Electron paramagnetic resonance (EPR) spin trapping applied to Chardonnay wines: impact of phenolic content and ethanol.

Chemical oxidation is the main cause of damage during the bottle aging of white wines. It is due to Fenton-type reaction mechanisms, i.e., a radical oxidation cascade which leads to the formation of 1-hydroxyethyl radicals (1-HER) from ethanol. An optimized electron paramagnetic resonance (EPR) approach, using the Fenton reaction and N-tert-Butyl-α-(4-pyridyl-1-oxide) nitrone as spin trap, was used to study the formation kinetic of 1-HER. Important parameters of the reaction were identified and optimized, i.e., temperature, dissolved oxygen and free bisulfite content. We found that acetaldehyde removed bisulfite without artifacts on 1-HER kinetics, making the protocol suitable for routine laboratory analysis. We then validated the log-normal model to describe the complex formation/degradation kinetic of 1-HER and used it to rank 69 Chardonnay wines in four categories. Low levels of phenolic acid, in particular the cinnamic acids such as caftaric acid, seemed to correspond to a specific formation kinetic of 1-HER (high level produced slowly). In addition, ethanol concentration (0-100 % vol.) greatly impacted its formation kinetics, whereas it had non-significant effect within the white wine range (12-15 % vol.). These results demonstrate that the response of white wines to radical oxidation in controlled and tuned conditions is a promising approach to evaluate aging potential and suggest that maturity level might modulate the 1-HER formation in Chardonnay wines.