The Effects of Physical and Chemical Parameters on the Adsorption of Tannin to Grape Cell Wall Material

Tannins are of paramount importance when evaluating red wine quality because they contribute to taste and mouthfeel. The purpose of this study was two-fold: (1) to investigate the partitioning of tannin with grape cell wall material before and after fermentation and (2) to characterize the effects of physical and chemical parameters normally encountered during commercial wine fermentation on tannin partitioning with grape cell wall mesocarp (CWM). A complete recovery of tannin before and after fermentation was accomplished and it was found that a significantly large fraction of tannin became bound to grape cell wall material by the end of fermentation, resulting in only 7.1% of the total available tannin extracted into wine. The effects of physical and chemical parameters on tannin adsorption to CWM were determined by measuring tannin present in solution before and after treatment with CWM. Adsorbed tannin was then experimentally separated into two fractions: fined and bound. The "fined tannin" is defined as the total amount of tannin removed from solution; the "bound tannin" was determined by desorbing tannin from CWM by acetone extraction and measuring the tannin that remained adsorbed to CWM. Physical parameters that were shown to decrease total adsorption were ethanol, sugar, and CWM particle size; parameters that had no effect on adsorption were pH and sulfite concentration; parameters that increased adsorption were ionic strength and contact time; temperature was shown to have opposite effects on the fined and bound tannin fractions - decreasing the fined fraction and increasing the bound fraction; and initial tannin concentration was shown to increase the fined fraction but the bound fraction reached a maximum and diminished at higher tannin concentrations. These results suggest that tannin interactions with CWM are mediated by hydrogen bonding and hydrophobic forces and are possibly also the result of covalent interaction.