- Top of page
- Materials and methods
Background and Aims: Grapevine smoke exposure has been reported to produce smoke aromas in wine, resulting in ‘smoke taint’. This study describes the application of smoke to field-grown grapevines between veraison and harvest to investigate the effect of timing and duration of smoke exposure on wine composition and sensory attributes.
Methods and Results: Smoke was applied to grapevines as either a single smoke exposure to different vines at veraison or at 3, 7, 10, 15, 18 or 21 days post-veraison or repeated smoke exposures to the same vines at veraison and then at 3, 7, 10, 15, 18 and 21 days post-veraison. Gas chromatography-mass spectrometry analysis of guaiacol, 4-methylguaiacol, 4-ethylguaiacol and 4-ethylphenol showed elevated levels in all wines produced from fruit from smoked grapevines. Repeated smoke exposures had a cumulative effect on the concentration of these compounds. A trained sensory panel identified the aromas of ‘burnt rubber’, ‘smoked meat’, ‘leather’ and ‘disinfectant’ in all wines derived from smoke-exposed grapevines but not in control wines.
Conclusions: Smoke application to field-grown grapevines between veraison and harvest can influence the accumulation of volatile phenols and intensity of smoke aromas in resultant wines. A peak period of vine sensitivity to smoke at 7 days post-veraison is identified. Repeated smoke exposures have a cumulative effect.
Significance of the Study: This is the first study to report the deliberate and controlled smoke application to field-grown grapevines demonstrating the timing and duration of smoke exposure to significantly affect wine chemical and sensory characters.
- Top of page
- Materials and methods
Postharvest smoke exposure of grapes has been shown to influence the chemical composition and sensory characteristics of wine with the potential to cause an apparent ‘smoke taint’ (Kennison et al. 2007). However, to date, smoke has not been deliberately applied to grapevines in a field situation to determine the impact on grape and wine composition under controlled conditions. Furthermore, the effect of timing and duration of grapevine smoke exposure on the development of smoke taint in wine has not been previously investigated. As such, scientific literature relating to the in-field exposure of grapevines to smoke in the development of smoke taint is scant despite the issue's high relevance to viticulture in Australia and overseas.
Smoke is a highly complex substance, comprising particulate matter, carbon monoxide, carbon dioxide, polycyclic aromatic hydrocarbons, ozone (O3), various oxides of nitrogen and sulfur as well as a multitude of volatile and semi-volatile organic compounds (McKenzie et al. 1994, Nolte et al. 2001, Radojevic 2003, Reisen and Brown 2006). The composition of smoke can vary greatly depending on both the fuel source and pyrolytic conditions, in particular combustion temperature, oxygen availability, and moisture content (Baltes et al. 1981, Maga 1988, Simoneit et al. 1993).
Smoke can impart desirable organoleptic properties to foods. These are largely attributed to the presence of smoke-derived volatile compounds including phenols, carbonyls, acids, esters, lactones, pyrazines, furan and pyran derivatives (Maga 1988, Wittkowski et al. 1990, McKenzie et al. 1994, Guillén et al. 1995, Guillén and Ibargoitia 1998, Fine et al. 2001). Of these volatiles, guaiacol and 4-methylguaiacol are considered to be key smoke components (Baltes et al. 1981, Wittkowski et al. 1992). They are derived from the thermal degradation of wood lignin during combustion and exhibit ‘smoky’, ‘musty’, ‘caramel’, ‘burning’, ‘sweet’, ‘phenolic’, ‘sharp’, and ‘smoked sausage’ aromas and flavours (Baltes et al. 1981, Boidron et al. 1988, Wittkowski et al. 1992, Rocha et al. 2004).
In wine, guaiacol and 4-methylguaiacol typically originate from oak barrel fermentation and/or maturation (Boidron et al. 1988, Maga 1989, Swan 2004) at concentrations of up to 100 and 20 µg/L for guaiacol and 4-methylguaiacol, respectively (Pollnitz et al. 2004). However, a range of volatile phenols, including guaiacol, 4-methylguaiacol, 4-ethylguaiacol and 4-ethylphenol, have recently been identified in juice, unwooded wine, acid and enzyme hydrolysates prepared from smoke-affected grapes (Kennison et al. 2007, 2008). Because these compounds were absent from the corresponding control samples (i.e., unsmoked grapevines), their origin was attributed directly to smoke exposure.
