Effects of Hyperoxidation on White Wine Phenolics, Varietal Thiols and Volatile Aromas

Abstract

Hyperoxidation is a pre-fermentative winemaking technique that involves deliberate O2 saturation of a non-sulphited grape must. This alternative winemaking practice is used to combat oxidative browning, decrease phenolic concentrations, reduce necessary antioxidant protection, increase shelf-life and improve mouth feel. Conventional white winemaking practices involve the promotion and preservation of fruity, floral & varietal aromas through the use of antioxidants in a reductive winemaking environment. Aim: Examine the effects of hyperoxidation on aroma and phenolic composition in three New Zealand white wine varietals; Chardonnay, Pinot gris and Sauvignon blanc. This research stems from requests within the wine industry, where hyperoxidation applications are being implemented in order to replace wine fining, modulate phenolics and reduce the browning potential of the final product. Methods: Grape musts were divided into four pre-fermentation groups, consisting of musts that were either treated using hyperoxidation, or left untreated, and in each case with and without antioxidant (SO2) protection; some commercially hyperoxidised Pinot gris and Sauvignon blanc must was also provided. Each treatment group was fermented in triplicate. Phenolics were assessed by HPLC-DAD. Varietal thiols were quantified using GC-MS by means of ethyl propiolate derivatisation. HS-SPME-GC-MS was used to analyse volatile aroma compounds. UV-Vis (A420) was used to quantify brown pigment concentrations. Findings: Hyperoxidation of unprotected musts reduce concentrations of phenolic compounds in white wines, with hydroxycinnamic acids being the most affected. Varietal thiol biogenesis also decreased as a consequence of the pre-fermentation technique. Volatile aromas were either increased (select fatty acids and higher alcohols) or decreased (select fatty acids, higher alcohols, terpenes, C6-compounds and esters) in concentration following hyperoxidation. Contrasting results have appeared between the commercially hyperoxidised wines and the laboratory scale hyperoxidation treatments in terms of volatile aroma concentrations; with esters showing the most conflicting outcomes. The majority of the hyperoxidised musts produced wines with higher concentrations of brown pigments. Conclusions: The results of this experiment have shown substantial evidence of the effects of hyperoxidation on the phenolic and aroma profile in New Zealand white wines. Future studies should investigate this experiment’s gaps in evidence, fine tune wine production and analytical analysis and investigate sensory analysis.