38 ESSENTIALS Of CHEMICAL PHYSIOLOGY 



produces coagulation in this solution, provided that a sufficient 

 amount of calcium salts is present. 



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 caseino- 

 gen ; the second action is that of the calcium salt, which precipitates 

 the altered caseinogen as casein. In blood also, calcium salts are 

 necessary for coagulation, but there they act in a different way, 

 namely, in the production of fibrin-ferment (see Coagulation of 

 Blood). 



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

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

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

 is, like globulins, 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 phosphorus- 

 rich material called miclein (see p. 29). 



The Fats 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, ^ ; palmitin, ^ ; stearin, ^^ ; 

 butyrin, caproin, and caprylin, j^,f. The old statement that each fat 

 globule is surrounded by a membrane of caseinogen has been shown 

 to be incorrect. Milk also contains small quantities of lecithin, a 

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

 its solubilities (see Bile), and a yellow fatty pigment or lipochrome. 



Milk Sugar or Lactose. — This is a saccharose (Ci2H220ii)- Its 

 properties have already been described in Lesson I. p. 13. 



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 produced are given on p. 14. When 

 souring occurs, the acid which is formed precipitates a portion of the 

 caseinogen. This must not be confounded with the formation of 

 casein from caseinogen which is produced by rennet. There are, 

 however, some bacterial growths which produce true coagulation like 

 rennet. 



