Program Book - Master Brewers Association of the Americas

P-137
Refermentation of aged beers: A new technique to elucidate
the contribution of flavor compounds to the aged flavor
DAAN SAISON (1), David De Schutter (1), Filip Delvaux (1), Freddy
Delvaux (1)
(1) Centre for Malting and Brewing Science, KULeuven, Belgium
Several quality aspects of beer are subject to changes during storage.
Alteration of the flavor profile in particular is of great concern to
brewers as flavor is considered the main quality parameter. Due to
the growing export and globalization of the market, this concern
has been emphasized, and the need for controlling flavor stability
has grown. Despite 30 to 40 years of research it is not yet clear
which chemical reactions and the corresponding flavor compounds
determine the aged flavor of beer. Formerly, (E)-2-nonenal was
regarded as the main cause of sensory changes during aging, but
now it is evident that a myriad of flavor compounds is responsible
for the overall aged flavor. The formation of these compounds is the
result of numerous chemical reactions, like oxidation of fatty acids
and higher alcohols, Strecker degradation, aldol condensation,
furanic ether formation, degradation of hop bitter acids, Maillard
reactions, etherification, terpenoid oxidation, glycoside hydrolysis
and synthesis of volatile esters. In this work, the effect of yeast on
volatile flavor compounds, which are suspected to contribute to the
aged flavor, was examined. Hence, beer was aged and subsequently
refermented. The advantage of this technique is that only those
volatile compounds which are relevant for the aged flavor are
considered. At first, sensory analysis of 3 pilsner and 2 specialty
beers was performed by an expert tasting panel. The intensity of
the aged flavor was rated as a whole and for separate typical aged
flavors. After aging, a very strong aged flavor was perceived in all the
selected beers. This aged flavor (eg. cardboard, caramel, ribes) was
reduced significantly, however, after refermentation. This indicates
that yeast was able to reduce at least a part of the compounds
responsible for these flavor notes. Volatile flavor compounds (fresh
and aging indicators) were analyzed with headspace SPME GC-MS.
A difference could be determined between carbonyl compounds that
can be reduced by yeast and non-reducible carbonyl compounds.
Other staling compounds and fresh flavor compounds (eg.
esters) were either unaffected by yeast or were formed during the
refermentation process. The observed decrease in the aged flavor
could not be fully explained, however, by the analysis of the known
staling compounds as flavor thresholds were mostly not exceeded
after aging. Therefore, flavor thresholds of several carbonyl
compounds were reconsidered, synergetic effects were studied and
other volatile compounds that were not yet linked to flavor stability
were examined.
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.
P-138
Pitching technology and oxygen supply with regard to yeast
physiology—Effects on fermentation performance and beer
quality
SVEN SCHÖNENBERG (1), Eberhard Geiger (1)
(1) Chair of Brewing Technology II, Technische Universität
München, Germany
Effective yeast management optimization as a key position within
the brewing process chain should include a minimization of starting
time, aeration and oxidative stress of the wort. This provides a
basis for obtaining more efficient fermentations and enhancing
beer quality as well as colloidal and flavor stability. The oxygen
supply during propagation and fermentation in particular in
“Drauflassverfahren”, which is subjected to brew cycles, is still
not adapted to the requirements of the yeast. Especially during
“Drauflassverfahren” the right oxygen supply and exact time are
determining factors for yeast growth and fermentation power. Under
brewing conditions oxygen in yeast metabolism is only required
for unsaturated fatty acids and sterol biosynthesis. As a result of
the Crabtree effect the citrate acid cycle is discontinued, and the
acetyl-CoA formed by the pyruvate dehydrogenase leads to product
repression because the yeast is not featured with carrier systems for
acetyl-CoA through the mitochondrial membrane. In this case the
PDH bypass is of vital importance for generated cytosolic acetyl-
CoA as a basis for lipid biosynthesis. The PDH bypass includes the
pyruvate decarboxylase, the acetaldehyde dehydrogenase and the
acetyl-CoA synthetase. By measurement of the specific enzyme
activities of this pathway two significant metabolism branching
points (pyruvate and acetaldehyde) for alcoholic fermentation and
biosynthesis were captured. The sugar concentration in pitched
wort as well as the oxygen level excite yeast metabolism pathways.
The Crabtree effect and the repression of maltosepermease and
maltase by glucose are two important phenomena. A successful
switch between glucose and maltose application is the basis for an
uninterrupted fermentation. The combination of flow cytometric
optical analysis of the DNA content, the measurement of certain
enzyme activities and the determination of sugar concentration
during propagation and fermentation starting time provided a
deeper insight into yeast physiology, the reaction of yeast to wort
parameters and oxygen supply. At the same time the effects of yeast
qualities like fermentation power were examined on beer quality
and flavor stability. The results of propagation tests and variations
of oxygen supply by “Drauflassverfahren” show the possibilities
of optimizing pitching technology. Oxygen itself provides an
opportunity to influence yeast physiology which increases
fermentation power and beer quality. In addition to this, yeast
physiology in combination with yeast technology is a key to reduce
costs of cooling systems and increase fermentation capacity.
Sven Schönenberg was born in 1975. Since 2005 Sven has been a
scientific assistant at the Chair of Brewing Technology II (Professor
Geiger), Technische Universität München. Sven studied brewing
and beverage technology (1999–2005) at the Technische Universität
München and graduated with a Dipl.-Ing. degree. Sven apprenticed
and worked as a brewer and maltster from 1995 to 1999 in an altbeer
brewery.
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