PHYSIOLOGICAL CHEMISTEY 329 



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

 globulin. 



II. The Carbohydrate Lactose. 



EXPERIMENT VII. 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. It occurs in two 

 stages as follows : 



C 12 H 22 O n + H 2 = 4CH 3 - CHOH - COOH. 



Lactose. Lactic acid. 



Some of the lactic acid is then further split up into butyric acid. 

 2CH 3 - CHOH - COOH = CH 3 - CH 2 - CH 2 - COOH + 2CO, + 2H 2 . 



Butyric acid. 



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 % of alcohol, and when clotted give a fine clot. For this 

 reason they have been recommended for invalids. 



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

 to Uffelmann's reagent, 1 when the dark purple colour of the latter will 

 be changed to yellow. Test for lactic acid (see p. 379). 



III. The salts of milk are chiefly phosphates and chlorides of the 

 alkalies and alkaline earths. A trace 0*00035 % of iron is also 

 present. 



EXPERIMENT IX. THE DETECTION OF PHOSPHATES AND CHLORIDES. 

 Add to 5 c.c. of protein-free whey half its bulk of nitric acid and 

 about twice its bulk of a solution of molybdate of ammonia in nitric 

 acid. Warm gently on the water bath, and a yellow precipitate of 

 phosphate forms. In rennet or acid whey the phosphates may be 

 precipitated by ammoniated magnesium citrate. Filter. Dissolve 



1 This reagent is made by adding a trace of ferric chloride to a 1 % solution 

 of carbolic acid. 



