K. ASADA (1), K. Takazumi (1), M. Sato (1), T. Oshima (1), T. Shigyo (1); (1) Sapporo Breweries Ltd., Yaizu, Japan
Yeast
Thursday, June 5 - 8:00 a.m.-9:45 a.m.
Level 4, Red Lacquer Ballroom
Fermenting yeast cells produce a wide variety of secondary metabolites,
including certain carbonyl compounds, sulfur containing compounds,
organic acids, higher alcohols, and esters. Volatile esters are only
trace compounds in fermented beverages such as beers, but they are
extremely significant for the flavor profile of beers. The
highest-impact flavor-active volatile esters in beers are ethyl acetate
and isoamyl acetate. Ethyl acetate smells like a solvent, whereas
isoamyl acetate, which is synthesized from isoamyl alcohol and acetyl
CoA, has a fruity banana aroma. So far, three different alcohol acetyl
transferases, which are responsible for the formation of certain acetate
esters, have been identified. Among these, ATF1 (alcohol acetyl
transferase) is the most effective gene for acetate ester formation
because 40% of ethyl acetate and 80% of isoamyl acetate are synthesized
by Atf1 protein. The amount of acetate ester is dependent on the yeast
strain, thus it is very important for brewers to select the appropriate
yeast. Sapporo Breweries Ltd. has more than 1,000 strains of yeasts,
including bottom-fermenting yeasts, top-fermenting, and wine yeasts. The
bottom-fermenting yeast SBC961 has strong aroma characteristics. The
characteristics of beer brewed with this yeast are a robust taste and a
well-balanced fruity and malty flavor. The fruity flavor of this beer is
caused by the high concentration of acetate ester emitted from SBC961.
In this study, we analyzed the isoamyl acetate-metabolizing system of
this special bottom-fermenting yeast using comprehensive methods, i.e.,
transcriptome and metabolome analyses. DNA microarray indicated that ATF1gene
expression of SBC961, which had been expected to be greater than that
of other strains, was about the same level as that of the control
strain, and that BAP2 (BCAA permease) and BAT2 (BCAA
transferase) gene expressions of SBC961 were higher than those of the
control strain. The fermentation tests supported the leucine uptake
ability of SBC961. The results suggest that isoamyl acetate formation
capacity and leucine uptake capacity are closely related. We cloned
several genes related to isoamyl acetate metabolism (BAP2, BAT2, and ATF1)
and obtained recombinant bottom-fermenting yeasts. During the
fermentation trials using them, we measured the quantity of low
temperature volatile compound (LVC) in the beer and that of
intracellular metabolites of SBC961 and the other recombinants
exhaustively by metabolomics analysis. These results suggest that SBC961
has a specific phenotype similar to both ATF1and BAP2
overexpression strains. Moreover, we calculated the contributing ratio
of isoamyl acetate biosynthesis between alcohol acetyl transferase and
isoamyl alcohol biosynthesis abilities.
Kei Asada received a master’s degree from the graduate school of
biostudies at Kyoto University, Japan. He began employment with Sapporo
Breweries Ltd. in 2008 as a microbiologist in the Frontier Laboratories
of Value Creation. From 2011 to 2012, he studied bottom-fermenting yeast
as a researcher in the Department of Biotechnology, University of
Tokyo, Japan.
