330 PRACTICAL PHYSIOLOGY 



caseinogen entangling some fat globules. 1 Its formation is due to 

 drying of the protein at the surface of the milk. 



The Chemical Constituents of Milk 



I. Proteins. The main protein of milk is a phospho -protein called 

 Caseinogen. This can be precipitated by adding to the diluted milk a 

 weak acid, or by saturating it with a neutral salt. (See Phospho- 

 proteins, p. 198.) 



EXPERIMENT III. Place about 5 c.c. of milk in a test tube, and 

 dilute with an equal bulk of water. To this diluted milk add, drop 

 by drop, a weak solution of acetic acid ; a precipitate of caseinogen, 

 entangling fat, falls down. Filter off this precipitate and wash it with 

 water. Now add to it a weak solution of Na 2 CO3 ; the precipitate 

 dissolves, and an opalescent solution of caseinogen, still, however, 

 containing some fat, passes through the filter. By repeated reprecipita- 

 tion and filtration comparatively pure caseinogen can be obtained, 

 from which the last traces of fat can be removed by treating with 

 ether. 



The chief property of caseinogen is its power to clot when treated 

 with rennin (a ferment contained in gastric juice) in the presence of 

 soluble calcium salts. (See Digestion, p. 232.) 



The fluid left after the clotting of the caseinogen is known as whey 

 in this case rennet whey. If the caseinogen be got rid of by acid, it 

 is known as " acid whey " ; if by " salting out," as " salt whey " ; if by 

 alcohol, " alcoholic whey," and so on. These wheys are different in 

 composition ; for instance, rennet whey and acid whey contain lact- 

 albumin, salt whey and alcoholic whey do not. 



EXPERIMENT IV. Apply the xanthoproteic reaction to some acid 

 whey : a positive result is obtained. Apply also the other protein colour 

 tests. Acidify some of the whey with acetic acid and boil ; the protein 

 is coagulated. The proteins are called lact-albumin and lact-globulin. 



II. The Carbohydrate Lactose. EXPERIMENT V. Boil some rennet 

 whey which has been weakly acidified with acetic acid. Filter off the 

 coagulated proteins. To the filtrate apply Trommer's or Fehling's 

 test ; reduction is effected. Barfoed's reagent is not reduced. 



Lactose does not, like dextrose, readily ferment with yeast, but it is 

 capable of undergoing a special fermentation, which changes it into 

 lactic acid. This is called the lactic acid fermentation. It depends on 

 the presence of a microbe, the bacillus acidi lactici. 



The presence of these free acids in the milk leads to the precipitation 

 of caseinogen, and this explains the production of the curd in sour milk. 

 It is quite a different thing from the curd which is produced by rennin. 

 Thus, it can be dissolved by means of a weak alkali, and if rennin be 

 added to the resulting solution true clotting will follow. 



Milk, however, will undergo alcoholic fermentation by a special 

 fungus, known as the kephir fungus. From cow's milk the drink 

 kephir is formed, from mare's milk the drink koumiss. They contain 

 from 1-3 per cent, of alcohol, and when clotted give a fine clot. 



EXPERIMENT VI. Take some sour whey. Add a few drops of it 



1 An emulsion of cod-liver oil in diluted blood-serum is given out ; warm 

 it to about 50 C., and a skin will form on the surface. Be careful not to heat 

 above 50 C., as then coagulation of the proteins will be produced. 



