COMPARATIVE ANALYSIS OF NITROGEN-TO-PROTEIN CONVERSION FACTORS FOR DETERMINING NET PROTEIN CONTENT IN SIX SUPERFOODS

Abstract

The true protein content of plant foods is best assessed directly by analyzing amino acids quantitatively or indirectly by using a nitrogen-to-protein conversion factor (NPCF) that is computed based on sums of amino acids. The practical way of determining protein in plant foods is by multiplying its total nitrogen (TN) value with a reliable plant-specific NPCF. In this study, three kinds of NPCFs were evaluated for measuring the net-protein content of chia, rye, quinoa, spelt, flax, and hemp. A universal factor of 6.25 was considered NPCF1, while NPCF2s and NPCF3s were based on the sum of amino acids with amide-nitrogen and computed quantities of Asn and Gln using a ratio method, respectively. The TN was minimal in rye (1.34% w/w) and maximal in flax (5.57% w/w). The values of NPCF2 ranged from 5.18 – 5.90 and from 5.45 - 6.22 for NPCF3. The net protein values obtained by NPCF2 were close to the values of direct quantitative analysis of amino acids as compared to other NPCFs. The use of computed Gln and Asn quantities incorrectly increased the amount of available nitrogen in the amino acids, especially in spelt, flax, and hemp; it was about 4% higher than the corresponding TN values, suggesting that the use of the computed method is not appropriate for these plants. NPCF1 overestimated the protein content in all the samples. The degree of overestimation is likely due to the inclusion of non-protein nitrogen in the calculation, which ranged from 5% - 22% in the samples tested.