Student Traces the Biochemical Pathways of Wine Spoilers
What are spoilage yeast and bacteria doing in wine, aside from spoiling wine? Lauren Schopp, an M.S. student in the Washington State University/University of Idaho School of Food Sciences, decided to investigate spoilage yeast and bacteria by tracing their biochemical pathways, or the ways they interact with chemicals already present in wine. She specifically looked at Brettanomyces bruxellensis, perhaps the Godzilla of spoilage yeast, and Pediococcus parvulus, a bacterium common to red wines but whose impact on quality isn’t well described. Although Brett and Pediococcus can grow independently, these two troublemakers flourish in one another’s company.
The food chain for Brett and Pedio starts with the grape, particularly the skin, which has naturally occurring compounds called hydroxycinnamic acids, often in the form of tartaric acid esters. Partly as a result of the winemaking process, a portion of these esters are transformed into acid forms that are precursors to spoilage compounds. Both Brett and Pediococcus metabolize those acid precursors, producing intermediate vinyl compounds. Brett then metabolizes the vinyl compounds to producing compounds in wine that taste or smell “off.”
Schopp, mentored by enology professor Charles Edwards, worked with samples of Merlot, Syrah, Cabernet Sauvignon and Pinot Noir donated by a Washington winery. Measuring levels of the precursors using high-performance liquid chromatography, Schopp found relatively high concentrations of caffeic acid in the Washington Merlot and Cabernet Sauvignon samples, higher than concentrations often cited in scientific literature. This should not immediately set off an alarm, however, because caffeic acid metabolizes into 4-ethylcatchol, which, while a spoilage compound, has a relatively high sensory threshold, meaning that wine would need high levels before it could affect flavor and aroma.
Concentrations of p-coumaric acid and ferulic acid, also precursors to spoilage compounds, were found to be within normal levels in the wine samples.
After introducing two strains of Pediococcus and four strains of Brettanomyces to various wine samples, Schopp determined that Pediococcus was quite partial to caffeic acid. It metabolized a smaller portion of p-coumaric acid, but didn’t bother with ferulic acid at all.
Brett behaved a bit differently, with its consumption of the precursors being strain-dependent. One of the strains of Brett failed to grow in the wine as its populations slowly declined over time. While utilization of caffeic acid and ferulic acid varied significantly depending on the strain, the remaining three strains showed a strong fondness for p-coumaric acid, using nearly all the available precursor. The decrease in p-coumaric acid and ferulic acid corresponded with an increase in their spoilage metabolites, 4-ethylphenol and 4-ethylguaiacol. However, not all results were this clear-cut. Significant variations in precursor metabolism often emerged depending on the type of wine and whether Brett, Pediococcus, or both were introduced to the sample. While Brett and Pedio flourished in each other’s company, this didn’t always result in increased metabolism of the precursors.
The good news for winemakers is that neither Brett nor Pedio could break down the tartaric acid esters of hydroxycinnamic acids, compounds which could have served as a large pool of precursors to foul-smelling compounds. Schopp’s research gives some insight into the behavior and interaction of Brettanomyces and Pediococcus in wine, although more work is necessary to get a better handle on controlling the spoilers. Her research was funded by the Washington Wine Advisory Board.
Beyond her lab work with wine and its spoilers, Schopp’s accomplishments include a first-place team award at the 2012 “Developing Solutions for Developing Countries” national competition. Schopp served on the student team that created “Mango Maandazi,” a fried bread product incorporating mangos to address harvest and nutrition issues in Kenya. For more information on the Mango Maandazi project, see http://bit.ly/KHX8h4.
For more information on the work in Edwards’ lab, see http://bit.ly/Sj5bU2.
-Bob Hoffmann