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Effect of Pre- and Postveraison Smoke Exposure on Guaiacol and 4-Methylguaiacol Concentration in Mature Grapes

^{1, 2} Research assistant and ³ Associate professor, Chemistry, Earth & Environmental Sciences, Irving K. Barber School of Arts & Sciences, University of British Columbia, Okanagan, 3333 University Way, Kelowna, BC, V1V 1V7 Canada.

¹ Present addresses: Red Rooster Winery, 891 Naramata Road, Penticton, BC, Canada;

² Calona Wines, 1125 Richter St., Kelowna, BC, Canada.

Acknowledgments: This work was supported by the British Columbia Wine Grape Council, the Investment Agriculture Foundation of British Columbia, and the Western Diversification Program. Vines provided by Mission Hill Winery.

The authors thank James Hopper for viticultural support.

^* Corresponding author (email: nigel.eggers{at}ubc.ca)

	Abstract

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Chardonnay, Merlot, and Pinot gris grapes were treated with smoke generated by the combustion of Ponderosa pine at preveraison, postveraison, and maturity. Guaiacol and 4-methylguaiaol concentrations were determined in the mature harvested grapes using a stable isotope dilution assay. Both guaiacol and 4-methylguaiacol were sorbed by the grapes during the smoke treatments and remained until the grapes were harvested. There was a general trend for increasing sorption of guaiacol and 4-methylguaiacol as grapes matured. A positive linear correlation was observed between the guaiacol:4-methylguaiacol ratio and guaiacol concentration for all smoke-treated grape samples that had concentrations above detection limits. Guaiacol concentrations ranged from 2 to 26 µg/L. These grapes could yield a wine where the concentrations exceed the detection threshold of guaiacol and the concentrations were of the same order as that resulting from contact with oak. An hour of smoke exposure would have an impact on the sensory characteristics of the resulting wines.

Key words: winegrapes, smoke taint, guaiacol, 4-methylguaiacol, smoke exposure

The occurrence of large wildfires is increasing in Western Canada and the United States (Westerling et al. 2006) and smoke taint of grapes and wines will likely become a serious problem. Combustion of wood produces source-specific organic compounds arising from pyrolysis of lignin, including substantial amounts of methoxyphenols (Fine et al. 2002, 2004). 4-Substituted methoxylated phenolic compounds and levoglucosan (a sugar anhydride) have been suggested as potential molecular markers for atmospheric particulate matter of the hundreds of organic compounds present in wood smoke. Methoxyphenols are abundant in wood smoke, their presence in atmospheric particulate matter is unique to biomass combustion, and they are relatively stable tracers (Simpson et al. 2005). The methoxyphenols guaiacol and 4-MG are also the major contributors to the aroma of wood smoke (Maga 1992), with smoky and burnt aroma, respectively (Boidron et al. 1988).

Toasted oak barrels also contain products resulting from the pyrolysis of lignin. Wine matured in oak typically contains between 10 and 100 µg/L guaiacol and between 1 and 20 µg/L 4-MG, although higher values have been determined (Pollnitz et al. 2004). A range of aroma thresholds for guaiacol has been reported, from 9.5 µg/L in a young red wine (Ferreira et al. 2000) to 75 µg/L in red and white wines (Boidron et al. 1988), while 4-MG has a reported threshold of 65 µg/L in red and white wines (Boidron et al. 1988). The AWRI reports that the sensory difference threshold for guaiacol in white juice was established as 6 µg/L or less, while some of the red wines affected by the bushfire smoke contained >70 µg/L (Høj et al. 2003). A recent article reported the aroma detection threshold in water to be 0.48 µg/L guaiacol (Eisele and Semon 2005).

Guaiacol, 4-MG, and vanillin were the only oak-derived volatile phenols found in wine at concentrations above their individual aroma thresholds (Chatonnet et al. 1992). Guaiacol has a negative effect on wine aroma above 80 µg/L (Rapp and Versini 1996), and it has been positively correlated with "smoky" character in a Chardonnay wine (Spillman et al. 1998).

