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In California and across many grape-growing regions of the world, fire and the resulting smoke pose significant risks to fruit quality at harvest. Beyond direct damage imposed from combustion or the impacts of ash-derived shading on vine respiration and photosynthesis, smoke taint is a primary concern of grape producers in high fire-risk areas. In these regions, prescribed burning may also be a common practice. Prescription fires are often applied to help reduce fuel loads in natural systems and decrease the risks of wildfire events that burn out of control. The implementation of prescribed burning may seem to contradict the goal of limiting smoke exposure in vineyards but is often applied with regard to crop developmental timing and with the goal of reducing risks of large-scale wildfire events.

Understanding Smoke Taint Risks in Vineyard Regions
The primary compounds that impart the emerging characteristic of smoke taint in wine are the phenolic diglycosides (PDs) bound within the molecular structure of the wine itself (Crews et al. 2022; Krstic et al. 2015). Some of the more common PDs are guaiacol and 4-methyl guaiacol, with others also capable of imparting “smoke taint” characteristics (Caffrey et al. 2019; Modesti et al. 2021). It is also known that some phenolic glycosides can naturally occur in grape berries but often at much lower concentrations than the values seen in smoke-tainted fruit (Hayasaka et al. 2010).

However, PDs are the “bound” version of these smoke-derived phenols. Prior to finding a binding site in berry flesh or skin, they persist in the smoke as volatile phenols (VPs) and are produced by combustion of lignocellulose and other compounds in plant matter (Crews et al. 2022). It is common for VPs to bind to sugars, including glucose and fructose, which can be found in fruit and persist through fermentation as PDs in the resulting wine (Bönisch et al. 2014; Caffrey et al. 2019). As a result, the amount of lignocellulose in the fuel source for any fire will affect the concentrations of VPs released during burning (Chira and Teissedre 2013).

Unfortunately, this relationship is not a straightforward one. While grasses and small vegetative species tend to have lower lignocellulose concentrations than perennial woody species, this is not true in all cases (Krstic et al. 2015; Simoneit 2002). Some work has shown that oats produce similar amounts of 4-vinyl guaiacol to pines when burned (Kelly et al. 2012). Taking past studies into consideration, the species composition of the fuel material and the VP production of each species should be considered when pursuing further information on smoke-taint risks to winegrapes.

The UC ANR Hopland Research and Extension Center (HREC) hosts more than 5,000 acres of natural systems with numerous studies examining a wide diversity of topics. In 2024, UCCE collaborated with HREC to assess smoke risk from prescribed burns in a preliminary study. Many prescribed burns occur in grassland or rangeland habitats in California consisting primarily of small vegetative species and few woody species. Many of these burn sites exist near vineyards, potentially posing a smoke-taint risk when controlled burns are implemented. Few existing studies specify smoke-taint risk from prescribed burns of grasslands or rangelands. Many recent studies focus on hardwood or conifer combustion and the risk posed by wildfires fueled by these sources.

Distance from the source of smoke is one factor that plays a significant role in the concentration of VPs that remain suspended in smoke (Krstic et al. 2015). Often, the farther from the source, the lower the VP concentration in the air. These VPs can be measured within the umbrella term of volatile organic compounds (VOCs), which include VPs and other compounds. The study conducted at the HREC station tested the concentrations of VOCs suspended in smoke at the source of the fire and at 465 m (0.29 miles) from that source. This was done to elucidate the output of VOCs from grassland/rangeland fuel loads and their persistence in the air at a considerable distance from the burn.

Preliminary Findings from Prescribed Grassland Burns
The following addresses data that have not been peer reviewed at this time and only represent preliminary work conducted by UCCE in Mendocino County.

A prescribed burn was conducted at HREC on Oct. 29, 2024, which covered rangeland consisting primarily of small vegetative plant species. During this burn, two PurpleAir Flex sensors were installed at 1 m and 465 m from the source of the fire, directly downwind. These sensors are capable of measuring VOCs suspended in the air and are powered by external batteries. While the sensors measure VOCs, which include VPs, they are unable to delineate between the various volatilized compounds and may include measurements of compounds that will not influence smoke-taint risk in winegrapes. As a result, the data collected represent the relative risk of VPs as a proportion of the VOCs directly measured. They do not represent an exact measurement of objective VP concentrations in smoke.

‘This preliminary study hints at the benefit of distance for reducing smoke taint risk from prescribed grassland/rangeland burns.’

