Program Book - Master Brewers Association of the Americas

O-39
High cell density fermentations: Promises and challenges
PIETER VERBELEN (1), Sofie Saerens (1), Filip Delvaux (1),
Freddy Delvaux (1)
(1) Centre for Malting and Brewing Science, Catholic University of
Leuven, Heverlee, Belgium
In the traditional production of lager beer, the fermentation
process takes about 1–2 weeks before entering a maturation period
of 1–3 weeks. As a consequence, fermentation and maturation
are the most time-consuming steps in the production of beer. To
improve the productivity of the beer fermentation process, several
strategies can be adopted. The interest in immobilized yeast for
primary beer fermentation seems to have dropped, but the essence
of this technique was to improve productivity by maximizing the
cell concentration in the reactor. Therefore, a promising strategy
could be the increase of suspended yeast cells in the fermentor.
In a first experiment, different pitching rates (10-20-40-80-120
million cells/mL) were applied in tall tubes (2 L) to investigate
the influence of this variable on yeast physiology and beer quality.
The fermentation speed was drastically increased when higher
initial cell concentrations were used. The net growth (maximum
cell concentration – initial cell concentration) decreased with
increasing pitching rate, which indicates that there must be a
growth limiting factor when using higher pitching rates. It was
hypothesized that the depletion of oxygen, needed for the formation
of essential membrane compounds, could be the limiting factor
for yeast growth at higher pitching rates, because less oxygen per
cell will be available. Throughout the fermentations, important
physiological parameters of the yeast were monitored, such as
viability, acidification power, glycogen, trehalose and fatty acids.
The results of these analyses revealed that physiological differences
exist between normal and higher pitching rates, although it
cannot be stated that higher pitching rates result in a poorer yeast
condition. In a second experiment, the yeast oxygenation conditions
were varied in the high cell density fermentations (80 million cells/
mL) and consequently altered growth profiles were observed. In
addition, high cell density fermentations were carried out at pilot
scale, and after reaching 80% attenuation, the beers were chilled
and GC headspace analysis was performed. No clear correlation was
observed between the higher alcohol/ester ratio and the pitching
rate or oxygenation condition which was used. In addition, the
outcome of tasting trials of the different beer types, performed by a
professional panel, showed no significant differences. These findings
show that the use of high cell densities in beer fermentations
provides promising opportunities, although challenges for this
technique remain to be investigated, such as the impact of high cell
density fermentations on the fermentation performance of different
yeast generations.
Pieter Verbelen graduated in 2005 as a bioengineer in chemistry,
with an option in food technology and industrial microbiology, from
the Catholic University of Louvain. For his M.S. thesis, he joined the
Centre for Malting and Brewing Science to study continuous primary
fermentation with immobilized yeast in a two- step system at the pilot
scale. After graduation, he started as a Ph.D. student (IWT-grant)
at CMBS, under the group of Professor Freddy Delvaux, where he’s
doing research on aspects of accelerated fermentations and yeast
physiology.
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O-40
Effect of the fermentation process on staling indicators in
order to influence the flavor stability of beer
DAAN SAISON (1), David De Schutter (1), Filip Delvaux (1), Freddy
Delvaux (1)
(1) Centre for Malting and Brewing Science, KULeuven, Belgium
Consumers consider flavor as the main quality parameter of beer.
However, the flavor profile is subject to changes during storage due
to many kinds of chemical reactions. As a beer ages, fresh flavor
notes diminish and several typical aged flavors appear. This lack
of flavor stability is of great concern for brewers as it is important
that a commercial beer is consistent and satisfies the expectations
of the consumer at all times. Despite extensive research, it remains
very difficult to control flavor stability. Since the fermentation
process has an enormous impact on many aspects of beer, it might
also influence flavor stability considerably. Biochemical processes
that occur during fermentation are not only responsible for flavor
formation due to the production and removal of flavor compounds,
but they might also influence flavor stability in several ways.
Although it has already been shown that the flavor stability is
different for beers produced with different yeast strains, research
on this effect is very limited. In this work, the effect of yeast strain
selection on staling indicators was studied in order to influence
flavor stability. Ten top fermenting Saccharomyces cerevisiae yeast
strains were compared in lab scale fermentations. The effect of
yeast on several parameters known to influence flavor stability
were evaluated. Additionally, volatile flavor compounds were
analyzed with headspace SPME GC-MS. The concentration of flavor
compounds able to mask aged flavors, sulfite content and pH of the
resulting beers were found to be yeast strain dependent. Next to this,
the reducing power of yeast appeared to be especially interesting. A
clear effect on several volatile carbonyl compounds and a substantial
reduction of precursors of aging reactions was observed. As these
factors have a considerable influence on flavor stability, the reducing
power of yeast was studied in more detail on lab and semi-industrial
scales. Two yeasts were selected for the fermentation of wort with
and without addition of volatile carbonyl compounds on a lab
scale. Additionally, these yeasts were used for the fermentation
of wort with low and high contents of volatile staling compounds
and precursors on a semi-industrial scale. The reduction of several
volatile carbonyl compounds was substantial and resulted in beers
with a similar content. Afterward, the beers were aged, and flavor
stability was evaluated by monitoring the evolution of volatile
staling compounds and by sensory analysis.
Daan Saison graduated as a bioengineer in food chemistry and
technology at the Catholic University of Leuven. He carried out
his masters thesis at the Centre for Malting and Brewing Science at
K.U.Leuven on the subject “Characterisation of Glycoside Hydrolase
in Brewers’ Yeast and the Influence on Hop Glycosides.” After
graduation, he started a Ph.D. program at the Centre for Malting
and Brewing Science.