Wild fires are a relatively frequent occurrence during hot and dry spells, and tend to occur with greater frequency in some parts of the world compared to others.Â Parts of Australia and the western United States, among others, are frequently prone to wild fires or bush fires, both places of which also are home to many vineyards.Â Is there an effect of the smoke from these fires on the grapevines?Â What about the wine that is produced from those grapes?
Studies have shown that postharvest smoke exposure on grape cluster change the chemical composition and sensory characteristics of the wine produced from them, which has become what is known as â€śsmoke taintâ€ť.Â What is relatively unknown, however, is whether or not smoke applied to the grape vines before harvest has any effect on the presence of smoke taint in the wines produced from the exposed vines.
â€śSmokingâ€ť some foods is commonplace around the globe, and often imparts desirable smells and flavors.Â This results from the presence of certain chemical compounds such as phenols, carbonyls, lactones, etc.Â Most important, however, are the volatile compounds guaiacol and 4-methylguaiacol, which are considered to be key smoke components.Â The desired â€śsmokyâ€ť effect in some wines result from the presence of these compounds, which usually come from oak barrel fermentation and/or aging; in concentrations of 100ÎĽg/L for guaiacol, and 20ÎĽg/L for 4-methylguaiacol.Â It has been shown that of all the compounds derived from smoke exposure, guaiacol has the lowest threshold (the value of which is not completely known), thereby making it the compound of greatest importance, and likely the compound which could most easily alter the sensory attributes of the wine produced from smoke-exposed grapes.
The purpose of the study reviewed today was to address the question of whether or not grape vines exposed to smoke alter the aromas of wine produced from those grapes, and whether or not the timing and exposure duration affected the quality of the resulting wine.
Â The vineyard for this experiment was located in Western Australia in the locality of Capel.Â This particular site was chosen due to itsâ€™ infrequent exposure to smoke caused by fire, and itsâ€™ far distance from forested areas.Â Several 6m x 2.5m x 2m greenhouses were constructed for each treatment, in order to enclose the smoke treatment on specific spots in the vineyard.Â The smoke treatment was created by burning dry barley straw in a closed metal drum for 30 minutes.Â Subsequently, an air pump was used to push the resulting smoke into the treatment greenhouses for grapevine smoke exposure.Â The grape vines used for the experiment were Vitis vinifera cv. Merlot.
Two different smoke treatments were applied during this study.Â The first was a single smoke exposure, and the second was a repeated smoke exposure for 8 consecutive exposure events.Â Each exposure was for 30 minutes in duration.Â Timing of exposure was tested by exposing the vines to their respective smoke treatment at 3, 7, 10, 15, 18, 21, or 24 days post-veraison.Â A control treatment was also established, whereby vines were not exposed to any smoke for any length of time.Â After harvest, wine was produced using the same winemaking techniques for all treatment types.
For the sensory analysis of the wines, 8 trained judges (4 male, 4 female) between the ages of 21 and 30 were recruited.Â Judges were selected if they had obtained at least 100hours of tertiary wine sensory education, were regular wine consumers, nonsmokers, of good health, and had the ability to detect smoke aromas in both red and white wines at a minimum threshold level.Â The wines were assessed for aroma only, and not taste.Â The wines were presented to each judge blindly, with the wines only identified by number, and presented in random order.Â Judges were allowed a 10-minute break after each sample tested, in order to avoid potential sensory fatigue.
- Â Â Â Total soluble solids (TSS) from control vine grapes (unsmoked) were higher (22.3 oBrix) than the grapes from vines that were exposed to smoke (19.3oBrix).
- Â Â Â Â Â Vines exposed to smoke had the lowest fruit yield (11kg/vine) compared to nonsmoked controls (15.3kg/vine).
- Â Â Â Repeated exposure to smoke resulted in necrotic lesions on the leaves of the grape vines, compared to no lesions in the single exposure and no exposure treatments.
- Â Â Â Free amino nitrogen (FAN) in the grapes at harvest time was significantly higher in repeated smoke exposure vines (134.4mg/L) than all other treatments (non-smoked control: 87.2mg/L).Â SO2 levels were also highest in the repeated smoke exposure than all other treatments.
- Â Â Â Fermentation rate was fastest for the repeated smoke exposure vines (8 days), compared to the control (non-smoked) vines (12 days).Â All other smoke treatments were also faster than the control (10-12 days).
