Non-protein (free and protein-bound) tryptophan content in cereal and legume seed flours

Cereals and legumes are the major source of protein in the human diet. There is little information on the presence of non-protein tryptophan, one of the essential amino acids less represented in the vegetable proteins. Our results show that tryptophan is present not only in the free-form but also is linked either to water soluble proteins or to proteins extracted at pH 8.9 both in cereal and legume flours. As regards the cereals, wheat and spelt contain the highest content of protein tryptophan and maize the lowest. Spelt, followed by barley and pearl millet flours, contains the highest amounts of free tryptophan and the rice the lowest. In addition, spelt flour shows the highest levels of tryptophan linked to both water soluble and buffered protein fractions, whereas rice contains the lowest amounts of protein-bound tryptophan. Among the legume flours, soybeans show the highest value in protein tryptophan and peas the lowest. Chick peas contain the highest concentrations of both free and protein-bound tryptophan water soluble fraction. This last fraction appears absent in broad bean, lentil and soybean flours. In addition, beans are shown to contain the highest levels of tryptophan linked to proteins extracted at pH 8.9, and peanuts the lowest values both as free and protein-bound forms.

Considering that tryptophan is one of the limiting amino acids of the biological value of vegetable proteins, the determination of non-protein tryptophan in food is very useful in calculating the score.