The aroma descriptors, aroma detection thresholds and wine concentrations reported for guaiacol, 4-methylguaiacol, 4-ethylguaiacol, and 4-ethylphenol are shown in Table 1. Because guaiacol exhibits the lowest aroma detection threshold (Boidron et al. 1988) and was the most abundant volatile phenol detected in smoke-tainted wines (Kennison et al. 2007, 2008), it is considered to be of greatest importance. Boidron et al. (1988) reported aroma thresholds for guaiacol in various media – 5.5 µg/L in water, 20 µg/L in model wine, 95 µg/L in white wine and 75 µg/L in red wine – whereas Simpson et al. (1986) reported a lower detection threshold, just 20 µg/L, for guaiacol in a dry white table wine. However, the detection thresholds for guaiacol and 4-methylguaiacol may in fact be even lower than these earlier data. Indeed, Eisele and Semon (2005) suggest that guaiacol is by far more potent. They determined the best estimate threshold for guaiacol to be 0.48 µg/L in water and 0.91 µg/L in apple juice, with even lower taste detection thresholds reported at 0.17 µg/L and 0.24 µg/L in water and apple juice, respectively. In a previous study involving the postharvest application of smoke to grape bunches, a perceptible ‘smoke taint’ was still evident after considerable blending to achieve sub-threshold concentrations of guaiacol and 4-methylguaiacol (Kennison et al. 2007). This suggests that guaiacol and 4-methylguaiacol might not be solely responsible for smoke taint in wine (Kennison et al. 2007).
Table 1. Aroma descriptors, aroma detection thresholds and wine concentrations reported for guaiacol, 4-methylguaiacol, 4-ethylguaiacol and 4-ethylphenol.
|Compound||Aroma descriptors||Aroma detection threshold (µg/L)||Wine concentration (µg/L)|
|Water||Model wine||White wine||Red wine|
|Guaiacol||Smoky†, phenolic§, chemical§||0.48§§ 5.5†||20†||95† 20‡||75†||0–100¶|
|4-ethylguaiacol||Smoky†, spicy†, toasted, bread§||25†||47†||70†||150† 110‡‡||2–437¶|
|4-ethylphenol||Horsy†, stable†, phenolic§||130†||440†||1100†||1200† 605‡‡||2–2200¶|
The effects of grapevine smoke exposure on the composition and sensory properties of wine are currently not well understood. This study was therefore undertaken to address this knowledge gap, and investigates the effect of both timing and duration of grapevine smoke exposure on wine quality.
- Top of page
- Materials and methods
Previous research has shown that the postharvest smoke exposure of grapes affects the chemical composition and aroma of wine, leading to the development of perceivable smoke taint aromas (Kennison et al. 2007). The current study demonstrates that field exposure of grapevines to smoke can lead to the development of smoke taint in wine, the timing of smoke exposure to grapevines can influence the chemical and sensory properties of resultant wine, and repeated smoke exposure has a cumulative effect on the concentration of smoke-derived volatile compounds in resultant wines.
The concentration of smoke taint indicator compounds (guaiacol, 4-methylguaiacol, 4-ethyphenol and 4-ethylguaiacol) measured in wine varied depending on the timing and number of smoke exposures that the vines received. Levels of these compounds in wines from grapevines subjected to a single smoke exposure at the onset of veraison were low, with guaiacol levels comparable to those found in control treatments (6.7 µg/L). However, smoke exposure at 7 days post-veraison resulted in higher levels of guaiacol (92 µg/L). Later smoke exposures resulted in significant levels of taint, but the concentrations of the indicator compounds were always less (63–93%) than wine corresponding to smoke exposure at 7 days post-veraison. The reasons for variation in sensitivity to smoke exposure during the post-veraison period are currently unclear. During veraison, changes occur in assimilate partitioning of sugar uptake and metabolism (Conde et al. 2007) and in phloem unloading from symplastic to apoplastic pathways (Zhang et al. 2006). The chemical and structural characteristics of grape cell walls also change during this period (Nunan et al. 1998, Mullins et al. 2000). The peak uptake of volatile smoke components by fruit following smoke exposure at 7 days post-veraison may be related to these changes in berry physiology. An alternative hypothesis is that the peak period of uptake identified in this experiment may be independent of ontogeny and could also relate to changes in sensitivity to uptake of the compounds at the leaf level associated with short-term environmental effects on vine physiology such as vine water status. These hypotheses are the subject of an ongoing study.
Repeated smoke exposures to field-grown grapevines led to accumulation of smoke compounds to high levels. Irrespective of when smoke exposures were applied to vines, from the period of veraison to harvest, the effects on the levels of guaiacol, 4-methylguaiacol, 4-ethylguaiacol and 4-ethylphenol in wines were additive. Indeed, the sum of smoke taint-related compounds detected in wines generated from single smoke exposure treatments closely approximates the compound levels detected in wines generated from repeated smoke exposures (Figure 5). These results imply that repeated or prolonged vineyard smoke exposure, which can occur from frequent fire events, over the post-veraison period will potentially have a cumulative negative effect on resultant wine quality and value.
Figure 5. Sum of guaiacol, 4-methylguaiacol (4-MeG), 4-ethylguaiacol (4-EG) and 4-ethylphenol (4-EP) concentration in eight wines made from Merlot grapes from vines that each received a single field-based smoke exposure (applied at veraison or at 3, 7, 10, 15, 18, 21 or 24 days post-veraison as indicated by bands) versus compound levels detected in wine from Merlot grapes from vines that received repeated smoke exposures (applied at veraison then again at 3, 7, 10, 15, 18, 21 and 24 days post-veraison).
