PEOTEINS. 235 



IX. Separation of the Proteins Digestible with Difficulty 

 from those Readily Digestible. Cheese contains only small 

 quantities of completely indigestible nitrogenous substances, 

 and it is, therefore, useful to determine the comparative 

 digestibility of the proteins. For this purpose, a process of 

 " interrupted digestion " is employed. In order to obtain 

 comparable results, care is taken to have constant (1) the amount 

 of nitrogen in the form of insoluble, but digestible, proteins ; 

 (2) the amount of gastric juice ; and (3) the acidity of the liquid, 

 the temperature, and duration of digestion. 



In each experiment so much of the sand mixture is taken 

 as contains 0-15 gramme of nitrogen in the form of insoluble, 

 but digestible, proteins, to which is added 150 c.c. of the gastric 

 juice, with 343 c.c. of water and 7 c.c. to 10 per cent, hydro- 

 chloric acid. The acidity of the total liquid (J litre) is exactly 

 0-20 per cent., the temperature is maintained at 37 to 40 C. 

 (99 to 104 F.), and the duration of the digestion is thirty to 

 sixty minutes. The liquids are warmed ^to 40 C. (104 F.) 

 before being measured and after mixing. At intervals of five 

 minutes during the digestion the liquid is stirred with a glass 

 rod ; and at the conclusion the total liquid is placed in two 

 large folded rapid niters, and the portion of the filtrate passing 

 through in the first five minutes taken for the determination 

 of the nitrogen. From the result a deduction must be made for 

 the nitrogen contained in the gastric juice, and for the nitrogen 

 in the cheese dissolving without the aid of the gastric juice 

 (amino, ammoniacal, albumose, and peptone nitrogen). 



Table XLII. gives the results of the analyses of three 

 varieties of cheese to illustrate the results of Stutzer's investi- 

 gation. The nitrogen in the proteins multiplied by 6-39 will 

 give, with fair accuracy, the amount of the proteins ; the nitrogen 

 of the albumoses and peptone multiplied by 6-39 will approximate 

 nearly to the amount of primary products of ripening. The 

 author has calculated from the nitrogen given in Stutzer's analysis 

 the proteins, primary and secondary products of ripening, in order 

 to compare the method given above with that previously described. 



For most practical purposes the author's method will give as 

 much information as that of kStutzer, if the following facts are 

 borne in mind : (1) The ripening of a cheese is shown by the 

 proportion of primary and especially secondary products ; and 

 (2) the digestibility of a cheese increases with its ripeness. 



Duclaux's Method. Duclaux has proposed the investigation 

 of the fatty acids developed by ripening as a means of judging 

 a cheese. The following are the methods used by him : 



Water, Fat, Alcoholic and Aqueous Extracts. Twenty grammes 

 of sand which has been previously dried, sifted, and ignited, 



