It is common practice for winemakers to do uninoculated fermentations on Chardonnay. Winemakers find that it can lead to increased complexity and mouthfeel attributes in their wines. Spontaneous fermentations, however, come with an increased risk of stuck fermentations and unpredictable aroma and flavour profiles. Correct management of uninoculated fermentations is therefore paramount to achieve the desired positive results.
The positive attributes associated with uninoculated ferments can be attributed largely to the greater percentage of non-Saccharomyces yeasts taking part in the fermentation, compared to single yeast inoculated fermentations. Non-Saccharomyces yeasts are easily influenced by winemaking conditions, with SO2 additions and aeration both having profound effects. In fact, one of the reasons for SO2 addition at crushing is to suppress wild yeast growth and to allow the inoculated Saccharomyces cerevisiae yeast to do its job without competition and interference.
A study conducted by Australian researchers over two vintages, 2019 and 2020, explored the effects of aeration timing, amount and duration on Chardonnay fermentations and aroma profiles. Whereas certain aeration regimes delivered positive outcomes, others delivered negative outcomes.
- Commercial Chardonnay juice was obtained from a winery in the Barossa Valley. Fermentations were conducted at 18°C in 2L containers. During 2019 aeration amount and duration was kept constant but timing was varied at 24, 48 and 72 hours after crushing. Sterile filtered air was used to sparge the fermentations.
- In 2020 timing and duration was kept constant and aeration amount was varied. (See the published open access article for exact details of the aeration regimes.)
- The microbial counts were monitored during fermentation and chemical analysis was done on the finished wines.
Effects on yeast populations
- From the 2019 results it was clear that the timing of aeration significantly affected fermentation duration, with the latest aeration (after 72 hours) resulting in the shortest fermentation time. This is most likely due to the aeration benefitting the strong fermenting Saccharomyces species that have started to proliferate at this stage. Fermentations were completed in 13 days as opposed to non-aerated fermentations that took 25 days.
- Aeration also significantly influenced yeast concentrations, with the aerated fermentations having much higher yeast counts than the non-aerated controls. The researchers attribute this difference to the higher numbers of non-Saccharomyces yeasts present in the former fermentations.
- In the 2020 fermentations aeration also shortened fermentation duration, but interestingly increased aeration amounts did not influence fermentation time. Whereas the control took 15 days to complete fermentation, the aerated fermentations took 11-12 days to complete.
- Total yeast count increased with aeration amount, being five, ten and 12 times higher than the control fermentation with no aeration.
- In the non-aerated fermentations (controls) Saccharomyces represented 40% and 50% of the yeast populations in 2019 and 2020 respectively. Once any form of aeration was used, Saccharomyces was no longer the dominant genus at the end of fermentation. In the 2019 aerated fermentations Hanseniaspora species dominated the yeast population and in 2020 Torulaspora delbrueckii dominated. The different Hanseniapora species also behaved differently with the different aeration regimes.
- It is important to note that it is not just aeration that influences the population sizes of different yeast species, but also the stimulatory or antagonistic effects yeast species can have on each other.
Effects on chemical composition
Compared to non-aerated control wines, aerated wines had:
- Lower total acetate ester concentrations (excluding ethyl acetate),
- Increased total higher alcohol concentrations,
- Lower medium chain fatty acids,
- Slightly lower ethanol concentrations in the 2020 wines,
- Higher succinic acid in the 72-hour aeration in the 2019 wines.
In the 2019 wines acetic acid was the highest in wines aerated at 24 hours and lowest in wines aerated at 72 hours. On the other hand, ethyl acetate (the other component of volatile acidity), increased with aeration timing, with the 72-hour aeration wine having the highest concentrations.
In the 2020 wines both acetic acid and ethyl acetate concentrations increased with increased aeration, except for where the lowest amount of aeration was applied, ethyl acetate concentration was slightly lower than the control.
TAKE HOME MESSAGE
The study tells us that the aeration of Chardonnay during fermentation can lead to increased non-Saccharomyces populations. Both the timing of aeration as well as the amount of aeration need to be strictly controlled, since it presents a double-edged sword. Up to a point the result can be very positive, with increased complexity of aromas and/or increased mouthfeel. However, after a certain point, things can go south, with volatile acidity developing to unacceptable levels. What makes determining the exact tipping point for each fermentation even more difficult is the fact that one does not know what species the grapes come in with. Some species are more prone to volatile acidity production than others. So, what worked this vintage might not work next vintage.
According to Hanneli Smit most of the young white wines Vinlab analysed after the past vintage, which presented with high volatile acidity concentrations, were from uninoculated fermentations. This can only mean that the aeration tipping point remains elusive for some winemakers. For some the high VA is a problem, others call it terroir.
Varela, C.; Cuijvers, K.; Van Den Heuvel, S.; Rullo, M.; Solomon, M.; Borneman, A.; Schmidt, S. Effect of Aeration on Yeast Community Structure and Volatile Composition in Uninoculated Chardonnay Wines. Fermentation 2021, 7, 97. https://doi.org/10.3390/fermentation7020097
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