Some white wines show premature ageing effects after relatively short bottle ageing, although there is nothing wrong with the vinification process and the levels of heavy metals (specifically iron and copper) are not higher than legally permitted. The question addressed by this study was if the faster ageing of white wine perhaps could be attributed to the levels and type of metals that act as catalysts of oxidation. The metal contents of beverages such as wines are determined, amongst other factors, by the mineral levels in the soil, agricultural practices, the raw materials used in production, conditions and type of processing and contact with machinery and processing equipment during production. Metals in wine are known to act as catalysts in oxidation processes, form complexes with various wine constituents, lead to haze formation, clouding and precipitation, cause wine instability at high concentrations, and impact on the organoleptic properties of wine. The aim of the project was to investigate the role of metals on the ageing potential and quality characteristics of white wines. A possible outflow would be to make recommendations on the optimum degree of reduction of the metallic content of specific white wines in the context of achieving a significant control over its tendency to deteriorate, and to implement strategies for wine quality improvement.
The role of metals in the shelf-life of white wine
In the first part of this study the optimal metal removal capacity of five metal-chelating resin(s)/agents, i.e. Divergan® HM, Lewatit® TP-207, Purolite® C100E, Purolite® C100S and potassium ferrocyanide was determined. Challenges facing a researcher in the selection and use of metal-chelating agents/resins include limited literature on metal-chelating agents/resins for the extraction of specific metals from wine. An extensive amount of time must, therefore, be spent on researching, selecting and procuring suitable resins. Winemakers might find it difficult to find resins locally as local suppliers are not familiar with metal-chelating resins. Resins are expensive. Each resin has unique properties, and advantages and disadvantages to be considered before application in wine is metal-selectivity, metal-removal efficiency, impact on other wine constituents (should be minimal), and whether or not it is safe for use in wine, regulated (i.e. potassium ferrocyanide) or FDA approved. Potassium ferrocyanide is a controversial fining agent which has already been used extensively in the wine industry, but under strict legal regulations due to its potential toxicity. Ion-exchange resins is also authorised in the case of alcoholic fruit beverage or grape-based liquor. The use of polyvinylimidazole/polyvinylpyrrolidone (PVI/PVP) copolymers in order to reduce the copper, iron and heavy metal content in wine is authorised and regulated under EC regulation. Chelating agents with a fine, powdery nature can make it difficult to clarify the wines once treated. The porosity of some chelating resins might affect the levels of impurities present, necessitating complex washing procedures to purify the resin. Some resins must first be activated before use, by washing it with a strong acid to convert it from a stable Na-form to its hydrogen (H) form and/or to get rid of impurities. Resins stored and transported in a stable Na form, needs to be activated before use. Care should be taken as activated resin (washed with acid and rinsed with deionised water) could impact on the pH of the wine after resin treatment, which would necessitate pH adjustments of the wine at the start of ageing to counteract imbalances in acid. Dose- and time-dependent studies have been performed in this investigation to ascertain the dose and time needed for optimal extraction of metals.
In this study the resins used singularly or in combination did reduce the levels of the various metals in wine to various degrees, i.e. 5% to nearly a 100%. A high-dosage, mixture of activated (acid-washed, where applicable) metal-chelating agents was the most effective choice in achieving an almost complete removal of metals from white wine. Metals analysed by inductively-coupled plasma optical emission spectroscopy (ICP-OES) included Al, B, Ba, Ca, Cd, Cu, Fe, Ge, K, La, Mg, Mn, Mo, P, Pb, Si, Sr, Tl, Zn and W.
Resin (Purolite® C100E and Purolite® C100S) treatments did impact on wine components, including a reduction in the total phenolic content. Resin-treated wines showed significantly less persistence, fruity aroma and lower overall quality. Although resin treatment impacted organoleptically and chemically on Chenin blanc wines, treated wines were less prone to deterioration or changes in colour quality, to turbidity and oxidative increases, and decreases in fruity character, persistence and overall quality under accelerated conditions at high storage temperatures. Treated wines, therefore, with reduced metal and phenolic contents, exhibited a reduced susceptibility to undergo negative colour changes under accelerated conditions.
The study aimed at investigating the role of metals in the shelf-life of white wine. Untreated or control, and treated wines from which metals were removed, were stored at different temperatures and evaluated chemically and organoleptically at different periods of ageing up to 16 months. Both Purolite resins used in this study demonstrated to be effective in reducing the content of various metals in white wines. However, significant losses were observed in the organoleptic characteristics of the treated wines. Treated wines were also significantly more acidic, with significantly lower pH than untreated wines. Although resin treatment to remove metals from wine impacted on the wine organoleptically and chemically, treated wines from which metals were removed, showed improved shelf-life potential compared to untreated wines, particularly under accelerated conditions at higher storage temperatures. In addition to treatment, storage temperature and time of bottle ageing also impacted on the quality and shelf-life of white wine, with wines stored at higher temperatures for longer periods of time described as having insufficient colour, presence of sulphur odours, VA, faulty acid balance, oxidative character, less fruity, lowest overall quality and more sediment. Future investigations should investigate other metal-chelating resins or methods for the removal or reduction of metals in wine in order to improve the shelf-life of the wine. Overall therefore, although resin treatment reduced the tendency of white wines to suffer deterioration under accelerated conditions, their use was of little practical value, since the Purolite resins adversely affected the organoleptic qualities of the wines.
The aim of the project was to investigate the role of metals on the shelf-life and quality characteristics of white wines. Dose-dependent studies performed initially indicated the amount of particular metal-chelating resin necessary to get optimal extraction of metals from Chenin blanc wine. Chenin blanc wine treated with optimal resin concentration to remove metals, and control wines not subjected to this treatment were then stored at different temperatures (i.e. 15°C, 25°C and 35°C) and analysed sensorially and chemically at time zero (unmatured) and at 4, 7 and 16 months of bottle ageing. Treatment, storage temperature and time of bottle ageing did impact on the sensory descriptors and chemical (including metals) data. Although resin treatment impacted organoleptically and chemically on Chenin blanc wines, treated wines were less prone to deterioration or changes in colour quality, to turbidity and oxidative increases, and decreases in fruity character, persistence and overall quality under accelerated conditions at high storage temperatures.
– For more information, contact Francois van Jaarsveld at VJaarsveldfF@arc.agric.za.