In West Coast climates, fuel load management in forests and areas of heavy foliage is achieved through many means. Of these methods, prescribed burning is an effective and implementable approach used across the state of California to mitigate the risk of wildfires. Fire is a perpetual risk factor in the production of perennial crops in the western agricultural regions of the United States. Concerns with fire range from direct damage due to combustion to residual effects of smoke exposure.
In the winegrape vineyards of northern California, fire and its associated smoke pose a unique threat in the form of smoke-taint damage to the fruit. As it is now, our use of prescribed burning as a fire prevention method may, or may not, pose a significant risk to winegrape crops; this is of great concern in winegrape vineyards where smoke damage may be carried over in the resulting wine as "smoke-taint."
Burning organic materials release various molecules as they burn, a subset of these are referred to as volatile organic compounds (VOCs) and are comprised of many different molecules. Some of these molecules, such as volatile phenols (VPs) bind readily to plant tissues in both the leaves and fruit. Once bound, they create phenolic diglycosides (PDs), which are strongly attached to the plant tissue and may require specific enzymes to separate or re-volatilize them. These PDs are the major contributing factor to "smoke-taint" damage in winegrape vineyards and the resulting wines. The phenols typically used to indicate smoke taint in must or wine are guaiacol and 4-methyl guaiacol with a handful of other phenols often measured as well. While this is how we identify smoke-taint damage in grapes and wine, the risk of smoke-taint being introduced in vineyards often depends on multiple, environmental factors; the distance from the source of smoke, wind conditions, foliar or human-made barriers between smoke and vineyard, and fuel species composition all matter.
The concentration and type of volatile phenols released from the combustion of organic matter varies depending on the fuel source and the amount of lignocellulose present in the organic matter being burned. Woody species such as oaks have a higher lignocellulose content than vegetative species like annual grasses and thus pose a greater risk of smoke-taint when burned. Prescribed burning may include both woody and vegetative species, but identifying the contribution of varying fuel sources to VOC production is an important step to addressing smoke-taint risk from prescribed fire.
Through a collaboration between UCCE Integrated Vineyard Systems Advisor Chris Chen and the Hopland Research and Extension Center (HREC), a preliminary study was conducted at the Hopland Research and Extension Center property to elucidate the effect of distance from prescribed burns on airborne VOC concentrations.
With funding from the Mendocino County Farm Bureau and HREC, two PurpleAir Flex sensors were installed during a prescribed, grassland burn event at the Hopland REC property in October 2024. One sensor was placed at the source of the fire and another placed within an oak grove 465 meters, or 0.29 miles, downwind from the fire. Airborne VOC levels were measured using these devices both before and throughout the duration of the burn event with no difference in initial values measured before the fire was started.
Throughout the burn, the concentrations of VOCs measured were 31% lower on average in the further sensor than the close sensor during the period where smoke was being released from the prescribed burn (18:00 h to 23:00 h); this represents an average 40 µg/m^3 decrease in airborne VOC concentrations at the more distant measurement point. These data indicate a reduction in airborne VOCs due to both distance and foliar barriers, which may absorb the volatile compounds as they move through the canopy. However, this study was unable to delineate the influence of distance from the effect of a foliar barrier.
While this preliminary study resulted in a statistically significant decrease in smoke-derived VOCs at a distance less than a half mile removed from the source fire, it does not provide enough evidence to make conclusive statements. The data collected does not delineate the effects of distance and barriers on VOC concentrations, nor does it describe the relationship between distance from a prescribed burn and airborne VOC concentrations in great detail. The reduction in airborne VOCs observed may not continue to decline in a linear fashion with distance and may decline at a different rate over longer distances; with the resolution provided by two sensors, we cannot determine the true impact of distance on airborne VOC levels. However, this preliminary study did demonstrate that distance from a source of smoke can significantly reduce the concentration of airborne VOCs which include the volatile phenols that directly contribute to smoke-taint in winegrapes.
Further studies are planned to improve our understanding of the effect of prescribed, grassland fires on smoke-taint damage in winegrape crops. A better understanding of the risks associated with distance and foliar barriers provides grape growers with the knowledge to better determine when a fire is a risk to their harvest.
Please note - these data and the included document do not currently represent peer-reviewed research.
Collaborators:
- Christopher Chen, Ph.D., Integrated Vineyard Systems Advisor, UC ANR Cooperative Extension
- John Bailey, Director, UC ANR Hopland Research and Extension Center
- Greg Solberg, Staff Research Associate, UC ANR Hopland Research and Extension Center
Funding Sources:
- Mendocino County Farm Bureau
- UC ANR Hopland Research and Extension Center
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
- 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