This document provides an overview of brewer's yeast (Saccharomyces cerevisiae) metabolism and its contribution to beer flavor. It discusses how yeast ferment sugars into ethanol and carbon dioxide through glycolysis and how various byproducts impact beer flavor. Researchers have studied how fermentation variables like pitching rate, oxygen levels, temperature, lipid content, and yeast strain impact metabolism and beer quality. High pitching rates can cause stress and off-flavors, but providing more oxygen can help. Temperature and yeast immobilization also impact fatty acid and diacetyl production in ways that could optimize high cell density fermentations. Understanding how these interconnected variables affect yeast metabolism and beer flavor can help improve efficiency and quality.

1. 1
Brewer’s yeast Saccharomyces cerevisiae metabolism and its
contribution to beer flavor
Kelly Jay
Butler University
April 8th, 2015
Introduction:
A Brief History
Beer is arguably one of the most popular and important accidental discoveries that has
happened in the last 10,000 years (Alba-Lois and Segal-Kischinevzky, 2010). The beverage has
had implications on religion, health, and everyday life since the time it was discovered and
continues to have implications in those areas as well as others. While the actual process of
fermentation by which beer is produced has remained the same, the brewing process has become
highly developed and the brewing industry has evolved into part of the global economy.
When people began enjoying beer they only knew that the savory product resulted from
fruits and grains left in covered containers for extended periods of time (Alba-Lois and Segal-
Kischinevzky, 2010). When it was observed that the product inside of the barrels bubbled, as
water does when it is boiled, the process of turning fruits and grains to wine and beer became
known as fermentation, stemming from a Latin root meaning “to boil.” People began to wonder
how fermentation occurred and what was involved in the process. It became apparent that if
containers sat too long they went bad, but if they sat for too short of a time they did not achieve
the desired flavors (Alba-Lois and Segal-Kischinevzky, 2010). From the moment people started
seeking answers to these questions, the science of alcoholic fermentation and brewing began.
Hundreds of years later, with the development of the microscope, researches were finally
able to observe microorganisms that were present in these closed containers during fermentation

2. 2
(Alba-Lois and Segal-Kischinevzky, 2010). Eventually, yeast was identified as the eukaryotic
fungus responsible for fermentation. Questions around fermentation emerged as it remained
unknown how these single celled organisms turn fruit into alcohol during fermentation (Alba-
Lois and Segal-Kischinevzky, 2010). Since the discovery of alcoholic fermentation and the
microorganisms responsible, a vast amount of time, work, and money has gone in to creating a
quality final product.
Brewing
Fermentation is only one aspect of brewing that is important to the final product of beer.
An understanding of the whole brewing process is important to recognize how all aspects affect
the way yeast can metabolize and ferment sugars into alcohol and other by-products. Sanitation
is important so that only desired microorganisms are influencing the final product (Palmer,
1999). A great deal of time and energy is put into sanitizing everything that will be involved in
brewing so that other microorganisms do not have adverse affects. The true process of brewing
starts with malting barley so that it germinates and begins converting starch into proteins, sugars,
and amino acids, all of which are essential to yeast metabolism and growth. Malted barley is
soaked in hot water to release its stored sugars, which are then boiled with hops for seasoning,
and cooled so that yeast can be added to begin fermentation. At this point in the process, the
water and malt sugars are commonly referred to as wort. As the yeast converts the sugars from
the malt into glucose it is fermented and results in CO2 and ethyl alcohol. When fermentation is
complete, the beer is bottled with the addition of more sugar to provide carbonation to the final
product since the CO2 produced from that point on is trapped inside the bottle (Palmer, 1999).

