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Brewing yeast
Brewer’s yeast
scepticism! The method was merely regarded
as a means of reducing infection by wild
yeasts and bacteria. In 1959 it was reported
that of 39 ale yeast cultures in use
commercially in Britain, 12 contained a single
strain, 16 had two major strains and the rest
contained three or more yeast strains.
Selection of the production strain(s) varies
depending on the beer type. Overall, the
properties that define good brewing yeast
cultures are:
• A rapid fermentation rate without excessive
yeast growth;
• An ability to withstand high alcohol and
osmotic stresses imposed during brewing;
• Reproducible flavour production;
• Ideal flocculation behaviour;
• Efficient use of maltose and maltotriose;
• Good handling characteristics, including
genetic stability.
A brief overview
There is an old German saying,
“Hopfen und malz, Gott
erhalt’s”, which translates to
“God save hops and malt”. So
perhaps if you provide the hops
and the malt, God will take care
of the rest. Archaeologists have
uncovered evidence of
breweries and bakeries that
used yeast over 4,000 years
ago. However it was not until the
17th century that the globular
structures in beer were first
observed using simple
microscopes. It took a further
120 years before the true role of
yeast in fermentation was
established.
by Anne E Hill and
Graham G Stewart
ICBD, Edinburgh, Scotland
I
n the interim, many brewers referred to the
foam used to pitch sweet wort as
‘godisgoode. By the mid 1800s the academic
mainstream had been convinced that yeast
was a living organism, rather than a chemical
catalyst and later in the century pure culture
techniques were developed at the Carlsberg
Laboratory in Copenhagen. Brewing research
on yeast has fluctuated since that time, with
periods of intense activity and times when
war or the economy has reduced the number
of active researchers. Here we briefly review
our knowledge of brewing yeast and discuss
yeast for 21st century beer production.
Outline of yeast
Around 1,500 species of yeast may currently
be described. These are predominantly singlecelled fungal micro-organisms (Figure 1) able
to grow in both the presence and absence of
oxygen. They are chemoorganotrophs,
meaning that they use organic compounds as
a source of energy and do not require sunlight
for growth. Cell sizes vary, depending on the
strain and the stage in the life cycle, from 3 to
14µ and have a mass of approximately 40ρg
when dry (40g of dried yeast typically
contains 1012 cells).
Most yeast reproduce asexually by budding
but some yeast species divide by binary
fission (the parent divides into two daughter
cells). The budding yeasts, or ‘true yeasts’,
are classified in the order Saccharomycetales,
Figure 1. Electron micrograph of a yeast cell
showing multiple bud scars.
the most common members belonging to the
species Saccharomyces cerevisiae.
Brewer’s yeast
Strains of Saccharomyces cerevisiae have
been used for thousands of years in both
baking and fermentation. The yeast cell is
also an important research model for genetics
and cell biology (also called an experimental
eucaryote), in addition to being useful in
energy generation through microbial fuel cells
and biofuel production. What, though,
distinguishes brewer’s yeast from the
hundreds of other yeast strains?
There are basically two major kinds of yeast
used in brewing: Saccharomyces cerevisiae
(ale) and Saccharomyces pastorianus, also
still called Saccharomyces uvarum
(carlsbergensis) by some, (lager). Ale yeast
operates at around room temperature (1822°C), ferments quickly, and produces the
‘fruitiness’ characteristic of many ales. They
are typically top fermenters and produce a
higher alcohol concentration than lager
yeasts. However, with the advent of
cylindroconical fermentation vessels, bottom
cropping ale yeast strains are not uncommon.
The use of centrifuges to crop yeast at the end
of fermentation has promoted the selection of
both ale and lager non-flocculent cultures.
Traditionally lager yeasts are bottom
fermenters. They work at colder temperatures
(8–15°C), ferment slowly and utilise more
wort sugars more efficiently, leaving a clean,
crisp taste. Around the world, ale and lager
yeasts predominate but there is an increase in
the number of breweries employing
Brettanomyces yeast, traditionally used in
Lambic beer production.
