234 THE ANALYSIS OF CHEESE. 



VI. Indigestible Nitrogeneous Substances. The fresh mucous 

 membrane of six pigs' stomachs is cut into small fragments 

 and mized with water and hydrochloric acid in a wicfe-necked 

 flask in the proportion of 5 litres of water and 100 c.c. of 

 10 per cent, (by weight) hydrochloric acid to each stomach. 

 At the same time, 2i grammes of thymol dissolved in alcohol 

 are added as a preservative. The mixture is left for twenty- 

 four hours, with occasional shaking, and then filtered through 

 flannel, coarse paper, and fine paper successively. If necessary, 

 the amount of hydrochloric acid in the extract is brought to 

 exactly 0-2 per cent. As thus prepared, the gastric juice remains 

 unaltered for months. 



Sand mixture, containing 5 grammes of cheese, is deprived of 

 its fat by extraction with ether, mixed with 500 c.c. of gastric 

 juice in a beaker, and the mixture warmed for forty-eight 

 hours in a thermostat at 37 to 40 C. (99 to 104 R). At 

 intervals of about two hours, 5 c.c. of 10 per cent, hydrochloric 

 acid are added, until the acidity of the whole reaches 1 per cent. 

 The liquid is filtered through paper or asbestos, the residue 

 washed with water, and the nitrogen in it determined. 



VII. Nitrogen in the Form of Albumose and Peptones. 

 A weighed quantity of the sand mixture, containing 5 

 grammes of cheese, is extracted by boiling with successive 

 portions (100 c.c.) of water, the liquid made up to 500 c.c. and 

 filtered ; and 200 c.c. of the clear filtrate (mixed with an equal 

 volume of dilute sulphuric acid) are precipitated by phospho- 

 tungstic acid. The precipitate is collected on a filter, washed, 

 and the nitrogen estimated by Kjeldahl's method. 



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

 extract is concentrated by evaporation, saturated with zinc sul- 

 phate, and filtered. Concentrated sodium hydroxide solution is 

 added to the filtrate until the zinc hydroxide dissolves, and 

 a few drops of a 1 per cent, solution of copper sulphate 

 added ; a violet-red colour (the biuret reaction) points 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. 



