Investigations on the nitrogen-to-protein conversion factor in organically produced crops

Summary

The accuracy of the common nitrogen-to-protein conversion factor (factor) of 6.25 for different plant and animal products is recurrently discussed. As a result, the factor 5.70 is for example already in use to predict protein content in bread wheat. Variations in the composition of products can lead to differences in the relationship between nitrogen (N) and protein. Therefore, the factor could differ between species and due to breeding progress even within species. Since the composition of organically produced crops can differ from conventional production, it is of interest to check the common factor for different organically produced crops.
Winter wheat (Triticum aestivum L., n=103), blue lupin (Lupinus angustifolius L., n=94), spring field pea (Pisum sativum L., n=74), and spring field bean (Vicia faba L., n=76) samples were collected from organic variety trials in Germany in three years. The samples were ground to pass a 1 mm sieve for CP analyses (according to VDLUFA, Kjeldahl N*6.25) or a 0.5 mm sieve for amino acid (AA) analyses. 18 AA were analyzed using HPLC. Assuming that total content of amino acids (TAA) is equivalent to CP content, the recovery was calculated by dividing CP by TAA and the factor by dividing TAA by N. Since the molar weight of asparagine (132.12 g/mol) and aspartic acid (133.1 g/mol) as well as glutamine (146.15 g/mol) and glutamic acid (147.13 g/mol) is similar, determining the content of NH3 to estimate amination rate was not necessary for the calculation.
The factors were compared against 6.25 and against one another. Pearson correlation analyses (proc corr, SAS 9.4) were conducted to test the relationship between N and TAA.
The determination of CP with Kjeldahl N analyses and the factor 6.25 recovered more than 100 % of TAA. Thus, the factor differed significantly from 6.25 for all four plant species (p<0.01). However, high contents of asparagine and glutamine can lead to an overestimation of protein since they contain more N than their acidic derivates. For organically produced wheat a factor was found that was similar to the one used in human nutrition and significantly lower than for field peas and blue lupins. Field beans had the significantly lowest factor. The prediction of protein based on N seems suitable especially for organically produced wheat, blue lupins and field peas considering very high correlation coefficients between N and TAA. However, the correlation is weaker in field beans maybe resulting of higher contents of NPN like lectins, vicin, convicin, L-DOPA, or nucleic acids.
The results demonstrate that the use of a standardized factor to calculate protein contents from N contents can overestimate protein contents of wheat and grain legumes in organic farming. It should be further discussed to adjust the factor for different species.