Identification of a sotolon pathway in dry white wines

Sotolon (3-hydroxy-4,5-dimethyl-2(5H)-furanone) is a chiral furanone, an aroma compound known to be responsible for premature-aging flavor in dry white wines. Sotolon generally results from mild oxygenation during bottle aging, and until now, its formation pathways had not been elucidated. The ability of the main precursors described in the literature under very different experimental conditions to produce sotolon was tested. In model wine solution maintained at 40 degrees C for 6 months, sotolon was produced by the oxidative degradation of ascorbic acid. By use of GC-MS, 2-ketobutyric acid, produced by the oxidative degradation of the ascorbic acid in the model wine solution, was identified as a potent precursor of sotolon in this pathway. Ascorbic acid is an exogenous compound, added before bottling, but 2-ketobutyric acid was found even in white wines that had not been supplemented. Consequently, this sotolon formation pathway is also valid in white wines with no added ascorbic acid. In addition, we showed that Saccharomyces cerevisiae strains were capable of producing variable concentrations of this ketone during alcoholic fermentation. In model wine solution, certain yeast strains released large quantities of 2-ketobutyric acid, similar to those found in oxidized dry white wines. In view of these results, the role of yeast strains in this premature-aging phenomenon of dry white wines is discussed. Finally, these investigations revealed that one chemical mechanism responsible for the low concentrations of sotolon found in prematurely aged white wines made from various grape varieties was an aldol condensation between 2-ketobutyric acid and acetaldehyde.