CASEINOGEN 13 



apparently higher. When milk is filtered through porcelain, 

 the acidity of the serum is usually approximately half that of 

 the original milk on direct titration with alkali, but Van Slyke 

 and Bos worth have shown that, if before determining the acidity, 

 the Hme salts are previously removed by precipitation with 

 neutral potassium oxalate, the acidity of the serum is equal to 

 that of the milk : in other words, the caseinogen calcium phos- 

 phate complex in milk is not acid to phenolphthalein but neu- 

 tral. Van Slyke and Bosworth ^^ filtered milk through porcelain 

 but instead of analysing the precipitate, compared the serum and 

 the original milk. This eliminates errors caused by the absorp- 

 tion of soluble salts if the first filtrates of serum are rejected. 

 Their results show that caseinogen exists in milk as neutral 

 calcium caseinogenate (caseinogen, Ca,^) and neutral dicalcium 

 phosphate. These are not in chemical combination as they 

 could be almost completely separated by mechanical methods. 



The reaction of caseinogen with rennin, a lab ferment, is 

 of considerable importance because of the information it yields 

 regarding the constitution of caseinogen, and also on account 

 of the presence of this ferment in the mucous fining of calves' 

 stomachs and the similarity of its action to that of the gastric 

 juices of the human stomach. Although this reaction has been 

 the subject of probably more investigations than any other sub- 

 ject in biological chemistry the modus operandi and the nature 

 of the reaction products are still comparatively obscure. 



It has long been known that fresh milk coagulates in the 

 stomachs of the higher animals. An aqueous extract of the 

 inner lining of the stomach of the calf causes curdfing and clots 

 milk producing a semi-solid mass. These facts have been 

 utilised since an early date in the manufacture of cheese. 



The earlier views concerning the nature of this change need 

 not be considered in detail as they have since been proved to 

 be entirely erroneous. The one most commonly accepted 

 regarded the action as one of decomposition of the milk sugar 

 into acids, which directly or indirectly produced the phe- 

 nomenon observed. The first important advance was made 



