Enzymes in saliva amplify an important aromatic component of wines made from the Sauvignon Blanc grape.
BIOCHEMISTS ARE VERY INTERESTED in the methods used for making wines, but they have rarely explored the physiology involved in tasting them. Recent results suggest that this situation may be changing. In wines made from the Sauvignon Blanc grape an odorant molecule has been found whose effect is registered only when the enzymes in saliva have separated it from its precursor. A few moments are needed, then, for the aroma to be perceived.
In 1995, Philippe Darriet and Denis Dubourdieu at the Institut d’Œnologie de Bordeaux discovered a molecule that gives Sauvignon wines a boxwood or broom note. Significantly, this simple molecule, whose skeleton is composed of only five carbon atoms, contains a sulfur atom. Darriet and Dubourdieu, observing that it appears during alcoholic fermentation, managed to identify its precursor. Additionally, they observed that the frequency with which this precursor is transformed into an odorant molecule depends on the strains of yeast responsible for fermentation. The Bordeaux chemists studied a class of enzymes capable of releasing the wine’s distinctive aroma, reasoning that if they could identify the particular enzyme at work in the case of Sauvignon they would have thereby determined the structure of the precursor.
In a related study, Claude Bayonove at the Institut National de la Recherche Agronomique station in Montpellier examined glycosidases, enzymes that dissociate odorant molecules of the terpene class from the sugar molecule to which they are bound. The molecules considered in the Montpellier study do not produce the odorant compound analyzed in connection with the Sauvignon grape because this compound is not bound to a sugar.
Amino Acids and Sulfur-Based Aromas
Darriet and Dubourdieu looked at enzymes that break the bonds between a carbon atom and a sulfur atom to see whether one of them causes Sauvignon’s aroma to appear. They were interested in an enzyme called lyase, produced by the intestinal bacterium Eubacterium limosum, that breaks down the sulfur derivatives of cysteine, an amino acid. The compound in Sauvignon was released in vitro from a nonvolatile extract of its must through the action of some product present in the medium obtained by grinding up Eubacterium limosum. They concluded that the precursor of the aroma in the must contains cysteine.
These results confirmed, first, that the Sauvignon grape has an aromatic potential that is brought out by the vinification process. This means that the wine maker must choose yeasts that are capable of developing the taste latent in the grape. The great gastronome Curnonsky (1872–1956) was famous for demanding that foods have “the taste of what they are.” Chemistry is a way of satisfying this requirement.
The Origin of Length
The Bordeaux chemists also showed that the aroma’s precursor is found in significant quantities in the wine itself, the aromatic molecule remaining bound to the nonaromatic part. The operation of enzymes in the saliva, which during tasting detach the cysteine from the sulfur aroma after a few seconds, is the chief mechanism underlying the notion of length, which is to say the length of time the sensation produced by a wine remains in one’s mouth. A special unit, the caudalie, has even been introduced to quantify the length of time this sensation persists once the wine has been swallowed. Certain Bordeaux wines produce a sensation that lasts several seconds (or caudalies) and, in a few exceptional cases, is capable of reasserting itself: Having disappeared, the sensation comes back. If the sulfur molecule discovered by the Bordeaux researchers does not actually explain this peacock-tail effect, it is the first to yield a length whose origin is understood.
What are the practical consequences of this discovery? Because the adulteration of wines is forbidden by law, oenologists (who often work for producers) will seek to optimize the concentration of a given aroma during vinification. But if you don’t have a lot of money and are willing to indulge in a little amateur chemistry, you can try adding the aroma to wines in your collection that don’t have enough of it. After all, the law doesn’t forbid lengthening the duration of one’s pleasure.