THE SECRETION AND PROPERTIES OF MILK 1387 



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 solu- 

 tions which are acid to litmus. When rubbed up with chalk it dis- 

 solves, displacing the carbonic acid and forming a calcium caseino- 

 genate. A solution of caseinogen in soda or potash is transparent 

 and passes easily through a clay cell. The calcium caseinogenate 

 forms only opalescent solutions. Apparently the compound is disso- 

 ciated 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 caseino- 

 genate solution is boiled it forms a pellicle on the surface in the same 

 way as milk does. On treating the caseinogen with rennet ferment it 

 is converted into a modification known as paracasein, which in the 

 presence of lime salts is thrown out as insoluble casein. To this 

 process is due the clotting of whole milk by rennet, which is made use 

 of in the preparation of cheese, the curd consisting of a network of 

 casein enclosing fat globules in its meshes. On allowing the clot to 

 stand it shrinks, pressing out a milk-serum. 



From the milk-serum or whey may be obtained two other proteins, 

 known as lactalbumen and lactoglobulin. These resemble very nearly 

 the albumen and globulin of blood-serum. They are coagulated on 

 heating. According to some authors a third protein is present in 

 the whey, to which the name whey-protein has been given, and which 

 is supposed to be split off from the caseinogen under the action of the 

 rennet ferment. 



Milk can be boiled without undergoing any coagulation. If it be 

 allowed to stand and become sour by the formation of lactic acid, at a 

 certain period boiling the milk causes its complete coagulation. Later 

 on the acid produced is sufficient in itself to precipitate the caseinogen. 

 Both these processes, namely, coagulation of half-sour milk by heating, 

 and spontaneous clotting of milk by the production of acid, are made 

 use of in different countries for the manufacture of cheese. 



MILK SUGAR.- The sugar of milk, or lactose, is most easily ob- 

 tained from whey, which, after separation of the clot, is boiled to pre- 

 cipitate the remaining proteins. On filtering and evaporating slowly, 

 the milk sugar crystallises out. Lactose is a disaccharide and has the 

 formula C 12 H 22 O n . It is only known to occur in milk. It may be 

 found in the urine of nursing women when the breasts are not kept 

 empty, so that there is reabsorption of the lactose formed in the 

 mammary glands. It is unaltered by ordinary yeast, so that the 



