Abstract
Taint in grapes and wine following vineyard exposure to bushfire smoke continues to challenge the financial viability of grape and wine producers worldwide. In response, researchers are studying the chemical, sensory and physiological consequences of grapevine smoke exposure. However, studies involving winemaking trials are often limited by the availability of suitable quantities of smoke-affected grapes, either from vineyards exposed to smoke or from field trials involving the application of smoke to grapevines. This study compared the accumulation of volatile phenol glycosides (as compositional markers of smoke taint) in Viognier and Cabernet Sauvignon grapes exposed to smoke pre- vs. post-harvest, and found post-harvest smoke exposure of fruit gave similar levels of volatile phenol glycosides to fruit exposed to smoke pre-harvest. Furthermore, wines made from smoke-affected fruit contained similar levels of smoke-derived volatile phenols and their glycosides, irrespective of whether smoke exposure occurred pre- vs. post-harvest. Post-harvest smoke exposure therefore provides a valid approach to generating smoke-affected grapes in the quantities needed for winemaking trials and/or trials that employ both chemical and sensory analysis of wine.
Highlights
When bushfires or prescribed burns occur near wine regions, vineyard exposure to smoke can affect the composition and sensory properties of grapes and wine [1], depending on the timing and duration of smoke exposure [2,3]
A significant pool of volatile phenol glycosides is still present after fermentation [6,11,17,18,19] and are thought to contribute to the “ashy” aftertaste associated with smoke taint [20]
Glycosides are relatively stable at wine pH and ambient temperature [21,22,23], with only small changes in volatile phenol levels being observed in smoke tainted wines following 5–6 years of bottle aging [23]
Summary
When bushfires or prescribed burns occur near wine regions, vineyard exposure to smoke can affect the composition and sensory properties of grapes and wine [1], depending on the timing and duration of smoke exposure [2,3]. Several studies have evaluated direct and/or indirect methods for determining free and bound volatile phenol concentrations in grapes and/or wine [12,24,25,26,27,28] Some of these methods are offered by commercial laboratories, enabling grape and wine producers to screen fruit from vineyards that might have been exposed to smoke, i.e., to assess the viability of proceeding with harvest and winemaking. One limitation associated with the use of ‘smoke tents’ is the number of grapevines that can be exposed to smoke in a single growing season, and the quantity of smoke-exposed fruit that can be generated for subsequent use in winemaking trials, or trials involving the amelioration of smoke taint in wine This impacts both the number and scale of treatments that can be evaluated, as well as the availability of suitable volumes of wine for sensory analysis. Kennison and colleagues found post-harvest smoke exposure of grapes yielded wines containing elevated levels of smoke-derived volatile phenols [4], but until now, glycosylation of smoke-derived volatile phenols in excised fruit has not been studied
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