Optimization of ecofriendly extraction of bioactive monomeric phenolics and useful flavor precursors from grape waste

Abstract

Grape marc, also known as grape pomace, is an underutilized biowaste composed predominantly of grape skin and seeds that is produced as a byproduct of winemaking on the million-tonne scale annually. The most important high-value current use of grape marc is in the production of oenological tannins, widely used additives in the food and beverage industry. More commonly, grape marc is simply either disposed of or used as feed or fertilizer. With recent evidence showing that extracts enriched in grape tannins contain significant amounts of the thiol precursors Cys-3SH and GSH-3SH and the possibility that these could influence food and beverage aroma, it was decided to investigate grape pomace extraction procedures in order to try to define extraction protocols that could maximize the recovery of these compounds from grape marc. Should such an extraction protocol be shown to be commercially viable, this could lead to further utilization of material that would otherwise be disposed of. Two thiol precursors and eight monomeric phenolics (quercetin 3-O-glucoside, quercetin, resveratrol, grape reaction product, trans-caftaric acid, catechin, epicatechin, and gallic acid) were identified and simultaneously extracted from Sauvignon Blanc grape marc using solid-liquid extractions. The optimal solvent ratio of acetone:water:EtOH was explored for each compound. Ten ternary diagrams were constructed, showing the effectiveness of extraction across 66 different solvent combinations using the aforementioned solvents. Effective extraction of thiol precursors was dependent on a high water content which is an advantage from an economic and environmental perspective, while for the most abundant phenolic, quercetin 3-O-glucoside, optimal extraction levels (1017 mg per kg of grape marc) were achieved using a 40:50:10 solvent mixture. In addition, this manuscript details the first extraction of thiol precursors from grape pomace which adds a significant potential commercial value to this underutilized byproduct.