Pinking of white wine is an undesired change potentially occurring over storage, leading to the turning of color from yellow into salmon-red hue. Recently, the appearance of pink color was associated to small concentrations of malvidin-3-O-glucoside (∼ 0.3mg/L) present in white wines produced under reducing conditions from Síria grape variety . Other suggested mechanisms were the polymerization of anthocyanins under oxidative condition, the combination of more than ten different monomers and polymeric compounds, the formation of a derivative from 2-S-glutathionyl-caftaric acid . However, this color modification has been not fully understood. This study aimed to clarify the molecular mechanisms and the compound(s) involved in the pinking of white wine.
The appearance of pinking was evaluated in model wine added with increasing concentrations of sulfur-containing compounds (i.e. glutathione, cysteine, mercaptoethanol), and fixed amounts of the phenolics (i.e. catechin and caffeic acid), singularly or in combination. An assay with copper, with and without phenolics, was also carried out. The oxidation was generated by adding p-benzoquinone in both oxic and anoxic conditions. The intensity of pink color was measured at 520 nm. A major compound associated to pinking was detected by UPLC-UV and its molecular weight and structure were investigated by High Resolution Mass Spectrometry (HRMS) and Nuclear Magnetic Resonance (NMR), respectively.
In most of the tested conditions, the pink color appeared and resulted more intense with catechin. On the contrary, the color was yellow-brownish in the absence of phenolics notwithstanding the presence of copper. Considering the single addition of the thiol compounds, the major pink intensity and the fastest appearance were due to cysteine. The pinking intensity was lower with glutathione and it was not detected with mercaptoethanol. Catechin was the phenolic mainly involved into the pinking. The rate of pinking formation was dependent on both the thiol/p-benzoquinone and catechin/p-benzoquinone molar ratios with the former playing a paramount role. Copper was also involved in this phenomenon The major formation rate was observed when thiol/p-benzoquinone molar ratio was about 0.7. The compound associated to the pink color showed a maximum adsorption at 505 nm, characteristic of anthocyanin-like moieties and its accurate mass ([M+H]+) was 450.0635 Da. NMR analysis evidenced three molecular forms in equilibrium. The estimated conversion yield was 5%.These data suggest that pinking phenomena, in our experimental conditions, is due to the oxidation of catechin with the aid of sulphur-containing compounds, the latter with a crucial role for this color change.
Authors: Ragg Enzio1, De Noni Ivano1 and Tirelli Antonio1
1Department of Food, Environmental and Nutritional Sciences (DeFENS), Università degli Studi di Milano, Via G. Celoria 2, 20133 Milan, Italy
*corresponding author: firstname.lastname@example.org
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 Gabrielli M., Fracassetti D., Romanini E., Colangelo D., Tirelli A., Lambri, M. (2021). Oxygen-induced faults in bottled white wine: A review of technological and chemical characteristics. Food Chemistry, 348, 128922. https://doi.org/10.1016/j.foodchem.2020.128922.
Keywords: Pinking, Oxidation, Quinones, Cysteine, Catechin