Factors affecting sparging efficiency (Part 2)

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

Introduction

The presence of dissolved oxygen in wine can lead to oxidation reactions with undesirable effects on the wine composition and quality. Wine producers often remove dissolved oxygen from wine using inert gas sparging techniques. Although sparging operations are common in the wine industry, factors affecting the efficacy of sparging regimes have been rarely investigated.

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

In Part 1 of this two-part article, the effect of inert gas flow rate, wine temperature and gas composition were summarised (experiments 1 and 2). In Part 2, the main effects of bubble size, repetitive sparging and extended sparging duration will be reported.

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) (see Part 1).

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 until 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 3: Testing the effects of a diffusion stone (bubble size) during sparging

 A wine was either sparged using a 15 μm diffusion stone or by using an open-ended hose from where the gas flowed freely. The same flow rates were applied to both treatments. The duration of sparging was set to the time required to bring the dissolved oxygen concentration to < 3 mg/L when using the diffusion stone.

Oxygen removal results

The rate at which oxygen was removed was significantly higher when using the diffusion stone when compared to the hose. Using a diffusion stone during sparging produced smaller bubbles, thereby increasing the total interfacial area between the gas and the liquid. Smaller bubbles also rise more slowly than larger bubbles, thus increasing the contact time between the gas and the liquid, and increasing the gas holdup.

Effect on carbon dioxide content

The dissolved CO₂ concentrations decreased significantly with and without the diffusion stone, however, the loss of CO₂ was greater when a diffusion stone was used.

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

Sparging efficiency can be greatly increased by using a diffusion stone. By using diffusion stones with the smallest applicable pore size, a greater surface of the inert gases is utilised, meaning that sparging will be more efficient in both time and resources.

Experiment 4: Testing the effect of repetitive sparging

 A wine was sparged using the general settings outlined above. The entire process was repeated alternating the sparging of O₂ and N₂ for a total of four cycles.

Effect on carbon dioxide content

Dissolved CO₂ concentrations decreased significantly after each successive N₂ sparging treatment.

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

Repetitive sparging did not alter the concentration of important compounds in wine (including the aromatic volatile thiols).

Experiment 5: Testing the effect of extended sparging times

For this experiment, no oxygen was added to the wine before sparging (thereby omitting any possible oxygen-induced effects). The wine (18°C only) was sparged for a duration of eight minutes (the average time for the experimental wines to reach dissolved oxygen levels < 0.5 mg/L), after which an additional 60 minutes of sparging was performed.

Effect on carbon dioxide content

After the first eight minutes of sparging, 40% of the dissolved CO₂ was removed. After the additional 60 minutes of sparging, the dissolved CO₂ was undetectable.

Effect on chemical composition

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

Conclusion

It seems that inert gas sparging does not significantly affect the aromatic compounds measured in this study. The impact of reduced carbon dioxide content should be carefully considered as too low carbon dioxide could result in a wine tasting flat and lifeless and could have further implications for the sensory experience of the wine.

Final remarks

The results from the current study would suggest that sparging regimes do not remove aroma compounds from wine and can therefore confidently be used to lower dissolved oxygen concentration in wine (provided that the lost dissolved CO₂ can be replenished).

The main factors affecting the efficacy of dissolved oxygen removal are bubble size and wine temperature (see Part 1). Winemakers should not be overly concerned with altering the chemical composition of wine when sparging. However, sparging should still be done with caution to ensure sparging procedures and parameters are in line with desired sensorial goals of any wine, especially in terms of the effects of dissolved CO₂. Differences in the outcomes of research scale studies vs industry scale studies can be significant. The static sparging experimental system is probably more efficient than in commercial settings due to automated homogenising mixer while sparging, smaller wine volumes and higher inert gas flow rate compared to industry practices. In-house sparging trials are recommended for each unique wine to ensure that any alterations in wine composition are advantageous with minimal to no undesirable changes.

The results from the study showed no immediate undesirable effect on wine composition, however, the study did not investigate the long-term effects of the treatment. Future studies should include the effect of sparging on the ageability of a wine. Future studies should also include sensory evaluation. In the current study, only a few important wine constituents were monitored chemically. Sensory evaluation will provide an overall assessment of the wine sensory composition and quality.

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.

Reference

1. Walls, J., 2020. Effect of oxygen management on white wine composition. Stellenbosch University.

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