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Taking advantage of the low oxidation potential of polyphenolic compounds, voltammetric techniques, such as cyclic voltammetry (CV) and differential pulse voltammetry (DPV) are used rather indiscriminately. In this work, we report Total Polyphenols results (TPP) obtained by these two techniques from a set of nine samples of red and Tawny Port wine. The CV and DPV voltammograms display significant correlations with the physical-chemical parameters used to characterize red and Tawny Port wines, particularly with polyphenols. Although data obtained from CV and DPV for a single polyphenol are directly proportional, important deviations are found between voltammetric results from wines. Results from CV tend to be larger than those from DPV. This difference, that can reach 50 % of the TPP value, was related to the presence of total sulphur dioxide. In view of the present study, the polyphenol quantification in wines should be performed by DPV to minimize the interference of SO2.

The development of a fingerprinting strategy capable to evaluate the "oxidation status" of white wines based on cyclic voltammetry is proposed here. It is known that the levels of specific antioxidants and redox mechanisms may be evaluated by cyclic voltammetry. This electrochemical technique was applied on two sets of samples. One group was composed of normal aged white wines and a second group obtained from a white wine forced aging protocol with different oxygen, SO(2), pH, and temperature regimens. A study of antioxidant additions, namely ascorbic acid, was also made in order to establish a statistical link between voltammogram fingerprints and chemical antioxidant substances. It was observed that the oxidation curve presented typical features, which enables sample discrimination according to age, oxygen consumption, and antioxidant additions. In fact, it was possible to place the results into four significant orthogonal directions, compressing 99.8% of nonrandom features. Attempts were made to make voltammogram fingerprinting a tool for monitoring oxidation management. For this purpose, a supervised multivariate control chart was developed using a control sample as reference. When white wines are plotted onto the chart, it is possible to monitor the oxidation status and to diagnose the effects of oxygen regimes and antioxidant activity. Finally, quantification of substances implicated in the oxidation process as reagents (antioxidants) and products (off-flavors) was tried using a supervised algorithmic the partial least square regression analysis. Good correlations (r > 0.93) were observed for ascorbic acid, Folin-Ciocalteu index, total SO(2), methional, and phenylacetaldehyde. These results show that cyclic voltammetry fingerprinting can be used to monitor and diagnose the effects of wine oxidation.

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The role of H þ on the steady-state voltammetric limiting current (I L ) of weak acid (HA) reduction was studied considering results from either lactic acid or acetic acid solutions at constant pH. For solutions with pH > pK a or pH ffi pK a the [H þ ] effect on the I L vs c HA slope was important even when the H þ current contribution was minor. In contrast, when the pH ( pK a high H þ current contributions were found; nevertheless the I L vs c HA slope was almost insensitive to pH. A general relationship between I L and c HA was derived considering both the H þ current contribution and the dissociation extent. This equation allows the analytical determination of weak acids, alone or in the presence of strong acids, from experimental I L and pH. Acid identification can also be performed through the expression presented, following the I L susceptibility to pH at constant c HA .