Wildfires have become an increasing problem in the United States in recent years. During the first half of 2017, 58,583 wildfires burned roughly 8.5 million acres, according to the National Interagency Fire Center.1 Compared to the 10-year average of approximately 6 million acres burned per year by wildfires, the 2017 land loss exceeds this by far. The effects are hazardous, with millions of homes threatened and financial losses that add up to billions of dollars over the past 10 years.

Smoke taint in wine
Unfortunately, more and more fires also strike in famous grapegrowing areas, such as the October 2017 firestorm in Sonoma, Napa and Mendocino counties in California. Apart from the direct danger of losing a vineyard to the fire, there is also an indirect risk of smoke exposure. When smoke from a wildfire is passing over vineyards, vinification of the affected berries can lead to unpalatable wines. These wines convey strong “ashy,” “burnt” and “smoky” aromas, with “an excessively drying” back-palate and retro-nasal “ash” perception.
     Recent studies report on the occurrence of this “smoke taint” in wines from California and Canada, but also from Australia, South Africa and Greece.5-8 The consequence is a significantly reduced market value, as consumers have been shown to reject smoke-tainted wines.

Smoke-derived volatiles in grapevines
Smoke from wood fire contains small volatile phenols such as guaiacol, 4-methylguaiacol, syringol, m-cresol, o-cresol and p-cresol (see “Smoke-Derived Volatile Phenols and Resveratrol” below) that contribute to the characteristic “burnt” odor.10,11 Of these compounds, guaiacol, m-cresol and 4-methylguaiacol have the lowest aroma thresholds of 23 µg/L, 20 µg/L and 30 µg/L (BET, or best estimate threshold), respectively, emphasizing why they are considered the primary chemicals associated with smoke taint.11 The U.S. wine industry has accepted the quantification of guaiacol and 4-methylguaiacol as indicators for smoke taint potential in grape juice and/or wine. It is suggested that they enter grapes mainly through the skins and the waxy cuticle rather than through the leaves.

     However, even when the level of indicators was low and the grape juice did not show a specific ashy note, the corresponding wines exhibited smoke taint.13,14 This is pointing toward the existence of a native grapevine-regulation system that processes the volatile phenols and masks their unpleasant smoky aroma.

Binding of volatile phenols to grape sugars
Juice from grapes affected by smoke was studied in laboratory analyses to discover what occurred with the volatile phenols.15 The experiments demonstrated that the plant had bound the smoky compounds to grape sugars (see “Uptake and Release of Smoke Taint in Grapevines” on page 61). The compounds lost their volatile properties, being now odorless and hidden within the juice. Why is the grapevine processing the smoky volatiles in such a way?

     Recently, one cause was discovered by researchers of the Technical University of Munich, Germany.3 Scientists found enzymes native to the grapevines that convey the binding of grape sugars to the smoky volatiles. These enzymes, called glycosyltransferases (GTs), transfer sugar molecules. In a laboratory environment, the activity of three GTs toward the most important smoke indicator compounds was tested. It was confirmed that the enzyme UGT72B27 processed all indicator compounds with high efficiency.

Natural glycosyltransferase activity
This leads to the question: Why is an enzyme native to the grapevine active on foreign smoky molecules? Here, the researchers found an explanation. They suggested that, under normal circumstances, the enzyme is binding glucose moieties to trans-resveratrol, a naturally occurring metabolite in grapevines.16 It has been detected in single and sugar-bound form in whole berries and in resulting wines and is usually released by the plant upon pathogen-infection or injury to counteract bacteria and fungi.

     Regarding the chemical structure of trans-resveratrol, certain similarities to the smoky volatiles can be observed immediately. Similar to guaiacol, syringol and o-cresol, trans-resveratrol possesses a basic phenolic structure and functional hydroxyl groups that enable the linkage of a sugar moiety. Therefore, the scientists at the Technical University of Munich argue that the grapevine enzyme cannot distinguish between its natural substrate trans-resveratrol and the foreign smoky molecules. It is processing both.

