The removal of Cu from wine by copolymer PVI/PVP

Cu removal from wine with copolymer PVI/PVP: Effects on Cu fractions and binders
The lower pH wines improved the total copper removal rate by PVI/PVP treatment.
Abstract
• PVI/PVP treatment effectively removes all forms of Cu in white wine.
• H2-bound Cu is more easily removed from red wine than organic acid-bound copper.
• H2 removes S with Cu, without removing other weaker Cu binders
Certain forms of copper in wine can affect the flavor and development of the wine. The copolymer polyethylimidazole/polyethylpyrrolidone (PVI/PVP) is known to remove copper from wine, but its effects on different forms of copper are uncertain. In this study, 3 Cu fractions in white wine were determined by colorimetric method, and 2 Cu fractions in red wine were determined by diatomite deep filtration and atomic spectrometry. PVI/PVP is formulated with silica or chitosan and reduces all three fractions of Cu measured in white wine, as well as sulfide-bound Cu in red wine. The low removal efficiency of organic acid-bound Cu in red wine is related to the higher pH value of red wine. After PVI/PVP treatment, the concentration of hydrogen sulfide in the wine was lower, but the weaker Cu binding changed little. These results show that PVI/PVP effectively removes the least desirable form of Cu present in wine, as well as its harmful binding agent (i.e., hydrogen sulfide).

introduce
Copper (Cu) is a common metal ion in all wines, with typical concentrations below 0.5 mg/l (Garcia-Esparza et al., 2006, Hirlam et al., 2019). Copper in wine may come from viticultural sources, including the application of biocides (Garcia-Esparza et al., 2006) and absorption by soil (Singh, 2006), or contamination during wine production (Clark, Wilkes, and Scollary, 2006). 2015) and the addition of Winemakers (Pyrzynska, 2007). In fact, copper is often deliberately added to wine to inhibit reducing odors, such as hydrogen sulphide induced odors (H2S), by forming non-volatile complexes with these adverse aroma compounds. Copper with excess hydrogen sulfide in wine can form copper organic acid complexes, and this form of copper can ensure that the wine is protected from the accumulation of hydrogen sulfide in the short term (Clark, Zhang and Kontoudakis, 2020). For residual copper sulfide complexes formed in wine during wine production, subsequent sheltering or filtration (0.45 or 0.20μm) is usually not successful in removing them (Clark, Grant-Preece, Cleghorn, and Scollary, 2015). Kreitman et al. (2016) studied the reaction 2S of Cu (II) and H in a model wine and identified the main product as copper sulfide (I). Alternatively, if this reaction occurs in the presence of a thiol compound, a range of disulfide and polysulfide can also be produced (Kreitman, Danilewicz, Jeffery, and Elias, 2017). Sulphide binding of copper and polythionane has been identified as a potential source of H 2 in the model wine aging process S under low oxygen conditions (Bekker et al., 2018, Ferreira et al., 2018, Kreitman et al., 2019). In addition, copper can be used as a medium for oxidation of wine (Danilewicz, 2016), and a mechanistic explanation for this role has been proposed (Danilewicz, 2016, Rousseva et al., 2016). Finally, excess copper in wine can cause copper fog (Ribereau-Gayon, Glories, Maujean and Dubourdieu, 2006), but this usually requires concentrations of copper much higher than those typically encountered in modern winemaking.
In order to limit the potential impact of copper on wine development, a number of strategies have been adopted to reduce its concentration during wine production. When particularly high copper concentrations in wine were more prevalent, more invasive copper removal protocols were developed. This includes using sodium sulfide (Ribereau-Gayon, 1935) with proteins (e.g., isinglass or bull’s blood) (Ribereau-Gayon, 1977) or using potassium ferrocyanide (i.e., blue clarification) (Ribereau-Gayon, 1935). Another strategy is to utilize bentonite, a conventional additive used in white wine production, but it is less efficient at removing copper than blue clarification (Catarino, Madeira, Monteiro, Rocha, Curvelo-Garcia, and deSousa, 2008). The use of polymer materials with targeted functional groups has been evaluated (Ben e ́tez et al., 2002, Loubser and Sanderson, 1986, Vibhakar et al., 1966), in which, Resin copolymers polyethylimidazole/polyethylpyrrolidone (PVI/PVP) have been shown to be particularly effective.
