Brett under control – but for how long?
Australian winemakers could be facing an all new fight in their battle against the spoilage yeast Brettanomyces bruxellensis (Brett), with concerns that sulfite tolerant strains are on the rise.
However, new Wine Australia-funded research from The Australian Wine Research Institute (AWRI) and the University of Adelaide could help the Australian wine sector stay a step ahead as it investigates the genetic basis for sulfite tolerance in Brett and works to identify better detection methods and possible alternative control strategies.
Project leader, AWRI Biosciences Research Manager, Chris Curtin said Australian winemakers got on top of Brett in the last decade, and major outbreaks are now less frequent.
‘But we still can’t eradicate this yeast from the winery, so whenever aspects of a working control strategy are changed, or the composition of strains present in the winery changes, there is risk of a Brett problem getting out of control’, Dr Curtin said.
Currently Brett yeasts can be controlled in the winery by a combination of good sanitation, minimisation of residual sugar, effective use of sulfite (SO2) and pH management.
‘But now an additional concern is that strains of Brett currently found in Australia vary in SO2 tolerance over a five-fold range, so evolution has already furnished some strains with a defence mechanism’, he said.
Dr Curtin said that 10-15 years ago, around 60 out of 100 Brett isolates from Australian wine were at the upper end of sulfite tolerance – more recently it’s 95 out of 100.
‘Particularly because improved sulfite management means wines have higher levels of molecular SO2, it’s a classic evolutionary bottleneck that eventually something will slip through’, he said.
In an attempt to get a clearer picture of what prompts the strains to evolve sulfite tolerance and the length of time it takes to develop, the researchers have created their own evolutionary bottleneck.
‘We’re taking multiple populations of the yeast strains and exposing them to increasing levels of sulfite to see when strains start showing more tolerance than they did in the beginning’, he said.
‘We’re also undertaking more genome sequencing to try and understand what’s happening at the genome level of the yeast.
‘If we can identify the genome indicators then we’ll aim to develop improved assays which could be used in a winery lab to identify if increased sulfite tolerance might be occurring.’
Dr Curtin said a weak point in many Brett control strategy implementations was a lack of appropriate monitoring tools.
‘Plating methods are laborious and slow, and some rapid methods infer the existence of viable but nonculturable (VBNC) populations of Brett that – under certain conditions – may resume growth and spoil the wine’, he said.
But in some cases the identified VBNC yeast could be dead and risk-adverse winemakers may be conducting costly and unnecessary processing.
As part of the project, researchers at the University of Adelaide are working to better identify VBNC populations and find improved diagnostic tools and/or interpretations of the results.
The four-year research project is titled Ensuring the continued efficacy of Brettanomyces control strategies for avoidance of spoilage, further information is available here.