Forest fires affect flavour of wine

Scientists from the Technical University of Munich (TUM) have discovered the reason why areas prone to forest fires produce unpalatable wine. They researched why the smoke aromas are stored in grapes and how this happens at a molecular level.

Previously, it has not been possible to determine whether a forest fire has occurred near the vineyard. The off-note 'smoky' taste is only evident in the finished product and is different to the taste of barrique-produced wines.

"The smell and taste of such a wine is then often described with the term ash or ashtray," said scientist Katja Härtl. "This leads to a strong reduction in the quality of the wine."

Published in the Journal of Agricultural and Food Chemistry, Professor Wilfried Schwab and his team from the Professorship for the Biotechnology of Natural Products at the TUM described how the smoky taste occurs.

Aromatic substances are attached to sugar and can be retained or stabilised with this sugary compound. The aromatic substance can then once again be detached from the sugar and released. This process is called glycosylation. It describes a series of chemical or enzymatic reactions, for example, in which carbohydrates are bound to small, hydrophobic compounds such as aromas. An enzyme called glycosyltransferase is responsible for this.

When exposed to bush or forest fires, the grape vine absorbs the smoky aromas via its leaves and fruits. In the plant, the off-notes are then linked with sugar molecules by a glycosyltransferase. This link with sugar molecules makes the smoky off-notes more water soluble and, as a result, the grape vine stores smoke aromas.

"Actually, the job of glycosyltransferase is to process the resveratrol," Schwab stated.

The enzyme should be processing resveratrol, which has health-promoting properties, but instead it is processing smoke aroma molecules as they are structurally similar.

As long as the grapes haven't been harvested yet, the smoke molecules are bound and the ashy smell and taste is difficult to detect. When yeast is added to ferment them, the process separates the sugar molecules and the smell develops.

"Therefore, it only becomes apparent in the finished wine that the vineyard was exposed to a fire and the final product is of poor quality," Härtl explained.

Schwab suggests that now they have established how the taste and smell develops, they can work towards preventing it in the future.

"In the next step, we can try to cultivate either grape vines with less glycosyltransferase. Or we'll add a second sugar to prevent the release of the bad aromas," he said.

Alternatively, other solutions could include using yeasts that do not release the smoke aromas during fermentation, or removing the gene responsible for this.

Understanding the formation of undesirable aromas in wine means areas prone to regular forest fires, such as Australia and Southern Italy, can now build on this research to eliminate the degradation in quality.