Adding enzymes to grape juice releases its flavors.
GRAPES ARE LIKE A LAZY STUDENT who can do better. In addition to odorant volatile compounds (members mainly of the class of terpenols—linalol, 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-rhamnopyranosidase 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.
The Enzymes of the Grape
The second stage in the glucopyranosidase-mediated hydrolysis of glyco-sides limits the release of terpenols from both the grape and the wine, for natural enzymes have little effect on the monoglucosides of tertiary alcohols (linalol, terpineol) that have a nonglucosidic part (called aglycone). By contrast, the beta-glucosidase found in yeasts used in winemaking shows weak activity for linalyl-beta-glucoside, one of the principal glucosides of the Muscat of Alexandria grape.
Whereas this grape exhibited noticeable beta-glucosidase activity, it presented only very weak rhamnopyranosidase activity and no activity whatever for arabinofuranosidase, which blocks metabolism in the first stage and limits its overall action on the grape’s terpenic glycosides.
Finally, because the grape’s glucosidase is unstable and inactive at the level of acidity found in the must and in the wine, it does not seem to be a good candidate for glycoside hydrolysis in either one. Would plant enzymes or microorganisms do a better job of hydrolyzing the terpenic glycosides than the enzymes of the grape? Plant enzymes hydrolyze only the glycosides of primary alcohols, such as geraniol, nerol, and citronellol; the beta-glycosides of tertiary alcohols, such as linalol and alpha-terpineol, were hydrolyzed by only one of the two enzymes studied, though with greater difficulty.
Today the Montpellier physical chemists are studying exogenous enzymatic preparations created by commercial researchers at Gist-Brocades, S.A., from the stock of legally approved microorganisms, seeking to identify organisms that display higher levels of activity at the temperatures and sugar and acid concentrations found in the juice of grapes and in musts.
Professional tasters are sensitive to the presence of intensified flavor in the juice of fruits or of “enzymed” wines, but the use of enzymatic preparations remains largely unexplored. The discovery of new glycosides (such as a recently identified apiosylglycodide) and corresponding enzymatic processes will be of particular importance in improving exogenous preparations.