Cereal Chem 48:321- 327.  |  VIEW ARTICLE
Tryptophan in Soybean Meal and Soybean Whey Proteins.

D. J. Sessa, K. J. Abbey, and J. J. Rackis. Copyright 1971 by the American Association of Cereal Chemists, Inc. 

Pronase hydrolysates of soybean meal, whole-whey protein, and heat-coagulated and supernatant proteins prepared from whey heated at 75 C. for 30 min. were analyzed for tryptophan by two different colorimetric methods. Dilutions of the hydrolysates were reacted with either p-dimethylaminobenzaldehyde or acetic acid containing ferric ions. In the second method, a reference standard was developed, based on isolated protein, that effectively blanked out high absorbance arising from reaction of concentrated sulfuric acid with soybean constituents. Values in g. tryptophan per 16 g. N were: 1.583 and 1.597 for soybean meal, 1.707 and 1.440 for whole whey, 2.433 and 2.380 for heat-coagulated protein, and 1.173 and 1.063 for supernatant protein. Standard deviation was 0.046 g. tryptophan per 16 g. N and least significant difference (95% level) between methods was 0.080. Tryptophan values for the whole-whey and supernatant proteins are significantly different by both methods. Calculations based on values of amino acid analysis of these soybean samples and the mean of the tryptophan values obtained by the two colorimetric methods show that soybean meal, whole-whey, heat-coagulated proteins, and supernatant protein contain 120, 113, 153, and 90% of the tryptophan content of the hen's egg essential amino acid pattern (mg. per g. total EAA).