The Effects of Microoxygenation on Pinot Noir Wines: The Timing of Oxygenation, Initial Phenolic Content, Yeast Activity, Acetaldehyde Production and Long-term Impact

Abstract

Microoxygenation (MOX) mimics the process of barrel ageing and has become a popular tool in winemaking. However, very few studies have investigated its impact on Pinot noir wines. Over the years, a series of factors, such as MOX timing and dosage, yeast activity, and initial phenolic content, have been shown to affect the outcome of MOX with deeper coloured wines. These all require further investigations on light-coloured Pinot noir wines. Therefore, this project aims to investigate the effect of MOX on Pinot noir wines, concerning the key influential factors anticipated from previous MOX studies. In the first vintage (2019), the timing and dosage of MOX were examined before and after malolactic fermentation. The oxygen was applied to wines at 10.8 and 52.4 mg/L/month for 30 days, doses that are close to the estimated total oxygen transmission through oak barrels for 12 months of barrel storage (i.e., 15 to 45 mg/L/year). Afterwards, a bottle ageing experiment was carried out to study the long-term impact of MOX, where these wines were bottled with screwcaps and stored in the upright position at 9°C for 12 and 18 months. In the 2019 vintage, an induced acetaldehyde production was observed during MOX, which was attributed to both chemical oxidation and a higher increase in yeast growth. Therefore, in the second vintage (2020), wines were sterile filtered to avoid microbial influence, and the effects of the wine's initial phenolic content were examined. MOX was applied for 14 days to two Pinot noir wines after malolactic fermentation (MLF), at 0.50 and 2.11 mg/L/day for wines of a lower and a higher phenolic content, respectively. The end point in both vintages was determined after SO2 addition (i.e., 90 mg/L in 2019 and 100 mg/L in 2020) to examine colour stability and resistance to SO2 bleaching. The results from the two vintages showed that MOX promoted a short-term/temporary colour improvement in the Pinot noir wines, increasing the absorbance at 520 nm and overall colour intensity. However, these were not significantly affected by the MOX timing and dosage. In the second vintage, the colour improvements were more apparent for the wines with a higher phenolic content. However, MOX did not guarantee colour stability, and much of the colour, except that of SO2 resistant pigments, was lost after SO2 addition. The lack of colour stability can be linked to the lower formation of acetaldehyde in the absence of yeast activity after sterile filtration. Thus, fewer acetaldehyde mediated anthocyanin reactions formed red pigments resistant to SO2 bleaching. For wines from the first vintage, negative impacts of MOX on wine colour development occurred after bottle ageing, with a considerable decline in the 520 nm absorbance, colour intensity and large polymeric pigments, which was greater in wines with MOX applied before MLF. It also appeared that MOX at the higher oxygen dosage lowered the wines’ natural preservative effect, seen by a significantly higher loss of total SO2 during bottle ageing, even though similar concentrations of free and total SO2 were obtained in all wines after bottle ageing. Therefore, for wines treated with MOX, the SO2 concentrations at bottling should be an important factor determining wine ageing ability. Impacts of MOX on monomeric phenolics were observed for both vintages. In the first vintage, wines with MOX applied after MLF had significant hydrolysis events for cis- and trans-coutaric acids and trans-caftaric acid at the end of MOX. These occurred after bottle ageing in the control wines and wines with MOX applied before MLF. These hydrolysis events were linked to cinnamoyl acid esterase (CE) activity of the inoculated Oenococcus oeni strain, which was promoted by MOX after MLF. Overall, with MOX, decreasing concentrations of the monomeric phenolics malvidin-3-glucoside and quercetin were observed. A decline of monomeric flavanols also occurred with MOX. However, the compounds and the degree of decline varied in the wines across the two vintages, possibly due to seasonal, viticultural, and winemaking influences on wine composition. Regarding tannins, MOX applied in both vintages increased the tannin concentration, although not always significantly different than the control treatments. There was a consistent decline in the degree of trihydroxylation (grape skin derived (-)-epigallocatechin extension units) in all MOX treatments from both vintages. In wines from the first vintage, this impact was observed even after bottle ageing for 18 months. These trends could be of concern as they might lead to an increase in astringency intensity, as grape skin tannins and (-)-epigallocatechin have been shown to restrain the coarse sensation in tannins. Overall, these results indicated that MOX should be applied to Pinot noir wines with caution.