Ruiwen Yang, Yanping L. Qian, Armando Alcazar, and Michael C. Qian*
*Oregon State University, 100 Wiegand Hall, Corvallis OR, Corvallis, OR 97330 (michael.qian@oregonstate.edu)

Climate change has increased the incidence of forest and bushfires in the west- ern United States. When grapevines are exposed to wildfire smoke, the grapes and grapevine can absorb smoke compounds and convert them into the corresponding glycosides or other bound forms. The amount of smoke compounds absorbed by the grapes depend on smoke intensity, smoke duration, grape variety, and many other factors. Guaiacol, 4-methylguaiacol, syringol, 4-methylsyringol, and o-, m-, and p-cresols have been suggested as indicators for smoke exposure. A rapid meth- od based on solid-phase microextraction gas chromatography-mass spectrometry with isotope compounds as internal standards was developed to analyze guaiacol, 4-methylquaiacol, 4-ethylguaiacol, o-cresol, m-cresol, p-cresol, and 4-ethylphenol in 30 min. Using this rapid analytical method, over 370 smoke-exposed red wines and 90 smoke-exposed white wines (including Rosé wine) were analyzed during the 2020 wildfire season. Total phenols (after acid hydrolysis, pH 1, 100 C/4 h) were also analyzed for all samples. Out of 370 smoke-exposed red wines, 84 samples had guaiacol concentrations of 0 to 5 μg/L and 159 samples had guaiacol concentrations of 5 to 10 μg/L. Most samples had less than 3 μg/L 4-methylguiacol. In smoke-exposed red wine, the average ratio of free guiacol to 4-methylguaiacol was 4.5. However, the average ratio of total guaiacol to total 4-methylguaiacol was 6.5, higher than the free forms. Except for m-cresol, levels of other total volatile phenols were approximately two to five times greater than the corresponding free volatile phenols. Distribution frequency was used to investigate the relationship between these smoke-related compounds. The highest frequency of total guaiacol/free-form guaiacol ratio was 2:4; the average was 3.3:1. The highest frequency of guaiacol/4-methylguaiacol ratio in free-form and total was 4:6, the average for free-form was 4.5:1, and the total was 6.6:1.

Funding Support: American Vineyard Foundation (AVF) Oregon Wine Board (OWB) Oregon Wine Research Institute (OWRI) China Scholarship Council (CSC)