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.
Highlights
Cereal protein is an important component in both animal feed and human diets, constituting approximately 16 to 45 grams per capita per day (Hackler, 1985)
Since the value of the nitrogen-to-protein conversion factor (NPCF) may vary with the concentration of nitrogen in grains, this study investigated 1) the amount of nitrogen recovered from anhydrous amino acids (NAAA) that included either amide-nitrogen or nitrogen from the computed quantities of Asn and Gln using a ratio method, 2) the universal factor of 6.25 (NPCF1) along with the plant-specific NPCFs based on NAAA with amide-nitrogen (NPCF2) or nitrogen from the computed Asn and
The results indicate that the amounts of Crude Protein 2 (CP2) were, on average, 17.39% ± 1.51(mean ± SD) higher than those of Crude Protein 1 (CP1) due to the inclusion of Asn and Gln in CP2 (Table 1)
Summary
Cereal protein is an important component in both animal feed and human diets, constituting approximately 16 to 45 grams per capita per day (Hackler, 1985). The quantification of proteins in foods is based on the total nitrogen (TN) content being multiplied by a 6.25 nitrogen-to-protein conversion factor (NPCF), as it was assumed that protein contains approximately 16% nitrogen (100/16 = 6.25) (Jones, 1931) This technique assumes that all nitrogen originates from protein, even though nitrogen exists in non-protein nitrogen (NPN) compounds such as nitrates, nitrites, chlorophyll, nucleic acids, and free amino acids (Lourenco et al, 1998). Foodspecific NPCFs have been determined for plants (Tkachuk, 1969), edible insects (Janssen et al, 2017), animal feeds (Boisen et al, 2000), and processed dietary foods (Salo-Vaananen and Kovistoinen, 1996) The results of these studies have demonstrated that the food-specific NPCFs were much lower (ranging from 3.24 to 5.51) than the traditionally used factor of 6.25. Since the value of the NPCF may vary with the concentration of nitrogen in grains, this study investigated 1) the amount of nitrogen recovered from anhydrous amino acids (NAAA) that included either amide-nitrogen or nitrogen from the computed quantities of Asn and Gln using a ratio method, 2) the universal factor of 6.25 (NPCF1) along with the plant-specific NPCFs based on NAAA with amide-nitrogen (NPCF2) or nitrogen from the computed Asn and
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