Problems for the winegrower
One might think that this intrinsic grapevine activity is an advantage, as the smoky volatiles are rendered non-smoky and masked by the binding to a sugar molecule. However, during winemaking, the bound smoke compounds are cleaved (hydrolyzed) from sugar molecules, thus allowing the smoky volatile phenol aromas and flavors to be released into wine (see “Uptake and Release of Smoke Taint in Grapevine”). Hydrolysis of the sugar bond can occur throughout the winemaking process including during fermentation, aging and storage. In fact, there can be a dramatic progressive release of the smoky compounds. It has been shown that the level of guaiacol can increase from 4.3 µg/L in control (unsmoked) wine up to 388.3 µg/L in wines exposed to smoke applications from juice stage to completion of malolactic fermentation.

     The hydrolysis reaction has been proposed to be executed primarily by enzymes such as β-glucosidase, which are native to wine yeasts and wine bacteria. However, acid hydrolysis cannot be ruled out as a possible mechanism for release of the smoky phenols. Furthermore, it has been shown that upon consumption of smoke-affected wine enzymes in human saliva can also cleave off remaining sugars, causing an unpleasant retro-nasal ash perception.8 This explains why wines can have ashy notes, even when the unfermented grape juice did initially not show hints of an unpleasant aroma.

Future prospects and possible solutions
Thanks to wine research, we now know how such a taste can develop. In the future, a grapevine variety could be bred to produce less GT enzyme. Alternately, a second sugar molecule could be added to prevent release of the smoky aroma, or yeasts not able to cleave the sugar-bound volatiles could be searched.
From a practical point of view, it would be desirable to develop a process specifically to remove the smoky volatiles or their sugar-bound forms from juice and wine without removing beneficial flavors and aromas. Unfortunately, many valuable flavor/aroma compounds are chemically highly similar to the smoky phenols, which is why the development of such a method would be very difficult.

     By means of genetic engineering, the grapevine gene responsible for production of the GT could be removed. However, consumers in certain areas such as Europe tend to reject genetically modified foods and beverages. Meanwhile, winegrowers and winemakers can take steps to minimize the sensory impacts of smoke exposure. Detailed advisory sheets are available online from the Department of Agriculture and Food in Western Australia and the Australian Wine Research Institute.

