Effect of oxygenation on polyphenol changes occurring in the course of wine-making
Introduction
Phenolic compounds play a major role in wine quality. They contribute to sensory characteristics of wines, in particular colour and astringency. Wine colour (WC) is strongly influenced by grape composition and enological practices, such as wine-making techniques, storage temperature, length of storage [1], [2] and oxygen exposure [3]. During conservation and ageing of red wines, the concentration of grape anthocyanins, initially responsible for WC, decreases because they progressively react with other phenolic compounds, particularly flavanols [4], [5]. This phenomenon is thought to result in the colour change from red-bluish of young wines towards the reddish-brown colour of matured wines, as well as in the decrease of wine astringency observed during ageing [6]. At wine pH values, anthocyanins exist as two forms in equilibrium, namely the red flavylium cation (A+) and the colourless hydrated hemiketal form (AOH), the latter being predominant [7].
Various pathways involving both anthocyanin forms have been proposed to explain the conversion of anthocyanins to new, more stable pigments [4], [5], [8]. The first process is a direct reaction between flavanols (T) and anthocyanins (A) [4], [5], [9], [10], [11], leading to two kinds of products, denoted A–T and T–A, according to the position of the anthocyanin moiety. Recent work in our laboratory [12] has revealed the presence in wine of both types of compounds which, respectively arise from addition of tannins onto A+ and from addition of AOH onto the carbocation formed by acid-catalysed cleavage of proanthocyanidin interflavanic bonds.
The second pathway concerns the reactions between anthocyanins and flavanols mediated by acetaldehyde. This mechanism, first described by Timberlake and Bridle [8] and finally demonstrated by Fulcrand et al. [13], starts with acetaldehyde condensation on flavanols leading to a carbocation intermediate, which reacts in turn with either another molecule of flavanol or the hydrated form of an anthocyanin. Actually, the detection of (+)-catechin ethyl-bridged dimers in wine [14], of ethyl-bridged (epi)catechin dimers and trimers and of (epi)catechin-ethyl-malvidin 3-glucoside in model solutions [15] proved the occurrence of this condensation reaction in red wines (Fig. 1).
The third type of reactions established in wines is a cycloaddition process between anthocyanins and some yeast metabolites possessing a polarisable double bond. In particular, vinyl phenol [16], pyruvic acid [17], acetaldehyde [18], were shown to react through this mechanism, leading to more stable pigments, structurally allied to pyranoanthocyanins (Fig. 2). The occurrence of these pigments has been demonstrated in wine [16], [19], [20], [21], [22] and their remarkable stability and resistance to sulfite bleaching has been established [20], [23]. Moreover, a similar structure resulting from reaction of B2 (epicatechin-4β-8-epicatechin) procyanidin dimer, acetaldehyde and malvidin 3-glucoside in model solution has been recently postulated on the basis of MS data [24]. This structure corresponds to a pyranoanthocyanin-tannin adduct, the formation of which might be relevant to a new stabilisation process of wine pigments.
All these reaction mechanisms may influence the colour and colour stability of wine, as well as its gustatory qualities related to the structure of the tannins. Their relative importance, as well as the structure of the end products, depends on the initial wine composition, but also on the presence of yeast metabolites and on oxygen exposure. Thus, a mild oxygenation process, referred to as micro-oxygenation, was proposed to improve wine quality, assuming that orientation of phenolic compound reactions towards the oxidative ways resulted in more coloured and less astringent products [3], [25]. Among the reaction pathways described above, only those involving acetaldehyde, i.e. formation of pyranoanthocyanins and of ethyl-bridged adducts, are expected to be favoured by the presence of oxygen. Acetaldehyde is a natural compound occurring in wines, produced either by yeast metabolism during fermentation [26] or by ethanol oxidation in presence of phenolic compounds [27].
The purpose of the present work is therefore, to evaluate the effect of oxygenation on red WC and phenolic composition and, to elucidate possible relationships between the formation of various types of derived anthocyanins and WC changes.
Section snippets
Wine sample
The wine analysed was a blended red wine, made from Vitis vinifera var. Cabernet Sauvignon (60%) and Tannat (40%), vintage 1999. The wine was distributed into six tanks; three were saturated with N2 (control) and three were dispensed continuously with a flow providing 5 ml l−1 pure oxygen through a microdiffuser (oxygenated). Thus, the experiment (including storage in tanks, fractionation and analyses) was performed in triplicate.
Fractionation procedure
A 2 ml of the wine was fractionated on a Toyopearl TSK gel HW-50 (F)
Results and discussion
The anthocyanin composition of the wine sample was established by LC/DAD and LC/ESI-MS analyses. The liquid chromatogram recorded at 520 nm for the oxygenated wine sample at 7 months time is presented in Fig. 3. The chromatogram shows the presence of 15 anthocyanins, originating from grapes. Peaks 1–5 correspond to the 3-glucosides of delphinidin, cyanidin, petunidin, peonidin and malvidin. Peaks 1′–5′ were identified as the corresponding 3-acetylglucosides and peaks 1″–5″ as the 3-p
Conclusions
Statistical analysis of the measured values indicated that significant changes took place throughout the storage time and as a result of 7-month oxygen exposure. The wines at t0month and t1month were characterised by higher levels of pigments non-resistant to SO2 bleaching, including in particular genuine anthocyanins. With time, their concentration progressively diminished and more stable structures such as pyranoanthocyanins gradually accumulated. Oxygen exposure enhanced anthocyanin
Acknowledgements
Thanks are due to Society Oenodev for providing the wine samples.
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