Factors affecting sparging efficiency (Part 1)

A recent study investigated various sparging parameters and assessed the effectivity, while monitoring certain wine constituents.

Introduction

Winemakers often use inert gas sparging to displace dissolved oxygen molecules in wine, thereby reducing the risk of oxidation. Even though sparging operations are regularly used in the cellar, there seems to be a lack of information regarding 1) parameters that can influence the efficacy of the sparging, and 2) the exact effects of sparging on the wine composition.

It is generally believed that the sparging of wine with the goal of reducing the dissolved oxygen content will inadvertently “blow off” aroma compounds, thereby reducing the aromatic intensity and altering the sensory perception of the wine. Theoretically, the volatile components can desorb into the gas during sparging in the same manner that dissolved gasses do, but the significance of potential losses needs to be qualified and quantified to assess if the effects have oenological importance.

Researchers from the South African Grape and Wine Research Institute, Stellenbosch University (James Walls, Steven Sutton, Dr. Carien Coetzee and Prof. Wessel du Toit) conducted a Winetech-funded study to investigate the effect of certain variables, such as inert gas flow rate, gas composition, duration of sparging, number of sparging cycles, wine temperature and the utilisation of a diffusion stone (bubble size), while also assessing the influence of the different sparging regimes on the wine’s composition. The findings of this study were published in an MScAgric thesis in 2020 titled, Effect of oxygen management on white wine composition by James Walls.¹ The main findings from this study will be reported in this two-part article.

Materials and methods

Sauvignon blanc (Stellenbosch) and Chenin blanc (Breede River Valley) wines were exposed to several sparging regimes. The wines were made according to the respective wineries’ standard practices and were considered bottle-ready at the time of collection. The experiments were conducted in four custom-built stainless steel tanks with a 65 L capacity each. Inert gas sparging was done using two types of gas; 1) nitrogen (N₂), and 2) a mixed gas consisting of 70% N₂ and 30% carbon dioxide (CO₂).

General settings (applicable to all experiments unless specified otherwise):

• Before sparging with inert gas, the dissolved oxygen concentration of the wines was increased to 3 mg/L using medical grade oxygen (99.8%).
• Sparging of inert gas was done at a flow rate of 120 mL N₂/(L of wine/minute).
• Sparging was performed using a 15 μm stainless steel diffusion stone.
• Inert gas sparging ceased when the dissolved oxygen concentration reached < 0.3 mg/L.
• The wine was continuously mixed using an automated homogenising mixer during sparging.
• Experiments were performed at two temperatures, 18°C and 10°C.

Experiment 1: Testing the effect of gas flow rate and wine temperature during sparging

The wine was sparged with N₂ using two gas flow rates:

• 120 mL N₂/(L of wine/minute).
• 280 mL N₂/(L of wine/minute).

Oxygen removal results

There was no significant difference in the rate of oxygen removal when comparing different inert gas flow rates (at either temperature tested). The lack of sparging improvement can be attributed to the fact that when applying a higher flow rate (in combination with the mixer) bubbles coalesce and rise to the surface more readily, resulting in less time for oxygen desorption to occur.³

Oxygen removal efficacy improved as the wine temperature increased. This is in accordance with Henry’s Ideal gas laws where, as the temperature increases, the difference in the partial pressure value required for the expulsion of dissolved oxygen, decreases, resulting in a decrease in gas solubility and faster removal of dissolved oxygen.^4,5

Effect on carbon dioxide content

The higher flow rate of inert gas sparging resulted in a greater loss of dissolved CO₂ compared to the lower flow rate (at either temperature tested).

The effect of temperature on the carbon dioxide removal needs to be carefully interpreted as the duration of sparging differed between the two tested temperatures. Due to the lower dissolved oxygen removal rate at 10°C, the sparging continued for four minutes longer (extra time needed to reach < 0.3 mg/L dissolved oxygen) when compared to the duration of sparging needed for the wine at temperature 18°C. As a result, the CO₂ concentration in the cooler wine was lower at the end of the sparging duration when compared to the CO₂ content in the warmer wine.

Effect on chemical composition

There were no significant differences in the free and total sulphur dioxide, glutathione or volatile thiol concentrations, as well as colour measurements between the different treatments under the conditions of this study.

Conclusion

The inert gas flow rates tested did not differ in oxygen removal efficiency, but it did affect the removal rate of dissolved CO₂. Lower flow rates in non-turbulent conditions should also be investigated.

Oxygen was more efficiently removed when the wine’s temperature was higher. More time and therefore more sparging gas is required to remove the same amount of dissolved oxygen when a wine is at 10°C compared to 18°C.

Experiment 2: Testing the effects of mixed gasses during sparging

Two types of gasses were tested; N₂ only and a mixed gas consisting of 70% N₂ and 30% CO₂.

Oxygen removal results

Sparging using N₂ gas was more efficient in removing dissolved oxygen when compared to sparging using the mixed gas.

Effect on carbon dioxide content

Sparging the wine with the mixed gas had varying effects on the CO₂ concentration, depending on the temperature of the wine. When sparged at 18°C, the CO₂ concentration did not change significantly. When sparged at 10°C, a significant increase in dissolved CO₂ levels was observed.

Effect on chemical composition

There were no significant differences in the free and total sulphur dioxide, glutathione or volatile thiol concentrations, as well as colour measurements between the different treatments under the conditions of this study.

Conclusion

Even though sparging with a mixed gas at higher temperatures was less efficient in terms of removing dissolved oxygen_, the retention of dissolved CO₂ when sparging with the mixed gas is an advantage not obtained when sparging with N₂ gas. It is therefore possible to remove dissolved oxygen, while maintaining or even increasing dissolved CO₂ in wine (effects are temperature dependent). Sparging with mixed gas can therefore potentially reduce production time as dissolved CO₂ will not have to be replenished after oxygen removal, thereby saving time and resources.

Final remarks

It is clear that factors such as wine temperature and gas composition can significantly affect sparging efficacy. In Part 2 of this two-part article, the effects of bubble size, repetitive sparging and extended sparging are reported.

Abstract

The sparging of wine using inert gas is often used in the wine industry to reduce the concentration of dissolved oxygen in especially white wines. Even though sparging operations are regularly used, there seems to be a lack of information regarding factors that will affect the efficiency of the process, as well as the effect of the technique on the wine composition.

References

1. Walls, J., 2020. Effect of oxygen management on white wine composition. Stellenbosch University.
2. Besagni, G., Gallazzini, L. & Inzoli, F., 2019. On the scale-up criteria for bubble columns. Petroleum 5(2): 114 – 122. https://doi.org/10.1016/j.petlm.2017.12.005
3. Kantarci, N., Borak, F. & Ulgen, K.O., 2005. Bubble column reactors. Process Biochemistry 40(7): 2263 – 2283. https://doi.org/10.1016/j.procbio.2004.10.004
4. Lyons, W.C., Plisga, G.J. & Lorenz, M., 2015. Standard Handbook of Petroleum and Natural Gas Engineering.
5. Agabaliantz, G., 1963. Bases scientifiques de la technologie des vins mousseux. Bulletin l’OIV 36: 703 – 714.

– For more information, contact Carien Coetzee at carien@basicwine.co.za.