3. 3
Fermentation
The type of yeast most commonly used in brewing beer is Saccharomyces cerevisiae
(Bokulich and Bamforth, 2013). S. cerevisiae is capable of quickly converting sugars to ethanol
in anaerobic conditions and grow rapidly in aerobic conditions; however, brewing capitalizes on
the anaerobic abilities of S. cerevisiae to complete alcoholic fermentation (Dashko et al., 2014).
The ability of yeast to complete fermentation directly affects their ability to survive under
oxygen-limited conditions (Bokulich and Bamforth, 2013).
Yeast use energy to grow, eat, and reproduce, which are all processes that drive
fermentation. Yeast cells gain the energy they need to survive by converting sugar (glucose) into
ethyl alcohol and carbon dioxide with ATP as an essential by-product (Alba-Lois and Segal-
Kischinevzky, 2010). The first step in fermentation is glycolysis, which does not require oxygen.
This part of respiration occurs at the beginning of the brewing process when yeast still have
available oxygen and are rapidly growing. Glycolysis is the metabolic pathway that converts one
glucose molecule into two pyruvic acid molecules while producing two ATP in the process. The
breakdown of these pyruvic acid
molecules is when fermentation begins.
Each pyruvic acid molecule is broken
down into two carbon dioxide
molecules and two ethyl alcohol
molecules, creating NAD+ in the
process. These two NAD+ molecules
are required for yeast to continue

4. 4
glycolysis since the production of ATP requires NAD+ to be replenished by fermentation (Alba-
Lois and Segal-Kischinevzky, 2010).
While the process of fermentation is vital to the survival of yeast, it is not a process that
has evolved as a long-term survival technique (Bokulich and Bamforth, 2013). Yeast cannot
tolerate excessive levels of alcohol in their environment. This is problematic since one of the by-
products of fermentation is alcohol and it remains in the liquid that the yeast live in. When yeasts
are trapped with too much alcohol and no oxygen the yeast cells die because the alcohol destroys
enzymes needed to continue to produce ATP. Most yeast strains can only tolerate up to 5%
alcohol before their enzymatic cascades begin to fail. For this reason, we normally see beers with
an alcohol content of approximately 5% (Bokulich and Bamforth, 2013).
Yeast Metabolism
In addition to ethanol and CO2, yeast produce a number of other by-products during their
metabolism. The lipid and free amino nitrogen (FAN) content of the barley used to make the
malt are necessary nutrients for yeast survival, but must be carefully proportioned to maintain
proper balance of by-product dependent flavors (Held, 2012). In addition, the temperature of
wort, oxygen conditions, and amount of yeast present affect the ability of yeast and other (less
desired) microorganisms to grow and survive (Smogrovicova and Domeny, 1998). A less than
desirable amount of any of these essential factors could cause the yeast to go dormant, while too
much of any one factor can lead to off-flavors in beer (Palmer, 1999).
Diacetyl is a compound that is the result of oxidation in wort (Smogrovicova and
Domeny, 1998). The compound is desired in small quantities (0.1-0.14mg/l), but with too much
oxygen the buttery flavor gives beer an unfavorable taste (Smogrovicova and Domeny, 1998).
Similarly, ester flavors are desirable to a certain threshold, but high levels can result in a fruity

5. 5
flavor that is unfavorable when it is too strong (Saerens et al., 2008). Other by-products also
affect the flavor of the final product of fermentation including fusel alcohols, ketones, phenolics,
fatty acids, and other trace minerals that allow yeast to carry out metabolic processes to produce
these by-products (Palmer, 1999).
Yet another desirable aspect of a final beer that depends on metabolism is a head of foam,
which is an indication of beer quality in some cases. This again results from details in the
brewing process that are unsuspected. The movement of CO2 bubbles to the surface of the beer
helps to move smaller lipids originally present in the barley to the surface (Leisegang and Stahl,
2005). Since these lipid molecules are hydrophobic they attach to the CO2 produced from
fermentation during bottling and rise to the surface. These lipids can drastically affect the flavor
and texture of beer (Leisegang and Stahl, 2005). Even the color of the foam is a direct result of
lipids that remain unused by yeast after fermentation (Marongui et al., 2015).
It is already clear that many variables go into creating a well crafted quality beer. Many
of these variables have already been studied in-depth and their affects have been clearly stated in
scientific literature. However, it remains unclear how many of these variables work together and
how the manipulation of one aspect affects the by-products produced by a different one.
Review of Significant Developments:
Increasing Efficiency
The motivation for advancement in this field is driven by economic gain through
increased production and higher quality beer. Since fermentation is the longest, and therefore
most costly, step in the brewing process it was proposed that increasing the pitching rate can
greatly increase the production of a single fermentation (Nagodawithana and Steinkraus, 1976;
Verbelen et al., 2009a). To test this Verbelen et al. (2009) tested five different pitching rates in

