Tuesday, Dec 19, 2017 | Last Update : 12:00 AM IST
During fermentation the wine yeast added by the fermentation process once again separates the sugar molecules and the smell develops.
Scientists have found why wine cultivated in areas where forest fires are frequent have aromas that make the alcoholic beverage unpalatable.
The findings may pave the way for growers to eliminate this degradation in quality.
Researchers at the Technical University of Munich (TUM) in Germany described the reason why the smoke aromas are stored in the grapes.
In the case of wine production, it is not initially clear whether there was a fire near the vineyard from which the grapes come. Only when opening a bottle of wine is a strong off-note perceived.
"The smell and taste of such a wine is then often described with the term ash or ashtray. This leads to a strong reduction in the quality of the wine," said Katja Hartl, scientist at TUM.
The smell is different from the barrique-produced wines, which taste "smoky" to some people.
The study, published in the Journal of Agricultural and Food Chemistry, got to the bottom of this smoky taste and is now described by Professor Wilfried Schwab and his team from the Professorship for the Biotechnology of Natural Products at the TUM.
Aromatic substances are volatile and in nature are attached to sugar, for example in plants. The aroma 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.
If grape vines are exposed to bush fires, as happens more often in Australia, Southern Italy and California, 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 - a protein that acts as a biocatalyst. This link with sugar molecules makes the smoky off-notes more water-soluble.
As a result, the grape vine stores smoke aromas that are no longer volatile.
"Actually, the job of glycosyltransferase is to process the resveratrol," said Wilfried Schwab, professor at the TUM.
Due resveratrol, a substance with a health promoting effect, is naturally contained in the grape vine and in terms of its structure resembles the smoky aroma molecules.
As long as the grapes have not been harvested yet, the stinking smoke molecules are bound and the ashy smell and taste can not be noticed.
During fermentation, however, the wine yeast added by the fermentation process once again 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," said Hartl