Consider the smell of smoke. It can be very comforting: think of sitting round a campfire, or logs burning in the grate of a country pub in winter. But in the wrong context it can be terrifying – the sign that something is badly wrong and danger is at hand. We have a similarly ambivalent relationship to the taste of smoke in our food and drink. Some people prize the distinctive bonfire and peat aromas of Islay whisky or German Rauchbier, while others dislike it. Rather more enjoy the taste of smoked salmon. And the flavour of barbecued beef brisket cooked for hours in a smoker is pretty much universally loved. But smoky wine? It has fewer followers.
Many wine regions are in areas that, from time to time, experience bushfires. These have been in the news a lot recently. In December and January 2020, Australia suffered devastating bush fires. ‘Australia is prone to fire,’ said Wine Australia’s Stuart Barclay, addressing journalists in London in a briefing about the bushfires as part of the Australia day tastings. ‘It is part of the natural landscape. But this year has been a monumental impact. 15 years of drought has made the country dry, and the fires have been almost impossible to manage.’ Barclay emphasizes that while 1500 hectares of vines are within fire-affected zones, this represents just 1% of Australia’s vineyard area. The Adelaide Hills, though, was particularly badly hit, with 648 hectares affected. ‘This ranges from complete devastation to partial devastation,’ says Barclay. ‘A major issue is the potential impact of smoke on grapes and wine.’
Australia isn’t the only country that has experienced severe bush fires in recent years. In October 2017 and October 2019 California had fires on a major scale that involved the destruction of some wineries and vineyards, although many of the grapes had already been harvested at this stage. Canada’s Okanagan Valley has also had issues with smoke tainted grapes in recent years. Forest fires in Portugal in June 2017 impacted vineyards. In light of the human and wildlife impact of these natural disasters it seems trivial to be thinking about the impact of bush fires on wine production, but the issue of smoke taint is something that is concerning many within the wine industry. And both those growing grapes and those buying grapes want a way of measuring whether or not grapes that have been exposed to smoke will produce tainted wines.
As a country where this is a recurring problem, Australia has led the way in this research. The Australian Wine Research Institute (AWRI) began looking at this issue in earnest in 2003 after a bad bush fire season. Of particular interest are the following questions. What are the components in smoke that can taint wines? Which are the vulnerable periods in the vine’s life cycle, where smoke exposure has the potential to cause problems? And how can you test grapes to see whether they are affected or not?
The smoke taint compounds
This first set of studies concluded that smoke taint was a real problem, with wines from smoke-affected vineyards displaying characters described in sensory work as “smoky,” “burnt,” “ash,” “ashtray,” “salami,” and “smoked salmon.” They also identified two compounds as key culprits: guaiacol and 4-methylguaiacol, although there probably were other chemicals involved, albeit at lower concentrations. They also showed that the taint compounds were found within the skins of the grapes, but not in the pulp or in the surface waxes on the skins.
Interestingly, it turns out that the chief chemical culprits in smoke taint are already present in many wines. Guaiacol and methylguiacol are formed by burning lignin, the structural component of wood. And when barrels are heated with fire to bend the staves, this results in the production of a number of flavour compounds from the wood that then find their way into wine. The concentrations typically found in barrel fermented or aged wines seem to be fine; It’s only when they are present at higher levels that they cause problems.
Further studies looked at the timing issue, and found that smoke exposure at any time between veraison (when the berries change colour and enter the final stage of ripening) and harvest can cause smoke taint, the most sensitive period being a week after veraison and afterwards. It is also worth noting that repeated smoke exposure builds up the level of smoke taint, as might be expected. The take-home message: the less smoke the better. Evidence suggests that the smoke taint compounds are absorbed by the skin of the berries (thus bigger, more developed berries are more at risk), where they bind to sugars in the plant tissue in a process known as glycosylation.
This is important, because it means that if you taste grapes that have just been picked, you can’t taste these bound smoke taint compounds. The sugars have to be cleaved off, either by enzyme activity or hydrolysis. So the only way to know whether grapes are smoke affected is to do a microvinification and then taste the results.
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