56 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



Milk Sugar or Lactose, This is a saccharose (0,211220! ,). Its 

 properties have already been described in Lesson II., p. 19. 



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. 19. 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. 



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. 19), and it is from these sugars that 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. 



EGGS 



In this country the eggs of hens and ducks are those particularly 

 selected as foods. The shell is made of calcareous matter, especially 

 calcium carbonate. The white is composed of a richly albuminous 

 fluid enclosed in a network of firmer and more fibrous material. The 

 amount of solids is 13'3 per cent. : of this 12'2 is protein in nature. 

 The proteins are albumin, with smaller quantities of egg-globulin 

 and ovo-mucoid (p. 45). The remainder is made up of sugar 

 (0*5 per cent.), traces of fats, lecithin and cholesterin, and 0'6 

 per cent, of inorganic salts. The yolk is rich in food materials for 

 the development of the future embryo. In it there are two varieties 

 of yolk-spherules, one kind yellow and opaque (due to admixture 

 with fat and a yellow lipochrome), and the other smaller, transparent 

 and almost colourless : these are protein in nature, consisting of the 

 phospho-protein called vitellin (p. 44). Small quantities of sugar, 

 lecithin, cholesterin, and inorganic salts are also present. 



The nutritive value of eggs is high, as they are so readily diges- 

 tible ; but the more an egg is cooked the more insoluble do its protein 

 constituents become. 



