Fractionation of copper and iron in wine: Assessment of potential macromolecule and sulfur binding agents
Abstract
Copper and iron are known to substantially impact wine stability through oxidative, reductive or colloidal phenomena. However, the binding of metal ions to different wine components under wine conditions, and the impact of this binding on the ability of the metal ions to induce spoilage processes, is not well understood. This study surveyed a range of red and white wines for an understanding of the variability of broad metal categories within the wines. The techniques utilized included an electrochemical constant current stripping potentiometry technique (ccSP), and solid phase extraction (SPE) fractionation of wine with subsequent analysis of the metal content of each fraction by inductively coupled plasma – optical emission spectroscopy (ICP-OES). The binding efficiency of specific classes of wine macromolecules and wine sulfur compounds for copper(II) and iron(II) was also assessed, and related to the metal categories found in the surveyed wines. The wine macromolecules examined included isolated white wine protein, white wine polysaccharide, red wine polyphenols (including procyanidins and monomeric phenolic compounds), and white wine polyphenols. The sulfur compounds included hydrogen sulfide, methanethiol, glutathione and thiol-substituted phenolic compounds. For the volatile sulfur compounds, the free and bound-forms were also measured by gas chromatography with sulfur chemiluminescence detection (GC-SCD). The binding was assessed by mixing the wine components with copper (II) (0.4 mg/l), iron (II) (3 mg/l) and two different metal ion mixtures (Fe 3 mg/l + Cu 0.4 mg/l and 3 mg/l + 0.2 mg/l) in a model wine system (pH 3.2) in low oxygen wine conditions. The results showed that in the wines surveyed the metal ions had significant variability in fractionation, with a higher proportion of bound copper than iron. From the binding studies, it was found that a component of the red wine polyphenol wine fraction demonstrated evidence of interaction with both copper and iron, whilst hydrogen sulfide was a significant binder of copper. Importantly, the binding between hydrogen sulfide and copper was shown to be reversible in wine conditions. The other wine macromolecules did not show any significant binding to the metal ions. The results demonstrate an important insight into the predominant forms of iron and copper ions in wine, and also insight into the main binders, especially from the perspective of wine macromolecules.
Issue: Macrowine 2016
Type: Article
Authors
*CSU/NWGIC