grapes are like a lazy student who can do better. In addition to odorant volatile compounds (members mainly of the class of terpenols—lin-alol, geraniol, nerol, citronellol, alpha-terpineol, linalol oxides, and terpenic polyols—whose very low threshold of olfactory perception plays an important role in giving wines their typicity), grapes also contain, in much greater quantities, terpenic glycosides, molecules composed of terpenols bound to sugars. These molecules are precursors of terpenols, but unfortunately they do not contribute to the flavor of wines.
Claude Bayonove and his colleagues at the Institut National de la Recherche Agronomique Laboratoire des Arômes et Substances Naturelles in Montpellier wanted to know whether the aromatic qualities of wines could be intensified by dissociating the two parts of these precursors (sugars and terpenols) by means of acids or enzymes. Because enzymatic hydrolysis seemed more promising than chemical treatment, in part because it gives a more natural aroma, they began by characterizing grape enzymes that release terpenols from their precursors.
Working with glycosidic extracts from grapes of the Muscat of Alexandria variety, the Montpellier team added thirty-four commercially developed enzymatic preparations (pectinases, cellulases, hemicellulases, and so on) to see whether some of them formed terpenols from their precursors. Five proved to be effective, releasing linalol or geraniol depending on the case. All the effective preparations contained beta-glucopyranosidase and alpha-rhamno-pyranosidase or alpha-arabinofuranosidase as active components, which were shown to carry out the enzymatic hydrolysis of the terpenic glycosides of the grape in two stages.
This analysis was followed by an in vitro replication of this hydrolysis using rhamnopyranosidase, arabinofuranosidase, and glucopyranosidase. These enzymes released not only the desired odorant terpenols but also norisoprenoids, volatile phenols, and benzylic alcohol, all compounds with very low perception thresholds and an agreeable smell.
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