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article imageFungal contamination changes the aroma of wine

By Tim Sandle     Jun 2, 2017 in Science
Geisenheim - There's been a long-running dispute about the extent that fungi which infect vines influence the flavor and aroma of wines. A new study shows this to be the case. Interestingly there are both positive and negative effects.
Fungal diseases are a major problem within viniculture, often leading to suppressed growth and damaged wines. Some wine growers maintain that the infections, while affecting yield, do not influence the taste and aroma of the wine produced. Others, however, say there are alterations. The problem has been put to the scientific test and the answer is that there is a reduction in wine quality.
The new study, from Hochschule Geisenheim University, Geisenheim, Germany, looks at two common types of fungal infection. These are bunch rot (caused by the fungus Botrytis cinerea) and powdery mildew (caused by Erysiphe necator). The influence of these microorganisms is to alter the aroma of wine, through changes in chemical aroma substance composition. The research looked at wine produced from the grape varieties White Riesling, Red Riesling and Gewürztraminer, as well as wine made from the unsprayed hybrid grape type Gm 8622-3.
The effects were, interestingly, mixed. Infection with bunch rot led to more positive aroma ratings. This was a consequence of increased levels of fruity- and vanilla-like notes. However, the impact of powdery mildew decreased the level of pleasant notes in the wine. The assessment was made in terms of chemical analysis, using a method called Aroma Extract Dilution Analysis (AEDA), and by drawing on the expert noses of 10 participants who had won awards for identifying different wine odorants. AEDA is widely used for the screening of aroma-active compounds in gas chromatography-olfactometry. With the process, once the odorants with the highest flavor dilution factors have been identified, their concentrations in foods are quantified and their odor activity values are calculated.
While dry results can be provided by chemical tests perhaps the most telling alterations come from those who tried the wines. Here the lead researcher Dr Andrea Buettner notes the changes are the "result of an interplay of subtler changes in multiple aroma active substances in each wine." Withe the positive effect, bunch rot infection increased the peach-like/fruity, floral and liquor-like/toasty aroma notes. Whereas, on the negative side powdery mildew led to a lack of positive aromatic notes.
The researchers hope the findings will help clarify how fungal infections affect wine under diverse climatic conditions. The research is detailed in the journal Frontiers in Chemistry, with the paper titled "Effects of Bunch Rot (Botrytis cinerea) and Powdery Mildew (Erysiphe necator) Fungal Diseases on Wine Aroma."
More about Wine, Fungi, Drink, Microbiology
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