As a result of work by Emil Hansen at the
Carlsberg Laboratory in the late 1880s, the
practice of employing pure strains in lager
production spread around the world. Aleproducing regions however, met this ‘radical
innovation’ with some opposition and
Living conditions
Compared to other media employed for the
production of fermentation alcohol (both
industrial and potable) wort is by far the most
complex. It is an intricate environment
because it consists of a number of simple
sugars, dextrins, amino acids, peptides,
proteins, vitamins, ions such as zinc,
magnesium, manganese, calcium, sodium and
potassium, nucleic acids and other
constituents far too numerous to mention. The
wort sugars include sucrose, glucose,
fructose, maltose and maltotriose together
with dextrin material. The individual amino
acids, ammonium ions and small peptides (diand tripeptides) found in wort are known
collectively as free amino nitrogen (FAN).
FAN is believed to be a good index for
potential yeast growth and fermentation
efficiency as adequate levels of FAN in wort
ensure efficient yeast cell growth and hence
desirable fermentation performance.
High-gravity brewing employs wort at
higher than normal concentration and
consequently requires dilution with
deoxygenated water at a later stage in
processing. High-gravity worts can influence
yeast performance with effects apparent upon
fermentation and flocculation. The increased
osmotic pressure, elevated alcohol
concentration and modified nutrient balance,
all have a profound influence on yeast
performance during their fermentation. For
example, when yeast is first pitched into high
gravity wort, passive diffusion of water out of
the cell occurs and this results in decreased
cell viability within the first 24 hours of
fermentation. The decrease in viability is
exacerbated with 20°Plato compared to
12°Plato wort. However, with both lager and
ale yeast the viability usually recovers later in
the fermentation. Another negative effect of
high gravity worts on yeast performance
concerns the number of generations (yeast
cycles) that can be fermented by a single yeast
culture. The number of cycles that can be
employed is reduced with increasing wort
gravity.
Brewer & Distiller International • June 2009 • www.ibd.org.uk
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Brewing yeast
Figure 2. Intracellular concentrations of
glycogen and lipids in a lager yeast strain
during fermentation of a 15°Plato wort.
Figure 3. Order of uptake of wort sugars by
yeast.
How does yeast work?
important in brewing, Scotch whisky
production and baking, since maltose is the
major component of brewing wort, spirit mash
in Scotland and wheat dough. A major limiting
factor in the fermentation of wort is the
repressing influence of glucose (and possibly
fructose) upon maltose and maltotriose uptake.
Only when approximately 50% (this is yeast
strain and wort composition dependent) of the
wort glucose has been taken up by the yeast
cells will the uptake of maltose commence. In a
similar manner, the presence of glucose will
repress the production of glucoamylase by S.
diastaticus thereby inhibiting the hydrolysis of
wort dextrins and starch. Repression of this
nature has a negative effect on overall
fermentation rate. A range of ale and lager
yeast strains have been employed in order to
explore the mechanisms of maltose and
maltotriose uptake from wort. A number of
studies have found that ale strains appear to
have greater difficulties completely fermenting
wort (especially maltotriose and particularly in
high gravity wort) than lager strains.
Recent studies on the utilisation of
nitrogenous wort components have confirmed
the early work by Jones and Pierce that amino
acid uptake can be divided into four groups
(Table 1) with amino acid uptake completed,
with the exception of proline, within the first
48h of fermentation. An important recent
finding is that yeast fermentation activity does
not cease when wort amino acids are depleted.
During fermentation, oligopeptides are
produced as a result of larger peptide
hydrolysis due to yeast protease
excretion/secretion. Both lager and ale yeast
strains can simultaneously use amino acids,
ammonium ions, and small peptides as sources
of assimilable nitrogen.
The life cycle of yeast is activated from
dormancy when it is pitched into the wort.
Growth follows four phases, which usually
overlap during fermentation: lag, log,
fermentation and sedimentation phases. To
grow successfully, yeast requires an adequate
supply of nutrients. For healthy fermentation a
ready supply of fermentable carbohydrate,
nitrogen, vitamins and minerals are needed.
