288 Report of the Department of Chemistry of the 



3. Casein is present in milk in combination with calcium as cal- 

 cium caseinate. It has not been definitely settled yet which par- 

 ticular compound is in milk, but it is probably either tetra-calcium 

 or tri-calcium caseinate. 



4. When the calcium caseinate of milk is acted on by rennin, it 

 is changed into another compound called calcium paracaseinate. By 

 this action one molecule of calcium caseinate is split into two mole- 

 cules of calcium paracaseinate. Thus, assuming for the sake of 

 our illustration, that the caseinate present in milk is the tetra- 

 calcium compound, we can represent the change from calcium 

 casinate to calcium paracaseinate in the following manner: 



Caj caseinate = Ca2 paracaseinate + Ca? paracaseinate. 



5. Paracasein, like casein, possesses acid properties, but has a 

 molecular weight of 4444, only one-half that of casein. Paracasein, 

 as an acid, has only one-half the combining power of casein; that is, 

 its highest combining power is equal to four equivalents of a mono- 

 valent metal; for example, Na 4 paracasein (sodium paracaseinate), 

 Ca2 paracasein (calcium paracaseinate). 



6. Calcium paracaseinate is less soluble than the corresponding 

 calcium caseinate present in milk from which it is formed, and, there- 

 fore, it is precipitated as a solid, or, in ordinary language, the milk 

 curdles. 



7. If rennin is added to a solution of sodium caseinate, the case- 

 inate is split into two molecules of sodium paracaseinate (for example, 

 Na 8 caseinate = Na 4 paracaseinate + Na 4 paracaseinate), but no 

 precipitation or curdling takes place. This is explained by the fact 

 that sodium paracaseinate is very soluble. If, however, to this 

 same solution of sodium paracaseinate we add a small amount of 

 some soluble calcium salt (calcium chlorid, for example), curdling 

 occurs at once, the curd being calcium paracaseinate. This pre- 

 cipitation or curdling is the result of a chemical reaction or double 

 decomposition, which can be illustrated in the following manner: 



Sodium paracaseinate (soluble) -f- calcium chlorid = calcium paracaseinate (insol- 

 uble) + sodium chlorid. 



This reaction or equilibrium can be made to proceed in either 

 direction at the will of the experimenter; for example, addition of 

 excess of sodium chlorid changes insoluble calcium paracaseinate 

 back into soluble sodium paracaseinate. These facts appeared to 

 us to furnish an explanation of the action of sodium citrate when 

 added to milk, in that there is formed calcium citrate and sodium 

 caseinate, which latter compound is converted by rennin into sodium 

 paracaseinate, a compound so soluble as not to curdle or form a pre- 

 cipitate. This hypothesis furnished the basis of our present investiga- 

 tion. 



The facts stated above raise the query whether or not sodium 

 citrate reacts with the calcium caseinate in milk to form sodium 



