Ethanol concentration influences the mechanisms of wine tannin interactions with poly(l -proline) in model wine

Changes in ethanol concentration influence red wine astringency, and yet the effect of ethanol on wine tannin–salivary protein interactions is not well understood. Isothermal titration calorimetry (ITC) was used to measure the binding strength between the model salivary protein, poly(l-proline) (PLP) and a range of wine tannins (tannin fractions from a 3- and a 7-year old Cabernet Sauvignon wine) across different ethanol concentrations (5, 10, 15, and 40% v/v). Tannin–PLP interactions were stronger at 5% ethanol than at 40% ethanol. The mechanism of interaction changed for most tannin samples across the wine-like ethanol range (10–15%) from a combination of hydrophobic and hydrogen binding at 10% ethanol to only hydrogen binding at 15% ethanol. These results indicate that ethanol concentration can influence the mechanisms of wine tannin–protein interactions and that the previously reported decrease in wine astringency with increasing alcohol may, in part, relate to a decrease tannin–protein interaction strength