Use of antioxidants in extraction of tannins from walnut plants

The radial diffusion assay was applied to estimate the amount of total tannins in extracts from walnut plant materials. We found that the protein-precipitating activity of the tannin extracts were increased by 30-75 % when antioxidants (ascorbic acid or sodium metabisulfite, 5.5 mM) were added in extraction solvents (50% aqueous methanol). The extracts obtained with Na2S2O5 were much less brown than control. Moreover, two-dimensional thin-layer chromatography revealed some additional polyphenolic compounds in the extracts made with Na2S2O5. These results suggest that the amount of tannins and/or their protein-precipitating property may be strongly affected by oxidation events during extraction procedure. The addition of antioxidants in the extraction solvents is useful to limit this problem.