- Â Â Â Ethanol content was up to 17% lower in wines made from vines with repeated smoke exposure (10.6% v/v) compared to the wines made from non-smoked vines (12.8% v/v).Â Wine made from grapes with single smoke exposure had ethanol concentrations between these two levels (between 10.9 and 12.5 % v/v).
- Â Â Â The highest levels of smoke-derived volatile phenols came from wines made from vines repeatedly exposed to smoke (388ÎĽg/L guaiacol, 93ÎĽg/L 4-methylguaiacol, 16ÎĽg/L 4-ethylguaiacol, and 58ÎĽg/L ethylphenol).Â In control wines, the guaiacol level was 4ÎĽg/L, with all other phenols <1ÎĽg/L (or undetectable).
- Â Â Â For experiments with a single exposure to smoke, the highest volatile phenol levels were found at 7 days post-veraison.
- Â Â Â Wines made from grapes of the repeated smoke exposure vines had the highest level of â€śoff-aromasâ€ť: burnt rubber, smoked meat, leather, and disinfectant/hospital.
- Â Â Â Wines made from grapes from the non-smoked vines had the highest level of â€śconfectionâ€ť and red berry fruit aromas.
- Â Â Â Wines made from grapes with a single smoke exposure exhibited both the â€śoff aromasâ€ť and the more pleasant wine aromas.
- Â Â Â For a Â single exposure of smoke, the wines made from grapes exposed to smoke at 7 or 10 days post-veraison led to the greatest level of â€śoff aromasâ€ť.Â At 18, 21, and 24 days, the more pleasant aromas were more prominent.
What does this all mean?
The primary result of this study was that field exposure of grape vines to smoke results in smoke taint aromas in wine.Â The timing of the exposure is also very important, and repeated smoke exposures result in an additive effect of those compounds that are markers for smoke taint.
Why is the timing of exposure so important?Â This study showed that with a single exposure to smoke on growing vines, the timing of exposure was critical in regards to smoke taint aromas in the resultant wines.Â Specifically, 7 to 10 days after veraison was the most sensitive time and the time at which more smoke taint properties were observed in the wine aroma.Â This timing effect could be due to specific changes in the physiology of the plants at this time, being that peak uptake of volatiles and other components by the berries happens in this window after veraison has commenced.Â Alternatively, it is possible that the leaves themselves are most sensitive during this period, thus allowing them to be more susceptible to smoke exposure and more likely to absorb those volatiles that result in the smoke taint aromas in the wine.
The presence of smoke taint in the wine from the smoke-exposed vines created â€śoff aromaâ€ť characteristics that overpowered the more pleasant fruit tones of non-smoke exposed wines.Â The judges were able to detect all four common volatiles, guaiacol, 4-methylguaiacol, 4-ethylguaiacol, and 4-ethylphenol, even at levels once thought to be below threshold, thus even a single smoke event at the right timing can have detrimental effects on the quality of the wine produced.
The increases in FAN that was reported in this study could be due to the necrotic/damaging effects that the smoke exposure had on the grape leaves themselves.Â It is likely a biochemical response to the damage, meaning when exposed to damaging levels of smoke, the plant released FAN compounds to the site of origin in order to help heal the damaged areas.
The study also showed that ripening was inhibited by the smoke exposure (regardless of when and how much was exposed), which would likely diminish the aromas of ripe fruit and berry, while allowing the aromas of smoke taint to overpower the nose even more.Â Also, the increased SO2 levels in the plants exposed to smoke induced stomatal closure of the leaves (aside:Â stomates are where the plants â€śbreatheâ€ť and exchange water/other gases), which could further inhibit grape development and ripening.Â Stomatal closure plus the physical damage of the leaf caused by smoke exposure would result in even more physiological damage and even less ripening of the grape itself.
Application of smoke to grape vines between veraison and harvest affects yield, grape composition (sugar and FAN), wine composition, wine sensory properties, and last but certainly not least, wine quality.Â For a single smoke exposure event lasting thirty minutes, the most damage was done when this exposure was 7 to 10 days after veraison.Â For multiple smoke exposure events, the damage was additive and cumulative, completely degrading the quality of the grapes and resulting wine.
In effect, while smoking certain food items may have positive, desired effects on the resulting food, smoking grape vines has the opposite effect on the resulting wine produced.Â Even though there are sometimes hints of smoky character in some red wines, even a one-time, relatively short, smoke event creates those same phenolic compounds in too high of levels that are unpalatable as a quality wine for consumption. Â
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