Kent Johnson (1), William Deutschman (1); (1) Westminster College - Salt Lake City, Salt Lake City, UT, U.S.A.
Yeast, Fermentation, and Microbiology
Maltotriose and higher order dextrins are significant constituents of brewing wort and are metabolized poorly by many strains of brewing yeast. The presence of residual extract in finished beer can have significant impacts on flavor, packaging stability, and resistance to spoilage bacteria. Recently the popularity of beers brewed or aged with non-traditional yeast strains, particularly in the Brettanomyces family, has risen dramatically, in part because yeast strains from the Brettanomyces family are reputed to be able to metabolize more complex sugars, thus “drying out” residual extract in high-gravity beers and leaving a beer with lower body and mouthfeel. While the maltotriose fermentation profile of more traditional yeasts such as Saccharomyces cerevisiae and S. pombe is well known, the ability to metabolize maltotriose or larger dextrins by yeasts in the Brettanomyces family is less well characterized, with conflicting information in the literature. There is very little information available on the metabolic ability of Brettanomyces yeast with respect to maltotetraose or larger glucose polymers. The objective in this study was to begin the process of determining the metabolic preferences and abilities of several commercially available Brettanomyces strains. Standard malt extract wort was prepared and inoculated with each yeast strain. The wort was aerated and fermentation was allowed to proceed to completion. The sugar profile of the fermentation mixture was monitored periodically using HPLC with an Aminex HPX-87H or Aminex HPX-42A column and ELSD detection. We found that the ability to metabolize maltotriose was much lower than had been previously reported under primary fermentation conditions. Additionally, the commercially available strains of Brettanomyces show little metabolism of larger glucose polymers, even with fermentations lasting several months. This work suggests that final fermentation or aging of finished beer in the presence of these yeasts may not be having the effects on residual extract that are commonly accepted and that the “drying out” of beer exposed to Brettanomyces may be due to other mechanisms.
Kent Johnson is currently pursuing a B.S. degree in biology, with a minor in chemistry, at Westminster College. At present, he is working on research in the fields of brewing science and institutional strategy, with plans to attend medical school after his undergraduate years.