combined with high ethanol tolerance, can
quickly produce levels of ethanol that are
toxic to most other organisms.
Carbon dioxide also gives a competi-tive
advantage to Saccharomyces yeast due
to the simple exclusion of oxygen in the
environment. This precludes the growth of
strict aerobic microbes and hinders other
microbes by forcing them to use less effi-cient
anaerobic ( fermentative) metabol-ic
pathways. For Saccharomyces yeast, on
the other hand, fermentative respiration is
actually quite efficient as they can ferment
even in the presence of oxygen.
Cellular communication
While Saccharomyces yeast cells are often
referred to as “simple single-cell organisms,”
they are capable of an incredible amount
of communication with other organisms
in the environment. Usually yeast produce
communication compounds in response to
environmental cues – such as high popula-tion
density or a lack of nutrients – and this
in turn directs other Saccharomyces cells to
adapt quickly to the new environment.
The most well-studied Saccharomyces
communication mechanism is in response
to nitrogen starvation – usually due to high
population densities. When Saccharomyces
reaches a certain cell concentration in
the wort, higher alcohols tryptophol (bit-ter
off-flavour) and 2-phenylethanol (rose
aroma) are produced, signalling to other
Saccharomyces yeast to form pseudohyphae
(a key characteristic stationary phase) and
cease cellular replication.
Acetaldehyde, as well as being a key
intermediate in ethanol fermentation, is
another compound known to synchro-nize
Saccharomyces yeast growth during
times of inconsistent carbohydrate sup-ply
in anaerobic conditions. Its small size
and volatile nature make it a prime sig-nal
molecule, but also another significant
cause of off-flavour with which brewers
must contend.
Selecting the right
Saccharomyces yeast
The interplay between Saccharomyces yeast
and wort is complex and dynamic, with
subtle changes in wort composition driving
distinct changes in yeast metabolism and,
ultimately, the production of flavour and
aroma compounds. Moreover, each unique
F E AT U R E
yeast strain will interact with its environ-ment
in different ways, expanding the com-plexity
of flavour and aroma produced dur-ing
brewing.
On one hand, this complexity results in
a unique and endlessly diverse set of beers
for consumers; but on the other, it can be
difficult for brewers to change just one
organoleptic aspect of beer through recipe
or process development alone.
For example, there are many meth-ods
of reducing VDK concentrations in
beer. Brewers commonly employ a diace-tyl
rest at the end of fermentation by
gradually increasing the temperature in
order to allow yeast to reabsorb diacetyl
and convert it to 2,3-butanediol (which
is less perceptible). However, increasing
(and then decreasing) the fermentation
temperatures in beer changes the fermen-tation
performance and flavour profile
of the finished beer. It can also be a cost-ly,
time-consuming process, especially at
larger scales.
Other methods of reducing VDKs
include reducing the pH of the beer, opti-mizing
wort free amino nitrogen (FAN)
content or supplementing the wort with the
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