DETERMINATION OF TRYPTOPHAN IN UNHYDROLYZED FOOD AND FEEDSTUFFS BY THE ACID NINHYDRIN METHOD

The optimum conditions for direct measurement of tryptophan in soybean meals, wheat, maize, barley, rye, sunflower, fish, and meat meals have been reported. The acid ninhydrin solution has been reacted directly with the protein matrix without any previous isolation or dissolution of the protein. Reproducibility and recovery studies showed that 10-100 μg of tryptophan-containing protein can be determined with a standard error of 2.7% or less. The decreases of tryptophan content of treated soybeans are essentially a function of the parameters applied. The remaining tryptophan contents, expressed in the percentages of the untreated one, were as follows: 81% HC1 + H3PO4(pH ~1.8), neutralized; 79% HC1 + H3PO4(pH ~2.0), neutralized; 67% HC1 + H3PO4(pH ~2.0), without neutralization; 63% microwave 5 min, 100 °C; 57% HC1 (pH ~2.0), without neutralization. © 1990, American Chemical Society. All rights reserved.