin of the 

 milk. There is also a small quantity of a new proteid called wliey- 

 proteid, which diff^ers from caseinogen by not being convertible into 

 casein. It is produced by the decomposition of the caseinogen 

 molecule during the process of curdUng. 



The curd formed in human milk is more finely divided than that 

 in cow's milk ; hence it is more digestible. In feeding children and 

 invalids on cow's milk, the lumpy condition of the curd may be 

 obviated by the addition of Ume water or barley wat«r to the milk. 



(Considerable discussion has taken place as to whether the 

 caseinogen of human milk may not be a different proteid from that 

 of cow's milk, especially in relation to the amount and manner of 

 combination of its phosphorus. The differences, however, appear to 

 be exphcable on the hypothesis that they are due to variations in 

 the amounts of calcium salts and of citric acid which are present. 



Caseinogen itself may be precipitated by acids such as acetic acid 

 or by saturation with neutral salts like sodium chloride. This, how- 

 ever, is not coagulation, but precipitation. The precipitate may be 

 collected and dissolved in lime water ; the addition of rennet then 



