Grape aroma, smokey wines and hot wines: Analytical measurements and applied wine production outcomes
Professor Kerry Wilkinson is a world renowned wine scientist in the School of Agriculture, Food and Wine at the University of Adelaide in Australia. Her research group studies the chemistry of grapes and wine – from consumer preferences for different styles of sparkling wine, to the application of novel technologies to improve winemaking efficiency and/or profitability – but she is best known for her research on smoke taint, which aims to understand and address the impacts of vineyard exposure to bushfire smoke. Her research findings underpin industry practice in response to vineyard smoke exposure – both the detection and remediation of smoke taint in grapes and wine.
Climate change has become a major challenge for grape and wine production around the world. Grapegrowers and winemakers are not only affected by increasing temperatures and prolonged drought, but by vineyard exposure to bushfire/wildfire smoke, which can taint grapes and wine, causing significant revenue losses where unpalatable smoky, ashy characters render wine unsaleable. Considerable research has therefore been undertaken over the past ~20 years to understand the compositional and sensory consequences of grapevine exposure to smoke.
Early studies measured guaiacol and 4-methylguaiacol as markers of smoke taint, because these compounds were oak volatiles (deriving from the thermal degradation of lignin during the toasting process of cooperage) known to impart smoke-related sensory attributes, and analytical methods existed for their quantitation in wine. Today, a suite of volatile phenols (guaiacols, cresols and syringols) are measured as smoke taint marker compounds – in both free and glycosylated forms.
Volatile phenols have been identified as constituents of smoke, and can be found in grapes immediately after smoke exposure, but are rapidly glycosylated, giving volatile phenol glucosides, gentiobiosides, diglycosides and rutinosides. During fermentation, some glycoconjugates are hydrolysed by yeast and/or enzymes, releasing volatile phenols into wine. However, a significant portion of the glycoconjugate pool remains after winemaking, and can contribute to the sensory perception of smoke taint due to in-mouth hydrolysis. Ideally, smoke taint risk assessments should therefore comprise determination of both volatile phenols and volatile phenol glycoconjugates, either directly (by GC-MS and LC-MS/MS, respectively) or indirectly (by GC-MS, before and after acid or enzyme hydrolysis).
The detection/evaluation of smoke taint is further complicated by the natural occurrence of some volatile phenols (and their glycoconjugates) in the fruit (and therefore wine) of some grape varieties, without smoke exposure, for example Shiraz and Petit Sirah. Research is therefore underway to establish the varietal, regional and temporal variation in naturally-occurring volatile phenol glycoconjugate concentrations, to better inform decision-making in the lead up to vintage, where vineyards are potentially impacted by smoke.
This presentation will provide an overview of the chemistry of smoke taint, the analytical methods available for determination of smoke taint, and the latest strategies for mitigation and amelioration of smoke taint in grapes and wine.
Tuesday, 5th of July 2022