Researcher: Heinrich du Plessis, ARC Infruitec-Nietvoorbij - 29 Apr 2016

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Researchers will determine the role of yeasts and bacteria in the release of odorous volatile phenols from their glycosylated (bound to sugars), non-volatile precursors.

Phenolic compounds released by wildfires into the atmosphere are absorbed by vines via the leaves and berries. The vine has a defence mechanism that binds these phenols to sugars to make them non-toxic and odourless. Research has shown that the concentrations of volatile (non-glycosylated) phenols, which do have a negative sensory effect, are much higher after alcoholic and malolactic fermentation than initially measured in the juice. This indicates that the sugars can be removed from the precursors through the winemaking process and as a result the phenols can become volatile and flavour active.

Some yeasts (including wild yeasts) can have glycosidase activity (a group of enzymes that can remove sugars). Under normal circumstances this is a very positive trait for white wine yeasts, where glycosidases can release monoterpenes from their glycosylated precursors in must to render wines fruitier. However, yeasts (white and red) with this activity must be avoided in musts suspected of smoke taint spoilage. Research on the role of yeast in enhancing smoke taint has already been done in Australia and different effects on volatile phenol concentrations were found. Nietvoorbij researchers plan to expand on this research and to include the evaluation of the most commonly used red wine yeasts in South Africa for its effect on volatile phenol precursors.

Some lactic acid bacteria (LAB) responsible for malolactic fermentation (MLF) also have glycosidase activity. This is often the reason why different sensory effects are observed with the use of different bacterial starter cultures. However, many LAB do not have glycosidase activity. Nietvoorbij researchers will evaluate the effect of the most commonly used commercial MLF cultures (as well as natural MLF) on volatile phenol concentrations. Yeast bacteria combinations will also be tested.

Practical winemaking guidelines will be made available to the South African wine industry for handling grapes suspected of smoke taint spoilage.


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