Understanding microoxygenation: Effect of viable yeasts and sulfur dioxide levels on the sensory properties of a Merlot red wine

Microoxygenation (Mox) is widely used in winemaking to improve color, in-mouth properties and aroma, but its use is not always predictable. Here we investigate the effect of Mox, (while monitoring viable yeasts and SO2 levels), on color, anthocyanin-derived pigments, tannins, aroma and in-mouth sensory properties as well as on hedonic rating by wine experts. Results on this Merlot wine show that the re-appearance of viable Saccharomyces cerevisiae yeasts modulates oxygen consumption rates, and dramatically increases acetaldehyde levels. This led to significant sensory changes, particularly for aroma. Mox reduced green-vegetable and reduction-related aromas, but also astringent mouth-feel properties related to tannins, and lower astringency was correlated with lower tannin activity. The Mox wines that exhibited yeast growth had higher hedonic scores from one group of expert tasters based on increased jammy/dried fruit flavors, while another group of tasters rated the non-Mox wines higher due to the green vegetable and spicy aromas. These results show that the chemical and sensory impact of a Mox treatment is highly dependent on the absence or presence of yeast growth, so it is important to monitor for yeast populations during Mox treatment.We thank the Spanish Ministry of Economy and Competitiveness (MINECO) and the European Union (FEDER Funds) for project funding (AGL2014-59840 and RTC-2015-3379). M.P.S.N. acknowledges the Spanish Ministry of Economy and Competitiveness (MINECO) for her postdoctoral fellowships (Formación Posdoctoral 2013: FPDI-2013-16332 and Juan de la Cierva Incorporación 2015: IJCI-2015-23444). We thank Constellation Brands for supporting the project and for providing the wine.Peer reviewe