References
1. National Interagency Fire Center (nifc.gov/fireInfo/nfn.htm; last accessed December 2017).
2. Verisk’s 2017 Wildfire Risk Analysis (verisk.com/insurance/visualize/key-findings-from-the-2017-verisk-wildfire-risk-analysis/; last accessed December, 2017).
3. Härtl, K., F.C. Huang, A.P. Giri, K. Franz-Oberdorf, I. Frotscher, Y. Shao, T. Hoffmann and W. Schwab. 2017 “Glucosylation of smoke-derived volatiles in grapevine (Vitis vinifera) is catalyzed by a promiscuous resveratrol/guaiacol glucosyltransferase.” J. Agric. Food Chem. 65, 5681-5689.
4. Hayasaka, Y., M. Parker, G.A. Baldock, K.H. Pardon, C.A. Black, D.W. Jeffery and M.J. Herderich. 2013 Assessing the impact of smoke exposure in grapes: Development and validation of a HPLC-MS/MS method for the quantitative analysis of smoke-derived phenolic glycosides in grapes and wine. J. Agric. Food Chem. 61, 25−33.
5. Høj, P., I. Pretorius and R.J. Blair. 2003 The Australian Wine Research Institute annual report. The Australian Wine Research Institute,, Australia, eds.
6. Sheppard, S.I., M.K. Dhesi, and N.J. Eggers. 2009 “Effect of pre- and postveraison smoke exposure on guaiacol and 4-methylguaiacol concentration in mature grapes.” Amer. J. of Enol. & Vit. 60, 98–103.
7. Chong, H.H. and M. Cleary. 2012 “Smoke taint aroma assessment in 2008 California grape harvest.” Qian, M. and Shellhammer, T.H., eds. 2008 Flavour chemistry of wine and other alcoholic beverages (American Chemical Society 67–79.
8. Krstic, M.P., D.L. Johnson and M.J., Herderich. 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.” Aust. J. Grape Wine Res. 21, 537−553.
9. Kennison, K.R., K.L. Wilkinson, H.G. Williams, J.H.  Smith and M.R. Gibberd. 2007 ”Smoke-derived taint in wine: Effect of postharvest smoke exposure of grapes on the chemical composition and sensory characteristics of wine.” J. Agric. Food Chem. 55(26), 10897-10901.
10. Fernández de Simón, B., L. Muiño and E.  Cadahía. 2010 “Characterization of volatile constituents in commercial oak wood chips.” J. Agric. Food Chem. 58, 9587−9596.
11. Parker, M., G. Baldock, Y. Hayasaka, C. Mayr, P. Williamson, I.L. Francis, M. Krstic, M. Herderich and D. Johnson. 2013 “Seeing through smoke.” Wine Viticulture J. 28, 42−46.
12. Kelly, D., A. Zerihun, D.P. Singh, C. Vitzthum von Eckstaedt, M. Gibberd, K. Grice and M., Downey. 2012 “Exposure of grapes to smoke of vegetation with varying lignin composition and accretion of lignin derived putative smoke taint compounds in wine.” Food Chem. 135, 787−798.
13. Dungey, K.A., Y. Hayasaka and K.L. Wilkinson. 2011 “Quantitative analysis of glycoconjugate precursors of guaiacol in smoke-affected grapes using liquid chromatography−tandem mass spectrometry based stable isotope dilution analysis.” Food Chem. 126, 801−806.
14. Singh, D.P., H.H. Chong, M. Pitt, M. Cleary, N.K. Dokoozlian and M.O. Downey. 2011 ”Guaiacol and 4-methylguaiacol accumulate in wines made from smoke-affected fruit because of hydrolysis of their conjugates.” Aust. J. Grape Wine Res. 17, 13−21.
15. Hayasaka, Y., K.A. Dungey, G..A. Baldock, K.R., Kennison and K.L. Wilkinson. 2010 “Identification of a β-D-glucopyranoside precursor to guaiacol in grape juice following grapevine exposure to smoke.” Anal. Chim. Acta 660, 143−148.
16. Careri, M., C. Corradini, L. Elviri, i. Nicoletti and I., Zagnoni. 2005 “Direct HPLC analysis of quercetin and trans-resveratrol in red wine, grape, and winemaking byproducts.” J. Agric. Food Chem. 2003, 51, 5226−5231.
17. Wenzel, E. and V. Somoza. 2005 “Metabolism and bioavailability of trans-resveratrol.” Mol. Nutr. Food Res. 49, 472−481.
18. Kennison K.R., K.L. Wilkinson, A.P. Pollnitz, H.G. Williams and M.R. Gibberd. 2011.” Effect of smoke application to field-grown Merlot grapevines at key phenological growth stages on wine sensory and chemical properties.” Aust. J. Grape Wine Res. 17(2), 5-12.
19. Department of Agriculture and Food in Western Australia and the Australian Wine Research Institute (awri.com.au/industry_support/winemaking_resources/smoke-taint/; last accessed December 2017).
 

Katja Härtl is lecturer and researcher in the Department of Biotechnology of Natural Products at Technical University of Munich in Freising, Germany. She specializes in enzymatic activities and the availability of single and sugar-bound secondary metabolites in crop plants including the grapevine.

Wilfried Schwab is professor and head of the Biotechnology of Natural Products Department at Technical University of Munich in Freising, Germany. His research interests are enzymes and secondary metabolites in plants, metabolic engineering and whole-cell biocatalysis of bioactive plant compounds.