Smoke taint compounds in wine: nature, origin, measurement and amelioration of affected wines
Compounds such as guaiacol and 4-methylguaiacol are formed by the pyrolysis of lignin as occurs during the toasting of oak (Nishimura et al. 1983, Sarni et al. 1990, Wittowski et al. 1992). Contact with such oak products in turn results in entry of these compounds into wine where they impart smoky, phenolic and burnt aromas (Boidron et al. 1988). In this way, guaiacol and 4-methylguaiacol occur in wines in the typical range of 10–100 µg/L and 1–20 µg/L, respectively (Pollnitz et al. 2004), by and large contributing positively to the complexity arising from oak treatment. The related compounds, 4-ethylguaiacol and 4-ethylphenol, are also produced by the action of Brettanomyces/Dekkera yeast (Chatonnet et al. 1995, Licker et al. 1998).
Because of the burning of plant material produces smoke that contains significant concentrations of these guaiacols and related compounds, smoke exposure of grapes is a means by which wines could accumulate higher than normal amounts of these volatiles. Such a possibility was no doubt suspected for some time, but it took the large-scale exposure of vineyards to smoke from extensive bushfire events before research confirming this link was initiated. Work at The Australian Wine Research Institute (Høj et al. 2003) revealed that grapes and the wines produced from these, which had been sourced from vineyards in Victoria and New South Wales affected by bushfires in early 2003, did in fact possess smoky, burnt, ash aromas, with guaiacol and 4-methylguaiacol being the most important compounds contributing to this taint. Taint compounds appeared to be absorbed by the leaves and grapes, particularly the skins of the latter. Grape pulp contained minimal guaiacols, but extended maceration with skins allowed marked extraction into the wines. Thus, manifestation of a smoke taint is much more likely in red wines where extended skin contact occurs. Mechanical harvesting introduced more leaf material into grape harvest bins and therefore produced wines with higher guaiacol content. Activated carbon reduced the guaiacol content of wine, albeit by only 5%. A limited study (two wines only) suggested that reverse osmosis could reduce guaiacol content by as much as one-third.
The occurrence of further bushfires in Australia has been the impetus for ongoing work on smoke taint by researchers at institutions including Curtin University of Technology, Department of Agriculture and Food Western Australia, The Australian Wine Research Institute and University of Adelaide. Of course, the global relevance of this issue was underscored by recent fire events in other viticultural regions of the world, including the Mediterranean, Africa, Canada and USA. In fact the frequency of such events has been increasing because of climate change (Overpeck et al. 1990) and is likely to continue to do so (Mira de Orduña 2010) with predicted warming scenarios (Schneider et al. 2007). The potential contribution of prescribed burning in forests, parks and reserves cannot be ignored either. Taken together, smoke taint would appear to be a key issue for the global wine industry.
Several papers on this topic have or will appear in The Australian Journal of Grape and Wine Research. These are offered as a virtual special issue with the most recent papers appearing in the second issue of volume 17. An overview of the topic and the content of these papers are provided here.
Research to date
Despite a recent appearance as a research priority, the understanding of issues around the nature, origin, measurement and fate of smoke taint compounds is well advanced, thereby leading to recommendations to viticulturalists and winemakers of strategies to minimise the negative consequences of smoke exposure of grapes. Some of the most recent findings are included as a series of five papers (Fudge et al. 2011, Kennison et al. 2011, Ristic et al. 2011, Singh et al. 2011, Wilkinson et al. 2011) featured online (VSI link) and in the following pages.
Perhaps the first peer-reviewed report on the topic comes from Kennison et al. (2007) in which postharvest grapes were exposed to smoke and shown to yield wines with an apparent smoke taint. Compounds exclusively associated with the smoke-treated wines included guaiacol, 4-methylguaiacol, 4-ethylguaiacol, 4-ethylphenol, eugenol and furfural. One hour of smoke exposure of Chardonnay vines is sufficient to yield up to 26 µg/L guaiacol in the harvested grapes (Sheppard et al. 2009). The timing and repeated exposure of vines to a similar treatment revealed differences in the impact on the resulting wine (Kennison et al. 2009). Peak sensitivity of the vine to smoke occurred at 7 days post-veraison with a simple cumulative effect evident upon repeated exposure. Guaiacol was the predominant volatile phenol ranging from 0 to 100 µg/L for single treatments or a total of ∼400 µg/L for repeated exposure. In a recent report (Kennison et al. 2011), smoke applied at 12 stages of development over three seasons revealed key periods of vine sensitivity to smoke taint in the wine. Thus, ‘low’ and ‘variable’ levels of taint were evident where exposure occurred from ‘shoots at 10 cm to full bloom’ and from ‘pea size berries to onset of veraison’, respectively. A peak of sensitivity occurring from 7 days post-veraison was confirmed. As for seasonal effects, no carryover of guaiacol or 4-methyl guaiacol was seen between seasons; however, yield appears to have been reduced within season by ∼24% and in the season following exposure by ∼50% (Kennison et al. 2011). Further research will determine if these effects on yield are reproducible.
