A Clean And Green Wine Industry: The impact of veld fires on wine quality

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A CLEAN AND GREEN WINE INDUSTRY
The impact of veld fires on wine quality
by Charl Theron
The impact of veld fires on the environment is not only visual, but may have massive repercussions for animal and plant life and obviously also agriculture if fields, vineyards or orchards are threatened or destroyed. This was again emphasised by recent veld fires in local winelands. The quality of the wine may also be impaired if the grapes are exposed to smoke taint. New wine regions that have been established in South Africa are surrounded by natural vegetation, which is obviously more exposed to veld fires. Australia, which is often ravaged by bushfires, has been doing extensive research to understand and possibly treat the impact these fires have on wine quality.
Smoke caused by the 2003 bushfires in Victoria, some of which blazed to within 100 metres of vineyards, had such a detrimental impact on the grapes that a unique taint has been identified in some wines; the off-odour has been described as a campfire that has just been extinguished, wet burnt paper or a house that had been on fire, smoky, dirty, soil-like or smoked meat (Ward et al., 2008). In some instances the aroma was reminiscent of a Brettanomyces character. As with many other wine compounds tasters differ markedly in their ability to detect the aroma and there are individual threshold values. At that stage the flavour could only be removed from the wine by means of a carbon treatment (White, 2007). During the 2006-2007 season in Victoria, Australia, 657 bushfires devastated more than 1 million hectares of veld. This amounts to 5,2% of the state’s total surface. Several well-known wine regions were threatened. The University of Adelaide and the Australian Wine Research Institute (AWRI) launched research projects to support the wineries in their attempt to solve the problem. Investigations conducted in 2003 confirmed the following:

The presence of atmospheric smoke may result in grape juice and wine with a smoky, burnt, ashtray or salami character which makes the wine unsuitable for consumption and consequently results in huge financial losses.
Guaiacol and 4-methylguaiacol have been found to be the most important compounds causing smoke taint. However, the latter is most probably not caused by these two compounds exclusively. What is more, both compounds are also encountered in wines that have been matured in toasted oak and may, in certain concentrations, be beneficial to the wine’s aroma.
The two compounds seem to occur mainly in the skin and not in the berry flesh, more specifically in the epidermis cells of the skin and not in the wax layer.
The sensorial threshold value of guaiacol in white juice is 6 pg/l or less, whereas it is 15 - 25 pg/l in red wines.
Different treatments of the wax layer on the foliage have been researched, without resulting in a reduction in guaiacol concentrations.
Increased skin contact results in higher guaiacol concentrations, which are also higher in mechanically harvested grapes than in hand-picked grapes.
Active carbon treatment of wines reduces the guaiacol content, but at the same time removes other compounds.
Increases in guaiacol content are almost linear during the first three to four days of alcoholic fermentation, where after it increases only slightly.
The Australian technology supplier Memstar has a patented membrane system that reduces the methylguaiacol content of wine (see www.memstar.com.au for more information). Columbit is the South African agent for Memstar (Krstic et al., 2007).
The stage at which vineyards are exposed to smoke, and the duration of such exposure, determine the impact on the chemical composition of the wine. Seven days after veraison is the peak period when key indicator compounds in wines are 4 to 22 times higher (Ward et al., 2008).
Current research in Australia focuses firstly on the accumulation in grapes and therefore wine, of volatile components that derive from smoke and secondly on the analysis of smoke-based volatile compounds in wine. Some smoke compounds appear to be bound in grapevines, only to be released at a later stage during fermentation. Apart from guaiacol and 4-methylguaiacol, which are currently the best-known indicators of smoke taint, there appears to be several bound precursors which are released under specific circumstances to create smoke taint. Besides the uncertainties surrounding smoke taint, very little is currently known about the treatment of vines damaged by fires. More research is currently under way (Turner et al., 2008).
References
Krstic, Mark; Martin, Steve & Lowe, Steve. 2007. Influence of bushfires in north-east Victoria on smoke taint issues. Australian Viticulture, March/April 2007: 31 - 33.
Turner, Bianca; Wilkinson, Kerry & Simos, Con. 2008. Smoke taint research under way to assist growers in the future. Australian Viticulture, May/June 2008: 100 - 101.
Ward, Glynn; Kennison, Kristen & Gibberd, Mark. Australian Viticulture, May/June 2008: 44 - 48.
White, Tim. 2007. Smoke taints perception on exact characteristic. CARTE blanche. Australian & New Zealand Grapegrower & Winemaker, June 2007: 65 - 66.
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