Chemical methods suitable for screening for protein content and quality in cowpea (Vigna unguiculata) meals.

Abstract

AbstractSeveral chemical methods have been tested for their suitability as screening procedures for estimation of the protein content and quality of cowpea (Vigna unguiculata) meals.Preliminary estimates of crude protein can be obtained by the UDY dye‐binding procedure, followed by the automated microKjeldahl (% N × 6.25) method using, for example, Carlo Erba equipment. Total amino acid nitrogen was estimated also with the Carlo Erba apparatus and for the cowpea varieties examined a positive correlation between the total amino acid nitrogen and total nitrogen contents was found. Total sulphur was evaluated as a measure of the sulpho‐amino acids in cowpea meals. Total sulphur was determined by wet digestion and subsequent turbidimetric estimation of barium sulphate. Methionine was determined in meal hydrolysates by a Locarte Autoanalyzer. Cysteine was determined either as cysteic acid on the Locarte Autoanalyzer or as bismuth sulphide.The results obtained for cowpea meals showed that there was a relatively small variation in the total sulphur and in the sulpho‐amino acid contents. Although the relation between total sulphur and sulpho‐amino acids was found to be significant at the 95% confidence level, the confidence limits were wide and at this stage a definite recommendation cannot be given as to the probable usefulness of the total sulphur‐screening procedure. A strong negative correlation was observed between both methionine and sulpho‐amino acids/16 g of N and % crude protein. Similarly, a negative relationship was observed between total sulphur/16 g of N and % crude protein.