Tryptophan in soybean meal and soybean whey proteins
1971
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 analysed for tryptophan by 2 different colorimetric methods. Dilutions of hydrolysates were reacted with either p-dimethylaminobenzaldehyde or acetic acid containing ferric ions. In the 2nd method, a reference standard was developed protein, that effectively blanked out high absorbance arising from reaction of conc. H2SO4 with soybean constituents. Values in g tryptophan/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. SD was 0.046 g tryptophan/16 NG 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 tryptophan values obtained by the 2 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/g total essential amino acids).
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