698 PHYSIOLOGICAL CHEMISTRY. 



lactoglobulin is precipitated. The filtrate contains lactalbumin, which can be 

 precipitated by saturating the solution with ammonium sulphate. 



Experiment 91. (Action of rennin on milk.} To 20 c.c. of milk add 4 c.c. of 

 a 0.1 per cent, solution of rennin, mix well, and digest at 40 C. (104 F.). 

 A coagulum consisting of casein and fat soon forms, while an aqueous fluid 

 (whey), containing proteins, milk-sugar, salts, and extractives, is pressed out. 

 After adding a drop or two of acetic acid heat a portion of the whey to boiling : 

 a voluminous coagulum of simple proteins is formed. 



Eepeat the above experiment with milk from which, by the addition of 2 

 c.c. of a 1 per cent, solution of ammonium oxalate, the calcium salts have 

 been removed. No coagulum occurs until calcium chloride is added in a 

 quantity sufficient to precipitate any ammonium oxalate left in solution, and 

 to furnish the calcium salt required for precipitation. 



Kepeat again, boiling the mixture in order to destroy the rennin before the 

 addition of the calcium solution ; clotting will occur, showing that calcium is 

 necessary only for the precipitation, not for the interaction between the casein- 

 ogen and the rennin. 



Milk-fat. It has been mentioned above that the fat of milk is 

 held in suspension as small globules, which are surrounded by a 

 protein envelope. The latter prevents the solution of fat when 

 ether is added directly to milk. If, however, a few drops of caustic 

 alkali be added with the ether, then the envelope will be destroyed 

 and the fat dissolves. Whenever a precipitate occurs in milk the fat 

 is carried down with the insoluble substance and the envelope is 

 generally destroyed. The fat of milk is a mixture of the glycerides 

 of several fatty acids, chiefly of palmitic and oleic, with small quan- 

 tities of butyric, caproic, caprylic, and stearic acids. Butter fat may 

 be recognized by liberating the butyric acid, which has a character- 

 istic odor. 



Experiment 92. (Liberation of butyric acid.) Use the mixed ethereal and 

 alcoholic filtrate from Experiment 89. Allow the ether to evaporate spon- 

 taneously, and add to the alcoholic solution of butter fat about 5 grammes 

 of potassium hydroxide. Heat the mixture on a water-bath until a drop of 

 it is found to be completely soluble in water, indicating complete saponifica- 

 tion. Evaporate until odor of alcohol has disappeared; add 30 c.c. of dilute 

 sulphuric acid, when the fatty acids are set free and butyric acid can be recog- 

 nized by its odor. 



Butter. Even in the thickest varieties of cream there is no cohe- 

 sion between the fat globules, while in butter the fat has actually 

 cohered. This change is accomplished by violently agitating (churn- 

 ing) the cream, when the fat particles gradually combine with each 

 other, while the liquid (buttermilk) separates. 



Chemically, butter is a milk-fat, containing a certain proportion^ 



