Influencing wine style through management of oxygen during winemaking
Effective management of oxygen during winemaking can help to create diverse wine styles, appealing to a range of consumers. Many approaches to oxygen management are currently practised; however understanding of the impact of oxygen management practices has largely been confined to those employed post-fermentation, during bottling and relating to the effects of closure selection on post-bottling development. The effects of oxygen management during the process of winemaking (from crushing through fermentation) are less well understood, with the limited information that exists relating mostly to the management of fermentation efficiency and reliability.
The role of oxygen during winemaking is likely, however, to have a profound effect on the final wine, and thus a significant opportunity exists for winemakers to use oxygen management before or during fermentation to influence critical aspects of winemaking, such as wine aroma, texture and post-bottling stability. This project combines small-scale laboratory trials and winery-scale experimental work, aiming to provide new insights into the timing and amount of oxygen required to achieve desirable outcomes in red and white winemaking, including the prevention of ‘reductive’ odours. The science and technique of adding oxygen in the early stages of fermentation is relatively new in wine science terms, and more widely accepted and used in countries such as France. Anecdotal evidence suggests, however, that small-scale work is being undertaken in this area by Australian wineries, with positive results.
Impacts on white wines of oxygen exposure during juice preparation and handling
The choice of pressing technique can have a noticeable effect on the resultant aroma and phenolic composition of white wine. Prevention of very early passive oxygen exposure during ‘inert’ pressing was compared with normal ‘aerobic’ pressing in Chardonnay wines. The large amounts of oxygen to which white grapes are exposed during pressing (in this case whole-bunch) resulted in a juice with lower phenolic load, increased higher alcohols, and modified fermentation esters, amino acids and volatile organic acids. Both resultant juices (inert and aerobic) were subsequently vinified using either reductive or oxidative handling techniques until the end of post-ferment racking. Compositional differences, resulting from either pressing mode (particularly for total phenolics) were far greater than the differences brought about by using reductive handling techniques (with extensive dry ice cover) compared to passive oxidative techniques. This was particularly the case for aerobically pressed juice where the chemical differences between handling techniques were not statistically valid. There were, however, subtle differences between reductive and oxidative handling techniques. Subsequent work from the 2016 vintage, looking at different oxygen concentrations (0, 5, 10, 15, 20%) during the pressing of Pinot Gris, indicates that increased oxygen exposure at pressing results in a longer time to complete fermentation.
Efficiency, chemistry and sensory impacts of oxygen exposure during white wine fermentation
Oxygen is a key nutrient in the context of fermentation despite wine fermentation being conducted largely anaerobically. Supplementation of ferments with oxygen has been shown to be beneficial to fermentation progress, especially if added at key growth stages. The effects of oxygen addition during white wine fermentation outside of these narrowly defined time points, have been examined, looking particularly at efficiency and chemistry impacts. Small (250 mL) and winery-scale (500 L) fermentations were used to evaluate the impacts of oxygen addition on fermentation performance and sensory characteristics of Chardonnay. Must oxygen concentration at the time of inoculation had no impact on fermentation duration or the concentration of yeast-derived aroma compounds in wine. However, aeration of ferments later than might normally be recommended still had a stimulatory effect on fermentation performance without negative consequences for wine sensory attributes. As such, if required (e.g. for stimulation of sluggish/stuck ferments), the use of oxygen outside the previously defined narrow window (24-72 hours post-inoculation) can be considered beneficial for ferment performance with limited risk to the sensory outcome.
Impacts of timing and duration of oxygen exposure during red wine fermentations
In 2012 it was established that stylistically diverse red wines could be created by use of macro-oxygenation during fermentation in rotary tanks using air (21% oxygen) or 40% oxygen. These wines showed bright red fruit characters, softer astringency, no reductive aromas and much lower residual metal concentrations than the wines made without oxygen. In 2016 macro-oxygenation was further explored using air in red fermentations (during pump-overs), examining the timing of addition as well as the duration or treatment. Results are currently being collated.