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

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