A new chemical criteria for white wine: The glutathione equivalent capacity

Use of Fe (II) and H2O2 along with Heating for the Estimation of the Browning Susceptibility of White Wine

The aim of the present study was to investigate if the use of Fe (II) and H2O2 (Fenton reaction) along with heating can be implicated for the rapid evaluation of the browning susceptibility of white wine. In this context, increasing concentrations of Fe (II) and H2O2 were added to two white wines (Roditis–Malagouzia and Debina), the samples were stored at 45 °C or 18 °C, and the absorbance at 420 nm (browning index) was measured. Moreover, total sulfur dioxide, total phenolics, flavanols, hydroxycinnamates and total free sulfhydryls were assessed during storage of Roditis–Malagouzia wine samples. The results showed that the addition of Fe (II) + H2O2 mixture in white wines increases rapidly the browning index at 45 °C, indicating the ability to develop a rapid test for the estimation of the browning susceptibility of white wines. Results also showed that the level of flavanols is a suitable index to follow the forced oxidative browning of white wine by adding Fe II + H2O2 at 45 °C. Moreover, results indicated that the levels of total free sulfhydryls may be a suitable index to follow the forced oxidative browning of white wine and in general, white wine oxidation.

Molecular Characterization of White Wines Antioxidant Metabolome by Ultra High Performance Liquid Chromatography High-Resolution Mass Spectrometry

The knowledge about the molecular fraction contributing to white wines oxidative stability is still poorly understood. However, the role of S- and N-containing compounds, like glutathione and other peptides, as a source of reductant in many oxidation reactions, and acting against heavy metals toxicity, or lipid and polyphenol oxidation as ROS-scavenger is today very well established. In that respect, the aim of the present study is to introduce an original analytical tool for the direct determination of the available nucleophilic compounds in white wine under acidic pH conditions. One step derivatization of nucleophiles has been realized directly in wines using 4-methyl-1,2-benzoquinone (4MeQ) as an electrophilic probe. Derivatization conditions considering probe concentration, pH, reaction time, MS ionisation conditions and adducts stability, were optimized using model solutions containing standard sulfur and amino compounds (GSH, Cys, HCys and Ser-Aps-Cys-Asp-Ser, Asp-Met, Met and Glu). Ultra-high-performance liquid chromatography coupled to a quadrupole-time of flight mass spectrometer (UHPLC-QqTOF-MS) analysis of up to 92 white wines from different cultivars (Chardonnay, Sauvignon and Semillon) followed by Multivariate analysis (PLS DA) and Wilcoxon test allowed to isolate up to 141 putative wine relevant nucleophiles. Only 20 of these compounds, essentially thiols, were detectable in samples before derivatization, indicating the importance of the quinone trapping on the revelation of wine unknown nucleophiles. Moreover, annotation using online database (Oligonet, Metlin and KEGG) as well as elementary formula determined by isotopic profile, provided evidence of the presence of amino acids (Val, Leu, Ile, Pro, Trp, Cys and Met) and peptides with important antioxidant properties. The complimentary set of MS/MS spectral data greatly accelerated identification of nucleophiles and enabled peptides sequencing. These results show that probing wines with 4-methyl-1,2-benzoquinone enhances thiols ionisation capacity and gives a better screening of specific S- N- containing functional compounds as part of the white wines antioxidant metabolome.