Indeed, some see yeast’s primary con-tribution
to beer as the production of etha-nol
and carbon dioxide, with the majority of
flavour and aroma coming from hops and/
or malt. Global brewing expert Dr. Charles
Bamforth confirms that, “Lots of people are
misguided and believe that it is hops alone
that are the basic raw materials of mak-ing
beer, with all of the alcohol and flavour
flowing from them.”
However, in reality this popular mis-conception
could not be further from the
truth: Yeast is actually a vital contributor
to the complex and delicate flavour and
aroma of beer. Simply put, without yeast
there is no beer.
As our understanding of chemistry,
microbiology and genetics improves, along-side
advances in high-throughput genetic
and biochemical technologies, scientists
and brewers alike are discovering the cen-tral
role Saccharomyces yeast plays, not
only in the production of ethanol and car-bon
dioxide, but also in the organolep-tic
(flavour and aroma) profile of beer.
It is now known that more than half of a
beer’s flavour and aroma are derived from
Saccharomyces yeast.
However, yeast do not produce organ-oleptic
compounds for the benefit of the
brewer or the beer. Instead, they produce
them to survive and thrive in the environ-ment
of a fermentation tank. We are actu-ally
quite lucky to have such a diverse and
complex palette of flavour and aroma com-pounds
made available by Saccharomyces
yeast. While the exact mechanisms and
interactions of such compounds are still
not completely known, Saccharomyces
yeast makes organoleptic compounds
for four main reasons: 1) metabolism; 2)
detoxification; 3) competition for resourc-es;
and 4) cellular communication.1
The primary goal of a Saccharomyces
yeast cell is to consume nutrients to
grow (metabolism), neutralize toxic and/
or dangerous compounds to produce a
more favourable growing environment
(detoxification), stop other organisms
from using resources it could use instead
(competition) and co-ordinate individual
cell activities so as to best survive as
a whole (communication). In this mini-review,
we introduce some yeast-derived
organoleptic compounds, discuss their
impact on the sensory profile of beer,
delve into the biological rationale to their
formation and, most importantly, provide
insights about how brewers can leverage
these strengths.
Metabolism
Like all life, Saccharomyces yeast cells must
take resources from the environment and
convert them into energy and cellular com-ponents,
such as carbohydrates, proteins,
lipids and nucleic acids. The metabolic pro-cess
perhaps most familiar to brewers is eth-anol
fermentation, whereby Saccharomyces
yeast convert carbohydrates such as malt-ose
into energy, with the by-products being
ethanol and CO2. Saccharomyces yeast is
one of the few organisms that can grow
effectively on ethanol alone, meaning it is
favourable for Saccharomyces yeast to pro-duce
high concentrations of ethanol as a
means of sequestering energy – energy that
can then be used when all carbohydrate
sources have been exhausted.
Acetaldehyde is a key intermediate
compound in ethanol fermentation, yet
high concentrations of acetaldehyde are
sometimes produced in stressful environ-mental
conditions; these are known to
cause growth inhibition and cellular stress.
In response, excess acetaldehyde is allowed
to diffuse out of the cell and into the envi-ronment,
ultimately causing a green apple
off-flavour in beer.
Vicinal diketones (VDK) are another
example of Saccharomyces yeast’s metabo-lism
contributing to the aroma of beer.
Saccharomyces yeast has the ability to pro-duce
the amino acids valine and leucine,
but the process is inefficient and some of
the by-products can cause cellular dam-age.
As a response, Saccharomyces yeast
excretes these by-products into the envi-ronment,
where they are converted into
VDKs and result in a butter or caramel off-flavour
in beer.
Detoxification
Saccharomyces yeast cells have adapted
to grow in some very extreme conditions,
due in part to their ability to neutralize or
convert various toxic or inhibitory com-pounds
in the environment in order to pro-mote
healthy growth. Indeed, these abili-ties
make Saccharomyces a good candidate
for bio-remediation applications outside of
brewing – including the removal of heavy
metals, agrochemicals from the environ-ment
and wastewater treatment.
In the context of brewing, phenolic
acids such as ferulic acid are a major com-ponent
of barley and wheat cell walls are
known to have some anti-microbial prop-erties,
including reducing cell growth and
colony formation in a variety of organisms.
Saccharomyces yeast possesses the ability
to convert ferulic acid into 4-vinylguaiacol,
4-vinylphenol and 4-vinylcatechol, which
are less toxic and produce the spicy, clove-like
aromas that typify wheat beers.
The production of esters like ethyl hex-anoate
(apple, anise) and ethyl octanoate
(sour apple) are also linked to detoxifica-tion.
Fatty acids normally come from the
wort and are required for proper cellular
growth. However, medium-chain fatty acids
(especially unsaturated ones) are known to
be toxic to Saccharomyces yeast, but can be
converted by into ethyl hexanoate and ethyl
octanoate2 and thereby rendered less toxic.
Competition
In order to dominate their environment
and ensure survival of their species,
Saccharomyces yeast also produce com-pounds
that inhibit other organisms from
growing and using nutrients. Again, etha-nol
is the most well-known to brewers and
Saccharomyces yeast is one of the fast-est
ethanol producers on the planet and,
F E AT U R E
Simply put, without
yeast there is no beer.
16 § POURED CANADA § www.poured.ca
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