SHIRAZ FLAVONOID EXTRACTABILITY IMPACTED BY HIGH AND EXTREME HIGH TEMPERATURES

Abstract

Climate change is leading to an increase in average temperature and in the severity and occurrence of heatwaves, and is already disrupting grapevine phenology. In Australia, with the evolution of the weather of grape growing regions that are already warm and hot, berry composition including flavonoids, for which biosynthesis depends on bunch microclimate, are expected to be impacted [1]. These compounds, such as anthocyanins and tannins, contribute substantially to grape and wine quality. The goal of this research was to determine how flavonoid extraction is impacted when bunches are ex-posed to high (>35 °C) and extreme high (>45 °C) temperatures during berry development and maturity. The sole effect of temperature was investigated on well-irrigated potted Shiraz grapevines grown in a glasshouse, where either the whole vine or bunches-only were heated using fans. For both experiments, berries were sampled at harvest, peeled, ground and total flavonoids were extracted using 60% acetone [2]. Two additional assays evaluated the potential temperature impact on subsequent wine composition using wine-like extraction (15% ethanol) [3] or micro-scale winemaking. Detailed tannin composition was primarily determined by LC-MS/MS after phloroglucinolysis [2], with complementary total tannin concentration (methyl cellulose precipitable assay). Secondary metabolites such as phenolic acid and anthocyanins were also analyzed.

The present work showed that short spells of high temperature may not impact on skin and seed tannin extractability when assessed on visually undamaged berries by harvest. Indeed, while total skin tannin concentrations, extracted with 60% acetone, were clearly reduced by a rise of temperature around véraison, skin extractable tannin (15% ethanol) and seed tannin concentrations were not impacted. In damaged berries at harvest, skin tannins were dramatically reduced while seed tannins were mostly preserved. Wine quality, made with a mix of heat-damaged and undamaged berries, was significantly reduced when about 20% (by mass) of the berries were visually damaged and necrotic, corresponding to about 50% of damaged berries (in number). Maintaining wine quality under a changing climate with more frequent extreme events leading to heat stress and/or water stress is challenging. However, this study showed that the impact of heatwaves in the vineyard may be compensated by a better extraction during winemaking and require further investigations at winery scales.