Postharvest smoke exposure of grapes influences the chemical composition and sensory characteristics of wine and causes an apparent smoke taint. Sensory studies have established a perceivable difference between smoked and unsmoked wines, with smoked wines described as exhibiting smoky, dirty, earthy, burnt, and smoked meat characters. 4-Methylguaiacol, 4-ethylphenol, eugenol, and furfural were identified in wines made from smoked grapes, but not in wines made from unsmoked grapes. Aroma thresholds for this smoke taint corresponded to dilutions of 1.6% for smoked free-run wine and 0.8% for smoked free-run juice fermented on skins wine (Kennison et al. 2008).

The spread of viticulture into regions prone to brush fire and the increase in forest fires in many of the world’s grapegrowing regions has led to the need for a greater understanding of how smoke exposure impacts viticulture. Information on how smoke contamination of grapes at various stages of berry development can affect the final amount of guaiacol and 4-MG in harvested grapes is anecdotal; one anecdote is that smoke contamination is carried over from season to season. The wildfire season in the Okanagan region occurs during July and August, while most grape varieties are harvested during September and October. Chardonnay, Pinot gris, and Merlot are major varieties grown in the Okanagan region. Our goal was to ascertain whether smoke contamination of grapes during the wildfire season produced elevated guaiacol and 4-MG in grapes at harvest.

	Materials and Methods

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 Materials   All solvents were reagent grade and obtained from Caledon Laboratories (Georgetown, ON, Canada). All chemicals were purchased from Sigma-Aldrich (St. Louis, MO) except when otherwise specified. 2-Methoxy-d₃-phenol (d₃-G) was purchased from CDN isotopes (Pointe-Claire, QB, Canada). Purity of all standards was verified by GC-MS before preparation of stock solutions.

 Synthesis of d₃-2-methoxy-4-methylphenol (d₃-4-MG)   Sodium (0.23 g, 10 mmol) was cut into small pieces and placed in a dish containing mineral oil to avoid exposure to air. The sodium was transferred into a 25-mL Shlenk tube that had been flushed with argon gas and the sodium was then washed twice with pentane. Deuterated methanol (1.6 mL, 40 mmol) was added to the tube via a syringe. 2-Bromo-4-methylphenol (0.46 g, 2.5 mmol) and dry dimethylformamide (5 mL) were combined and added to a three-neck round-bottom flask containing copper (I) chloride (10 mg, 0.10 mmol) under an argon atmosphere. The deuterated sodium methoxide from the tube was then transferred to the flask using a canula and the resulting solution was refluxed at 130°C for 3 hr or until completion (followed by GC-MS). The dimethylformamide was then allowed to evaporate off overnight and the product was subsequently acidified with concentrated hydrochloric acid to pH <4.5 as indicated by litmus paper. The solution was extracted with ether and dried over magnesium sulfate. The ether was distilled off and the product passed through a of 3.7-g silica column. The product was washed through the column with 25 mL dichloromethane, which was removed by distillation, leaving d₃-4-MG (0.23 g, 66% yield, 100% pure, and coeluted with 4-MG by GC-MS). Mass spectrum: m/z 141 (M⁺, 100%), 123 (34%), 95 (9%), 67 (23%).

 Grape samples   Vines of Vitis vinifera L. cv. Chardonnay, Pinot gris, and Merlot located at Mission Hill Family Estate Vineyard, Oliver, British Columbia in the Okanagan wine region were used as test subjects.

 Experimental protocol   Vines were exposed to smoke at three different stages of growth, corresponding to preveraison, postveraison, and maturity. As the maturation of the grape depends on variety, the dates on which the vines were smoked varied (Table 1), and replicate smoke treatments were performed within 48 hr of each other. Two vines of each variety, located four rows apart, were each treated with smoke at each stage of growth. Control vines were two rows from smoked vines and at least nine vines apart. Smoke treatment began at dawn. The temperature was monitored inside the smoke treatment apparatus and was maintained between 20 and 30°C.

 Smoke exposure   Smoke was generated by burning 500 g of homogeneous chipped Ponderosa pine in a modified barbeque. To ensure that the smoke temperature did not exceed 30°C, the smoke was passed through a condenser composed of galvanized steel ducting (3 m length, 7.6 cm diam) containing coiled copper tubing (13 m length, 0.6 cm diam) through which ice water was pumped. The smoke entered a box (0.76 m depth, 1.22 m width, 1.83 m height) enclosing a vine; the box was framed with pine wood and sealed with vapor-barrier polyethylene. The plastic was attached to the wood frame using multipurpose glue and a staple gun. To seal the box around the vine support wires, the 0.76-m side was split down the center and the box was positioned over the vine. The open sides were sealed with Velcro.