6. 6
lab fermentations and measured various physiological and beer quality markers. They found that
many genes involved in stress responses were expressed in yeast used in fermentations with
higher pitching rates. This suggested that higher concentrations of yeast were limited in some
way compared to those with a lower pitching rate. The buildup of unsaturated fatty acids was
lower than normal in the high pitching rate batches. Higher pitching rate also resulted in final
products with higher diacetyl levels, trehalose concentrations, and lower viability of yeast. The
low fatty acid concentration at the beginning of fermentation is concerning because yeast require
fatty acids and sterols for normal growth during fermentation. This result was confirmed when
fluorescent dye used during fermentations confirmed that cell viability and growth were lower
throughout fermentation as pitching rate increased. The accumulation of trehalose suggested that
the yeast in the high pitching rate fermentations where not getting sufficient nutrients, since
trehalose is known be stored when yeast are entering their dormant phase. Finally, diacetyl was
not reduced to the flavor inactive compounds (acetoin and 2,3-butanediol) to which it is normally
reduced to. Its high level is responsible for a buttery off-flavor in many beers. While the total
fermentation rate did decrease with pitching rate there were clearly many adverse affects to the
final product (Verbelen et al., 2009a).
Balancing Variables
This study lead to another by an almost identical group of authors that looked to stabilize
the adverse affects that high pitching rate has on the final product of beer. The study considered
the impact of oxygen during high cell density fermentations on growth rate as well as
physiological markers and by-products (Verbelen et al., 2009b). Different oxygen conditions
were applied to high cell density (high pitching rate) fermentations in the form of wort aeration,
wort oxygenation, preoxygenation of yeast and combinations of these things. Improved oxygen

7. 7
conditions lowered the glycogen and trehalose levels that were measured at the end of
fermentation, suggesting that the yeast cells were more viable at higher pitching rates with access
to more oxygen. Some genes that are expressed under high oxidative stress became active in high
cell density fermentations (HCD) with increased oxygen. This suggests that oxygen also has a
threshold in fermentation which can be damaging to the yeast if surpassed. Cell density had a
higher affect on fermentation rate as shown by a significant decrease in time to reach total
fermentation in all HCD fermentations, but relatively no difference between differing oxygen
concentrations. Net growth, viability, and FAN consumed were all positively affected by the
addition of oxygen to HCD fermentations. Sufficient amounts of oxygen are required for yeast to
produce the fatty acids that they require to grow and with better oxygen conditions fatty acids
returned to approximately normal levels. Diacetyl levels were measured above the 80ppb
threshold, suggesting that the oxygen conditions are not sufficient to return diacetyl levels in
HCD fermentations to normal levels. From this study it is clear that oxygen content is one
limiting factor in HCD fermentations, but others still exist. The combination of wort aeration and
yeast preoxygenation appears to give the most satisfying results by yielding a final product of
HCD fermentation most similar to that of a normal fermentation. Other limiting factors in HCD
fermentations need to be identified and studied in order to create a HCD fermentation that results
in an adequate final product (Verbelen et al., 2009b).
Based on the two previous studies it is clear that pitching rate and oxygen both affect
many factors of fermentation and its by-products (Verbelen et al., 2009a). These studies also
mention the importance of lipid content on yeast growth before and during fermentation, as well
as harmful effects on the final product of beer flavor and foam (Verbelen et al., 2009b). The
oxidation of unsaturated fatty acids can directly cause a stale aroma in beer, while small

