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.
Impact of rate, length and timing of oxygen addition during fermentation
The effect of oxygen addition, both its quantity and timing, was explored in a series of laboratory fermentation trials. The primary finding was that fermentation performance and wine chemistry were predominantly influenced by the total amount of oxygen consumed by the fermentation, not the duration over which it was delivered. Ferments that received a large amount of oxygen in a short period and those that received a small amount over a longer period, with equivalent overall consumption, exhibited similar performance and chemical profiles.
Specifically it was observed that:
- volatile acids such as acetic, octanoic, and decanoic acids, normally associated with negative sensory attributes in wine, were reduced with increasing oxygen dose
- branch chain acids and their associated esters, such as 2-methyl butanol and ethyl-2-methyl butanoate increased proportionally with oxygen treatment
- the concentrations of branch chain esters in particular were modulated around their aroma thresholds and therefore may change sufficiently to influence sensory qualities
- significant stripping of oxygen by CO2 occurred, with oxygen uptake rates inversely proportional to CO2 production rates, at least at low oxygen input concentrations.
The extent of must aeration at the time of inoculation, which can be influenced by tank filling operations, had minimal impact on fermentation performance and production of yeast-derived volatile compounds. This suggests that variations in must oxygen concentration at the time of inoculation are unlikely to have an effect on wine sensory attributes.
In addition to investigations on the effects of total oxygen consumption, the effect of oxygen addition timing was also explored. From a fermentation performance perspective, oxygen additions when ferments had reached 80% to 60% of initial sugar had the biggest impact, which is consistent with the work of others. Fermentation duration was still reduced by treatment at 40% initial sugar, but was substantially longer than observed for the earlier treatments. No difference in fermentation duration was found between ferments treated at 20% initial sugar or no oxygen treatment. The effects on wine chemistry largely mirrored those on fermentation performance, with exposure of fermentations to oxygen at 80% of initial sugar having lower concentrations of medium chain fatty acids and later additions showing increasing concentrations of these acids.
In summary, laboratory experiments demonstrated that oxygen additions between 80 to 60% of initial sugar for a period of 2 to 48 hours, depending on the concentration and flow rate of gas used, have maximal impact on fermentation outcomes and that these parameters can be modulated to shape the extent of the effect.
Two important pilot-scale (500 L) vintage trials were conducted over the first two years of the project, focusing on oxygen exposure in white wine. The first trial in 2014 investigated the effect of passive oxygen additions during white winemaking, that is the oxygen that gets into wine during pressing and handling but is not actively bubbled into the juice or ferment. By separating the very early oxygen exposure that occurs at pressing from the later exposure which happens through different ways of handling juice or wine after pressing until the end of fermentation, it was possible to find out at which stage oxygen has the greatest effect. In the trial two pressing modes (inert and aerobic) and two forms of post-pressing handling (reductive or oxidative) were used to create four distinct wines, allowing the effects of oxygen timing to be closely examined. Analysis of aromatic compounds and phenolic composition showed that oxygen exposure during the phase when grapes are first burst open by pressing, as a controlled proxy for mechanical harvesting or crushing, is significantly greater than the oxygen exposure during post-pressing handling. Sensory analysis of the wine from this trial showed important differences, with colour and ‘tropical’ fruit aromas being statistically different. Analysis of chemical data indicated variations in certain aroma compounds (methanethiol, methional, furfural, benzaldehyde, ethyl propanoate and ethyl octanoate) were significantly impacted only through the very early oxygen exposure during pressing, while other compounds (glycine, glutamic acid, tyrosine, 2-methylpropyl acetate, and hexyl acetate) were only affected by oxygen introduced through oxidative handling. A number of other amino acids and volatile esters were influenced by both pressing mode and handling.
The vintage 2015 trial examined the effect of oxygen additions during active fermentation. Short (2 hours) and long exposure (20 hours) treatments were applied when sugars had dropped to 80% of their initial concentration and a long exposure treatment was applied when sugars had dropped to 20% of initial level. Results so far indicate that both early and late additions (20 hours) increased fermentation rate which should also have an impact on the fermentation volatiles. This will be validated by sensory and chemical analysis over the coming year.
Several optical-based dissolved oxygen (DO) measurement tools were used in both the pilot-scale experiments and a large commercial winery. It was found that process-grade probes in specialist housings are best suited for DO measurement during pump-overs or transfers due to their fast equilibration and response time and that they have appropriate configuration for use in a commercial winery. Hand-held meters are equally adaptable to measuring in-tank DO during racking operations. Techniques for introducing oxygen into an active ferment were also assessed. A Venturi injector was trialled in industry and proved a simple and effective device.