Karen Fortmann (1), Christine Hughey (2), Kearney Foss (2); (1) White Labs, San Diego, CA, U.S.A.; (2) James Madison University, Harrisonburg, VA, U.S.A.
Brewer’s yeast is known to contribute over 500 flavor-active compounds to beer. Here we profiled a 20-bbl batch of an amber ale that was equally divided and fermented by White Labs with Bedford British ale yeast, Dusseldorf alt yeast, Tennessee whiskey yeast and Abbey ale yeast. Untargeted metabolomics profiling was conducted by positive and negative ion ESI LC q-TOF MS. Mass Profiler Professional was used to align and filter molecular features (MFs, unique mass and retention time) found in all replicate samples (P ≤ 0.05). Retained MFs were searched against a KEGG-curated metabolite database for Saccharomyces cerevisiae with an allowed mass error of ≤15 ppm. Database hits were used in a multi-omics experiment to simultaneously match assignments in the positive and negative ion data to metabolites in known yeast pathways. Pathways related to carbohydrate and amino acid metabolism, which are known to contribute to flavor and aroma in beer, were of primary interest. A sensory panel determined that the yeasts imparted distinctly different flavor profiles (from fruity to spicy) to the beers. These differences are likely reflected in their different MS profiles. Principal component analysis (where P1 and P2 accounted for ~55% of the variance) revealed that the Tennessee whiskey yeast was least similar to the other strains in P1, largely due to the increased number of features unique to this yeast (~18% of all retained MFs). The other three strains clustered together in P1 but were separated in P2. Approximately 32-39% of MFs were found in all strains and replicates (n = 12). Tentatively identified metabolites matched to 41 pathways, with 8 and 19 pathways related to carbohydrate and amino acid metabolism, respectively. Initial efforts have focused on five pathways in which 25-40% of the metabolites have been matched to the KEGG-curated library and 12-18% have been RT matched to standards. Polyamine synthesis and the methionine salvage pathway are of particular interest since they involve the metabolite 5-methylthioadenosine (5-MTA), which has been proposed as a marker for beer oxidation during storage (Food Chem. 135:1284-1289, 2012). Its concentration was constant in all four fresh beers (0.8% RSD). Other matched metabolites in these pathways include spermidine and methionine (confirmed). Spermidine (8% RSD) and methionine (6% RSD) were found in all samples except one; spermidine was absent in the Abbey ale yeast. As we work to increase the number of confirmed metabolite identifications, we hope to better understand how of these and other metabolites vary as a function of yeast strain and, ultimately, contribute to beer flavor.
Karen Fortmann is a senior research scientist at White Labs, Inc., a yeast manufacturing company is San Diego, CA. She received her Ph.D. degree in biochemistry from the University of California, San Diego, in 2014. After completion of her degree, Karen strayed from cancer research and turned to the world of fermentation sciences. She joined White Labs in July 2014 and has been working on discovering the oddities of “omics” in yeast since then.