Program Book - Master Brewers Association of the Americas

I-11
Control of flavor production in yeast
HIROYUKI YOSHIMOTO (1)
(1) Kirin Brewery Co., Yokohama-shi Kanagawa, Japan
Acetate esters, such as isoamyl acetate and ethyl acetate, are
major flavor components. While hydrogen sulfide is an off-flavor
with an aroma of rotten eggs, sulfite is an antioxidant and plays
an important role in maintenance of freshness. These flavor
compounds are produced during fermentation and are the
most important determinants of beer quality. Acetate esters are
synthesized from acetyl CoA and the corresponding alcohols by
alcohol acetyltransferase (AATase), and hydrogen sulfide and
sulfite are produced during reductive sulfate assimilation in yeast.
To understand and control their production in yeast, we have tried
a variety of approaches (genetic, gene expression, metabolite
levels, and breeding). First, results of the genetic approach have
suggested that the reduction in acetate ester production by aeration
or the addition of unsaturated fatty acids is due to a reduction in
enzyme synthesis resulting from transcriptional suppression of
the ATF1 genes responsible for most AATase activity. Promoter
analysis of the ATF1 gene identified an 18-bp element containing
a binding consensus sequence of transcription factor Rap1p,
which is essential for transcription activation and suppression
by unsaturated fatty acid. Oxygen inhibited the ATF1 transcript
through the Rox1p-Tup1p-Ssn6p hypoxic repressor complex at
the binding consensus sequence of Rox1p. ATF1 expression is
activated by nitrogen sources through a protein kinase, Sch9p.
Second, we performed a comprehensive analysis of gene expression
and levels of sulfur metabolites containing sulfite and hydrogen
sulfide. DNA microarray was used to evaluate gene expression, and
concentrations of metabolites were measured using GC, HPLC, and
CE-MS (capillary electrophoresis-mass spectrometry). This analysis
suggests that O-acetylhomoserine (OAH) is the rate-limiting factor
for production of both sulfite and hydrogen sulfide in bottomfermenting
yeast. Third, we have developed a high sulfite-producing
bottom-fermenting yeast strain by integrated gene expression and
metabolite levels analysis. Based on the results obtained from gene
expression and metabolite level analysis, we hypothesized that
sulfite levels could be increased and sulfide levels decreased if the
flux from aspartic acid to OAH and the flux from sulfate to sulfide
were increased simultaneously. Appropriate genetic modifications
were then introduced into a prototype strain to increase metabolic
fluxes from aspartate to OAH and from sulfate to sulfite, resulting
in high sulfite and low hydrogen sulfide production. To select
spontaneous mutants of a bottom-fermenting yeast strain in high
sulfite and low hydrogen sulfide production, a mutant resistant to
both methionine and threonine analogs were selected and analyzed
for similar metabolic fluxes. One promising mutant produced
much higher levels of sulfite than the parent, but parental levels of
hydrogen sulfide. Finally, findings from our approaches suggested
a model for investigating the mechanisms that control flavor
production in yeast.
Hiroyuki Yoshimoto received a Ph.D. degree in engineering from
Hiroshima University, Japan, in March 1992. He was employed
with Kirin Brewery Company, Limited in April 1992 as a researcher
for yeast in the Central Laboratories for Key Technology. He also
studied yeast technology in Stanford University, CA, from June
1999 to September 2001. Since March 2007, he has been working in
the Research Laboratories for Brewing, Kirin Brewery Company,
Limited.
52
I-12
Investigation of consumer preferences for beer by combined
sensory and instrumental analyses
ASAKO TAKEUCHI (1), Yutaka Miyamoto (1), Kenkichi Aoki (1),
Tetsuo Aishima (2)
(1) Asahi Breweries Ltd., Ibaraki, Japan; (2) Chemometrics and
Sensometrics Laboratory, Tokyo, Japan
Many brands of beer are available on the Japanese market. However,
up until several years ago, most top-selling brands belonged to
the pilsner category and appeared similar in taste to one another
compared with the wide variety of beers produced around the
world. Recently, along with normal beers and low-malt beers
(happoshu), non-malt beer-flavored beverages with a broader range
of characteristics have been distributed throughout Japan. This
presentation reports on a study that evaluated the preferences of
consumers who usually drank one particular brand of non-malt
beer-flavored beverage. The consumers were recruited as tasters
after screening, and their overall preferences for 16 commercial
Japanese beers and beer-flavored beverages were evaluated using
a seven-point hedonic scale. Additionally, we obtained data from
chemical analysis and quantitative descriptive sensory analysis in
order to identify the character of each sample. Based on a cluster
analysis of the preference data for each sample, the consumers
were grouped into several different categories, including a class of
individuals who preferred the non-malt beer-flavored beverages. We
created a preference map for each of the categories by combining the
consumer preference data and the sensory quantitative descriptive
sensory analysis data. Our results demonstrated that these
consumers showed a range of preferences for beers and beer-flavored
beverages, even though they all usually drank one particular brand.
In addition, we were able to examine the correlations between the
components, the sensory attributes, and the consumer preferences.
Asako Takeuchi is a researcher at the Development Laboratories
for Alcoholic Beverages of Asahi Breweries, Ltd. She joined Asahi
Breweries Ltd. in 1998 after receiving her M.S. degree in applied
microbiology from Mie University, Japan. She was transferred to the
research section in October 2000, where she began work on brewing
microbiology. Since 2005, her research has focused on sensory
evaluation.