The occurrence of bush fires near wine regions in Australia, Canada, South Africa and the USA has caused significant financial losses for grape growers and winemakers. During grapevine exposure to smoke, volatile phenols, such as guaiacol, syringols and cresol, are taken up by leaves and fruit, but these volatiles are quickly glycosylated and accumulate in glycoconjugate forms. Hydrolysis of glycoconjugate precursors by yeast during fermentation results in the release of volatile phenols, but a significant proportion of the glycoconjugate pool remains in the final wine. Smoke tainted wines can exhibit objectionable ‘smoky’, ‘ashy’, ‘burnt’ and ‘medicinal’ aroma and flavor attributes, with an excessively ‘drying‘ character on the back-palate, which diminishes their palatability and thus, market value. The extent to which smoke taint develops in wines made from smoke-affected grapes of different grape varieties was investigated. Chardonnay, Sauvignon Blanc, Pinot Gris, Shiraz, Cabernet Sauvignon, Merlot and Pinot Noir grapevines were exposed to smoke under experimental conditions (for 1 hour at approximately 7 days post-veraison). The concentrations of smoke derived volatile phenols and their glycoconjugates were measured by GC-MS and LC-MS/MS respectively, to determine the impact of grapevine exposure to smoke on wine, while descriptive analysis determined the impact on wine sensory properties. Analyses were then repeated after bottle aging to ascertain the stability of volatile phenol glycoconjugates with time. In an addtional trial selected varieties were exposed to smoke with and without addition of heat to determine the effect of temperature on uptake of smoke-derived compounds. The impact of grapevine smoke exposure on wine composition and sensory properties was evident in all varieties, but the intensity of smoke taint varied by variety, with Pinot Gris and Cabernet Sauvignon exhibiting the highest levels of taint for white and red grape varieties, respectively. Glycoconjugate precursors were surprisingly stable during bottle ageing, but the sensory perception of smoke taint in bottle aged wines was influenced by the loss of fruit intensity and/or evolution of developed characters with ageing. Improved understanding of the uptake, accumulation and stability of smoke derived volatile compounds in grapes and wine will enable industry to better manage the impact of bushfires on grape and wine production.

Uptake of smoke derived volatile phenols by grapevines and their impact on wine composition and sensory properties

255th National Meeting (2018)

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