Researcher: Dr Astrid Buica - 29 Apr 2016

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Researchers from the Department of Viticulture and Oenology (DVO), Stellenbosch University (SU) will develop a method to measure the odourless smoke taint precursors in juice and wine. Historically wildfires during our harvest were not very common, but the 2015 season as well as the start of the 2016 harvest season saw devastating fires in our winelands. Wine produced from grapes that were exposed to wildfire smoke can have a smoke-like character, which is unacceptable to consumers. Smoke particles (phenols) released into the atmosphere during a wildfire are absorbed by vines via the leaves and grapes. The vine responds with an immune response and binds sugars (glycosylation) to these “foreign” phenols to render them non-toxic and odourless.

The sugars can be released during fermentation by skin contact, enzymes, yeasts, bacteria and the acidic medium during ageing. Once the phenolic compounds are free they become volatile and flavour active. Volatile phenols are measured using a gas chromatograph (GC-MS) in the same way as Brettanomyces spoilage (4-ethylphenol and 4-ethylguaiacol - also volatile phenols) are measured. Both SU via its Central Analytical Facility (CAF) and Vinlab can perform volatile phenol analysis. Unfortunately, however, it gives one no indication of the extent of the smoke taint spoilage as it cannot measure the non-volatile (non-gas) smoke taint precursors. A result indicating a low volatile phenol concentration in a wine does not mean you can continue to bottle your wine and sell it. You might get a big surprise after six months or a year as the low pH in the wine gradually removes the sugars from the phenols. It is therefore of critical importance in the light of drier weather conditions, water restrictions, climate change and potential higher occurrence of wildfires that we can measure the iceberg under the water in grapes and wines that were exposed to fires.

The method for measuring the glycosylated phenols involves a very sophisticated (and extremely expensive) machine (LC-MS/MS), which is not available in wine analytical laboratories. It is available at CAF where the method development will take place. At the same time researchers will try to implement a previously published method using enzymes or low pH treatments to remove the sugars from the glycosylated phenols. This will make it possible to use the GC-MS as the phenols will be volatile after treatment.

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