The inclination to deny the possibility of smoke taint must be sorely tempting, particularly considering the great promise which this harvest is reported to have shown, but logic says there must be some consequences. Trouble is, just how significant will those consequences be?
The recent devastating fires in the Stellenbosch wine region, specifically in the Helshoogte Pass, Helderberg and Constantia areas, exposed vineyards at varying stages of ripeness, to heavy concentrations of smoke for extended periods of time. In some instances, entire blocks of vines were either destroyed or damaged badly enough to make the grapes unusable, and to necessitate removal and replanting. The condition of the remaining grapes which were apparently undamaged, but exposed to smoke for varying lengths of time is questionable, considering the Australian experience.
Little if any research into this phenomenon has been conducted locally, probably because the incidence of fires at the crucial time of berry development - from veraison stage to harvest - has been uncommon, but that is changing. Marriane McKay, lecturer in oenology at Stellenbosch University's department of viticulture and oenology (DVO), says that it is a newer problem for the local industry, particularly over the last two years, as the fires have come so close to the vineyards and it is now much more of an issue.
She notes that the Australian wine industry has far more experience with smoke taint than South Africa. A brief search amongst academic databases reveals a number of Australian studies conducted so far, and a
Google search leads one to the web site of an Australian company that has developed a successful patented treatment, which is also locally available.
The Australian Wine Research Institute (
AWRI), attached to Adelaide University, is an industry funded body that focuses on research, development and support for the greater benefit of the industry, has closely monitored the impact of smoke taint in wine in Australia over the years. Significantly, it offers analytical services for the detection of smoke taint indicator compounds - guaiacol, 4-methylguaiacol and eugenol being the most commonly observed - as does the Grape and Wine Research and Development Corporation (
GWRDC), another industry funded body.
It should be noted that two of these compounds do occur normally in oak matured wines at concentrations of up to 100 micrograms/L for guaiacol and 20 micrograms/L for 4methyguaiacol, since they are derived from the lignin degradation during the cooperage toasting process.
Sensory detection of guaiacol shows smoky, phenolic, aromatic, burnt, and burnt bacon and 4-methyl guaiacol shows smoky, leather, and spicy aromas. Any winemaker who gets a whiff of these in their wine ought to be justifiably concerned.
A 2007 study [1] indicates that the smoke-taint compounds, of which there are believed to be in excess of 3,000, enter the vine through the leaves and are actively translocated to the fruit.
It is clear from a review of the available literature in Australia, both formal studies and anecdotal dissertation, that smoke taint is a reality. The nature and extent of damage caused are well documented in two research papers, published in 2007 and 2008.
The 2007 study [2] exposed harvested grapes to smoke for a controlled period prior to making wine, both free run and free run on skins to emulate white and red wine making. Wine was also made from a control sample of grapes not exposed to smoke, for comparison purposes. The results were unequivocal - chemical analysis of the resulting wine clearly showed significant concentrations of the most common smoke taint indicators - guaiacol, 4-methyguaiacol and to a lesser extent, 4-ethyl guaiacol, 4-ethyl phenol and eugenol - in the wine made from grapes exposed to smoke.
The study notes that wine fermented on skins was expected to show higher concentrations of the compounds, based on the results of previous studies, but concentrations were higher in smoked free run wines, suggesting the permeation of smoke into the grape berry, a postulate supported by Marriane McKay, who suggested that the compounds may be absorbed through the stomata in the grape skin. Interestingly, the study also noted that the wine made from smoked grapes completed fermentation much earlier - 9 days in fact - than the control sample, and also had higher alcohol content.
A further study conducted in 2008 [3] but this time with grapes in a vineyard, yielded more detailed results.
Merlot grapes were exposed to smoke at intervals in a controlled environment from veraison to harvest. Wine was made from the grapes, and samples were taken at various stages of the winemaking process for analysis.
The smoke taint compounds were least evident in the free run juice of smoked grapes, increasingly rapidly during malolactic fermentation, with the highest concentration in the finished wine. Significantly, wine sampled 12 months post bottling still reflect marginally reduced levels of guaiacol and 4-methyguaiacol but increased levels of 4-ethyl guaiacol, 4-ethyl phenol and eugenol.
Mitigation measures include the use of carbon - considered too aggressive - and reverse osmosis after the completion of the wine making process, since the research indicates that the compounds are bound to sugars in the juice stage, and therefore difficult if not impossible to extract.
Memstar, an Australian company based in Oakley, Victoria has developed a reverse osmosis process which has been used successfully to remove smoke taint compounds from wine, but it is no silver bullet. Memstar MD David Wollan likens the use of the process to radical surgery on a cancer patient. The patient may not die, but they will not be in the same state as they would have been if they had not contracted cancer. The reason for this is quite simple: the undesirable compounds and some desirable compounds are chemically very close, and the reverse osmosis process is therefore going to strip out some desirable compounds along with the undesirables. He says the treated wine will not be the equivalent of what it would have been had it not been smoke tainted. Having said that, the system is routinely used in Australia, and more recently in California, to treat smoke tainted wine with much success.
A number of local winemakers who are grappling with the possibility of smoke tainted wines, notably Wolfgang von Loeper of Wedderwill Estate, and Reino Thiart of Idiom (De Capo vineyards), and Miles Mossop of Tokara have indicated that they might consider using reverse osmosis if smoke taint compounds are evident in finished wines.
Vergelegen cellar master Andre van Rensburg, is however, unequivocal that he will not consider reverse osmosis since "forcing the wine through a membrane" conflicts fundamentally with his winemaking practice of gravity flow and no pumping at all.
Hans Losch of
Columbit, the local representative of Memstar, says that the portable Memstar unit is capable of processing up to 700 litres per hour, at an average cost of 80c/l, depending upon how many passes through the system are required. He notes that in about 5% of cases, smoke taint has re-emerged in treated bottled wine up to a year or more after bottling.
Stellenbosch based
Thales Wine Cellar Services are also investigating a mitigation method based on one of their current treatment applications, but consultant oenologist Cornea Cilliers would not be drawn on the detail, except to invite any producer who might suspect smoke taint, to contact them directly in order to get involved with the project.
With climate change now an accepted fact, increased summer temperatures are likely to result in more wild fires in future years, thus greatly increasing the likelihood of smoke damage to future harvests. Proximity to the fire itself is not a determinant of the likelihood of smoke damage. In 2007, smoke from fires 100km distant from the Yarra Valley was blown into the valley by prevailing winds, resulting in significant smoke damage to grapes.