34 THE BOOK OF CHEESE 



The coagulum formed in the case of milk includes fat 

 and calcium phosphate. The slight pellicle which coats 

 over milk when it is warmed is of the same composition. 



49. Use of acid. A commonly accepted explanation 

 of the precipitation of casein by acids is that the casein is 

 held in solution by chemical union with a base (lime in 

 the case of milk) ; that added acid removes the base, 

 allowing the insoluble casein to precipitate; and that 

 excess of acid unites with casein, forming a compound 

 which is more or less readily soluble. 



50. Robertson's theory. According to Robertson's 

 conception, in a soluble solution of a protein or its salt, 

 the molecules of the protein unite with each other to a 

 certain extent, in this way forming polymers. The re- 

 action is reversible, and the point of equilibrium between 

 the compound and its polymeric modification varies 

 under the influence of whatever condition affects the 

 concentration of the protein ions. Addition of water, 

 or of acid, alkali or salt, or the application of heat has 

 such an effect, and consequently alters the relative num- 

 ber of heavier molecule-complexes. Robertson's experi- 

 ments give evidence that one of the effects of increase 

 of temperature on a solution of casein is a shifting of the 

 equilibrium in the direction of the higher complexes. 

 He explains coagulation as being a result of these molec- 

 ular aggregates becoming so large as to assume the prop- 

 erties of matter in mass and to become practically an 

 unstable suspension and then a precipitate. The acid 

 curd then is casein or some combination of casein with 

 the precipitant acid. 



51. Rennet curd. Rennet extract and pepsin coagu- 

 lation differs from coagulation by acids, and cannot be 

 looked on as a simple removal of the base from a caseinate. 



