ASEV Brettanomyces Symposium Highlights
June 18, 2018
Written by: Matthew Glynn
After fourteen years, the American Society of Enology and Viticulture hosted another Brettanomyces Symposium in Monterey, California, to share advanced understanding of the history, current state, and future of the infamous wine yeast. The program was well-balanced between academic research and practical approaches for wine producers.
Known equally as a component of terroir, complexity, and microbial spoilage, Brettanomyces bruxellenis has long been at the center of stylistic controversy. Traditional European producers, on one hand, feel that the yeast contributes to the matrix and complexity inherent in wine, and the new world intelligencia, on the other hand, define Brettanomyces as microbial spoilage – a fault in wine.
Understanding of the wine yeast Brettanomyces has increased since the last symposium in 2004: genomic discoveries have illuminated strain relatedness and geographic distribution; insights from industry laboratories continue to assist with Brettanomyces management; and more data has been generated to support barrel sanitation practices to manage the impact of the yeast on wine.
New studies have explored cellular inhibition of Brettanomyces growth. A Brettanomyces Aroma Wheel was created. What has not changed? Brettanomyces remains a resourceful, scavenging yeast; chances of eradication are slim to none; and utilizing a methodical process to manage Brettanomyces is still the best approach concerning wine pH, SO2, racking, sampling, filtration and winery sanitation.
New insights into the history and diversity of Brettanomyces were highlighted by Dr. Ileana Vigentini (University of Milan, Italy) and Dr. Chris Curtain (Oregon State University, Corvallis). The speakers shared newly identified lineages among an impressive 1,488 isolates. Increased knowledge of the genetic composition of Brettanomyces has led to new possible explanations for the varied metabolic potential of the yeast. In many organisms, humans for example, cells contain two copies of each chromosome (i.e. these organisms are diploid). Of the 1,488 isolates of Brettanomyces described, 50% of them have a third set of chromosomes (i.e. they are triploid).
Dr. Linda Bisson (Symposium moderator, University of California, Davis) asserted that these additional chromosomes could allow Brettanomyces to respond differently in varied environmental situations and aid the yeast’s scavenging ability and diverse aromatic expression. Geneticists also are seeking an explanation for the SO2 resistance of some strains. Genome coding has led to the identification of the SSU1 gene, which is believed to be associated with the creation of a sulfite pump that is embedded in the plasma membranes of cells. This pump could allow Brettanomyces cells to “detoxify” themselves by pumping sulfites out of the cells. More research is needed on this subject, as the SO2 resistance is not as simple as hoped. However, it is known that there is a lineage of Brettanomyces strains - distributed throughout winegrowing regions of the world - with resistance to increased levels of sulfites.
Dr. Richard DeScenzo, of ETS Laboratories, discussed Brettanomyces from the perspective of an independent laboratory to provide an overview of frequently asked questions. He discussed the detection of Brettanomyces, impact on sensory, risk in the barrel, and preventing Brettanomyces spoilage in the bottle. Brettanomyces cells settle in barrels – always stir barrels before collecting samples for analysis! He discussed how controls are used with both plating and PCR (polymerase chain reaction) to ensure quality results. Although 4-ethylphenol (4-EP) and 4-ethylguaiacol (4-EG) are tested as markers associated with Brettanomyces growth, he noted that there are more than 40 compounds that have been associated with Brettanomyces growth in wine. DeScenzo noted a common observation of volatile phenol (4-EP and 4-EG) production beginning when the Brettanomyces cell density reaches ~100,000 cells/mL. The sensory threshold for 4-EP is usually in the 450 to 500 µg/L range, compared to 4-EG with a threshold closer to 50 µg/L, and the ratio of 4-EP: 4-EG has a significant effect on the perceived aromas of the compounds. His data indicates that steam cleaning barrels is the most effective method for killing Brettanomyces cells due to the ability of steam to penetrate the porosity of the oak surface. Control and intervention methods include: SO2, racking, filtration, Chitosan, Velcorin and temperature. Even with quality data, it is difficult to determine if Brettanomyces will bloom in the bottle.
“Assume the worst,” to avoid surprises. DeScenzo recommends practices including: sanitation, checking used barrels, check incoming wine, and reduce the airborne load of Brettanomyces cells. For management from an analytical perspective he recommends early detection, regularly monitoring for 4-EP/4-EG levels, identifying the cell population at the onset of 4-EP/4-EG production, and setting a target intervention point for further action. DeScenzo discussed the concept of Quorum Sensing (a mode of cellular communication using signal molecules, which allows cell populations to behave as a group), and speculated the presence of this phenomenon in Brettanomyces populations. It will be exciting to learn where this line of research leads.
