Extraction of Phenolic compounds during white grape pomace contact

Abstract

The majority of white grapes in Australia are machine harvested. Machine harvesting inevitably results in berry breakage as grapes are shaken from vines. Contact between the expressed juice and grape skins prior to winery draining and pressing results in extraction of phenolic compounds from grape skins. These compounds can persist throughout the winemaking process and produce inferior bitter and astringent white wines with a greater tendency to brown. In this work laboratory studies of phenolic extraction were performed with three different batches of grapes, over a range of relevant temperatures, times, potassium metabisulfite additions (an additive commonly used with machine harvested grapes in vineyards) and degrees of berry breakage. Specific experiments were performed using full factorial designs with three levels of temperature, six levels of time, and either three levels of potassium metabisulfite addition or berry breakage. Two different juice fractions (corresponding with different yields) were collected for phenolic analysis. Time, temperature, potassium metabisulfite addition and berry breakage all significantly increased phenolic extraction. The relationship between phenolic extraction and the explanatory variables differed for the two juice fractions. Plots of phenolic extraction are presented together with results from multiple regression analysis.