During the wildfire season in the Okanagan region, daytime temperatures range from 30 to 35°C, and can approach 38°C. The clear plastic smoke chamber behaves as a greenhouse and temperatures inside reach 60°C during the hottest time of day, which is sufficient to kill the vines. Consequently, all smoke treatments started at dawn, the coolest part of day, when temperatures were ~20°C. Vines were enclosed in the apparatus for one hour, although the smoke usually dissipated before that time. Smoke dissipation appeared to depend on the intensity of the fire and the environmental conditions.

 Analytical methods   Three grapes were independently analyzed from each collected cluster, giving a total of 12 grapes analyzed per smoked vine. Each grape was weighed and transferred to a 5-mL test tube. The grape was crushed using a glass rod and 20 µL internal standard (1.21 mg/L d₃-G and 1.31 mg/L d₃-4-MG) was added. Diethyl ether (2 mL) was added to the test tube and the test tube was shaken for 20 sec. A cork was added to the test tube and the sample was stored overnight (16–24 hr) at 4°C. A 1-mL portion of the organic phase was transferred to a 2-mL vial, capped, and analyzed using the GC-MS method.

Calibration standards   Solutions containing 200, 100, 50, 25, 12.5, 6.25, and 3.125 µg/L guaiacol (G) and 4-MG were prepared in ethanol. The calibration standards were prepared by transferring 1 mL of the G/4-MG solution to a 2-mL vial and adding 20 µL of the mixed 1.21 mg/L d₃-G and 1.31 mg/L d₃-4-MG internal standard solution.

Instrumental analyses   Samples were analyzed using a Thermo Electron Trace GC coupled to a DSQ quadrupole MS operating in the positive ion electron impact ionization mode at 70 eV (Thermo Electron, Waltham, MA). The GC was fitted with a 30 m x 0.25 mm fused capillary column DB-1701 Agilent/J&W (Santa Clara, CA), with 0.25-µm film thickness. The carrier gas was ultra-high-purity helium at a constant flow rate of 1.2 mL/min without vacuum compensation. A 2-µL liquid sample was injected using a Thermo Electron AI 3000 autosampler. The GC injector was operated in the split/splitless mode, with a splitless time of 1.00 min followed by a split flow of 50 mL/min. The oven temperature began at 40°C, held for 1 min, then increased to 260°C at 8°C/min, and held for 1 min. The GC injector temperature was 220°C. Under this temperature program, the analyte retention times (RT) were: d₃-G, RT = 12.72 min; G, RT = 12.75 min; d₃-4-MG, RT = 14.55 min; 4-MG, RT = 14.58 min.

The MS ion source temperature was 200°C and the GC-MS transfer line temperature was 250°C. MS scans were obtained in the selected ion monitoring (SIM) mode and the ions monitored in SIM runs were m/z 81, 95, 109, 123, 124, 127, 138, and 141. The qualifying ions (ions used to confirm identity) were m/z 81 and 109 for guaiacol and m/z 95 and 123 for 4-MG.

 Data analysis   For statistical analysis, concentrations that fell below the method detection limits (MDL; 0.2 µg/kg for guaiacol and 1.5 µg/kg for 4-MG) were set at the MDL. A one-way analysis of variance (ANOVA) was tested on each group first. The Tukey test with pair-wise comparisons was used to determine if the difference between groups was significant for those groups with ANOVA p values < 0.05. Tukey pair-wise comparisons were also performed, treating concentrations below the method detection limits as zero instead of the MDL, and the same results were obtained.

	Results

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 d₃-4-Methylguaiacol synthesis   d₃-4-MG was synthesized by a modification of a previous method (Dills et al. 2006). A minimum of three deuterium are necessary as the natural isotopic abundance will give rise to M + 1 (14% M) and M + 2 (1% M) peaks in the mass spectrum, and these two peaks would interfere with the internal standard. Thus these three deuterium were placed on the methoxyl group as these deuterium are unlikely to exchange. Comparison of the mass spectrum of d₃-4-MG with 4-MG showed that only the molecular ion was 3 mass units higher (141 and 138 m/z). The ratios of the ions m/z 139, 140, 142 to the molecular ion m/z 141 in d₃-4-MG and the ions m/z 136, 137, 139 to the molecular ion m/z 138 in 4-MG were the same, indicating that d₃-4-MG is isotopically pure.