The survivability – or cell viability – of Brettanomyces in two environments was presented by Dr. Charles Edwards (Washington State University, Pullman). His team analyzed cell viability in both pomace in the vineyard and in varying depths of oak barrel staves. Pomace samples returned to the vineyard contained viable cells for up to 130 months, and heat treatments to kill the cells in pomace are effective at 50° C for 10 minutes (with decreasing time requirements as the temperature increases). Edwards’ studies of cell viability in oak barrels indicate that cells were found 4-6 millimeters deep in American oak barrel staves and 6-8 millimeters deep in French oak barrel staves. Thus active management of pomace and barrels is needed to prevent the carryover of viable Brettanomyces cells.
Brettanomyces and its origin in vineyards was discussed by producers. Some producers reported no Brettanomyces populations are coming from their vineyards – that cell growth begins in the winery and is associated with the barrel aging process. Other producers have vineyard blocks with consistent Brettanomyces growth in subsequent wines. Following the insights shared by Edwards’ work on cell viability in pomace, a risk of (re)introducing cells into the winery from the vineyard appears real. Additionally, producers discussed the need to pay careful attention to winery waste streams – especially if reverse osmosis waste, which may contain viable Brettanomyces cells, is sent to winery wastewater ponds and ultimately to vineyards.
Winemakers Steve Peck (J. Lohr Vineyards and Wines) and Marcia Torres Forno (Jackson Family Wines) shared their perspectives managing Brettanomyces in wineries. They focused on training their teams to build capabilities, developing methodical monitoring programs, and consistently making well-informed winemaking decisions. Industry trends indicate that wine producers are embedding the cost of Brettanomyces monitoring programs into their annual budgets due to the value added. A cost-effective new tool for Brettanomyces detection – vinoBRETT, by Invisible Sentinel – brings PCR technology in an easy-to-use format into the winery. This tool has proven to be cost-effective and provides quick results. By incorporating best winemaking practices and monitoring regularly for Brettanomyces presence, producers are able to address issues as they arise and achieve their target wine style.
Several studies focused on both the inhibition of Brettanomyces growth by Oenococcus oeni and the liberation of volatile phenol precursors by certain O. oeni isolates have been conducted by Dr. James Osborne (Oregon State University, Corvallis, Ore.) and his students. One study indicates that the O. oeni cells present at the end of malolactic fermentation can have an inhibitory effect on Brettanomyces cell growth. In a separate study he noted that one strain of O. oeni decreased the concentration of coutaric acid (the tartaric-bound form) and increased the p-coumaric acid (the unbound form) thus creating volatile phenol precursors for further metabolism by Brettanomyces. Osborne has launched new studies to help understand the strain variability in both observations. Clearly, the context of the microbiome is a rich area for further research.
The Brettanomyces Aroma Wheel (modelled from the original Anne Noble Wine Aroma Wheel) was presented by Lucy Joseph, (University of California, Davis). Joseph’s research included the exhaustive identification of desirable aromas produced by specific Brettanomyces isolates in wine. The inherent nature of Brettanomyces to alter its metabolic behavior based on the subtle differences in the wine chemical environment and microbe populations, results in continually varying aroma compound production.
Joseph’s presentation highlighted the differences in complexing spoilage aromas, which result from Brettanomyces and those which result from bacterial (often lactic acid or acetic acid bacteria) infection. She recommends suspecting Brettanomyces when sensing aromas including Band-Aid, smoke, earth and barnyard. Bacteria is more commonly associated with mousy, metallic, butter, animal fur, vinegar and nail polish aromas.
The symposium was well-organized and informative. The history of the ASEV National Conference as a place of collaboration and information sharing was apparent during this event. The advances in the comparative genomics of Brettanomyces continues to progress rapidly, along with further understanding of cell population growth and implications for volatile phenol production. For wine producers, the benefits of improved monitoring tools and services will enable informed choices for winemaking, and the known practices that have proven effective in managing Brettanomyces were reviewed while incorporating new insights.
Matthew Glynn, MS, an established Napa Valley and Sonoma County winemaker, is the principal of a winemaking consulting business, Vinetarium Consulting, based in Napa, CA.