378 OTHER MILK PRODUCTS. 



Qualitative Test for Peptones. — A portion of the hot- 

 water extract is concentrated by evaporation, saturated with 

 zinc sulphate, and filtered. Concentrated sodium hydroxide 

 solution is added to the filtrate until the zinc hydroxide dis- 

 solves, and a few drops of a 1 per cent, solution of copper sul- 

 phate added ; a violet-red colour (the biuret reaction) poiats 

 to the presence of peptone. If desired, this may be estimated 

 by evaporating 200 c.c. of the filtrate to 50 c.c, saturating with 

 zinc sulphate, filtering, and washing with saturated zinc sulphate 

 solution ; the precipitate, which consists of albumoses, is treated 

 by Kjeldahl's method. The nitrogen in the peptone is the 

 difference between that in the albumoses and that in the pre- 

 cipitate formed by phospho-tungstic acid. 



VIII. Proteins. — The nitrogen present in these substances, 

 which are insoluble in boiling water, is obtained by subtracting 

 from the total nitrogen the amounts found in IV., V., VI., and 

 VII. It is not advisable to use the residue from VII. for this 

 purpose, on account of the large amount of sand present. 



IX. Separation of the Proteins Digestible with Diffi- 

 culty from those Readily Digestible. — Cheese contains only 

 small quantities of completely indigestible nitrogenous sub- 

 stances, 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. of 10 per cent, hydro- 

 chloric acid. The acidity of the total liquid (i 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 filters, 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). 



