Investigating Differences in Tannin Levels Determined by Methylcellulose and Protein Precipitation

Two different published methods for determining condensed tannin levels in grapes using protein (BSA) and methylcellulose precipitation have led to inconsistent results. We hypothesized that these inconsistent findings were due to the differences in the methodologies. Thus, we modified the published protocols to use the same extraction solvent, resuspended the precipitant, and quantified the tannin concentration by reaction with ferric chloride (FeCl₃). Pellets of Shiraz and Cabernet Sauvignon grape skin tannin precipitated with both protein and methylcellulose were readily resuspended in an aqueous buffer containing triethanolamine (5% v/v) and sodium dodecyl sulfate (10% w/v) adjusted to pH 9.4. Consistent with previous research, 50% ethanol in water (v/v) was a less effective extraction solvent than 70% acetone. In all instances, the calculated tannin concentration in grape skin was lower using methylcellulose as a precipitant and the proportional difference between methylcellulose and protein as a precipitant was not consistent across samples, extraction solvents, or methods of quantification. These results suggest that there are significant differences in the nature of the tannins precipitated by methylcellulose and protein, although the nature of these differences has yet to be fully explored.