A NEW METHOD FOR THE DETERMINATION OF THE FOOD 
VALUE OF PROTEINS, WITH APPLICATION 
TO CYNOSCION REGALIS. 
By GEORGE F. WHITE and ADRIAN THOMAS. 
It is frequently desired to compare the rate and course of digestion of various 
proteins by enzymes according to methods which, while not complicated, will give 
distinctive and reliable results. Van Slyke’s® method for the determination of amino 
acids has been applied by White and Crozier* with evidently great success to a compari- 
son of the tryptic proteolysis of beef and several fish meats. The ease of manipulation 
of the apparatus, the brief time required for a determination, and the regularity of the 
experimental data, make the process generally useful. The conclusions drawn also 
conform with those from metabolism experiments, making the results of still greater 
value. 
The method which Sorensen'^ has proposed for the estimation of amino acids and 
polypeptides, by titration with caustic soda after the addition of formaldehyde, by its 
simplicity suggests itself for artificial digestion processes. It is the object of this article 
to show its application to the hydrolysis of Cynoscion regalis (squeteague, weakfish) 
by trypsin, and to compare the results with those obtained by Van Slyke’s method. 
The squeteague was boiled in water for a quarter of an hour, allowed to drain from 
excess of liquid, and preserved ice-cold. x\n analysis of two samples gave an average 
of 4.52 per cent nitrogen. 
The digestion was carried on in 250 c. c. volumetric flasks placed in a thermostat 
kept at temperature of 37.5° C. Enough meat to furnish 1 .5 g. of nitrogen was weighed 
out, ground up with water together with i g. of trypsin, and this mixture poured into 
the flask ; 25 c. c. of N/io sodium hydroxide solution was added, and the whole made up to 
250 c. c. with water. Trypsin is presumably most active in a medium made alkaline 
with sodium carbonate, but the presence of this salt would interfere with the titration 
for the amino acids where phenolphthalein must be used as an indicator, so that the 
alkalinity was insured by the presence of the hydrate. Separate mixtures were made 
o Van Slyke, D. T>.: A method for quantitative determination of aliphatic amino ^oups. Journal of Biological Chemistry, 
vol. IX, p. 185-204, 1911. Baltimore. 
b White, G. F. &Crozier, W.: Comparative proteolysis experiments with trypsin. Journal of the American Chemical Society, 
vol. 33, p. 2042-2048, 1911. Easton, Pa. 
c Sorensen, S. P. L.: Fermentstudien. Biochemische Zeitschrift, bd. 7, p. 45-101, 1907. Berlin. 
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