The aim of this study was to investigate the effect of oxygen during pressing and handling in modulating the metal ion composition in a Chardonnay wine throughout vinification, and as such determine whether certain metal ions preferentially accumulate in specific wine fractions (i.e., grape solids post-pressing, wine lees, or in the finished wine supernatant). In this study, the different levels of oxygen exposure were achieved through inert and oxidative press conditions, as well as the protected or oxidative handling of juice and just-fermented wine. The metal ions investigated included macro metal ions, metal ions known to have catalytic abilities, as well as metal ions introduced into wine through metal winemaking equipment.
- Chardonnay grapes from the Barossa region (2014 vintage) were hand-picked and whole-bunch pressed (3 tonnes per load) using a Bucher-Vaslin XPF50 Inertys®.
- Two pressing protocols were followed, inert mode and aerobic (or normal) mode.
- The inertly pressed or aerobically pressed juice samples were subdivided into juice samples that were to be handled either “reductively” or “oxidatively” during winemaking and these samples were transferred into 500 L temperature-controlled tanks.
- All juice samples received a standard addition of 35 mg/L sulfur dioxide (SO2) in the press holding tank before transportation to the Hickinbotham–Roseworthy Wine Science laboratory at the University of Adelaide, where the inert and aerative juice treatments were cold settled.
- The wines were handled “reductively” by creating an inert atmosphere in the source and receival tanks (blanketing with inert gas or dry ice) during transfer and racking operations, resulting in four combinations of pressing and handling: inert pressing–reductive handling (IR), inert pressing–oxidative handling (IO), aerobic pressing–reductive handling (AR), and aerobic pressing–oxidative handling (AO). All treatments were conducted in triplicate.
- Bentonite was added to all samples during active ferment when the total soluble solids dropped by 1 Bé to give a concentration of 1 g/L.
- The reductive handling technique was continued until after post-fermentation racking, at which point 60 mg/L SO2 was added. The tank headspace was regularly sparged with inert gas to maintain a headspace oxygen concentration below 0.5% air saturation.
- For the oxidative handling, the same processes were followed as for the reductively handled wines; however, no inert gas was used during the winemaking process until after post-fermentation racking.
- After post-ferment racking, all four treatments were treated identically, with all subsequent winemaking operations carried out in a standard reductive manner using inert gas cover to minimize oxidation.
- Wines were analysed for the following metal ion concentrations: Ag, Al, As, Au, Ca, Cd, Co, Cr, Cu, Fe, Hg, K, Mg, Mn, Na, Ni, Pb, Pd, Pt, Se, Sn, and Zn by Flinders Analytical, Flinders University (Adelaide, Australia) using an Agilent 7500 cx inductively coupled plasma mass spectrometer.
- Oxygen exposure significantly impacted 13 metal ions at different phases of winemaking.
- However, only the concentrations of Cr, Cu, and Fe were impacted by early oxygen exposure during pressing, with lower Cr and Cu concentrations in wines that were aerobically pressed and lower concentrations of Fe in wines that were inertly pressed.
- The sequestering of Al, Cu, Ni, and Zn by wine lees was significantly affected by oxygen treatment, with lees collected from wines that were treated oxidatively sequestering significantly greater amounts of Cu and Zn and removing these metals from the wine supernatant.
- The metal ion that was most affected by oxygen exposure during pressing and handling was Cu, with significantly lower Cu measured in wines that were produced under oxidative conditions.
SIGNIFICANCE OF THE STUDY:
Of all the metal ions evaluated in this study, Cu was the most significantly impacted by oxygen exposure throughout the winemaking process. Importantly, oxygen exposure significantly impacted the amount of Cu bound and subsequently removed by juice solids and wine lees, and this was reflected in the Cu concentration in wines post-bottling. The effects of oxygen exposure on residual metal ion concentrations in wine have important implications for the organoleptic qualities of wine as well as for wine authenticity studies, given that the metal ions most affected by oxygen exposure are known to be important for aroma development and wine oxidation. Previous studies have shown that oxygen exposure modulates the formation of “reductive” aroma compounds. Considering that Cu is known to be directly involved in modulating “reductive” aromas, this study provides a putative mechanism for the lower prevalence of ‘reduced’ characters in wines produced oxidatively by demonstrating that significantly less residual Cu remains in wines exposed to oxygen during vinification. This decreases the risk of Cu being involved in redox reactions in these wines.
Marlize Z. Bekker, Martin P. and Paul A. Smith (2019). Changes in Metal Ion Concentrations in a Chardonnay Wine Related to Oxygen Exposure during Vinification. Molecules, 24(8), 1523; https://doi.org/10.3390/molecules24081523
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