An assessment of voltammetry on disposable screen printed electrodes to predict wine chemical composition and oxygen consumption rates.

The present work aimed at determining the applicability of linear sweep voltammetry coupled to disposable carbon paste electrodes to predict chemical composition and wine oxygen consumption rates (OCR) by PLS-modeling of the voltammetric signal. Voltammetric signals were acquired in a set of 16 red commercial wines. Samples were extensively characterized including SO2, antioxidant indexes, metals and polyphenols measured by HPLC. Wine OCRs were calculated by measuring oxygen consumption under controlled oxidation conditions. PLS-Regression models were calculated to predict chemical variables and wine OCRs from first order difference voltammogram curves. A significant number of fully validated models predicting chemical variables from voltammetric signals were obtained. Interestingly, monomeric and polymerized anthocyanins can be differently predicted from the first and second wave of the first derivative of voltammograms, respectively. This fast, cheap and easy-to-use approach presents an important potential to be used in wineries for rapid wine chemical characterization.

Formation and Accumulation of Acetaldehyde and Strecker Aldehydes during Red Wine Oxidation.

The main aim of the present work is to study the accumulation of acetaldehyde and Strecker aldehydes (isobutyraldehyde, 2-methylbutanal, isovaleraldehyde, methional, phenylacetaldehyde) during the oxidation of red wines, and to relate the patterns of accumulation to the wine chemical composition. For that, eight different wines, extensively chemically characterized, were subjected at 25°C to three different controlled O2 exposure conditions: low (10 mg L-1) and medium or high (the stoichiometrically required amount to oxidize all wine total SO2 plus 18 or 32 mg L-1, respectively). Levels of volatile aldehydes and carbonyls were then determined and processed by different statistical techniques. Results showed that young wines (<2 years-old bottled wines) hardly accumulate any acetaldehyde regardless of the O2 consumed. In contrast, aged wines (>3 years-old bottled wines) accumulated acetaldehyde while their content in SO2 was not null, and the aged wine containing lowest polyphenols accumulated it throughout the whole process. Models suggest that the ability of a wine to accumulate acetaldehyde is positively related to its content in combined SO2, in epigallocatechin and to the mean degree of polymerization, and negatively to its content in Aldehyde Reactive Polyphenols (ARPs) which, attending to our models, are anthocyanins and small tannins. The accumulation of Strecker aldehydes is directly proportional to the wine content in the amino acid precursor, being the proportionality factor much higher for aged wines, except for phenylacetaldehyde, for which the opposite pattern was observed. Models suggest that non-aromatic Strecker aldehydes share with acetaldehyde a strong affinity toward ARPs and that the specific pattern of phenylacetaldehyde is likely due to a much reduced reactivity toward ARPs, to the possibility that diacetyl induces Strecker degradation of phenyl alanine and to the potential higher reactivity of this amino acid to some quinones derived from catechin. All this makes that this aldehyde accumulates with intensity, particularly in young wines, shortly after wine SO2 is depleted.

The kinetics of oxygen and SO2 consumption by red wines. What do they tell about oxidation mechanisms and about changes in wine composition?

This work seeks to understand the kinetics of O2 and SO2 consumption of air-saturated red wine as a function of its chemical composition, and to describe the chemical changes suffered during the process in relation to the kinetics. Oxygen Consumption Rates (OCRs) are faster with higher copper and epigallocatechin contents and with higher absorbance at 620nm and slower with higher levels of gallic acid and catechin terminal units in tannins. Acetaldehyde Reactive Polyphenols (ARPs) may be key elements determining OCRs. It is confirmed that SO2 is poorly consumed in the first saturation. Phenylalanine, methionine and maybe, cysteine, seem to be consumed instead. A low SO2 consumption is favoured by low levels of SO2, by a low availability of free SO2 caused by a high anthocyanin/tannin ratio, and by a polyphenolic profile poor in epigallocatechin and rich in catechin-rich tannins. Wines consuming SO2 efficiently consume more epigallocatechin, prodelphinidins and procyanidins.

Oxygen and SO2 Consumption Rates in White and Rosé Wines: Relationship with and Effects on Wine Chemical Composition.

