696 PHYSIOLOGICAL CHEMISTRY. 



contain of carbon dioxide 7.06, of oxygen 0.1, of nitrogen 0.7 

 volumes. 



Milk contains several enzymes, whose natures vary with their 

 sources. One of these, an oxidizing ferment (oxidase, catalase), is a 

 constant constituent, and is of importance because its absence shows 

 that the milk has been heated for preservation. 



The presence of an oxidizing ferment in milk can thus be shown : Shake 10 

 c.c. of milk with 1 c.c. of tincture of guaiac, 5 c.c. oil of turpentine, and 5 c.c. 

 of solution of hydrogen dioxide. A blue color is developed when the ferment 

 is present. 



Milk-proteins. The proteins of milk are caseinogen, lactoglobulin ^ 

 and ladalbumin. Lactoglobulin and lactalbumin closely resemble the 

 globulin and albumin of the blood-serum, and are believed to be de- 

 rived from them with little constitutional change. Caseinogen is, on 

 the other hand, a specialized protein containing phosphorus and be- 

 longing to the group of phospho-proteins. It is present in milk 

 either in solution or perhaps in combination with phosphates in a 

 partially insoluble form, and this combination may be responsible for 

 some of the opacity of the milk. When milk is acted upon by rennin 

 there is a coagulation of the caseinogen and the formation of a clot. 

 It may readily be shown that this process takes place in two steps. 

 First, the caseinogen is changed by the rennin to a form called para- 

 casein. This substance remains in solution, and the nature of the 

 change is not understood. In the second stage the paracasein forms 

 a combination with the calcium salts of the milk and is precipitated 

 as casein (calcium-casein). The calcium enters only in the second 

 step and has no part in the formation of paracasein. The significance 

 of this coagulation of milk in the stomach is not known. It is fre- 

 quently stated that a peptone is split off from the caseinogen in the 

 production of paracasein. The process may be hydrolytic in nature, 

 and a preliminary step in the digestion of caseinogen. As implied 

 above, no coagulation will take place if the calcium salts be removed 

 from the milk. 



Caseinogen occurs only in milk ; it is a phospho-protein, yielding 

 on hydrolysis a pseudonuclein. When dry it is a fine, white powder, 

 insoluble in water, but soluble in dilute salt solution and in water 

 containing a little alkali. 



Caseinogen resembles the alkali albuminates in dissolving in water in the 

 presence of calcium carbonate with evolution of carbon dioxide. The solution 

 is precipitated by hydrochloric and acetic acids, the precipitate being soluble 

 in slight excess, and reprecipitated by a large excess of the acid. The solution 



