Introduction
As Australia is facing a warming climate with increasing bushfire incidences the issue of smoke derived taint in grapes and wine has become a more regular occurrence. Losses from smoke taint vary from year to year due to the unpredictable nature of bushfire events and significant financial losses have occurred in Western Australia, Victoria and New South Wales with isolated incidences in other states. Where significant smoke exposure occurs during sensitive periods of vine development the resultant wine is unfit for purpose. Smoke taint has also occurred in wine-grapes grown in Southern Europe and the USA.
Wines made from grapes exposed to smoke exhibit ‘burnt’, ‘smoked meat’, ‘leather’, ‘disinfectant’, ‘charred’, ‘ashtray’ and ‘salami’ aromas. Smoke tainted wines are usually unpalatable and unfit for purpose therefore resulting in financial losses for the wine industry.
Due to the emerging nature of this issue there are few sources of published information. The following summary endeavours to address the lack of information on this issue and compiles recent developments in the area of smoke taint research. Readers should note that these findings are limited as they have been developed under experimental conditions at a single location, with one grape variety (Merlot) using a model fuel source (straw) to generate smoke. Conjecture on the effects of smoke on grapes and wine continues with research ongoing.
Components of smoke
Smoke is a highly complex substance that contains a multitude of compounds including carbon monoxide, carbon dioxide, ozone, polycyclic aromatic hydrocarbons, oxides of nitrogen and sulphur, volatile and semi-volatile organic compounds and particulate matter (McKenzie et al. 1994, Radojevic 2003). Many authors have found smoke to impact the organoleptic properties of foods and this is attributed to smoke derived volatile compounds, including phenols (Maga 1988, Guillén et al. 1995). Of these phenols, guaiacol and 4-methylguaiacol are important in smoke analysis due to their ‘smoky’, ‘toasted’, ‘ash’, ‘chemical’ and ‘phenolic’ aromas in food and in wines originating from oak barrel fermentation / storage (Baltes et al. 1981, Boidron et al. 1988, López et al. 1999). The analysis of guaiacol and 4-methylguaiacol in grapes, wine and vine material provides a mechanism for detecting the exposure of smoke to grapevines although it is noted that these are not the only compounds in smoke that may contribute to the sensory smoke taint. Analysis of guaiacol and 4-methylguaiacol is conducted by gas chromatography-mass spectrometry (GC-MS) equipment.
Field research
To investigate the issue of smoke derived taint in grapes and wine, researchers have developed a novel apparatus for the application of smoke to field-grown grapevines. This apparatus has been developed to enclose grapevines within tents whilst smoke is pumped from a smoke generating device into the tent that encloses the vines (Figure 1) (Kennison et al. 2008a). Tent frames are made of galvanized steel and covered with a greenhouse grade film (Solarweave) to allow the transmission of light to grapevines. Smoke is generated in a 50 L metal drum and forced into the tent by a remote controlled variable speed air pump.
Figure 1. Field smoke application apparatus showing smoke tent with vines enclosed (left) and smoke generator (right).
Research results
The research had demonstrated a direct link between exposure to smoke for field-grown grapevines and the detection of smoke-like compounds (guaiacol and 4-methylguaiacol) in grapes and final wines (Kennison et al. 2008b). The presence of guaiacol and 4- methylguaiacol in grape samples is attributed to smoke exposure as these compounds were not found in samples from unsmoked (control) vines. Smoke exposure to grapevines also resulted in the detection of smoke-like aromas of ‘burnt rubber’, ‘leather’, ‘smoked meat’ and ‘disinfectant’ in resultant wines. These aromas were not discovered in wines made from fruit of unsmoked (control) vines.
Further investigation has revealed that taint-related compounds such as guaiacol and 4- methylguaiacol are released throughout the fermentation process resulting in elevated levels in wine (Kennison et al. 2008a). The level of potential smoke taint can therefore be underestimated in fruit samples. Ongoing research is investigating methods of sample preparation that provide a more accurate estimate of the potential level of smoke taint from grape samples.
Research has investigated the process by which smoke is assimilated and translocated by the vine to understand how smoke compounds can result in the grape berry and the final wine. Interestingly, initial research has found smoke compounds to reside within the skins of grape berries only and not within berry pulp (AWRI 2003, Kennison et al. 2009).
Timing of grapevine exposure to smoke
Grapevines appear to vary in susceptibility to smoke uptake throughout the growing season and this has been of major focus of research. Recent research has investigated the effect of smoke application to grapevines at key growth stages (Kennison et al. 2009). Smoke was applied to Merlot grapevines at the stages of shoots 10 cm, flowering, berries pea size, beginning of bunch closure, veraison, intermediate sugar, berries not quite ripe and harvest to investigate the sensory and chemical effects of smoke exposure. From this research, an understanding of the key timings of grapevine sensitivity to smoke exposure and the development of smoke taint in wine has been described (Figure 2).
There are three distinct periods of grapevine sensitivity to smoke exposure. The first period (P1) is characterized by a low potential for smoke uptake early in the growing season when shoots are 10 cm in length and at flowering. The potential for smoke uptake is variable during the second period (P2) from when berries are pea size through to 3 days post veraison. Grapevines show a low to medium sensitivity to smoke uptake during P2. During the third identified period (P3) from 7 days post veraison through to harvest grapevines have a high potential for the uptake of smoke compounds.
Cumulative effect of smoke
Vignerons have questioned the quantity of smoke exposure that is required to create a smoke taint effect in grapes and wine. Although research is ongoing in this area, it is interesting to note that a single heavy exposure of smoke (30% obscuration/m) to grapevines for 30 min is sufficient to result in smoke taint in wine (assuming that the exposure occurs during a period of sensitivity). Research has further investigated the effects of repeated smoke applications to the same vines and has found repeated smoke applications to have a cumulative effect on the levels of smoke compounds and smoke aromas in wine (Kennison et al. 2008b). Therefore, repeated smoke exposures or smoke exposures for a long period of time results in an accumulation of smoke aromas and compounds in resultant wines.
Figure 2. Key periods of grapevine sensitivity to smoke exposure and the development of subsequent smoke taint aromas and compounds in wine. Information is derived from research investigating the direct application of smoke to field grown Merlot grapevines.
Author
Kristen Kennisonab , Kerry Wilkinsonb and Mark Gibberd
