Evaluation of the ESCA technique as a screening method for the estimation of protein content and quality in seed meals

The ESCA (Electron Spectroscopy for Chemical Analysis) technique has been evaluated as an analytical screening method for the determination of protein content and quality of grain meals of some legumes and cereals. To this end the grain meals of a number of different species were analysed for total nitrogen, meals of varieties of cowpea (Vigna unguiculata) were examined for sulpho‐amino acid content, and the levels of basic amino acid in meals of some maize and barley varieties were measured by this technique; some amino acids, polyamino acids and pure proteins were also examined as standards. The results obtained for 15 grain meals by the ESCA technique, expressed as the ratio of the intensity of the N1s photoelectron signal to that of the C1s signal, were compared with %N as determined by a semi‐automated micro‐Kjeldahl method, and the correlation coefficient between the results from these two methods was r = 0.98. The binding energies of S2p electrons were determined for some amino acids and proteins, and the amino acid sulphur was differentiated from oxidised sulphur. The N1s/S2p intensity ratios for four pure proteins correlated very well with the calculated stoichiometric N/S ratios (r = 0.999). The N1s/S2p intensity ratios were also determined for seven cowpea varieties and the correlation coefficient between these values and the ratios N/Saa, measured by chemical methods, was r = 0.77. The N1s photoelectron peaks of some varieties of barley and maize meals showed a pronounced shoulder on the high binding energy side. On deconvolution into two components the main peak was assigned to nitrogen in the peptide bond and the smaller peak was assigned to the protonated nitrogen on the side chains of the basic amino acid residues.