Download figure to PowerPoint
Quantitative descriptive wine aroma analysis demonstrated an increase in smoke-related aromas described as ‘burnt rubber’, ‘smoked meat’, ‘leather’, and ‘disinfectant and hospital’ in wines from the repeated smoke exposure experiment. These aromas clearly dominated the wine's sensory profile, overpowering any ‘confection’ and ‘red berry fruits’ aromas (Figure 3). Smoked wine from the repeated smoke exposure experiment contained guaiacol and 4-methylguaiacol at levels well in excess of the highest published aroma detection thresholds for red wine (of 75 and 65 µg/L, respectively), although 4-ethylguaiacol and 4-ethylphenol were present at sub-threshold concentrations (Boidron et al. 1988). In contrast, the volatile phenols were either not detected or detected at trace levels only in control wines. Therefore, as in previous studies, the source of volatile phenols can be attributed directly to smoke (Kennison et al. 2007, 2008). Furthermore, the accentuation of smoke taint in wine derived from the repeatedly smoked grapevines is correlated with the increased levels of guaiacol and 4-methylguaiacol, with little or no contribution from 4-ethylguaiacol or 4-ethylphenol.
Smoke-like aromas were also present to various degrees in all wines vinified from the single smoke exposure experiment regardless of smoke application timing and resultant compound level in wine. The levels of guaiacol, 4-methylguaiacol, 4-ethylguaiacol and 4-ethylphenol were detected by the panellists in all these wines below reported aroma thresholds for red wine (Boidron et al. 1988, Chatonnet et al. 1992) except for smoke application at 7 days post-veraison that produced wine with guaiacol above the reported aforementioned aroma detection threshold in red wine of 75 µg/L. Panellists detected elevated smoke aromas in the 7 days post-veraison wines, although smoke aromas were also detected in all other wines made from fruit of vines exposed to single smoke applications – even though their compound levels were below published aroma thresholds. It should be noted that there is some conjecture regarding the detection threshold for guaiacol; certainly there is disagreement between published thresholds, i.e., 75 µg/L reported by Boidron et al. (1988) and 20 µg/L reported by Simpson et al. (1986). A recent workshop demonstrated that all 60 delegates presented with 70 unidentified wines including a control and a wine spiked with guaiacol (20 µg/L) were able to discern the wines, giving descriptors for the latter such as ‘smoky’ and ‘burnt bacon’ (Mark Sefton, pers. comm., 2007). Given the intricate and complex nature of smoke, indicator compounds play an important role in assessing the extent and impact of smoke taint on wine quality. In the present study, the volatile phenols, together with sensory analysis, have been proven as effective smoke taint markers. However, it is acknowledged that with time, additional volatile compounds will likely be identified as components of smoke, which are also responsible for the discernable aroma attributes of smoke-tainted wine.
Grapevine smoke exposure leads to increased levels of FAN in grapes, an effect most evident following repeated smoke applications. Interestingly, grapes harvested from repeatedly smoked grapevines also fermented the most rapidly, in agreement with previous studies (Kennison et al. 2007). While the increase in ferment rate may be associated with the increased FAN in must (Henschke and Jiranek 1993, Bell and Henschke 2005), the basis for this increase is unclear. Some direct contributions from nitrogenous smoke compounds is possible as research has demonstrated the uptake and assimilation, by nitrite reductase, of nitrogenous compounds (NO and NO2) by plants (Hosker and Lindberg 1982, Nussbaum et al. 1993, Stulen et al. 1998, Takahashi et al. 2001); however, it is likely that any such contribution would be very minor based on simple mass balance. Increased FAN may also be linked to the injury response of grapes following high levels of smoke exposure, for example, a biochemical response to necrotic lesion that developed on laminae following repeated smoke treatment (Heath 1980).
Field-based smoke exposure to grapevines showed an adverse effect on grape ripening (e.g., sugar accumulation) irrespective of the timing and duration of smoke application. In a recent study, the stomatal conductance, CO2 assimilation rate and intercellular CO2 levels of Chrysanthemoides monilifera were reduced for 5 h following smoke exposure for 1 min, with 24 h required to achieve physiological recovery to control levels (Gilbert and Ripley 2002). Additionally, the presence of SO2 and O3 in smoke has been shown to induce stomatal closure in grapevines (Rosen et al. 1978). It is therefore conceivable that the photosynthetic capacity of grapevines decreases following smoke exposure, which in turn inhibits grape maturation and ripening. Furthermore, these physiological effects would be further exacerbated by the loss of photosynthetically active leaf area because of the formation of necrotic lesions on laminae (Heath 1980), as occurred in the current study with grapevines subjected to repeated smoke treatments.
In summary, the deliberate application of smoke to field-grown grapevines between veraison and harvest affected yield, grape composition (sugar accumulation and FAN), wine composition, wine sensory properties, and most importantly, wine quality. The volatile phenol levels and intensity of smoke taint in wine was influenced by both the timing and the duration of grapevineexposure to smoke. For single smoke treatments, the highest levels of volatile phenols were observed in wines corresponding to smoke exposure 7 days post-veraison. For repeated smoke treatments, a cumulative effect on smoke-derived volatile phenol concentrations was observed.