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.
Exploring influence of aeration during fermentation on simultaneous MLF
Aeration of ferments and simultaneous MLF inoculation are two winemaking techniques that have shown benefits in recent years. However, until now, the interaction of these two techniques had not been tested. During the 2017 vintage, a 2 x 2 factorial experiment was carried out looking at timing of MLF inoculation and a repeated oxygen addition. Chemical and descriptive sensory analyses were conducted six months after bottling. Statistical analysis revealed five sensory attributes that differed significantly among treatments. Generally, oxygen had a large effect on wine colour intensity, and wines sparged with oxygen during primary fermentation were rated relatively low in opacity and higher for sweetness and ‘fruit’ aftertaste. The MLF inoculation treatment effect had a relatively small influence only on opacity and ‘boiled potato’ aroma, with wines from the sequential inoculation treatment rated highest in both attributes. The interaction of the oxygen treatment with the inoculation treatment had a moderate effect on reductive aroma, indicating that sequential inoculation without the addition of oxygen, and simultaneous inoculation with the addition of oxygen may both result in some level of reductive aromas.
Non-Saccharomyces yeast use oxygen differently
In ferments conducted by Saccharomyces cerevisiae, oxygen availability helps drive the formation of yeast biomass by providing the resources for lipid biosynthesis. However, even in the presence of oxygen, S. cerevisiae will ferment sugar to ethanol. When oxygen is made available to many non-Saccharomyces yeast species, sugar metabolism can be directed towards endpoints other than ethanol. To explore this concept, experiments were conducted to evaluate conditions that might enable aerobic sugar consumption by non-Saccharomyces yeasts. In a collaboration with Università Politecnica delle Marche, Italy, several strains were evaluated under a range of aeration conditions. Ethanol reductions of between 0.8% and 1.8% v/v, depending on the degree of aeration, were observed without significant production of acetic acid. Given that non-Saccharomyces yeasts are already available for commercial use, this work is one example of how altered production practices may be used to lower alcohol concentration in wine.
Changes in tannin structure and interaction with protein after exposure to oxygen
Wines sparged with oxygen during fermentation were previously shown to be less astringent than reductively treated wines, and had meaningful changes in tannin concentration and structure, both of which might have contributed to the sensory effect (Bekker et al. 2016). To further investigate this, a collaboration was initiated with Dr Aude Watrelot from the University of California, Davis, to characterise tannin which had been isolated from wines undergoing either oxygen or nitrogen sparging during fermentation. Tannins extracted from red wine exposed to oxygen had a higher percentage of galloylation and were more pigmented than tannins from reductively treated wine. Particle size of tannins was analysed using NTA and the interaction of tannins with a model protein (poly-L-proline) was measured by isothermal titration calorimetry. The interaction of tannins from oxygen-treated wine with protein involved fewer hydrogen bonds than hydrophobic interactions, suggesting that oxidised tannins were more hydrophobic. The assessment of particle size suggested that oxidised tannins were prone to greater self-association, leading to a larger overall particle size than the tannins from the nitrogen treatment. The study has shown that the structural changes in tannins conferred by oxygen can result in significant changes in their colloidal behaviour and their interaction with protein. This may have implications for the perception of astringency, and is being further investigated.
Bekker, M.Z., Day, M.P., Holt, H., Wilkes, E., Smith, P.A., 2016. Effect of oxygen exposure during fermentation on volatile sulfur compounds in Shiraz wine and a comparison of strategies for remediation of reductive character. Aust. J. Grape Wine Res. 22(1): 24-35.