Bainian Chen, Arran Rumbaugh,* Ron Runnebaum, Chen Liang, Annegret Cantu, and Hildegarde Heymann
*USDA-ARS, 595 Hilgard Lane, RMI North, room 3156, Davis, CA, 95616 (arran.rumbaugh@usda.gov)

Within California’s wine industry, wildfire smoke poses a significant threat to vineyards by imparting negative sensory attributes to wines, termed “smoke taint.” Hence, predicting the emergence of such tainted wines is crucial. This study evaluated the efficacy of a proof-of-concept approach to reproducibly expose grapes to smoke conditions. The long-term aim is to establish threshold levels of smoke-derived compounds in grapes that cause negative sensory attributes in the final wines. Around 250 kg of Cabernet Sauvignon grapes was harvested at 25 Brix from the Tyree vineyard at the University of California, Davis. Four experimental groups were exposed to various levels of smoke generated from red oak chips: control-exposure (CE, no smoke exposure), low exposure (LE, 0.5-hr exposure), medium exposure (ME, two-hr exposure), and high exposure (HE, four-hr exposure). Each experiment was conducted in an in-house constructed smoking chamber. Following smoke exposure, duplicate fermentations of each treatment were performed in stainless steel buckets. Volatile phenols (VPs) and their glycoconjugates (bound) were measured in grape and wine samples using gas chromatography-tandem mass spectrometry and liquid chromatography-tandem mass spectrometry, respectively. In addition, sensory differences in wines were determined utilizing descriptive analysis (DA). Free and bound VP concentrations in grapes and wines correlated positively with smoke exposure duration and intensity, as expected. Results indicated no significant sensory differences between LE and CE wines. ME samples were described as “medicinal” and significantly lower in fruity aroma than LE and CE samples. HE samples were rated even lower in fruity aromas and higher in “barbeque” and “ashy” aromas than all other treatments. Overall, ME wines had a significant sensory affect compared to CE wines, suggesting a potential range for future threshold experiments. The study was a successful proof-of-concept to develop threshold levels of free and bound VPs in grapes that will lead to unfavorable sensory attributes in the final wines.

Funding Support: USDA-ARS Funded Research