188 PEACTICAL PHYSIOLOGY 



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 Bacterium lactis. It occurs in two 

 stages as follows : 



C 12 H M 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 + 2C0 2 + 2H a . 



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. 



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. 207). 



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

 alkalies and alkaline earths. A trace 0'00035 per cent. of iron 

 is also present. 



EXPERIMENT IX. THE DETECTION OF P 9 0,. Add to five cubic centi- 



2 



metres of proteid-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. 

 EXPERIMENT X. THE DETECTION OF CALCIUM SALTS. To some 

 whey, freed from proteid by boiling, add a few drops of a solution of 

 potassium oxalate a white haze of calcium oxalate results. 



IV. The Fats of Milk. Examine a thin film of milk under the 

 microscope, and note that the fat consists of small spherical bodies, 

 which are transparent and do not adhere to one another. 



The fat can be removed by shaking the milk with ether after the 

 addition to it of a few drops of weak NaOH solution. 



EXPERIMENT XI. To about five cubic centimetres of milk in a 

 test tube add two drops of caustic potash (20 per cent.), and then about 

 five cubic centimetres of ether. Cover the top of the tube with the 

 thumb and shake the mixture, occasionally lifting the thumb 

 slightly to allow the vapour of ether to escape. The ether will 

 dissolve the fat, and the milk will become much less opaque. By 

 adding alkali, a certain amount of the caseinogen is changed in its 

 physical condition, so that the caseinogen molecules, which lie 



iThis reagent is made by adding a trace of ferric chloride to a 1 per cent, 

 solution of carbolic acid. 