This Article addresses the study of O2 and SO2 consumption rates of white and rosé wines, their relationship to the initial chemical composition, and their effects on the chemical changes experienced by wine during oxidation. Eight wines were subjected to five consecutive air-saturation cycles. O2 was monitored periodically; SO2, color, and antioxidant indexes were determined after each cycle, and the initial and final compositions of the wines were thoroughly determined. Wines consumed oxygen at progressively decreasing rates. In the last cycles, after a strong decrease, consistent increases of oxygen levels were seen. Oxygen consumption rates were satisfactorily modeled, being proportional to wine copper, quercetin, and kaempherol contents and negatively proportional to cinnamic acids. SO2 consumption rates were highly diverse between wines and were positively related to free SO2, Mn, and pH, among others. In the last saturations, SO2 consumption took place regardless of O2 consumption, implying that SO2 should reduce chemical species oxidized in previous saturations. Some volatile phenols seem to be the end point of radical-mediated oxidation of polyphenols taking place preferably in the first saturation.

Gas chromatography-mass spectrometry strategies for the accurate and sensitive speciation of sulfur dioxide in wine.

Understanding the chemistry of wine oxidation requires the accurate and sensitive quantitative determination of the most important molecular species which SO2 can form. An analytical strategy based in three independent static headspace GC-MS determinations is proposed in order to obtain information about the total, nominally free and truly free levels of SO2. Nominally free forms are directly determined after sample acidulation, total forms require the previous incubation at 100°C, and truly free forms are determined after preconcentration of the headspace of the undisturbed sample in an alkaline solution. The two first determinations provide results equivalent to those reported by the aeration-oxidation (A-O) method, with lower limits of detection (1mgL-1) and better repeatabilities (<4.0%). Results from the analysis of different wines revealed that levels of nominally free are systematically in excess than those of truly free SO2, which suggests that free SO2 determined by any method using previous acidulation includes at least two different species of SO2, which may have different antioxidant behavior.

Oxygen Consumption by Red Wines. Part I: Consumption Rates, Relationship with Chemical Composition, and Role of SO2.

Fifteen Spanish red wines extensively characterized in terms of SO2, color, antioxidant indexes, metals, and polyphenols were subjected to five consecutive sensor-controlled cycles of air saturation at 25 °C. Within each cycle, O2 consumption rates cannot be interpreted by simple kinetic models. Plots of cumulated consumed O2 made it possible to define a fast and highly wine-dependent initial O2 consumption rate and a second and less variable average O2 consumption rate which remains constant in saturations 2 to 5. Both rates have been satisfactorily modeled, and in both cases they were independent of Fe and SO2 and highly dependent on Cu levels. Average rates were also related to Mn, pH, Folin, protein precipitable proanthocyanidins (PPAs), and polyphenolic profile. Initial rates were strong and negatively correlated to SO2 consumption, indicating that such an initial rate is either controlled by an unknown antioxidant present in some wines or affected by a poor real availability of SO2. Remaining unreacted SO2 is proportional to initial combined SO2 and to final free acetaldehyde.

Oxygen Consumption by Red Wines. Part II: Differential Effects on Color and Chemical Composition Caused by Oxygen Taken in Different Sulfur Dioxide-Related Oxidation Contexts.

Chemical changes caused by oxidation of red wines during 5 consecutive air-saturation cycles have been assessed. In order to investigate the existing relationship between the effects caused by O2 and the levels and consumption rates of wine SO2, the total oxygen consumed by the wines (16-25 mg/L) was subdivided into different nonmutually exclusive categories. The ones found most influential on chemical changes were the O2 consumed in the first saturation without equivalent SO2 consumption (O2preSO2) and the O2 consumed when levels of free SO2 were below 5 mg/L (radical forming O2). Chromatic changes were strongly related to both O2 categories, even though anthocyanidin degradation was not related to any O2 category. Radical forming O2 prevented both formation of red pigments and reduction of epigallocatechin and other proanthocyanidins, induced accumulation of phenolic acids, and caused losses of ß-damascenone and whiskylactone without evidence of acetaldehyde formation. O2preSO2 seemed to play a key role in the formation of blue pigments and in the decrease of Folin index and of many important aroma compounds.