Thi Nguyen and Andrew Waterhouse*
*Department of Viticulture and Enology, University of California, Davis, 1 Shields Ave., Davis, CA 95616 (alwaterhouse@ucdavis.edu)

Postfermentation formation of acetaldehyde through practices such as microoxygenation seldom occurs predictably. Recent studies have suggested that acetaldehyde, while normally thought of as a product of chemical oxidation, may be produced by Saccharomyces cerevisiae in oxidative conditions. It was first hypothesized that providing oxygen after fermentation induces a switch to respiratory metabolism, during which ethanol is reverted to acetaldehyde. Dry synthetic wines made with various strains of S. cerevisiae, to avoid loss of acetaldehyde in reactions with natural wine constituents, and to evaluate the strain-dependence of acetaldehyde production, were oxygenated postfermentation. While acetaldehyde dynamics were strain-dependent, no accumulation was observed, with concentrations instead unchanging or declining for all strains tested. It was then hypothesized that acetaldehyde production is not due to respiratory metabolism, but rather to the fermentation of residual sugar stimulated by the provision of oxygen. Following the addition of 3 g/L glucose to otherwise dry synthetic wine made with S. cerevisiae strain EC1118, an acetaldehyde increase concomitant with the consumption of glucose was observed when the wine was oxygenated. Glucose was also consumed in non-oxygenated wine, though no acetaldehyde accumulation was observed. It is proposed that fermentation predominates regardless of oxygen availability, but oxygenation precludes the need for ethanol production normally required to maintain the redox balance of NAD+/NADH, thus effectively halting the fermentative pathway at acetaldehyde. These experiments help explain the erratic acetaldehyde production observed during postfermentation oxygenation, though additional experiments with real wines are necessary to verify these findings.

Funding Support: Gail E. and Ruth M. Oliver Scholarship, Adolf L. and Richie C. Heck Research Fellowship, Mario P. Tribuno Memorial Research Fellowship, Harold Berg Memorial Fund