Oxygen contribution to phenolic evolution during aging of red wines

Abstract Red wine aging is essentially an oxidative process mainly regulated by wine phenolic composition and storage conditions. Wines contain hydroquinones such as catechol derivatives that undergo redox reactions, reducing oxygen to hydrogen peroxide. These reaction are catalized by metals. Iron(II) species present react with hydrogen peroxide to form hydroxyl radicals in the Fenton reaction. These radicals can then react with alcohols to form aldehydes. Because ethanol is the predominant alcohol present in wine, acetaldehyde is the major product of this reaction. Acetaldehyde and other oxidation by-products activate a great series of reactions changing wine quality. Generally, a slow oxygenation improves quality of red wines, while an excessive oxidation causes a dramatic quality loss. Red wine phenolics, by trigging and driving oxidative reactions, strongly affect the outcome of wine oxidation and the acetaldehyde production. Although numerous studies deal on wine oxidation the role of initial wine phenolic composition on the outcome of oxidation is still not known. Apart polyphenols (e.g. anthocyanins, proanthocyanidins and pH,) also exogenous factors such as sulfur dioxide, hydrolysable tannins and glutathione worth investigation for the important implication for red wine quality and longevity. In this PhD thesis four studies were made to contribute to deep knowledge on wine oxidation chemistry. The first study provides clear evidence for a connection between anthocyanin/tannin ratio of red wine and the effect of oxygenation. Contact between wine and oxygen influences wine quality and the results obtained in this study highlight that, when dealing with wine rich in anthocyanins, the lower the anthocyanin/tannin ratio, the higher the positive effect of oxidation. Different anthocyanin tannin ratios during an oxidative process affects the copigmentation reaction causing a shift towards higher intensities (hyperchromic effect). In particular, the increase in polymeric pigments resistant to SO2 discoloration during oxidation is enhanced lowering the anthocyanin/tannin ratio. Tannin compounds have a similar behavior with oxidation: the three wines showed a decrease in VRF (monomers dimers and trimers) and an increase in BSA reactive tannins indicating a possible increase of the degree of polymerization. The higher the quantity of tannins added in wines, the lower the acetaldehyde produced by Fenton reaction. This is the first time that a strong link between the formation of acetaldehyde and the anthocyanins/tannins ratio in red wine has been observed. Further studies are still required to advance knowledge in the use of different tannins to improve wine characteristics during practices such as micro and nano-oxygenation. Starting from results obtained in the first study, a successive experiment was carried out aimed to evaluate the effect of the addition of grape tannins and hydrolysable tannins (gallic and ellagic tannins) of wood origin. All enological tannins preparations used increased the level polymeric pigments in wine at the end of the treatment. Among them, ellagitannins increased drastically the production of polymeric pigments already during the first phases of oxidative stress improving the stability of the color of the wine and causing a shift towards higher intensities (hyperchromic effect). During first phases of oxidation tannins reactive towards BSA increase for all wines, suggesting that during oxidation the high reactive tannins are involved in numerous condensation reactions. Acetaldehyde was quickly produced by Fenton reaction and, interestingly, all exogenous tannins determine a greater production of this highly reactive compound. For all wines analyzed a dramatic consumption of acetaldehyde was observed. These results confirm the key role of acetaldehyde in wine oxidation as trigger compound for reactions of stabilization of colour and condensation of tannins. In a third experiment the protective effect of sulfur dioxide and glutathione GSH on malvidin 3-monoglucoside degradation was evaluated. The use of GSH alone determined an increase in the degradation of malvidin 3-monoglucoside regardless of pH in model solution and in real wine. Results obtained in this study showed that the possibility to use GSH to prevent anthocyanins oxidation is not linked to its capability to quench hydrogen peroxide but only, in the first steps of oxidation, to act on quinones chemistry and limit the reduction of oxygen to hydrogen peroxide. When in wine is present hydrogen peroxide GSH is not able to scavenge it and contrast Fenton reaction nor alone and not in combination with SO2 at concentration usually proposed during winemaking. Taking into account these results and a recent study showing no protective activity of GSH to prevent white wines oxidation after one year of aging in bottles (Panero, et al., 2015), the use of this tripeptide as an alternative to SO2 has to be revised and the chemistry of action of this compounds in wine conditions better understood. The last experiment was performed applying a controlled nano-oxygenation to three monovarietal wines Aglianico (AGL), Casavecchia (CAS) and Pallagrello (PALL) very rich in tannins. In this work a general trend for polyphenols evolution during aging has been observed and it resulted into the loss of native anthocyanins, the formation of small and large polymeric pigments and the decrease in reactivity of tannins towards salivary proteins. The oxygen transmission rate (OTR) of closures can influence this trend but in different entity for the three wines considered. AGL wines resulted more affected by oxygen uptake than PALL and CAS ones. Because AGL wines showed lower SO2 protection and total phenolic content, these results underline that with low phenolic content a wine is more susceptible to oxidation and needs a special care on oxygen management. Wines differed for total anthocyanins content more than for total tannins content and AGL showed a anthocyanin/tannin ratio almost 3 times higher than CAS and PALL. This consideration and results obtained after 15 months of bottle aging suggest that the ratio between these two classes of compounds may have an important part on wine evolution. Further study can help to elucidate the role played by each phenolic class during NOx. The effect of OTR on sensory profiles of wines was more relevant than on polyphenols because for all wines, the closure with the highest oxygen ingress determined a higher intensity of red fruit notes. For CAS and PALL an antagonist effect between fruity notes and reduction ones was also observed and OTR of closure strongly affect this balance shifting wines towards reduction off-odours, especially with the lowest oxygen ingress. These data indicate that the selection of a closure that allows only specified amounts of oxygen into the wine over time is a useful tool to improve red wine quality taking into account its expected lifetime in bottle. All these results highlights the importance of anthocyanin/tannin ratio and of phenolic composition for the oxygen tolerance of a wine. Further studies should be aimed to find the relationship between the phenolic compounds variations, the acetaldehyde production and the formation of odorous volatiles compounds linked to oxidative spoilage of wine. With all these information a method to evaluate wine oxygen tolerance and to better manage wine shelf-life could be pointed out. During wine production and aging, the evaluation of acetaldehyde as well as the analysis and the use of SO2 resulted really critical and should be routinely monitored. GSH is instead not good to prevent Fenton reaction but only to its nucleophilic reaction with quinones. At this regard, the application of MOx in the post-fermentatives phase should be performed with low levels of SO2. Only when the desired stabilization is reached the wines should be properly preserved from further oxidation using SO2 and taking into account the level of acetaldehyde in wines.