Introduction
In the May 2007 issue of Grapegrower and Winemaker we discussed the impact of smoke from bushfires in the North East and Eastern regions of Victoria on grapes grown in these and surrounding regions (1). This current vintage has seen an unprecedented incidence of bushfires in Victoria, in several cases involving vineyards being burnt. The consequences for the 2009 vintage are, at the time of writing this article, not totally determined but are expected to be more severe than 2007.
The 2009 Victorian bushfires
The horrific bushfires that have occurred in Victoria, starting on 7th February, have seen the greatest loss of human life and houses ever recorded in Australia for such an event.
The impact these fires may cause on the quality and production levels of wine from the 2009 vintage does seem trivial when compared to this incredibly tragic loss of life., however, those of our colleagues living in the fire affected regions have to get on with their lives and earn an income, and so for them, at least, this is not a minor matter.
Figure 1: Fire affected vineyard at …
We must also bear in mind the predictions of the United Nations Intergovernmental Panel on Climate Change (IPCC) in their 2007 report. This states that we should expect more numerous and more severe bushfires in the Southern areas of Australia.
These areas are predicted to be subject to periods that will experience drier and hotter conditions over the ensuing decades. It appears we will have to learn to live with these more extreme conditions and manage our vineyards and winemaking with this in mind.
Connecting smoke taint of fruit to smoke taint in wine
Little research had been done on smoke taint before the 2003 season when fires devastated many parts of Victoria, NSW and the ACT. The 2003 bushfires had a disastrous effect on some vineyards in these States. Since then several studies have produced more detail on this fairly recently recognised wine taint.
There is now conclusive evidence that there is a link between the exposure of grapes to smoke and the incidence of smoke taint in the resultant wine <a href=”#ref2″>(2)</a>.
In one 2007 study, Verdelho grapes were harvested at 24 Brix, one portion of bunches was placed on wire racks and exposed to the smoke coming from straw for one hour. Another portion of the fruit was kept separate and was not exposed to the smoke.
Two wines were produced from the “smoked” grapes, one without skin contact and one with skin contact. These differing treatments were performed to mimic white and red grape processing. Two similar wines were made from the “unsmoked” grapes.
The resulting wines were then judged by a panel of 24 tasters using the triangle test. The smoked wines were found to be significantly different at the 99.9% confidence level to the “unsmoked” wines. Of the 24 judges, all were correct for the skin contact “smoked” wine, whilst 22 obtained correct results for the non skin contact “smoked” wine.
How is smoke taint quantified?
As we have previously reported (1), the compounds guaiacol and 4 methylguaiacol (G & 4MG) are considered good marker species for smoke taint. These are measured by Gas Chromatography Mass Spectrometry (GCMS) at part per billion (ppb) levels with a detection limit of 1 ppb. See Figure 1. This analysis has been offered at our Victorian laboratory for several seasons now and since this year’s “Black Saturday” bushfires exploded into our world we have been performing this test very frequently.
Figure 2. Steve Byrne, Vintessential Lab Manager, with GCMS instrument used for smoke taint analysis.
In the abovementioned Verdelho study (2) all four wines were analyzed by GCMS for smoke taint compounds. Guaiacol and 4-methylguaiacol were found in the wines made from the “smoked” grapes, but were not detected in the wines made from the “unsmoked” grapes. In the “smoked” wine, the G & 4MG were 1470 and 326 ppb in the non skin contact wine, and 969 and 250 ppb in the wine made on skins.
What levels of G & 4MG are considered acceptable?
The aroma thresholds for G & 4MG have been reported as 95 and 65 ppb in white wine and 75 and 65 ppb in red wine respectively <a href=”#ref2″>(2)</a>. As can be seen above, the trial wines in question were well above these limits.
Another report (3) quotes the sensory threshold for guaiacol in white juice as 6 ppb or less. The same tasting panel was also reported to have a sensory difference threshold for guaiacol of 15 to 25 ppb for a red wine that had a level of 37 ppb of guaiacol.
Another recent report (4) involving work done after the 2007 North East Victorian bushfires quotes anecdotal evidence from winemakers involved in the study that guaiacol was a good marker for smoke taint and that most tasters could detect the presence of guaiacol in a dry white wine at the 6 ppb level. The report did recommend that further work needs to be done to ascertain if this threshold level was relevant for all grape varieties.
What treatment is possible for smoke tainted grapes?
The DPI study (4) investigated a number of processing options for minimizing the effect of smoke tainted fruit on the finished wine. These included the temperature of the fruit being processed, the levels of G & 4MG in different press fractions, whether the fruit was harvested by hand or mechanical means, and the use of different fining agents post fermentation.
Although there is some good data in the report, obviously much more work needs to be done to find consistent ways to minimize the impact of smoke affected fruit on wine quality.
Conclusion
The 2009 vintage is going to cause severe smoke taint issues for a number of wine regions in Victoria. The use of smoke taint analysis based upon the levels of guaiacol and 4 methylguaiacol (G & 4MG) is likely to be widely employed to help determine the suitability of fruit from a number of regions that have been affected by bushfire.
It appears more research needs to be done to set reasonable guidelines as to the level of taint on fruit and the concomitant levels in wine. Methods of reducing the impact of smoke tainted fruit still also need to be fully explored.
References:
1. Howell, G & Vallesi M; “The Ashes we didn’t want – smoke taint in Vintage 2007”, Grapegrower and Winemaker, May 2007
2. Kennison K.R; et al, “Smoke derived taint in wine: effect of postharvest smoke exposure of grapes on the chemical composition and sensory characteristics of wine”, J Agric. Food Chem; 55, 10897-10901, 2007
3. Godden, P; et al “Investigations conducted during 2003 and 2004 into the nature and amelioration of taints in grapes and wine, caused by smoke resulting from bushfires” AWRI Information Services
4. Whiting, J, Krstic, M; “Understanding the sensitivity to timing and management options to mitigate the negative impacts of bush fire smoke on grape and wine quality – scoping study”, Dept of Primary Industries Victoria, 2007
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