Interference of phenolic compounds in Brassica napus, Brassica rapa and Sinapis alba seed extracts with the Lowry protein assay

The protein content of aqueous extracts of Brassica napus, Brassica rapa and Sinapis alba meal was determined by the Lowry and Kjeldahl nitrogen assays. Phenolic compounds interfered with the Lowry method to different extents based on the lines studied as well as the extraction procedure used. Three ways to correct for this interference were studied; acid precipitation of the protein before analysis, analyzing in the presence and absence of copper and the binding of free phenolics using non-ionic, porous polystyrene (Amberlite XAD-4). Analysis in presence and absence of copper, and using the difference in absorption at 660 nm between these analyses, proved to be the best way to correct for phenolic interference in the Lowry assay. Extractability of Cruciferae seed phenolics may be pH dependant thus the contribution of phenolics to the Lowry protein assay varies with the pH used for extraction.