The effect of distance on VOC concentration was statistically significant in these data and much lower in the sensor placed farther from the source of the smoke. When comparing the farther sensor readings to the readings at the source of the smoke, the concentrations of VOCs measured were 31% lower on average during the burn period (10 a.m. to 6 p.m.). This was the time frame where smoke was released from combustion of the fuel materials. These results were statistically significant across time and identified an average 40 µg/m³ decrease in airborne VOC concentrations at the more distant measurement point. The maximum decrease in VOC concentrations at 465 m occurred at the height of the burn (≈3:00 p.m.) and represented a 49% lower VOC value at the distant sensor when compared with the sensor close to the fire.

This preliminary study hints at the benefit of distance for reducing smoke taint risk from prescribed grassland/rangeland burns. However, these results do not represent the true risk of smoke taint on winegrapes at greater distances. Many other factors are likely to play a significant role in the final PD concentrations in musts and wine, including physical and foliar barriers, stage of fruit development, scion cultivar and more. This study is planned to continue in 2025 and will increase the number of sensors placed to identify effects of both distance and foliar barriers on VOC persistence in airborne smoke.

References
Bönisch, F., Frotscher, J., Stanitzek, S., Rühl, E., Wüst, M., Bitz, O., & Schwab, W. (2014). A UDP-Glucose:Monoterpenol Glucosyltransferase Adds to the Chemical Diversity of the Grapevine Metabolome. Plant Physiol, 165(2), 561-581. https://doi.org/10.1104/pp.113.232470

Caffrey, A., Lerno, L., Rumbaugh, A., Girardello, R., Zweigenbaum, J., Oberholster, A., & Ebeler, S. E. (2019). Changes in Smoke-Taint Volatile-Phenol Glycosides in Wildfire Smoke-Exposed Cabernet Sauvignon Grapes throughout Winemaking. American Journal of Enology and Viticulture, 70(4), 373-381. https://doi.org/10.5344/ajev.2019.19001

Chira, K., & Teissedre, P.-L. (2013). Relation between volatile composition, ellagitannin content and sensory perception of oak wood chips representing different toasting processes. European Food Research and Technology, 236(4), 735-746. https://doi.org/10.1007/s00217-013-1930-0

Crews, P., Dorenbach, P., Amberchan, G., Keiffer, R. F., Lizama-Chamu, I., Ruthenburg, T. C., McCauley, E. P., & McGourty, G. (2022). Natural Product Phenolic Diglycosides Created from Wildfires, Defining Their Impact on California and Oregon Grapes and Wines. Journal of Natural Products, 85(3), 547-561. https://doi.org/10.1021/acs.jnatprod.2c00028

Hayasaka, Y., Baldock, G. A., Parker, M., Pardon, K. H., Black, C. A., Herderich, M. J., & Jeffery, D. W. (2010). Glycosylation of Smoke-Derived Volatile Phenols in Grapes as a Consequence of Grapevine Exposure to Bushfire Smoke. Journal of Agricultural and Food Chemistry, 58(20), 10989-10998. https://doi.org/10.1021/jf103045t

Kelly, D., Zerihun, A., Singh, D. P., Vitzthum von Eckstaedt, C., Gibberd, M., Grice, K., & Downey, M. (2012). Exposure of grapes to smoke of vegetation with varying lignin composition and accretion of lignin derived putative smoke taint compounds in wine. Food Chemistry, 135(2), 787-798. https://doi.org/https://doi.org/10.1016/j.foodchem.2012.05.036

Krstic, M. P., Johnson, D. L., & Herderich, M. J. (2015). Review of smoke taint in wine: smoke-derived volatile phenols and their glycosidic metabolites in grapes and vines as biomarkers for smoke exposure and their role in the sensory perception of smoke taint [https://doi.org/10.1111/ajgw.12183]. Australian Journal of Grape and Wine Research, 21(S1), 537-553. https://doi.org/https://doi.org/10.1111/ajgw.12183

Modesti, M., Szeto, C., Ristic, R., Jiang, W., Culbert, J., Catelli, C., Mencarelli, F., Tonutti, P., & Wilkinson, K. (2021). Amelioration of Smoke Taint in Cabernet Sauvignon Wine via Post-Harvest Ozonation of Grapes. Beverages, 7(3), 44. https://www.mdpi.com/2306-5710/7/3/44

Simoneit, B. R. T. (2002). Biomass burning — a review of organic tracers for smoke from incomplete combustion. Applied Geochemistry, 17(3), 129-162. https://doi.org/https://doi.org/10.1016/S0883-2927(01)00061-0