Controlled oxygen addition during fermentation is beneficial for especially red wines and is applied by pump overs, punch downs and rack and return practices. According to some winemakers these are not sufficient and other methods are used for oxygen addition.
Research at the Australian Wine Research Institute (AWRI) focused on Chardonnay and Shiraz to evaluate the impact of aeration at crushing and during fermentation. The addition of oxygen during white wine fermentation improved the kinetics of fermentation, but had limited positive or negative effects on the sensory characteristics of resulting wines. In the case of Shiraz aeration during fermentation did however lead to wines with more aged character, better hue, less anthocyanins and tannins and also smaller tannins. Further research confirmed that oxygen addition during fermentation also stimulates the fruit flavours formation and reduces the negative sulphur flavours, which may suppress it.
During pump overs and plunging practices a layer of carbon dioxide on the surface limits the introduction of air, although the cap is broken and mixed. By using somebody to spray the fermenting must over the cap or using a splash tub aeration can be improved, but is still not sufficient. In the pilot scale research of the AWRI additional oxygen spargers were used in the circuit of pump overs or sinters at the bottom of the fermenter. By doing that the oxygen addition was entangled from the cap management in order to evaluate the effect of aeration as such. The easiest way to add oxygen is aerating the fermenting must via a sparger, connected to a compressor, in the pump over pipeline or connecting a venturi in the pipeline to suck air into the liquid stream. Instead of using air as aerating agent pure oxygen can be used. Oxygen cylinders equipped with regulators and flow control connected to sinters can be used to add the oxygen in tub pump overs. Sufficient aeration can be determined by adjusting the oxygen flow. Aeration or oxygenation must however be done when the fermentation is already running and is usually done during the first quarter of fermentation. Oxygenation is done daily or twice daily at a rate of two to 10 litres per minute, but it appears that five litres per minute is optimal. Some Australian winemakers prefer oxygen instead of air, because it is easier to measure and control the addition. The use of oxygen is also not expensive. The setup cost for an annual production of about 300 000 litres red wine is about AUS $1 000 and afterwards it is only the cost of the oxygen.
The practices aeration or oxygenation are applied by different Australian winemakers for different red cultivars. The benefits are however more prominent in the case of Shiraz, which tend to have more reductive flavours due to sulphides. All agree that it has twofold benefits namely the improvement of the fermentation and sensory characteristics of the resulting wine. A healthy yeast culture is obtained during fermentation, resulting in quicker completion to complete dryness. Less sulphide compounds are not only formed during fermentation, but will also be removed by the aeration. Fruity flavours become more prominent and polymerisation of tannins are favoured to create softer palates (Logan, 2021).
Logan, S., 2021. Realising the benefits of aeration during fermentation. Australian Grapegrower and Winemaker, December 2021: 60 – 66.