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​O-6. Pure culture fermentation characteristics of Brettanomyces yeast species and their use in the brewing industry

Presenter: Chad Yakobson, Crooked Stave Artisan Beer Project, Denver, CO.

This study was conducted to evaluate the pure culture fermentation performance of eight strains of Brettanomyces yeasts with the objective to find characteristic compounds produced during fermentation. Colony formation and growth was initially observed on media agars recommended for use in the brewing industry, with strong growth observed on MYPG, WLN, and CuSO4 agar medias. Cell growth during semi-aerobic batch culture showed 2-phase and 3-phase growth patterns, with maximum cell counts reached after 168 to 192 h of propagation. Pure culture fermentations were conducted with multiple pitching rates, and variation was observed in the different abilities of strains to achieve adequate attenuation after 35 days. Longer time was needed for most strains except B. bruxellensis (BSI-Drie), which achieved 82.16% apparent attenuation with a pitching rate of 6×106 cells/mL. When the strains were pitched at a rate of 12×106 cells/mL, a correlation was observed between final pH and final apparent attenuation, with a lower final pH resulting in greater attenuation. Fermentations were further conducted with wort preacidified with lactic acid at concentrations of 100, 500, 1,000, and 3,000 mg/L. Higher initial concentrations of lactic acid had a significant effect, increasing the level of attenuation observed in each strain, while generally decreasing the secondary metabolites produced. Compound analysis of the fermented beers showed ethyl acetate, ethyl caproate, and ethyl caprylate were the significant esters produced, with no isoamyl acetate detected throughout the study. Of the esters produced, ethyl caproate and ethyl caprylate were produced at levels previously unseen in fermented beer.

Chad Yakobson is the owner and brewer of Crooked Stave Artisan Brewery and a member of MBAA District Rocky Mountain. He obtained a B.S. degree in horticulture science before going on to study viticulture and oenology in New Zealand. He then switched his focus to the brewing industry, concentrating on various microorganisms present during barrel aging of sour beer. In 2010 Chad completed his master’s thesis, "Primary Fermentation Characteristics of Brettanomyces Yeast Species and Their Use in the Brewing Industry," and was awarded an M.S. degree in brewing and distilling from Heriot-Watt University and the International Centre for Brewing and Distilling in Edinburgh, Scotland.