Dr. Carien Coetzee
19 August 2020
It is widely accepted that the presence of dissolved carbon dioxide will help protect the wine from oxygen pickup (and subsequent oxidation reactions). Often, wines with higher dissolved carbon dioxide content, such as perlé or sparkling wines, are considered to have sufficient protection and preventative measures to exclude direct contact between oxygen and the wine is neglected. To what extent this protective mechanism is in force is not widely known.
The gas-liquid equilibrium is not simple and is further complicated by the fact that both oxygen and carbon dioxide are usually present in wine simultaneously. Two studies focussed on the protective role of dissolved carbon dioxide and how the presence of this inert gas can stop oxygen from dissolving in wine1,2. The current blog post will summarize some of the main findings from these publications.
Specific experiments using the simultaneous or individual injection of air and/or gaseous carbon dioxide were performed. In most cases, the experiments were performed on synthetic solutions, however, part of the results was validated using real wine1.
STUDY A. Oxygenation1
The researchers prepared two wines with different dissolved carbon dioxide concentrations:
1) a wine which was initially free of carbon dioxide (by sparging with nitrogen gas)
2) a wine which was saturated with carbon dioxide (1.4 g/L at 20°C)
Both wines were oxygenated with pure oxygen and the dissolved oxygen and carbon dioxide concentrations were monitored over time.
Results showed that there was a significant decrease in the oxygen dissolution rates when carbon dioxide was present. The oxygen dissolution decreased from 77% in the sample which had no initial carbon dioxide to 9% dissolution in the sample that was initially saturated with carbon dioxide. It is important to note that during this oxygenation process, the carbon dioxide concentration progressively decreased until it reached 0 g/L and eventually the wine was completely saturated with oxygen.
STUDY B. Oxygenation with a constant source of carbon dioxide2
In a second study, the carbon dioxide content of three litres of degassed synthetic solution (12% v/v alcohol, 5 g/L tartaric acid and pH 3.5) was kept constant by continuous injection of gaseous carbon dioxide. Simultaneously the wine was oxygenated by injecting synthetic air (20% oxygen and 80% nitrogen). The two gasses were injected using two separate porous diffusers and the liquid was continuously stirred. Different flow rates were tested. This scenario is similar to a situation of active fermentation (providing a continuous source of carbon dioxide) where the fermenting must is in contact with air.
Results showed that the greater the flow rate of the gas, the higher the final equilibrium concentration (concentration at which the amount of dissolved gas stabilised) was.
In experiment 1, air was injected at a flow rate of 0.026 L/min, while carbon dioxide was injected at a flow rate of 0.27 L/min (Figure 12). After about 25 minutes, the equilibrium concentrations were reached: 0.6 mg/L for oxygen and 1430 mg/L for carbon dioxide.
In experiment 2, the flow rate of the air was increased to 0.14 L/min, while the flow rate of carbon dioxide remained at 0.27 L/min. In this experiment, the equilibrium concentration of the oxygen increased to 3.6 mg/L, whereas the equilibrium concentration of carbon dioxide decreased to 1050 mg/L when compared to experiment 1 (Figure 12).
Discussion and Conclusion
- The presence of dissolved carbon dioxide in wine strongly affects the efficiency to which oxygen is dissolved. A higher carbon dioxide concentration decreases the potential amount of oxygen that can be dissolved as well as the speed of which the oxygen is dissolved1.
- In the case where there is no continuous production of carbon dioxide, but the wine has a high initial dissolved carbon dioxide content, the carbon dioxide can protect the wine by reducing the rate of oxygen dissolution2. The oxygen will, however, eventually, saturate the wine due to the progressive removal of dissolved carbon dioxide and therefore its protective effects. A higher initial carbon dioxide concentration will only delay oxygen saturation. In situations where brief contact of air is experienced, a high dissolved carbon dioxide concentration will be able to protect the wine from excessive oxygen dissolution.
- During fermentation, the high concentration of dissolved carbon dioxide and the continuous formation of the gas provides complete protection against the dissolution of oxygen2. In these conditions, the amount of oxygen dissolved will depend on the rate of carbon dioxide production and the air flow rate. A high rate of carbon dioxide formation during fermentation might render micro-oxygenation completely inefficient2.
There is a relationship between the maximum possible dissolved oxygen in respect to a given dissolved carbon dioxide concentration. The researchers2 derived the following equation.
O2 max = -0.005*CO2 + 7.9 mg/L
This can be used as a guideline, however, keep in mind that many of these experiments were performed in synthetic model wine solutions. In a real wine situation, the reaction/consumption of oxygen by some wine compounds present could have a significant influence on the dissolved oxygen concentration.
- Temperature will play a critical role in the concentration of dissolved gas as the solubility of gas increases as the temperature decreases. Figure 22 shows a representation of the relationship between dissolved oxygen, dissolved carbon dioxide and temperature when a wine is in contact with air.
(1) Devatine, A., Chiciuc, I., Poupot, C., Mietton-Peuchot, M. Micro-Oxygenation of Wine in Presence of Dissolved Carbon Dioxide. Chem. Eng. Sci. 2007, 62 (17), 4579–4588.
(2) Devatine, A., Chiciuc, I., Mietton-Peuchot, M. The Protective Role of Dissolved Carbon Dioxide against Wine Oxidation: A Simple and Rational Approach. J. Int. des Sci. la Vigne du Vin 2011, 45 (3), 189–197.
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