8. 8
hydrophobic lipids are responsible for a stable head of foam as they bind to CO2 as it moves
toward the surface of beer released from the bottle (Leisegang and Stahl, 2005). Whereas small
lipids are beneficial, un-metabolized longer chain fatty acids can actually cause the foam to
collapse and become unstable, leading to adverse affects on flavor (Bravie et al., 2009). Bravie et
al. (2009) again confirmed that pitching has an effect on the lipid profile of wort and thus affects
both foam and flavor. In a normal cell density pitching, increased lipid content allowed for
increased growth of yeast cells since the lipids provide essential nutrients. Since a majority of
fatty acids are found within the yeast cells, the removal of yeast biomass decreased the lipid
content of final products (Bravie et al., 2009). Since oxidation of fatty acids is what causes an
off-flavor in some fermentations, it could be beneficial to look at the removal of yeast biomass
after HCD fermentations with increased oxygen conditions.
Variables Connect
Another way that scientists have looked to speed up the brewing process is by increasing
the temperature at various points of fermentation. Smogrovicova and Domeny (1998) used gas
chromatography to determine the amount of ethanol and volatile compounds in beer. Total
nitrogen, FAN, bitterness and diacetyl concentration were all measured along with cell
concentration in order to determine growth rate of yeast under different temperature
fermentations. These measures indicated how flavor can be affected by temperature changes.
Temperatures from 5-20oC were used (15oC being a typical fermentation temperature) and yeast
were either free or immobilized in calcium pectate, ӄ-carrageenan, or on DEAE-cellulose. The
concentration of diacetyl increased with temperature for free and DEAE-cellulose immobilized
batches, but decreased as temperature increased in calcium pectate and ӄ-carrageenan
immobilized batches. Higher temperatures for entrapped yeast batches showed an increase in

9. 9
most fatty acids. This combination of fatty acid increase and diacetyl decrease with higher
temperatures for entrapped yeast batches could potentially yield important results (Smogrovicova
and Domeny, 1998). Since yeast growth in HCD fermentations was mostly affected by low
levels of fatty acids at the beginning of fermentation, immobilized strains at higher temperatures
could be utilized with HCD to compensate for fatty acid production. Additionally, the decrease
in diacetyl concentrations can be beneficial in the same way to counteract HCD effects. The
production of esters was also affected by immobilized yeast at higher temperatures
(Smogrovicova and Domeny, 1998). The concentrations of esters increased, giving rise to a
fruitlike or floral taste that comes with the aromas that they produce.
Little is known about what affects the production of ethyl esters, since such low
quantities of it are produced (Saerens et al., 2008). The factors that affect ethyl ester production
have been identified as the concentrations of unsaturated fatty acids in wort during fermentation,
the carbon/nitrogen ratio, and the temperature of fermentation. A decrease in ethyl ester
production was shown when levels of unsaturated fatty acids were increased in wort during
fermentation. This was determined by filtering wort to change the beginning lipid content and
measuring ethyl ester production in wort that contain different lipid concentrations. To measure
the nitrogen-carbon ratio, the FAN content and sugar content respectively were manipulated.
Carbon was varied while nitrogen was kept constant, and vice-versa. The results showed that
specific ethyl esters were produced in different quantities at different ratios. These results did not
seem to impact the overall flavor effect of ethyl esters. However, increases in fermentation
temperature significantly decreased the ethyl ester production, as shown in previous studies.
Together, this information shows that ester production varies with a wide range of temperatures.

