The stability of tryptophan, 5-methyl-tryptophan and α-methyl-tryptophan during NaOH hydrolysis of selected foods

This study evaluated the use of 5-methyl-tryptophan, α-methyl-tryptophan or synthetic tryptophan to correct for the losses of protein-bound tryptophan in foods during NaOH hydrolysis. Synthetic tryptophan and each protein source was incubated in 4.5 M NaOH containing 5-methyl-tryptophan and α-methyl-tryptophan in nitrogen gas-sparged Teflon vials for 0-144 h at 110 °C. The hydrolysis and loss rates of protein-bound tryptophan, 5-methyl-tryptophan, α-methyl-tryptophan and synthetic tryptophan were predicted using least-squares nonlinear regression. Using 5-methyl-tryptophan or synthetic tryptophan to correct for hydrolytic losses of tryptophan overestimated the tryptophan content by 8.2-19% and −0.3-8.8% respectively, while correction using α-methyl-tryptophan underestimated tryptophan by between 0.2% and 8.1% across the protein sources. Correction using α-methyl-tryptophan or synthetic tryptophan was more accurate than using 5-methyl-tryptophan, but when highly accurate tryptophan composition data are required, least-squares nonlinear regression is the best approach as it removes the need for a hydrolysis correction factor.