Effect of Heat on Cow's Milk. 



141 



The results secured in this experiment have a bearing, also, on 

 the alleged effect of heat on the calcium salts of milk as affecting 

 the coagulability of milk by rennet. It seems evident that the 

 only calcium salts affected by heat are colloidal calcium salts and 

 the question is therefore raised as to the possibility of the colloidal 

 CaHP0 4 of milk playing a part in the rennet coagulability. 



In order to determine whether this is true or not, two 200 c.c. 

 portions of fresh whole milk were dialyzed in collodion bags 

 against running distilled water for 48 hours, using 1 per cent, 

 toluene as preservative. When rennet was added to this milk 

 there was no coagulation even after several hours. One drop of 

 4 molar CaCl 2 solution added to 100 c.c. of the rennet treated 

 milk caused instant coagulation. The same result followed the 

 addition of 2 or 3 drops of dilute HC1 solution. The addition of 

 10 c.c. of the colloidal gelatin solution of CaHP0 4 to 100 c.c. of 

 the rennet treatment milk was, however, without any effect. 



It is apparent that the colloidal CaHP0 4 of the milk does not 

 play any part in the rennet coagulation. The indications are, 

 also, that the effects of heat on rennet coagulation which can 

 apparently be overcome by the addition of soluble calcium salts 

 are not to be explained on the grounds of an effect of the heat on 

 the calcium salts of the milk but rather on the grounds of an effect 

 of heat on the casein itself. . Just what this may be is not definitely 

 clear, as yet. The author has this problem under investigation 

 and hopes to be able to present definite data on it at a later date. 

 It will be sufficient to point out at this time that the explanation 

 of this phenomenon involves the fact that rennet coagulation is 

 unquestionably both a chemical and a colloidal reaction. The 

 calcium caseinate of milk is in colloidal solution. Rennet appears 

 to hydrolyze the calcium caseinate into two molecules of calcium 

 paracaseinate. The clotting of the calcium paracaseinate is a 

 secondary phenomenon which is a gellation, perhaps of the nature 

 of a crystallization of colloid — in this case a hydrophyllic colloid 

 in a state of hydration. The conditions which govern what is 

 regarded as a normal clotting of the calcium paracaseinate are 

 evidently disturbed by the application of heat to the colloidal 

 calcium caseinate of the milk. Zoller 1 has recently shown how 



Roller, H. F., J. Ind. Eng. Chem., 1921, xiii, 510. 