 Guaiacol and 4-MG concentrations   Concentrations of guaiacol and 4-MG for each smoke treatment and control grape samples with all varieties combined are shown (Table 2). Both guaiacol and 4-MG were sorbed by the grape from the smoke treatments and remained until the grapes were harvested. There was a general trend for increasing sorption of guaiacol and 4-MG as the grapes matured. Mean and standard error for each variety and smoke treatment were calculated (Table 3), as were Tukey pair-wise comparisons for guaiacol and 4-MG concentrations between smoke treatments (Table 4), and Tukey pair-wise comparisons between varieties (Table 5).

 Skin thickness   The mean of the guaiacol concentrations measured in the grapes treated with smoke at preveraison, postveraison, and maturity was calculated for each variety, giving average concentrations independent of the stage of growth at which the grapes were smoke treated. The mean concentration of guaiacol was 3.1 µg/kg in Merlot, 3.7 µg/kg in Pinot gris, and 9.8 µg/kg in Chardonnay. The thickness of the grape skins of each variety was determined as the average of 24 grapes (not treated with smoke) from four clusters, with 0.202 mm for Chardonnay, 0.219 mm for Pinot gris, and 0.272 mm for Merlot. There was a trend between skin thickness and guaiacol concentration; the thicker the skin, the less guaiacol absorbed.

	Discussion

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The AWRI has reported that although few vineyards were actually damaged by the 2003 fires in Australia, smoke taint in wines was widespread. The Institute advised that smoke resulting from controlled burning of bush might result in smoke-tainted grapes in nearby vineyards (Høj et al. 2003). Winemakers and grapegrowers in the Okanagan region observed that smoke taint in wines appears to occur when smoke from fires many miles away settles around the vines.

In 2006, Okanogan County in Washington State, which borders Canada and is ~50 km from the Similkameen Valley vineyards, had massive forest fires that burned from late July to late September. The Similkameen Valley runs north to south, with its southern end almost reaching the U.S. border and Okanogan County. It is conceivable that throughout the summer into fall, Similkameen grapes were subject to increased atmospheric smoke. Wine made from Similkameen fruit (5.5 ± 0.7 µg/L) had significantly different concentrations of guaiacol than wines made from Okanagan fruit (3.4 ± 0.4 µg/L, p < 0.05) (Pearson 2008).

The pyrolysis of lignin caused by wildfires generates sizeable amounts of guaiacol. In a study characterizing fine particulate emissions from the combustion of different wood species, results showed that Ponderosa pine produced 0.22 mg guaiacol per gram of organic carbon released (Fine et al. 2004). Thus the 500 g of Ponderosa pine combusted in these experiments would have generated ~10,000 µg guaiacol, yet only very low concentrations were measured in grapes.

Once smoke dissipates from a fire source, the temperature decrease would result in the condensation of most of the guaiacol, as the vapor pressure of guaiacol is 0.015 kPa at 25°C (www.sigmaaldrich.com). The guaiacol formed by a wildfire could condense on the carbon particles of smoke. When these carbon particles settle on grapes, the guaiacol may transfer to the grape. It appears that guaiacol is then absorbed into the skin of the grape (Høj et al. 2003).

In our study, the smoke was contained in the box for less than one hour. Anecdotal evidence from Okanagan grapegrowers suggests that smoke taint arose from vines that were immersed in smoke for at least 8 hr. Thus, it is not surprising that the levels of guaiacol and 4-MG found in this study were low.

Both guaiacol and 4-MG were sorbed by the grape from the smoke treatments (Table 2), and concentrations were higher in the more mature grapes. Because grapes on average double in size between veraison and harvest, any guaiacol or 4-MG that was sorbed by the grape before veraison would be diluted by the harvest date (Kennedy 2002). The small berries before veraison would have a smaller surface area and consequently would sorb less guaiacol and 4-MG from the smoke. As these berries grow in size, the amount sorbed would be diluted. The difference in guaiacol and 4-MG concentrations between the control grapes and those treated with smoke at maturity was significant (Table 4). There were significant differences in concentrations for varieties at different treatment times; guaiacol concentration in Pinot gris was significantly different between all pairs of treatments except maturity/control and maturity/preveraison, while in Chardonnay it was significantly different between maturity/control, maturity/preveraison, and maturity/postveraison. Some comparisons were not significant, particularly for Merlot; a longer smoke exposure may have shown a significant difference.