PVI/PVP consists of polymerized N-vinylimidazole and n-vinyl-2-pyrrolidone monomers with a molar ratio of 9:1 (Supplementary Figure 1) (Mira, Leite, Catarino, Ricardo da-Silva and Curvelo-Garcia, 2007). Commercial products may include other ingredients, such as silica or chitosan, the latter of which is proposed to help remove metal ions as well as act as a clarifying agent. PVI/PVP has some similar properties to polyethylene polypyrrolidone (PVPP), which is commonly used to remove phenolic compounds from wine, but PVI/PVP is more efficient at removing metals, including copper, iron (Fe), lead (Pb), cadmium (Cd), and aluminum (Al) (Hirlam et al., 2019, Mira et al., 2007, Schubert and Glomb, 2010). Studies have shown that metal removal efficiency varies depending on the type of PVI/PVP used (i.e., with or without other additives). Copper removal from white wine has been reported to be more effective than removal from red wine (Hirlam et al., 2019), but the reasons for this difference are uncertain. Residual pyrrolidone was detected in PVI/PVP and PVPP treated wines, however, it was found that pyrrolidone could be formed in yeast metabolite wines regardless of copolymer treatment (Schubert et al., 2010).
In determining the efficiency of removing Cu from wine by PVI/PVP, the effect of PVI/PVP on the different forms of Cu in wine has not been given insight. That said, its effects on the potentially harmful sulfide-bound form of Cu or on the potentially beneficial Cu-organic acid form are unknown. In fact, many studies have added PVI/PVP shortly after adding 0.5-6 mg/l Cu (II) to wine (Friedenberg et al., 2018, Mira et al., 2007). Such a large amount of copper addition may facilitate the removal of one form of copper rather than another (i.e. copper-organic acid with copper-sulfide complexes). In addition, it is uncertain whether PVI/PVP removes Cu alone or in combination with its complexing agent. For example, if sulfide is released from Cu during PVI/PVP treatment, the accumulation of 2S of uncomplexed H will be detrimental to wine quality. Various analytical techniques and methods have been validated for the measurement of different forms of copper in wine, including electrochemistry (Clark, Kontoudakis, Barril, Schmidtke and Scollary, 2016), solid phase extraction fractionation (Pohl&Sergiel, 2009), Colorimetry and diatomaceous earth deep filtration (Clark et al., 2020). Using these different methods, H2S and mercaptan compounds were identified as the most critical wine components capable of converting Cu from its organic acid form to Cu (Clark et al., 2020).
The aim of this study was to evaluate the effects of two different types of PVI/PVP on the removal of different forms of Cu in wine and model wine. A preliminary analysis was performed to explain the increased Cu removal efficiency in white wines compared to red wines. The copper form is then measured by previously validated electrochemical, colorimetric and deep filtration techniques. To gain insight into whether the Cu binder remained in the wine after PVI/PVP treatment, the concentrations of residual hydrogen sulfide and methylmercaptan were measured and the ability of the treated wine to rebind Cu was assessed. Finally, a model wine study was conducted to evaluate the activity of PVI/PVP in removing copper-thinning byproducts (i.e., polythioalkane).
Chemical and wine samples
Copper (1000mg/L, ICP grade), sodium sulfide (99.5%), sodium methionate (90%), L-cysteine (98%) and L-glutathione reduction (98%) were purchased from Sigma-Aldrich (Castle Hill, New South Wales, Australia). Regenerated cellulose (RC) 0.2μm membrane syringe filter (Phenex) is provided by Phenomenex (LaneCoveWest, NSW, Australia). Ultrapure water (18.2MΩcm) is produced by the Milli-QPlus purification system (MerckMillipore, Bayswater, VIC, Australia).
A 12% (v/v) ethanol solution is used
Total copper removal rate of PVI/PVP: white wine versus red wine
As reported by Hirlam et al. (2019), the relationship between wine pH and total copper removal was investigated in order to determine the reasons for the improved total copper removal efficiency in white wines compared to PVI/PVP. The pH of wine is of particular interest, as it is known that red wines generally have a higher pH than white wines. Although the original study by Hirlam et al. (2019) did not publish pH values for wines treated with PVI/PVP, the collection of this dataset (supplementary Table 2),
conclusion
The removal of copper by PVI/PVP is affected by the pH of the wine, so the removal is significantly more effective at lower pH values. PVI/PVP treatment is effective in reducing all Cu content in white wines, but in red wines, PVI/PVP is more effective in removing Cu associated with sulfides than Cu associated with organic acids. This result in red wine is likely due to the higher pH value of red wine which hinders the removal of organic acid-bound Cu components.