Program Book - Master Brewers Association of the Americas

discussed in this presentation. We characterized, under different
wort conditions, three popular lager beer yeast strains with different
ethanol tolerances. The strains were characterized at an average
gravity of 14°Plato and at high gravity—21° and 24°Plato. The
higher gravities were achieved with the addition of glucose or
maltose syrups to the basic wort. The fermentations with wort
at 21°Plato were also compared with fermentations at 21°Plato
supplemented with different nitrogen sources. In all fermentations,
samples for both transcriptome and intra- and extracellular
metabolome analysis were collected from early exponential
and stationary phases of the fermentations. Metabolome and
transcriptome analysis of the samples from the stationary phase,
especially in the case of the less ethanol tolerant strain showed
significant differences between the samples grown in glucose versus
maltose supplemented wort. The main metabolites that contributed
to this separation were central carbon intermediate metabolites and
metabolites associated with the pyruvate and phosphenol pyruvate
metabolism. Comparison between the 21°Plato fermentation
supplemented with glucose versus maltose syrups showed
significantly changed genes associated with amino acid metabolism,
cell organization and stress response. When comparing the samples
from the 21°Plato control fermentations versus the 21°Plato
fermentation supplemented with different nitrogen sources, the
analysis showed increased amino acid content for the yeast cells
both intra- and extracellularly, improved fermentation performance
and a more favorable flavor profile of the final beer. In conclusion,
both metabolome and transcriptome analysis can be used as tools to
determine the physiological state of brewer’s yeast in high gravity
beer fermentations and help us further improve its fermentation
performance.
Maya P. Piddocke received a M.S. degree in biology from Sofia
University, Bulgaria, in 1998 and a M.S. degree in biotechnology
from the Center for Microbial Biotechnology at the Technical
University of Denmark in 2004. After the completion of her masters
thesis she began work on her Ph.D. degree on the topic of “High
Gravity Beer Fermentations for Low Calorie Beer Production” under
the supervision of Professor Lisbeth Olsson and in collaboration
with Novozymes A/S, Denmark. Her research focused on studying
the response of brewers’ yeast to various stress factors imposed
during high-gravity beer fermentations, with emphasis on extensive
intra- and extracellular metabolome and transciptome analysis and
focus on glucose repression and nitrogen limitation. The project also
involved work on the physiological characterization of brewers’
yeast strains under different gravity conditions, studying nitrogen
supplementation in beer fermentations, as well as the flavor and
aroma profile of the resulting beer. Currently she is in the final stage
of completing her Ph.D. thesis.
P-136
Influence of fermentation temperature and high-gravity
brewing on the synthesis of yeast-derived volatile aroma
compounds
SOFIE SAERENS (1), Kevin Verstrepen (2), Johan Thevelein (3),
Freddy Delvaux (1)
(1) Center for Malting and Brewing Science, Catholic University
Leuven, Belgium; (2) Harvard FAS Center for Systems Biology,
Harvard University; (3) Laboratory of Molecular Cell Biology,
Department of Molecular Microbiology, VIB, Catholic University
Leuven, Belgium
As far as consumers are concerned, the aroma and flavor of beer
are among the main characteristics that determine its quality and
value. The aroma of beer is a unique mixture of volatile compounds
originating from the malt, hop and secondary products formed
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during fermentation. The aroma complexity dramatically increases
during alcoholic fermentation as a result of the synthesis of
important volatile compounds by Saccharomyces yeast species.
The nature and amount of these compounds depend on multiple
factors, such as wort composition, fermentation temperature and
yeast strain. The aim of this study was to quantify the differences in
the synthesis of yeast-derived volatile compounds resulting from the
difference in fermentation temperature and high-gravity brewing
conditions. Volatile compounds were quantified at different stages
of the fermentation and compared to the expression of 12 genes
involved in aroma biosynthetic pathways. The volatile compounds
synthesized by beer yeasts include higher alcohols (marzipan and
floral aromas), acetate esters and ethyl esters (fruity and floral
aromas) and carbonyl compounds (buttery aromas) among others.
Higher alcohols can be synthesized either from intermediates of
sugar metabolism, through anabolic reactions, or from branchedchain
amino acids, through a multistep catabolic reaction, the
Ehrlich pathway. Ester compounds are produced by condensation
of an alcohol and a coenzyme-A-activated acid (acyl-CoA). Hence,
in S. cerevisiae, acetate esters result from the combination of acetyl-
CoA with an alcohol, by the action of the alcohol acetyl transferases
Atf1 and Atf2. Correspondingly, ethyl esters are generated from
acyl-CoA and ethanol by the action of Eht1 and Eeb1. Diacetyl is
another important compound in beer. The final concentration of
diacetyl in beer depends on three factors, namely synthesis and
excretion of α-acetolactate, the immediate precursor of diacetyl,
conversion of this precursor into diacetyl, and removal of diacetyl
by yeast. The capacity of yeast to synthesize these compounds varies
between different yeast strains. Although their exact contribution
is not completely clear, fermentation temperature and the use
of high-gravity worts are additional variables that affect the final
concentration of yeast-derived aroma compounds in beer. As the
enzyme activity of Atf1 and Atf2 is the limiting factor for acetate
ester production, like the activity of Bat1 and Bat2 for higher
alcohol production, we investigated if there is a correlation between
the biosynthesis of these compounds and the expression of the
corresponding genes, especially when a higher temperature or
higher wort density was applied. Taken together, our study reveals
whether the expression level and activity of the biosynthetic
enzymes could be prime targets for flavor modification by alteration
of process parameters.
Sofie Saerens graduated with a degree in applied bioscience and
engineering, with an option in biomolecular engineering from
K.U.Leuven, Belgium. For her M.S. thesis, she joined the group of
Professor Freddy Delvaux at the Centre for Malting and Brewing
Science, Catholic University Leuven to study transcriptional
regulation of the ATF1 ester synthesizing gene in the brewers’ yeast
Saccharomyces cerevisiae. A year later, she started a Ph.D. program
at the Centre for Malting and Brewing Science. Between 2003 and
2007, Sofie has investigated the biochemical background and control
of flavor-active ethyl ester formation in brewers’ yeast. After earning
her Ph.D. degree, Sofie was appointed as a post-doctoral research
fellow in the Centre for Malting and Brewing Science. She now
studies novel technologies for polygenic analysis and modification in
yeast.