Given the undesirability of smoke taint in wines, special consideration needs to be given to fruit potentially contaminated with volatile phenols. While these compounds can be evident in the grapes (Sheppard et al. 2009), the concentration can increase during fermentation, thereby making initial assessments of the degree of damage difficult. In fact, monitoring guaiacol concentrations in grapes from smoked Merlot vines from free-run juice through fermentation and up to 12 months post-bottling, demonstrated a progressive increase from 1 µg/L to a peak of 388 µg/L in the finished wine (Kennison et al. 2008). Most recently, Singh et al. (2011) show that guaiacol and 4-methylguaiacol can continue to accumulate in wines from smoke affected vines for at least 3 years during bottle storage. The fact that volatile phenol concentrations can be increased in free-run juice from smoked vines by acid or enzyme (β-glucosidase) hydrolysis supports the notion that volatile phenols accumulate in smoke-affected grapes in a glycosylated form (Kennison et al. 2008).
Application of deuterated guaiacol to vine leaves and berries revealed that guaiacol actually took seven conjugated forms, predominantly diglycosides (Hayasaka et al. 2010a). The conjugates were found in trace amounts in untreated vine material, but all seven species could be detected in grapes and leaves from grapevines exposed to bushfire smoke. Once absorbed by vine leaves or berries, very limited translocation of the conjugates between these treatment sites was evident. In expanding the search for glycosylated derivatives of the key volatile phenols identified in smoke (i.e. guaiacol, phenol, p-, m- and o-cresols, methylguaiacol, syringol and methylsyringol), all such compounds could be identified in treated grapes (Hayasaka et al. 2010b,c). Their extraction into wine was incomplete (78% for Chardonnay, 67% for Cabernet Sauvignon), but up to 92% of those present in wine could be removed through acid hydrolysis. In this case, little of the aglycones were typically detected suggesting their decomposition.
Quantitative determination of glycoconjugates of guaiacol is possible through of an HPLC-MS/MS-based SIDA (stable isotope dilution assay) method (Dungey et al. 2011). Responding to the need to more reliably quantify the diversity of volatile phenol aglycones and glycoconjugated, and hence aid prediction of the degree of contamination of wine with smoke taint, Wilkinson et al. (2011) compared recently published analytical methods. Strong correlations were found between the amounts of guaiacol and 4-methylguaiacol glycoconjugates and the aglycones released through hydrolysis. Using such techniques, grape growers can determine the degree of contamination of grapes with free volatile phenols and the potential for subsequent development of smoke taint through glycoconjugate hydrolysis during winemaking and post-bottling ageing. Wineries can therefore determine whether to reject such fruit. If grapes are accepted, strategies are needed to minimise or eliminate the formation of readily apparent smoke taint.
The final featured papers explore the influence of winemaking techniques on the development of smoke taint (Ristic et al. 2011) and the utility of reverse osmosis with solid phase adsorption as a means of reducing the content of volatile phenols in wines from smoke-affected grapes (Fudge et al. 2011). Given that guaiacol glycoconjugates predominantly accumulate in grape skins (Dungey et al. 2011), winemaking techniques that minimise skin contact can reduce extraction of glycoconjugates and, hence, the potential for taint formation (Ristic et al. 2011). The use of different yeast strains to conduct fermentation produces significant differences in the amount of guaiacol in the finished wines. Choice of yeast also influenced the intensity of smoke and cold ash aromas, smoky flavour and ashy aftertaste. In a similar fashion, tannin treatments could alter a panel's perception of these same attributes. While further clarity is still needed, the study does offer a range of techniques that might be used to reduce volatile phenol content of wines and in turn their perception. Where smoke taint remains objectionable despite steps taken during grape processing and winemaking, reverse osmosis and solid phase adsorption offer promise as a means of removing volatile phenols (Fudge et al. 2011). Thus, the content of undesirable volatile phenols was dramatically reduced by the treatment such that treated wines could be readily distinguished for untreated wines. Small increases in the residual concentrations of guaiacol and 4-methylguaiacol appeared to occur with extended ageing (30 months) of the wines.
Thus, the collection of papers included in this feature spans the problem of smoke taint from the grape to the glass and provides insights into the chemistry and biology at play. Importantly, this knowledge not only provides options for evaluating the severity of smoke damage and its amelioration, but also raises important future research questions.