CH. XXX.] VARIETIES OF MILK 483 



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, calcium 

 salts are necessary for coagulation, but there they act in a different 

 way, namely, in the production of fibrin-ferment (see p. 449). 



Caseinogen is a phospho-protein (see p. 429). In milk it is com- 

 bined with calcium to form calcium caseinogenate ; when acetic acid 

 is added, we therefore get calcium acetate and free caseinogen. 



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

 (butter) is very like that of adipose tissue. 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, 

 f ; palmitin, J ; stearin, J ; butyrin, caproin, and caprylin, y T . The 

 old statement that each fat globule is surrounded by a film of 

 protein is, according to Kamsden's recent observations, correct. 

 Milk also contains small quantities of lipoids (lecithin, cholesterin, 

 and a yellow fatty pigment or lipochrome). 



Milk Sugar, or Lactose. This is a disaccharide (C 12 H 22 O n ). Its 

 properties have already been described in Chap. XXYI1L, p. 411. 



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 an enzyme 

 secreted by micro-organisms, and would not occur if the milk were 

 contained in closed sterilised vessels. Equations showing the change 

 produced are given on p. 411. When souring occurs, the acid formed 

 precipitates a portion of the caseinogen. This must not be con- 

 founded with the formation of casein from caseinogen, which is 

 produced by rennet. There are, however, some bacteria which, like 

 rennet, produce true coagulation. 



Alcoholic Fermentation of Milk. When yeast is added to milk, 

 the sugar does not readily undergo the alcoholic fermentation. Other 

 somewhat similar fungoid growths are, however, able to produce the 

 change, as in the preparation of koumiss ; the milk sugar is first 

 inverted, that is, dextrose and galactose are formed from it (see p. 411), 

 and it is these sugars from which alcohol and carbonic acid originate. 



The Salts of Milk. The principal salt present is calcium phos- 

 phate; a small quantity of magnesium phosphate is also present. 

 The other salts are chiefly chlorides of sodium and potassium. 



It is an undoubted fact that the milk provided by Nature for the 

 growing offspring is different in the various classes of the animal 