10. 10
Ester production can be varied in many different temperature fermentations which can provide
insight into flavor profiling (Saerens et al., 2008).
Yeast Strain
Since the strain of yeast can affect production of final by-products, it is very important to
have strains of yeast available that are known as good starters for brewing and that produce the
desired products under a set of established conditions. For this reason it is of vital importance
that breweries properly store and maintain their strains to ensure the brewery’s future ability to
produce a quality product. Of interest are strains that can add depth to the collection of known
brewing strains. Marongiu et al. (2015) considered untraditional starter strains that can introduce
genetic variation to ensure longevity of brewing strains. The study considered 12 isolated S.
cerevisiae strains from homemade sourdough breads (Marongui et al., 2015). To produce quality
by-products, each sourdough strain had to be able to withstand the boiling of wart and convert
maltose, the sugar usually used in brewing, into ethanol in a reasonable amount of time. The
study showed that many of these 12 strains are capable of producing craft beer through
fermentation. The beers obtained from the test brews were analyzed for alcohol content,
bitterness, and other flavor markers. The sourdough starter strains were successful because they
were able to ferment maltose and trehalose with comparable levels of previously mentioned
markers. Only strains S10, S15, and S16 were not able to ferment maltose, and were nullified as
potential starter strains. The baker strains were also able to multiply and grow in the beginning of
fermentation when maltose was present, another marker of success as a brewing strain. All the
sourdough strains were also able to ferment hopped wort, sometimes at a higher rate than normal
brewing strains. Based on wort fermentation rates the strains of sourdough were classified as

11. 11
strong, mild, and weak fermentors, where mild fermentors matched the fermentation strengths of
normal craft beer starter strains.
The use of strains that are very similar but differ genetically allows brewers to select
strains that may be more durable in different environments, and that can lead to differentiating
factors in final beer products. The four strains that fell into this mild characterization (S38-F2,
S38-S38, S33-F2, and S33-38) were all tasted by a panel of test consumers using 12 assessors.
The test consumers were trained and asked to describe the “presence, color and persistency of the
foam, fruity, malt and yeast character for the odor, bitter and acid for the taste and freshness and
fullness for the mouth-feel sensations.” The sensory profile reported on this scale was equivalent
for all of the sourdough strains in addition to the normal brewing strains. This suggests bakers’
yeast strains are a viable option for growing the collection of S. cerevisiae starter strains for
brewing (Marongiu et al., 2015).
Summary of the Current State of Knowledge:
Factors that affect yeast metabolism are clearly stated throughout literature on
fermentation and beer brewing and are becoming highly understood. Many of the
aforementioned studies illustrate that altering the conditions of yeast growth and the following
fermentation can drastically change the final products that yeast produce. These changes are so
important because they affect the flavor of beer. Since many of the aspects affecting by-products
such as lipids, oxygen, temperature and pitching rate are now known, scientists are now
considering ways to optimize the brewing process. It is known that temperature and pitching rate
increase the fermentation rate, but scientists are now considering how to balance the adverse
affects on other compounds to maintain stable by-products during these faster processes (Bravie
et al., 2009; Smogrovicova and Domeny, 1998; Verbelen et al., 2009b).

12. 12
Some factors negatively affect yeast growth, viability, and desired by-products from
yeast during fermentation (Nagodawithana and Steinkraus, 1976). Some are similar to the
compounds mentioned above that are necessary for a balanced fermentation. However, if they
surpass a certain threshold they become detrimental to the flavor profile of beer. For example,
the normally desired diacetyl compound produces an unfavorable buttery flavor in high
concentrations. High cell density fermentations are desirable because they are fast, but they
result in high diacetyl concentrations. As mentioned before this can be balanced by the addition
of oxygen, but only in moderation since oxygen can too cause adverse affects such as the
production of too many esters, a fruity flavor that again is undesirable past a threshold (Bravie et
al., 2009; Leisegang and Stahl, 2005; Verbelen et al., 2009b).
Other scientists have even begun to understand that the manipulation of the yeast strains
themselves is an effective way to change metabolism and the by-products produced during
alcoholic fermentation. Oxygenation during fermentation is no longer limited to wort aeration
and oxygenation, but preoxygenation can increase oxygen availability by directly oxidizing yeast
cells (Krogerus et al., 2015). Additionally, initial studies have been done on yeast hybridization.
Specifically, two lager yeasts were crossed in a hope to maintain favored properties from each
parent strain, S. cerevisiae and Saccharomyces ubayanus. The hybridization’s success was
illustrated by the presence of favorable markers considered in beer profiles, but some
inconsistencies in aroma did persist. It is also essential to further study DNA inheritance, and the
viability of future strains before this information can be fully utilized (Krogerus et al., 2015;
Styger et al., 2011).
Newer flavors can also come from isolating yeast strains from other fermented origins
such as bread. These new strains can provide slight flavor enhancements, but are still able to

