As has been obvious in the last decade, grape growing regions of the world (America, Spain, Chile, Australia, and South Africa) have been plagued by devastating wildfires. While fires may not directly burn vineyards, the masses of smoke belching out into the surrounding area contains a range of compounds with aromas that range from sweet, smoky and toasty to ashtray, char and tarry (definitely unpleasant). Guaiacol, 4-ethyl guaiacol, 4-methylguaiacol, and 4-ethyl phenol are some of the volatile phenols that are used by scientists as chemical ‘markers’ for the presence of smoke taint in wine. These compounds and others like them are not only absorbed by exposed grapes, but have been shown to increase as a result of extraction during the winemaking process.
Australian researchers are at the forefront of grappling with this problem, due to the frequency and magnitude of wildfires in their country over many years. To the Aussies’ credit, they have managed to figure out a lot of the issues around smoke taint, including the presence of so-called ‘glycoconjugate precursors’ (sugar-bound non-volatile compounds), which are a result of the grapevine detoxifying and storing the smoke volatiles out of harm’s way. These glycoconjgates are water-soluble, and are extracted efficiently during winemaking. Unfortunately, the sugar-bond can be broken under acidic conditions or through the action of enzymes, and the volatile smoke compounds re-released into the wine, which brings back the smoke taint. Of course, the Department of Viticulture and Oenology (DVO) at Stellenbosch University (SU) has been working on the smoke taint issue for a while now as this problem impacts the quality of any South African wines made from smoke-affected grapes.
Sensory issues experienced in our unpublished study, supported by Aussie work, suggest strongly that an ‘ashy/smoky’ palate in wine results from unhydrolysed glycoconjugates which may remain behind even if the wine is thoroughly treated through fining or reverse osmosis to remove smoke volatiles. Unfortunately, bacterial hydrolysis enzymes are likely to be present in the mouth of every consumer (no matter how thoroughly they floss), which means that the risk of a horrible tasting experience is there for them, as long as glycoconjugates are lurking about in wine.
To date, even the well-funded Aussies haven’t come up with a strategy for targeted removal of the glycoconjugates, so that option is not currently on the table. Research, funded by Winetech, is currently underway at the DVO. This includes the methodology to measure volatile phenols in grapes, juice, must and wine through a GC-MS method developed at the Central Analytical Facility at SU by Mr Lucky Mokwena. We are currently monitoring volatile phenols in a range of South African wines using the CAF method, an aspect of which is that we are tracking fire events in the Western Cape over time, and seeing if there are correlations with volatile phenol content in wines. We are also busy with a number of other projects looking at interactions between volatile phenols and with other commonly occurring taint compounds with a specially trained panel. We are investigating amelioration of smoke taint through relatively simple winemaking interventions (release-and-removal strategies), and where possible, we are helping industry make decisions about affected wines with our expert smoke taint panel.
So, even though 2018 has been fairly free of terrible veldfires near our vineyards, we are prepared for next year when we may not be so lucky. The smoke taint team is happy to accept any enquiries related to smoke taint, smoky or rubbery taints, samples for GC-MS analysis (grapes, juice, must and wine) that may be affected, and give advice in any way we can.
Please feel free to contact Marianne McKay (marianne@sun.ac.za) (phone 021 8083774) or Nongcebo Langa (langan@sun.ac.za) to discuss.