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O-5. Metabolism of flavor compounds in Brettanomyces bruxellensis during secondary fermentations at varying pH

Presenter: Andrew Sandstrom, Colorado State University, Fort Collins, CO. Coauthor(s): Chad Yakobson, Crooked Stave Artisan Beer Project, Fort Collins, CO; Pat Murfin, New Belgium Brewing Company, Fort Collins, CO; Jack Avens, Corey Broekling, and Matthew Lewis, Colorado State University, Fort Collins, CO.

There is variability in genome arrangement between strains of Brettanomyces (“Brett”) yeast. These variations have been found to cause different characteristics in the yeast. Everything from cell morphology, colony growth, and metabolism vary between strains. By understanding the metabolism of different strains of Brett, certain flavor characteristics could be targeted in the production of beer. The effects of pH on seven-day secondary fermentations with two strains of Brettanomyces bruxellensis were studied. New Belgium Brewing Company (Fort Collins, CO) provided both strains; one was their production Brettanomyces bruxellensis (type 1), while the other originally came from White Labs Inc. (San Diego, CA) (type 2). Seven-day secondary fermentations were chosen in hopes of understanding the metabolism of flavor compounds by these two strains under New Belgium Brewing Company’s process. "Fat Tire" (New Belgium Brewing Company) beer acidified with lactic acid (95%) was used for fermentations. During the acidification and transfer of the beer, some aeration occurred. Using gas chromatography with flame ionization detector (GC/FID), differences in metabolism between strains were seen. The variability due to pH differences can be attributed to the spontaneous creation of flavor compounds with the addition of acid. A pH-independent two-tailed t test with equal variance found yeast dependent compounds. Yeast type 1 had significantly less isoamylacetate, ethyl octanoate, and acetaldehyde compared to the control and type 2, while type 2 had a large increase in acetaldehyde, 3.6 times the amount of the control and 5.76 times the amount of type 1. A pH independent multivariate analysis was consistent with the findings of the two-tailed ttest, it shows that the yeasts vary the most with respect to acetaldehyde. The reduction of esters was seen in type 1. This ester metabolism is most likely the activity of esterase enzymes that break down the esters into acid and alcohol components. Esterase activity increases the fatty acid content of the beer and affects the “goaty” flavor characteristic of Brett. In type 2, a large increase in acetaldehyde was due to a backup in the metabolism of ethanol or sugar. These differences could be used to create a specific flavor profile based on aging with different strains of Brettanomyces.

Andrew Sandstrom studied biomedical sciences at Colorado State University, Fort Collins, from 2006 to 2010. He started home brewing in 2006 when he moved to Fort Collins. In his senior year at CSU, Andrew took brewing science and technology, spending his last semester working with specialists in a variety of fields. Brewers Chad Yakobson (Crooked Stave Artisan Beers Project) and Pat Murfin (New Belgium Brewery) assisted on experimental design and carrying out the experiment. Cultures were grown, plated, and propagated for fermentations in CSU’s Food Science Brewing Lab under Jack Avens. Samples were chemically tested in the Proteomics and Metabolomics facility at CSU under Corey Broekling and Matt Lewis. New Belgium Brewery provided yeasts and other lab supplies. Andrew has been a member of MBAA since 2010.