COAGULATING MATERIALS 33 



produced sample from the very choicest single stomach, 

 but it gives a uniformity of result which Improves the 

 average product greatly. 



47. Pepsin. Pepsin is on the market in several 

 commercial forms, as a liquid, scale pepsin and in a 

 granular form known as spongy pepsin. Some commer- 

 cial concerns put out a preparation which is a mixture 

 of rennet extract and commercial pepsin. 



48. Chemistry of curdling. The chemistry of casein l 

 and of curd formation under the influence of acid and 

 rennet extract and pepsin has been the subject of many 

 years' research. While many points remain unsettled, 

 the general considerations together with a large mass of 

 accepted facts may be presented and some of the unsolved 

 problems pointed out as left for future researches. 



Casein is a white amorphous powder, practically 

 insoluble in water. It is an acid and as such readily 

 dissolves in solutions of the hydroxides or the carbonates 

 of alkalies and alkaline earths by forming soluble salts. 



Pure casein salt solutions and fresh milk do not 

 coagulate on boiling, but in the presence of free acid 

 coagulation may take place below the boiling temperature. 



1 The paragraphs on the chemistry of casein and on rennet 

 action have been selected from a complete discussion of the 

 subject by E. B. Forbes and M. H. Keith in Ohio Exp. Sta. 

 Tech. Bui. 5 entitled, "A review of the literature of phosphorus 

 compounds in animal metabolism." The original references 

 cited in this discussion are given at the end of the chapter in the 

 order of their citation in the text. 



See also, Van Slyke, L. L., and D. D. Van Slyke, I, The 

 action of dilute acids upon casein when no soluble compounds 

 are formed; II, The hydrolyses of the sodium salts of casein, 

 N. Y. (Geneva) Exp. Sta. Tech. Bui. 3, pages 75-162, 1906. 



Sammis, J. L., S. K. Suzuki and F. W. Laabs, Factors control- 

 ling the moisture content of cheese curds, U. S. Dept. Agr. Bur. 

 An. Ind. Bui. 122, pages 1-61, 1910. 



