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.
How much aeration is too much? Testing the limits of white and red fermentations’ capacity to tolerate aeration.
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 the experimental work to date, characteristics associated with oxidation related to excessive aeration have not been observed, in either white or red fermentations. In 2019, pilot-scale (500 L) experiments were undertaken to investigate the aeration capacity limits of red and white fermentations. Sparging of ferments was undertaken at a rate that achieved 40%, 10% and 1% air saturation. These treatments were initiated four days post-inoculation when sugar was at 80% of the initial concentration. Aeration treatments were maintained for 48 hours at a constant input flow rate of air.
In white ferments the minimal aeration treatment was sufficient to stimulate fermentative activity, reducing overall fermentation time by four days. No further reduction in ferment duration was observed with increasing oxygen treatment. A lesser impact on fermentation kinetics was observed in red fermentations, with their duration reducing by two days with the highest oxygen treatment. Signs of oxidation did become evident, especially in the whites, with 10% and 40% air saturation treatments resulting in wine quality downgrades. Red wines were more robust toward oxygen treatments at all treatment levels. Chemical and sensory analysis will be used to quantify the impacts of the aeration treatments. These experiments highlight the practical differences between white and red winemaking for use of oxygen in the winery and demonstrate the limits of using aeration as a tool in winemaking.
Aeration of wild ferments
Aeration of inoculated ferments, as described above, is usually timed to maximally intersect with growth of Saccharomyces cerevisiae in order to either maximise the benefit to its growth for white ferments, or to provide opportunity to interact with the wine matrix in the case of red ferments. Non-inoculated fermentations introduce a different combination of possible interactions. Modulating the timing of aeration affords the 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.
The question of how aeration could be used to modulate non-inoculated fermentation was approached using freshly prepared Chardonnay must without SO2. Three different treatments were applied, all of the same intensity (2.5% air saturation) and duration (24 hours) but applied at 24, 48 or 72 hours post-must preparation. The effect of aeration on the duration of uninoculated fermentations was more pronounced than for inoculated ferments, being reduced by as much as seven days depending on when the aeration was applied. The effect of microbiological community structure is being assessed using a metagenomic approach. Whether aeration will prove to be as useful to the management of non-inoculated fermentations as it is proving to be for the management of inoculated ones will continue to be of interest within this project and will form part of ongoing work.
Oxygen transfer to ferments
Whether transfer of oxygen to fermenting must occurs in large red fermenters that use air mixing equipment (most commonly used in larger wineries for cap management) remains an unanswered question. The large bubble size produced by this sort of equipment should preclude any useful gaseous exchange. To understand whether this was the case, several dissolved oxygen (DO) data loggers were suspended in four 150-tonne fermenters equipped with Pulsair® mixing at 1, 3 and 5 metres from the bottom of the fermenter for the duration of the 2019 vintage (three to four fills). The DO peaked at 17% air saturation between 3 and 5 metres but this effect lasted only ten minutes per cap management cycle. Over the duration of fermentation this intake is potentially sufficient to induce oxygen-mediated effects in the finished wine. This work suggests that equipment already installed in many wineries could be used to implement an aeration strategy during fermentation.