25 January 2006 - by -
South Africa, Australia and California are notoriously prone to wildfires. Fire is a feature of the environment and often essential to maintain the natural ecosystem. But fire and wine don't mix, writes Frank Smith from Australia, where research on 'smoke taint' is underway.
Australian grape growers claim wildfires cost them more than $7.5 million in lost revenue during the 2003 and 2004 vintages. The Canberra bushfires of 2003 resulted in heavy smoke damage, while growers in Victoria suffered at least $4 million losses from fires in adjoining national parks.
The Australian Wine Research Institute (AWRI) says the smoke taint issue was the single largest problem they dealt with in 2003-4, both in terms of product value and numbers of growers affected. Smoke-affected wines and juices showed characters described by AWRI's sensory panel as ‘smoky', 'burnt', 'ash', 'ashtray', 'salami' and 'smoked salmon’. Viticulture consultant Jim Campbell-Clause is more blunt. He describes smoke-affected wine as having ‘an unpalatable septic tank pong.’
The AWRI has established that guaiacol and 4-methylguaiacol are the most important compounds contributing to smoke taint. These compounds commonly occur in oaked wines; they are formed by the degradation of lignin when the oak is toasted.
However, the tasting panel found a back palate of ash character that was more pronounced in smoke-affected wine samples than in juices or wines spiked with similar concentrations of guaiacol. They concluded that other smoke-related compounds were present, although at very low concentrations.
In January 2003, the Alpine and King Valley regions of Victoria suffered 40 consecutive days of smoke pollution as a result of bushfires in neighbouring national parks. No one took much notice, until the first batches of fruit were harvested for sparkling wine. The juice had a smoky aroma and a dusty character on the palate.
Consultant Jill Kuchel of Vignoble Monitoring Services set up a series of experiments aimed at removing as much of the smoke taint as possible from the fruit before harvesting commenced. This was the first research undertaken anywhere in the world on the effects of smoke pollution on grapes or wine. ‘We assumed that the smoke taint in the berries was due to the presence of guaiacol,’ she says. ‘This was partly based on the fact that smoky characters in wine were caused by guaiacol compounds.’
She experimented with washing the fruit in various ways, in the hope that the guaiacol had only penetrated the waxy cuticle layer of the berry and not the interior of the fruit. ‘Hot water did result in a slight trend to reduce guaiacol, but this was not a practical treatment in the vineyard. There was no significant difference between any of the other washing treatments.
‘Guaiacol was only present in the macerated juice samples, suggesting that the guaiacol was present in the inner skin layers rather than the outer berry cuticle layer.’
Kuchel also tried washing the canopy and fruit zone with irrigation water applied as an overhead spray. But there were no significant differences in guaiacol levels between treatments and control.
She thinks that leaves around the bunch zone contained guaiacol and that these leaves may be picked during harvest due to their close proximity to the fruit. Analyses showed that there were small amounts of guaiacol in the leaves. ‘We recommend growers remove leaves from bins and apply extra air to the bunch zone while harvesting,’ she says.
In the winery, guaiacol was present in the skins, but not the pulp. Guaiacol in the must increased during fermentation up to day four and then levelled out. The smoky aroma decreased during fermentation, but a dry characteristic was still present on the palate, she says.
In the Pemberton district of Western Australia fires were lit deliberately by the Department of Conservation and Land Management (CALM) to reduce the risk of wild fires. Local growers are up in arms. ‘In 2001 we had a contract to supply grapes to Palandri from our Treenbrook vineyard, depending on the results of the first year,’ says Greg Banfield, director of Barwick Estates. ‘Three months after we had delivered them, Palandri's winemaker called us in. The wine made from our Cabernet Sauvignon had an acrid back-palate and horrible, lasting taste. We lost the contract.’ Barfield and other growers are suing CALM for $1.5m.
The risk may vary with the stage of grape ripening when the fruit are exposed to smoke. Research has begun at Western Australia's Curtin University to establish if there are times when smoke causes minimal damage to fruit.
Grape growers in South Africa will no doubt watch developments with interest.
Frank Smith is a retired agricultural scientist and freelance journalist based in Perth, Western Australia. He specialises in science and human interest stories and enjoys the odd glass of Pinot Noir.
Also see Harvest Report 1 - Fires in focus by Kim Maxwell
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