If wine is cultivated in an area where forest fires occur more often, such as in Australia or Southern Italy, aromas can develop in the finished product which make the alcoholic drink inedible. So far, it was not known why this is so and what happens at the molecular level. A team at the Technical University of Munich (TUM) now describes the reason for the storage of the smoke aromas in the grapes and thus points the way for breeders to eliminate this quality degradation.
In the case of wine production, it is not clear whether there is a fire or fire near the vineyard from which the grapes come. Only when opening a bottle of wine is a strong misjudgment perceived. “The smell and taste of such a wine is then often described as” ashes or ashtrays, “says scientist Katja Härtl” This leads to a severe deterioration of the quality of the wine. ”
Whoever now thinks of barrique-grown wines, which for some” smoky “tastes, learns from the scientist that it is not comparable. But what happens in the fruit? For a study, a team at TUM went to the bottom of this smoke smell.in the journal ” Journal of Agricultural and Food Chemistry “.
GIVE THE FLAVOR SUGAR!
Odoriferous substances are volatile and are coupled to sugars in nature, for example in plants. With this sugar compound, it is possible to hold or stabilize the aroma. Later, the odorant can then be decoupled from the sugar and released. This process is called glycosylation. It describes a series of chemical or enzymatic reactions in which carbohydrates are bound, for example, to small, water-deficient compounds such as aromas. An enzyme called glycosyltransferase is responsible for this.
When grape vines are exposed to bush fires, as happens more often in Australia, Southern Italy or California, the vine leaves the smoky aromas of its leaves and fruits. In the plant, the faults are then linked with sugar molecules by a glycosyltransferase – a protein that acts as a biocatalyst. This connection with sugar molecules makes the smoky annotations more water-soluble. As a result, the grapevine stores the smoke no longer volatile.
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But why does the glycosyltransferase process these foreign substances? Professor Schwab’s team has also made a statement: “Actually, the task of glycosyltransferase is to process the so-called resveratrol.” Resveratrol, a substance with a health promoting effect, is naturally contained in the grapevine and resembles the structure smoky aromamolecules. As long as the grapes have not yet been harvested, the stinking smoke molecules are bound and the ashy smell and taste can not be noticed.
During the fermentation, however, the yeast added by the fermentation process cleaves the sugar molecules and the smell unfolds. “For the first time in the finished wine, the vineyard has been exposed to a fire and the final product is of poor quality,” explains author author Katja Härtl, because the glycosyltransferase has ensured that foreign flavors are not absorbed by the smoke molecules by the consumer are desired.
“We now know how such a taste can develop,” says Prof Wilfried Schwab. “In the next step, we can try to breed either vines with less glycosyltransferase, or we can attach a second sugar to the release of the poor flavors prevent.” Alternatively, yeasts could also be used during the fermentation, which are not capable of releasing the smoke aromas. The gene responsible for this can also be removed, which, however, tend to reject consumers.
The elucidation of the molecular mechanism leading to the formation of undesirable aromas in wine now allows winegrowers to take various counter-measures to ensure the quality of the alcoholic beverage.