Inadequate nutrition can result in poor beer
stability, generation of mutant yeasts, poor
flocculation, off flavours and incomplete
fermentations. Yeast physiology, activity and
nutrition requirements vary between strains
and additional nutrients can be used to ensure
consistent fermentations or to avoid
fermentation problems.
At the beginning of fermentation, build-up
of unsaturated fatty acids and sterols, at the
expense of the intracellular storage
carbohydrate glycogen, is essential for a
normal growth pattern of the yeast population
during the rest of the fermentation process
(Figure 2). Yeasts are unable to synthesise
unsaturated fatty acids and sterols under
strictly anaerobic conditions, so in practice
oxygen is supplied during pitching.
Optimisation of the dissolved oxygen content
of wort during fermentation is important to
achieve a good fermentation and a high quality
product.
One of the major advances in brewing
science during the past 40 years has been the
elucidation of the mechanisms by which the
yeast cell utilises, in an orderly manner, the
plethora of wort nutrients. In the normal
situation, brewing yeasts are capable of
utilising sucrose, glucose, fructose, maltose
and maltotriose in this approximate sequence
(or priority), although some degree of overlap
does occur, leaving maltotetrose and the other
dextrins unfermented (Figure 3). Brewer’s
yeast is also capable of utilising sugars such as
galactose but not lactose unless it is hydrolysed
to its constituent monosaccharides – glucose
and galactose. In addition, there is a closely
related species (regarded as the same species
by some) to S. cerevisiae, designated as S.
diastaticus. This yeast species produces an
extracellular glucoamylase that is capable of
utilising wort dextrins to glucose units which
are metabolised. The transport, hydrolysis and
fermentation of maltose are particularly
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One of the major factors when considering
important characteristics during brewing or
other ethanol fermentations is flocculation. The
ideal brewing strain is one which in a typical
fermentation, without the use of a centrifuge,
remains in suspension as fermenting single
cells until close to the end of fermentation
when the wort sugars and most amino acids, as
well as vicinal diketones are reduced. Only
then should the culture rapidly flocculate and
settle out of suspension to be harvested and
repitched into wort for a subsequent
fermentation.
Genetic manipulation of brewer’s
yeast strains
Over the years, considerable efforts have been
devoted to a study of both the biochemistry and
genetics of brewer’s yeast (and other industrial
yeast strains). The objectives of these studies
have been two fold:
(1) to learn more about the biochemical and
genetic make-up of brewing yeast strains;
and:
(2) to improve the overall performance of such
strains, with particular emphasis being
placed on broader substrate utilisation
capabilities, increased ethanol production,
improved stress tolerance to environmental
conditions such as high osmotic pressure,
ethanol, temperature, salt and physical
shear and to understand the mechanism of
flocculation.
The behaviour, performance and quality of a
yeast strain is influenced by two sets of
determining factors, collectively called naturenurture effects. The nurture effects are the
responses made (i.e. the phenotypes) to the
environmental factors which the yeast is
subjected from pitching onwards. On the other
hand, the nature influence is the genetic makeup (i.e. the genotype) of a particular yeast
strain.
Spontaneous yeast mutations are a common
occurrence throughout the growth and
fermentation cycle, but they are usually
recessive, due to functional loss of a single
gene. The characteristics that are encountered
resulting from mutations that are harmful to
wort fermentation are:
• The tendency of yeast strains to mutate from
flocculent to non-flocculent;
• The loss of ability to ferment maltotriose;
• The presence of respiratory deficient
mutants.
Table 1. The order of wort amino acid uptake during fermentation
Group A
Fast absorption
Group B
Intermediate absorption
Group C
Slow absorption
Glutamic acid
Aspartic acid
Asparagine
Glutamine
Serine
Threonine
Lysine
Arginine
Valine
Methionine
Leucine
Isoleucine
Histidine
Glycine
Phenylalanine
Tyrosine
Tryptophan
Alanine
Ammonia
Brewer & Distiller International • June 2009 • www.ibd.org.uk
Group D
Little or no absorption
Proline
Brewing yeast
production by fusion of a
flocculent strain with sake
yeasts;
• Construction of strains with
improved osmotolerance by
fusion of S. diastaticus or S.
rouxii (an osmotolerant yeast
strain).
