The Tryptophan Content in Protein of Cereal Grains and Legume Seeds as a Function of Nitrogen Content

Abstract

The tryptophan content ([Trp]) in the dry matter (DM) of wheat, maize, barley, sorghum, rice, pearl millet grains, pea and broad bean seeds is evaluated from samples, with various nitrogen contents ([N]_DM) and genotypes, using a procedure that has been shown to be strictly quantitative.

The determination of linear correlations between [Trp]_DM and [N]_DM, and between the tryptophan content in protein ([Trp]_N) and [N]_DM or 1/[N]_DM for every species leads to the following observations: (1) [Trp]_DM and [N]_DM are linearly related. The data show that previous reports of similar relationships underestimate tryptophan by 10 ± 5% owing to tryptophan degradation during alkaline hydrolysis preparatory to analysis; (2) the linear correlations between [Trp]_N and 1/[N]_DM, resulting from linear relationships [Trp]_DM and [N]_DM display coefficients of determination (r²) far lower than 1 and similar to those found for linear correlations between [Trp]_N and [N]_DM; (3) [Trp]_N increases with [N]_DM increasing for rice and pearl millet while it decreases for all other species.

In conclusion: (1) linear relationships between tryptophan and nitrogen have a low predictive value; (2) the nutritional score of tryptophan of foods and feeds, as calculated from the determination of tryptophan using a procedure involving alkaline hydrolysis, is generally underestimated by 10%.