CH. XXVIII.] MILK 463 



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. 414). 



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 com- 

 pound of a proteid with the proteid-like but phosphorus-rich material 

 called pseiido-nuclein (see p. 402). In milk it is combined with 

 calcium to form calcium casein ogenate ; 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. 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, f ; palmitin, J ; stearin, J ; 

 butyrin, caproin, and caprylin, y T . The old statement that each 

 fat globule is surrounded by a film of caseinogen is, according to 

 Ramsden's recent observations, correct. 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 12 H 22 O n ). Its 

 properties have already been described in Chap. XXV., p. 390. 



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. 391. 

 When souring occurs, the acid 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 bacteria which, like rennet, produce true coagula- 

 tion. 



Alcoholic Fermentation in 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. 391), 

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



The Salts of Milk. The chief salt present is calcium phosphate ; 

 a small quantity of magnesium phosphate is also present. The other 

 salts are chiefly chlorides of sodium and potassium. 