• Recombinant DNA techniques
can also be used to make
thousands of copies of the
same DNA molecule to
amplify DNA, thus generating
sufficient DNA for various
kinds of experiments or
analysis. ‘Improved’ yeast
strains using recombinant
techniques include those with:
• Glucoamylase activity from
the fungus Aspergillus niger;
Figure 4. Respiratory sufficient (red colonies) and
• Glucanase activity from the
respiratory deficient (white colonies) mutants – triphenyl
tetrachloride overlay.
bacterium Bacillus subtilis, the
fungus Trichoderma reesii and
The respiratory deficient (RD) or ‘petite’
barley;
mutation is the most frequently identified
• Acetolactate decarboxylase activity from
mutant found in brewing yeast strains. This
the bacteria Enterobacter aerogenes and
mutant usually arises spontaneously when a
Acetobacter spp for diacetyl control;
segment of the DNA in the mitochondria
• Extracellular protease for enhancing beer
becomes defective to form a flawed
physical stability beer; and
mitochondrial genome. The mitochondria are
• Modification of yeast’s flocculation
then unable to synthesise certain proteins.
properties.
This type of mutation is also called the
‘petite’ mutation because colonies of such a
Prospects for the use of recombinant DNA
mutant are usually much smaller than the
with brewer’s yeast and their use in the
respiratory sufficient (RS) culture (also called
brewing industry are still uncertain. It is
‘grande’) (Figure 4). Deficiencies in
surprising that recombinant brewer’s yeasts
mitochondrial function result in diminished
are not commercially in use today in both
ability to function aerobically and beer
brewing and distilling. Permission was
produced with a yeast culture that is RD is
granted over a decade ago from the
likely to have flavour defects and
appropriate authorities in the United
fermentation problems. For example, beer
Kingdom for the use of a brewing strain,
produced from these mutants may contain
cloned with DNA from S. diastaticus that
elevated levels of diacetyl and higher alcohols secretes glucoamylase to utilise wort dextrins
and residual maltose and/or maltotriose.
and produce low calorie beer. However public
The advent of ‘new biotechnology’ has
opinion does not yet support the use of such
stimulated the development of novel methods
manipulated strains.
of genetic manipulation, such as spheroplast
(protoplast) fusion and recombinant DNA
The future
techniques. Examples of successful fusions
The economic significance of brewer’s yeast
with commercial brewing and related strains
cannot be overestimated. With the importance
are:
of the environment and the consequent need
• The construction of a brewing yeast with
to improve efficiency and reduce fermentation
amylolytic activity by fusion of S.
losses, it seems timely to take a more
cerevisiae and S. diastaticus;
considered look at the yeast we employ and
• A polyploid strain capable of high ethanol
the environment they are placed in.
In March this year, the complete genome
sequence of a Saccharomyces pastorianus
strain was published. Analysis of the
sequence shows that lager yeast is a hybrid of
the ale yeast Saccharomyces cerevisiae and
another yeast, Saccharomyces bayanus. New
developments in gene sequence technology,
such as this, will enable advances in the way
yeasts are used. The sequencing of the S.
cerevisiae genome (published in 1996) in
conjunction with gene expression analysis has
already enabled the identification of genes
that have altered gene expression patterns in
response to stressful environmental
conditions. A ‘stress model’ has been
developed to assess yeast stress resistance and
evaluate the suitability of a specific strain for
use in industrial ethanol fermentations. This
‘model’ could potentially be used for
screening candidate yeast strains for relative
stress resistance in the fuel ethanol industry
and other industries where yeast encounters
similar stresses. In addition, the creation of
large libraries of yeast strains combined with
progress in yeast screening will enable the
rapid identification of strains that match
desired brewery fermentation targets (such as
flavour and flocculation characteristics).
Over the past century we have come a long
way from early perceptions of fermentation
and the tools are now available to make
significant improvements. Despite the
increased wealth of our knowledge of
brewer’s yeast though, there is still something
of the Divine in the work they do! ■
■ The authors
Annie Hill is a Lecturer in Microbiology and Graham
Stewart is Emeritus Professor in Brewing and
Distilling both at Heriot-Watt University.
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