Evaluating Diagnostics and Mitigation of Smoke Exposure in Grapes and Wine

Abstract

Smoke taint refers to the unpleasant smoky and ashy attributes that can characterise wines made from grapes exposed to bushfire smoke. As outlined in Chapter 1, there is an urgent need for strategies that detect smoke exposure in grapes and mitigate its effect on wine sensory profiles, especially given that climate change models predict more frequent bushfires in the future. Current diagnostics of smoke exposure in grapes involve the measurement of volatile phenols and their glycoconjugates by GC-MS and HPLC-MS/MS, respectively. While their presence is an essential indicator, their concentrations are inherently dynamic and must be contextualised with respect to their natural abundance in grapevines leaves and fruit, the timing, density, and duration of smoke exposure, the fuel from which smoke is derived, and the timing of sampling for smoke taint analysis of grapes. Critical gaps in these domains confound our interpretation of current diagnostics and to achieve clarity, they need to be addressed. Chapter 1 concludes with a statement of research aims underpinning this thesis. To begin, this thesis investigates the volatile phenol glycoconjugates that are most indicative of smoke exposure in grapes and wine across distinct wildfire seasons in California (Chapter 2). The complex biochemistry of current smoke taint markers prompted investigation into their accumulation in grapevines following smoke exposure. This thesis demonstrates a temporal gap between the depletion of smoke-derived volatile phenols and the subsequent appearance of glycoconjugates, as well as the potential use of particulate matter sensors to monitor vineyard smoke exposure (Chapter 3). In parallel, this thesis also responds to the challenges of high analytical demand during the peak of the 2019/2020 bushfire season in Australia. Measuring volatile phenols and glycoconjugates is a resource-intensive process, and this thesis explores fluorescence spectroscopy as a rapid detection tool (Chapter 4). Beyond acquiring more knowledge of existing smoke taint markers and rapid methods of detecting them, further progress in smoke taint diagnostics may be achieved with the of alternative markers. Current diagnostics measure fewer than ten volatile phenols, but far more exist in smoke. In addition, smoke carries ozone, a known oxidant that induces a biochemical stress response in plants. Using an untargeted metabolomics workflow, this thesis unveils both endogenous and exogenous metabolites that differentiate smoke-affected grapevines at different time points, from 2 hours to 20 days, following smoke exposure (Chapter 5). Fortifying the grape and wine industry against the risks of grapevine smoke exposure requires not only improved diagnostics, but also novel mitigation strategies. Current remediation practices involve reducing volatile phenols and their glycoconjugates from wine or masking their unpleasant smoky, ashy attributes. While essential, these approaches prolong mitigation efforts until wines are at or near completion. More importantly, they present risks of diminishing positive attributes in wine. Ideally, preventative strategies should be employed in the vineyard. This thesis describes the development, evaluation, and application of a smoke box for conducting mitigation trials with improved efficiency and demonstrates the potential of activated carbon fabric to protect grapes from contamination by smoke (Chapter 6). To conclude, this thesis reflects on the enclosed chapters and other possible directions for future smoke taint research (Chapter 7).