Abhishek Singh, Ann-Dorie Webley, Larry A. Lerno, Jesse Skratt, Jeff McCord, Anita Oberholster, and Susan E. Ebeler*
*University of California Davis, Department of Viticulture & Enology, 1136 Robert Mondavi Institute North, 595 Hilgard Lane, Davis, CA, 95616 (seebeler@ucdavis.edu)

In recent years, the United States West Coast has witnessed increased wildfires that have impacted the grape and wine industry. Wildfires release large amounts of volatile phenols from burning woods that can absorb into nearby grapes, negatively impacting grape quality. Absorbed volatile phenols are glycosylated in grapes as part of the vine defense mechanism. These smoke-related volatile phenolic compounds also occur naturally in grapes and can confound quantification of actual smoke-tainted grapes during exposure to wildfire smoke. This work screened a wide range of non-smoke-exposed red and white grape varieties for free and glycosidically bound volatile phenols to create a database of typical baseline ranges for these compounds. Benchmarking baseline levels will help the industry make harvest and winemaking decisions during smoke events. We have measured free and total levels of guaiacol, 4-methylguaiacol (creosol), phenol, 4-ethylguiacol, o-, m-, p-cresol, 4-ethylphenol, 4-methylsyringol, syringol, and their glycosylated precursors in non-smoke exposed grapes from different regions of California over two harvest years, 2021 to 2022. Air quality measures (PM 2.5 and Air Quality Index) were consistent, with no major smoke events in the regions sampled, indicating that the samples provided a reasonable estimate of baseline levels. GC-MS/MS and LC-MS/MS were used to quantify the phenolic compounds. Results showed a wide variation in the distribution of volatile phenols, not only in the different compounds analyzed, but also across multiple sites. The data showed no correlation between free and total levels, meaning both measurements are essential for the baseline database. Interannual (2021 to 2022) comparison of volatile phenols in Cabernet Sauvignon and Chardonnay grapes indicated that the individual phenolic compound concentrations were highly variable. This study found a high degree of variability in levels and distributions of volatile phenol compounds across growing regions, varieties, and years, so robust baseline levels will require a large, multiyear database.

Funding Support: American Vineyard Foundation