THE SECRETION AND PROPERTIES OF MILK 1239 



bacillus lacticus. In some cases the milk may undergo a species of alcoholic 

 fermentation, as in the formation of kephir, which is made by the fermenta- 

 tion of mares' milk. 



The opaque appearance of milk is due chiefly to the presence of multitudes 

 of fine fatty particles. On allowing the milk to stand the particles rise to 

 the surface, forming^cream, and by mechanical agitation, especially if the 

 milk is slightly sour, they may be caused to run together with the formation 

 of butter. Much discussion has arisen as to the reason why the fat globules 

 do not run together naturally. By many authors it has been imagined that 

 they are clothed with a special protein membrane (haptogen membrane) 

 originating from the protoplasm of the cell in which the fat globules were 

 originally formed. It must be remembered that in any protein solution, 

 such as that in which the globules are suspended, the protein tends to aggre- 

 gate, with the formation of a pellicle, at the surface, so that an emulsion 

 once produced in such a fluid will tend to be more or less permanent. There 

 seems therefore no reason to assume the presence of a distinct membrane 

 differing in composition from the proteins present in the surrounding fluid. 

 The fats of milk consist for the greater part of the neutral glycerides, tripal- 

 mitin, tristearin, and triolein. In smaller quantities it contains the tri- 

 glycerides of myristic acid, butyric acid (?), and capronic acid, with traces 

 of caprylic, capric, and lauric acids. 



The milk plasma, the fluid in which the fat globules are suspended, 

 contains various proteins, a carbohydrate, lactose, and inorganic salts, 

 with a small amount of lecithin and nitrogenous extractives. 



THE PROTEINS OF MILK. The chief protein of milk is caseinogen, 

 belonging to the class of phosphoproteins. Like other bodies of this class 

 it presents distinct acid characteristics, being precipitated by acids and 

 soluble in dilute alkalies. It may be prepared from separated milk by the 

 addition of weak acids. A convenient method is to dilute one litre of milk 

 with ten litres of distilled water and add to the mixture 10 c.c. of glacial 

 acetic acid. The precipitate which is formed rapidly sinks to the bottom 

 and may be washed two or three times by decantation. It may be purified 

 by solution in dilute ammonia and precipitation by acetic acid two or three 

 times. The precipitate finally obtained is extracted with alcohol and 

 ether, and the dried caseinogen prepared in this way forms a snow-white 

 powder which is practically insoluble in water and dilute salt solutions. It 

 is easily dissolved on the addition of a little alkali, when it yields solutions 

 which are acid to litmus. When rubbed up with chalk it dissolves, displacing 

 the carbonic acid and forming a calcium caseinogenate. A solution of case- 

 inogen in soda or potash is transparent and passes easily through a clay cell. 

 The calcium caseinogenate forms only opalescent solutions. Apparently 

 the compound is dissociated by water with the formation of caseinogen acid 

 which is in a state of partial solution as swollen-up aggregates. It is 

 impossible therefore to filter calcium caseinogenate through a clay cell. It is 

 mainly in this form that caseinogen is contained in milk, hence the opalescent 

 appearance of the milk- plasma. When calcium caseinogenate solution is 



