Sulfite tolerant strains of Brettanomyces bruxellensis, or Brett, are on the rise, new research suggests.
But just how widespread the increasing tolerance is remains unknown.
Speaking at the Australian Wine Industry Technical Conference – where she was named a Fresh Science winner – Australian Wine Research Institute (AWRI) scientist Caroline Bartel said recent monitoring suggests that sulfite (SO2) tolerance in Brett could emerge as an issue for winemakers.
Brett is a wine spoilage yeast that results in unpleasant band-aid, barnyard and horsey aromas in wine.
Current practice is to control the yeast through the addition of SO2 post malolactic fermentation (MLF). But with the growing trend of increasing pH in red wines it is sometimes difficult to get the dose right to kill Brett off.
‘Higher wine pH combined with current winemaking trends to minimise SO2 use, may result in “sub-lethal” concentrations that could lead to strains with increased tolerance to SO2’, Caroline explained.
More than 200 Australian B. bruxellensis isolates were acquired from three time periods (2000–2004, 2010–2014 and 2016–2018) for the research project.
The isolates from 2000–2004 and 2010–2014 were obtained from the AWRI Wine Microorganism Culture Collection, while the 2016–2018 isolates were acquired from 24 wine samples from 10 different wineries.
The team found that there was greater SO2 tolerance among the 2016–2018 isolates.
‘We found that the distribution of SO2 tolerance had widened, with more isolates clustering above the average. These isolates were able to grow at higher concentrations of SO2 than two strains we already consider as highly tolerant.’
Caroline said it was unknown whether the emergence of SO2 tolerance would lead to a situation similar to the rise in Brett-related off flavours that was experienced in the late 1990s through to the early 2000s – and more samples from a wider group of wineries were needed.
‘We know a lot more about Brett than we did then. We know what techniques to use to mitigate against and minimise the effects of spoilage. The maintenance of winery cleanliness and adherence to correct barrel sanitation techniques are highly important at preventing initial contamination. Inoculation with MLF strains shortens the length of MLF, so that SO2 can be added sooner, giving Brett less time to take hold.’
'Enough emphasis cannot be placed on the importance of wine pH when adding SO2'– Caroline Bartel, AWRI
‘As the pH of wine increases, the amount of the active, antimicrobial form of SO2 dramatically decreases. When adding SO2 use the Sulfur Dioxide Addition calculator and the Molecular Sulfur Dioxide Addition calculator on the AWRI website. This will ensure that sufficient SO2 is added to the wine.’
On a positive note, Caroline said while Brett remains an issue in the winery setting, so far the problem has not translated into finished, commercially available wines.
She said a study conducted by the AWRI, investigating the levels of the Brett-induced volatile phenol, 4-EP, in commercially available Cabernet Sauvignon wine showed that 4-EP levels had reduced from a mean of over 1200 micrograms per litre (μg/L) in 1997 to 29 μg/L in 2015. This level is well below the sensory threshold.
‘A greater emphasis on sterile filtration at bottling could be a factor in this reduction.’
Caroline said the Australian wine sector should carefully consider alternative strategies for controlling Brett, including the additive chitosan or the biocidal processing aid dimethyl dicarbonate (DMDC).
In the meantime, good ‘housekeeping’ in the winery was a key to staying on top of Brett.
‘Prevention is better than cure. That means maintaining winery cleanliness, following correct barrel sanitation regimes; and considering wine pH when calculating SO2 additions.’
Caroline is actively looking for new Brett isolates to determine how widespread the emergence of SO2 tolerance is. If you can assist please contact Caroline at caroline.bartel@awri.com.au.