CH. xxvni.] MILK. 459 



vertible into casein ; this is produced by the decomposition of the 

 caseinogen molecule during the process of curdling. 



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

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

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

 viated by the addition of lime water or barley water to the milk. 



The addition of rennet produces coagulation in milk, provided 

 that a sufficient amount of calcium salts is present. If the calcium 

 salts are precipitated by the addition of potassium oxalate, rennet 

 causes no formation of casein. The process of curdling in milk is a 

 double one ; the first action due to rennet is to produce a change 

 in caseinogen ; the second action is that of the calcium salt which 

 precipitates the altered caseinogen as casein. In blood also, cal- 

 cium salts are necessary for coagulation, but there they act in 

 a different way, namely, in the production of fibrin ferment (see 

 p. 411). 



Caseinogen is often compared to alkali-albumin. The latter, 

 however, does not clot with rennet, and is, unlike caseinogen, 

 readily soluble in acids. Caseinogen is not a globulin, though it 

 is, like globulins, readily precipitated by neutral salts. It differs 

 from a globulin in not being coagulated by heat. It is a nucleo- 

 proteid ; that is, a compound of a proteid, with the proteid-like 

 but phosphorous rich material called nuclein (see p. 400). 



The Pats of Milk. The chemical composition of the fat of 

 milk (butter) is very like that of adipose tissue. It consists 

 chiefly of palmitin, stearin, and olein. There are, however, smaller 

 quantities of fats derived from fatty acids lower in the series, 

 especially butyrin and caproin. The relation between these 

 varies somewhat, but the proportion is roughly as follows : 

 Olein, i ; palmitin, ^ ; stearin, ^ ; butyrin, caproin, and 

 caprylin, ^. The old statement that each fat globule is sur- 

 rounded by a film of caseinogen is not now regarded as true by 

 most authorities. Milk also contains small quantities of lecithin, 

 a phosphorised fat ; of cholesterin, an alcohol which resembles fat 

 in its solubilities, and a yellow fatty pigment or lipochrome. 



Milk Sugar, or Lactose. This is a saccharose (C^H^O,^. 

 Its properties have already been described in Chap. XXV., p. 388. 



Souring of Milk. When milk is allowed to stand, the chief 

 change which it is apt to undergo is a conversion of a part of 

 its lactose into lactic acid. This is due to the action of micro- 

 organisms, and would not occur if the milk were contained in 

 closed sterilised vessels. Equations showing the change pro- 

 duced are given on p. 388. When souring occurs, the acid 

 formed precipitates a portion of the caseinogen. This must not 



