Influencing wine style and efficiency through management of oxygen during wine production
This project will use both model systems and pilot-scale fermentations to investigate the impacts of oxygen exposure at crushing or during fermentation on fermentation efficiency and wine style. Shiraz and Chardonnay have been the primary varieties investigated to date, and it is of value to establish whether the impacts observed with these varieties also translate to other varieties and styles such as Merlot, Cabernet Sauvignon, rosé, sparkling wines, alternative varieties, and also spontaneous fermentations.
The project will also monitor wines with known oxygen exposure as they age, to assess oxygen-related chemical changes that occur after fermentation. Factors that modulate these changes (e.g. metals, pH, time and duration of oxygen exposure) are critical to delivering the best quality product to the market and the consumer.
Finally, practical knowledge about methods for appropriate delivery of oxygen to fermentations, and dose, remains a limiting factor affecting the uptake of oxygen use by industry. In collaboration with industry partners, this project will address this limitation by exploring different approaches to oxygen delivery and developing knowledge and advice to pass on to winemakers.
Finding the right aeration options
Previous work has shown that introduction of air or oxygen during the active phase of fermentation can be beneficial. In white grape ferments, aeration can be used to stimulate fermentation rate, especially in low-turbidity musts, without significant impact on the sensory qualities of the finished wine. In red grape ferments, aeration has a minimal impact on fermentation kinetics; however, stylistic changes associated with tannin structure and abatement of ‘reductive’ characters, particularly as wines age, have been observed. In 2019 the capacity of red and white fermentations to withstand increasingly extreme aeration treatments was tested and the limits of aeration, beyond which oxidative damage becomes apparent, were defined.
In general, either long-duration, low-intensity aerations or repeated short-duration, high-intensity aerations during fermentation have been required to produce beneficial changes in red and white wines without causing oxidative damage. Such regimes may suit winemaking approaches where limited resources are available or where aerations might be undertaken during scheduled pump-overs. However, a question remained as to whether a single well-timed, short-duration high-intensity aeration would be able to elicit the same result. This question was addressed in the 2020 vintage.
Short-duration, high-intensity aerations were compared to long-duration, low-intensity aerations in both Chardonnay and Shiraz fermentations at pilot scale. The amount of air delivered during each of the treatments was calibrated such that each of the ferments was exposed to equivalent amounts of oxygen regardless of the treatment format. Preliminary results suggest that a single well-timed, short-duration high-intensity aeration does not ‘pack the same punch’ as the other modes of delivery (repeated and long, low-intensity aerations). In low-YAN Chardonnay ferments the long-duration, low-intensity treatment reduced fermentation time by 15 days compared to the short-duration treatments and by almost 20 days compared to the no-treatment control. Consistent with earlier observations, there were no differences in ferment duration in the red fermentations. Whether or not these treatments had an impact on wine composition will be determined through post-bottling analysis in 2021. These experiments reinforce previous observations, highlight the practical differences between white and red winemaking when using aeration during fermentation, and provide practical detail about how aeration can be implemented in the winery for maximum benefit.
Aeration of wild ferments – finding the sweet spot
Aeration of non-inoculated fermentations introduces a different combination of possible interactions. Modulating the timing of aeration provides an opportunity to interact with different members of the microbial community that can be dominant during the early phases of a fermentation and potentially enhance the survival of selected members. Earlier work showed that long-duration, low-intensity aeration of non-inoculated ferments had the effect of reducing fermentation times, similar to that observed in inoculated ferments. This was true irrespective of when during the first three days of fermentation the aeration was applied. Aside from suppression of medium-chain fatty acid production, few other variations to wine composition were observed.
This year, understanding of how wild fermentations respond to aeration was extended by maintaining a long-duration treatment format but increasing the intensity of the treatment. The effect of aeration on fermentation progress was similar to that observed in the previous year; however, marked changes in the volatile profile of the wines were observed, even for the lowest intensity treatment. An exploration of the effect on microbial community structure is currently being undertaken in collaboration with the AWRI’s bioprospecting project. It is of great interest to understand whether the changes observed in volatile profiles are underpinned by alterations to microbial populations. The potential benefits of aeration in the management of non-inoculated fermentations will continue to be explored within this project.
Exploring the mechanics of aeration – moving beyond the why to the how
Over numerous vintages the impacts of aeration on red and white fermentations have been shown to be distinctly different. But what about the process of aeration itself? Is it necessary to calibrate air input rates depending on whether the ferment is Shiraz or Chardonnay? Is the same dissolved oxygen response achieved from the same input of air in the two systems? Given a target dissolved oxygen value, do the air input rates required to achieve the target scale in a predictable way with fermenter size? These are questions that are beginning to be addressed through analysis of laboratory-scale and pilot-scale dissolved oxygen data covering six years of vintage experiments. It is anticipated that this will provide important foundational data to inform winemakers how macro-aeration can best be managed in the winery.