The impact of oxygen exposure before and after bottling on the polyphenolic composition of red wines

The impact of moderate oxygen exposure after bottling on wine phenolic composition and colour properties has been investigated on four Grenache red wines showing high and low phenolic contents, obtained by flash release (FR) and traditional soaking (Trad), respectively, and processed with (Mox, 4.6 mg l⁻¹ O₂) or without (noMox) micro-oxygenation. Four oxygen transfer rate (OTR) conditions (0.8, 1.9, 8.0, and 11.9 μl oxygen/bottle/day) were ensured by using synthetic closures with controlled oxygen permeability and storage under controlled atmosphere. Wine phenolic composition and colour were monitored by triplicate analysis of bottles sampled at bottling (T0) and after 5 and 10 months of ageing. Phenolic composition was determined by reverse-phase HPLC-DAD–MS analysis, performed directly on the wines and after phloroglucinolysis for proanthocyanidins. Colour was evaluated by UV–visible spectrophotometry and tristimulus colorimetry. At T0, FR wines contained larger amounts of catechins, proanthocyanidins and hydroxycinnamic acids and slightly lower amounts of anthocyanins than the Trad wines while the Mox wines showed higher levels of some derived pigments, especially carboxypyranoanthocyanins, resulting from reactions of anthocyanins with pyruvate. During ageing, a progressive decrease of proanthocyanidins and hydroxycinnamic acid concentrations was observed, independently of OTR. In contrast, the loss of free SO₂, flavan-3-ol monomers and the conversion rate of anthocyanins to new pigments, especially into sulphite bleaching resistant pigments, increased with OTR levels. Among derived pigments, carboxypyranoanthocyanins levels increased both with micro-oxygenation and with OTR and can be considered as oxidation markers while flavan-3-ol–anthocyanin adducts were formed in larger amounts with lower oxygen exposure.