There were significant differences in guaiacol concentrations between Chardonnay/Merlot and Chardonnay/Pinot gris when all smoke treatments were combined (Table 5). The postveraison smoke treatment produced significant differences for all three combinations; Chardonnay/Merlot, Chardonnay/Pinot gris, and Merlot/Pinot gris.

Control grapes also had low levels of guaiacol and 4-MG, although we could not determine if these levels came from guaiacol in environmental particulate matter. Previous studies have demonstrated that guaiacol is found as a glycoside in Tempranillo and Grenache berries (Lopez et al. 2004), Shiraz berries (Wirth et al. 2001), and Merlot juice (Sefton 1998).

Previous work at the AWRI demonstrated that washing of smoke-tainted grapes with water did not lower the concentrations of guaiacol or 4-MG, and consequently the authors attempted to determine the location of guaiacol in smoke-tainted grapes. Their experiments indicated that guaiacol and 4-MG were not located in the wax-bloom on the surface of the grapes or in the pulp beneath the skin, but were located in the skin (Høj et al. 2003). Our results indicate that the thinner-skinned grape varieties had higher concentrations.

The correlation between the ratio of guaiacol to 4-MG and the guaiacol concentration (G:4-MG concn ratio = 0.297 x G concn + 0.198; R² = 0.89) may result from the temperature of the Pondorosa pine combustion, or it could be a differential absorption effect of guaiacol and 4-MG into grape skin. The ratio resulting from the combustion of Ponderosa pine has been reported as 1.85 (Edye and Richards 1991) and from oak (Quercus sp.) 1.8 and 1.6 (Guillén and Manzanos 2002, 2005). The ratio reported in wine after 12 months in French oak was ~10 (Moreno and Azpilicueta 2007). Treatment of Verdelho grapes with smoke from the combustion of dry straw yielded a ratio of 4.5 in wine made from free-run juice and 3.9 for wine made from free-run juice fermented on skins (Kennison et al. 2008). The average ratio for all samples analyzed with guaiacol and 4-MG concentrations above the detection limit was 2.2; as this ratio is close to that reported for the combustion of Ponderosa pine and oak, the ratio of G:4-MG is probably determined by the temperature of combustion.

Guaiacol in wine is considered to arise from oak treatments, and the concentrations are similar to our results. Chardonnay wines macerated with American oak chips had 13 to 20 µg/L guaiacol (Guchu et al. 2006). Guaiacol concentration in 12 red single variety wines after different oak treatments ranged from 2 to 10 µg/L (Ortega-Heras et al. 2007), and in 52 monovarietal red wines it ranged from 1 to 11 µg/L. In a study carried out in this laboratory, guaiacol concentration in 44 oak-barreled red Merlot wines had a mean of 3.8 ± 0.3 µg/L (± SE) and ranged from 2 to 10 µg/L (Pearson 2008).

A recently reported aroma detection threshold of guaiacol in young red wine is 9.5 µg/L (Ferreira et al. 2000). Less information is available in the literature on the detection threshold of 4-MG; one reported threshold is 65 µg/L in red and white wine (Boidron et al. 1988).

In this study, where grapes were exposed to smoke for less than 1 hr, guaiacol concentration ranged from 2 to 26 µg/L. These grapes could yield a wine where the concentrations exceed the detection threshold of guaiacol and are of the same order as that resulting from contact with oak. The vines were treated with smoke at dawn, during a period of low photosynthetic activity; the potential for assimilation and translocation of smoke throughout the vine and into fruit is much greater during active functioning. Measurable concentrations of guaiacol and 4-MG were obtained and this study provides a conservative minimum exposure time for grapes. One hour of smoke exposure would be expected to have an impact on the sensory characteristics of the resulting wines.

	Conclusion

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These results show that one hour of smoke contamination of grapes at various stages of grape development can lead to concentrations of sensory characteristics and 4-methylguaiacol in grapes harvested at maturity that approach the aroma threshold of these compounds in the resulting wines.

Manuscript submitted June 2008; revised October 2008

Accepted for publication November 2008

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