AbstractBACKGROUNDMicro‐oxygenation (MOx) is a common winemaking treatment used to improve red wine color development and diminish vegetal aroma, amongst other effects. It is commonly applied to wine immediately after yeast fermentation (phase 1) or later, during aging (phase 2). Although most winemakers avoid MOx during malolactic (ML) fermentation, it is often not possible to avoid because ML bacteria are often present during phase 1 MOx treatment. We investigated the effect of common yeast and bacteria on the outcome of micro‐oxygenation.RESULTSCompared to sterile filtered wine, Saccharomyces cerevisiae inoculation significantly increased oxygen consumption, keeping dissolved oxygen in wine below 30 µg L–1 during micro‐oxygenation, whereas Oenococcus oeni inoculation was not associated with a significant impact on the concentration of dissolved oxygen. The unfiltered baseline wine also had both present, although with much higher populations of bacteria and consumed oxygen. The yeast‐treated wine yielded much higher levels of acetaldehyde, rising from 4.3 to 29 mg L–1 during micro‐oxygenation, whereas no significant difference was found between the bacteria‐treated wine and the filtered control. The unfiltered wine exhibited rapid oxygen consumption but no additional acetaldehyde, as well as reduced pyruvate. Analysis of the acetaldehyde‐glycerol acetal levels showed a good correlation with acetaldehyde concentrations.CONCLUSIONThe production of acetaldehyde is a key outcome of MOx and it is dramatically increased in the presence of yeast, although it is possibly counteracted by the metabolism of O. oeni bacteria. Additional controlled experiments are necessary to clarify the interaction of yeast and bacteria during MOx treatments. Analysis of the glycerol acetals may be useful as a proxy for acetaldehyde levels. © 2017 Society of Chemical Industry