13. 13
successfully ferment sugars into ethanol. New strains not only provide the opportunity for slight
differentiation in flavor, but provide the brewing industry with viable starter strains if there were
to be damage to existing strains (Marongui et al., 2015).
Suggestions for Future Research:
It is evident that scientists already know which buttons they can push to manipulate the
metabolism of S. cerevisiae. Further research can be done on balancing the effects of these
manipulations to create the highest quality and most optimized fermentation. We already know
that a higher pitching rate can be slightly counteracted by the benefits of oxygenation of wort and
preoxygenation of yeast (Verbelen et al., 2009b). Since the oxygenation is only able to
reintroduce some of the fatty acids required for adequate yeast growth before it starts to oxidize
diacetyl and other undesired targets it would be reasonable to consider if filtration for lipid
content could be used to compensate for the rest of the fatty acid required for yeast growth. We
know that lipid content can be altered so that wort starts with a higher content, so manipulating
the lipid content and oxygen conditions in response to higher pitching rate could potentially help
high density fermentations flourish with normal flavors (Bravie et al., 2009).
The genetics involved in aroma and flavor profiles also provide an interesting outlet for
new information. It is clear through the hybridization done in Krogerus et al., (2015) that
genetics have the ability to change metabolism. In another study, specific genes involved in
precise aroma profiles have been identified as well (Styger et al., 2011). Since we know that
genes involved in aroma profiles can be isolated, it is reasonable to expect that this information
can be used to determine which yeast strains would be good hybrids. For example, one strain that
has high expression of certain genes that create a desirable flavor can be crossed with another
that has high expression of other desirable genes. The question would remain if all the beneficial

14. 14
highly expressed genes would be passed from the parent to the hybrid, but the discovery of how
to cross yeast strains in this way could provide new starter strains that are highly desired by
brewers.
Finally, it would be economically beneficial to compare all of the known approaches and
any newly discovered ones for efficiency in time, resources, and cost. Analyzing which
approaches are most realistic with technologies available in a brewery and the level of training
for those who work in breweries are important to putting these discoveries into practice. This
analysis will help to find the most cost effective ways to implement laboratory science into
actual breweries to provide tangible economic savings. The potential for breweries to make beer
more effectively and more precisely control the flavor would lead to even more innovation in the
industry.

15. 15
References
Alba-Lois, L., C. and Segal-Kischinevzky. 2010. Yeast fermentation and the making of beer and
wine. Nature Education. 3(9): 17.
Bokulich, N.A., and C.W. Bamforth. 2013. The microbiology of malting and brewing.
Microbiology and Molecular Biology Reviews. 77(2): 157-172.
Bravie, E., G. Perretti, P. Buzzini, R.D. Sera, and P. Fantozzi. 2009. Technological steps and
yeast biomass as factors affecting the lipid content of beer during the brewing process. J. Agric.
Food Chem. 57: 6279-6284.
Dashko, S., N. Zhou, C. Compagno, and J. Piskur. 2014. Why, when and how did yeast evolve
alcoholic fermentation. FEMS Yeast Res. 14(6): 826-832.
Held, Paul. 2012. Chemical and biochemical means to detect alcohol- determination of ethanol
concentration in fermented beer samples and distilled products. Biofuel Research. 12: 29-35.
Krogerus, K., F. Magalhaes, V. Vidgren, and B. Gibson. 2015. New lager yeast strains generated
by interspecific hybridization. J Ind Microbiol Biotechnol. DOI 10.1007/s10295-015-1597-6.
Leisegang, R. and U. Stahl. 2005. Degradation of a foam-promoting barley protein by a
proteinase from brewing yeast. J. Inst. Brew. 111(2): 112-117.
Marongui, A., G. Zara, J.L. Legras, A. Del Caro, I. Mascia, C. Fadda, and M. Budroni. 2015.
Novel starters for old processes: use of Saccharomyces cerevisiae strains isolated from artisanal
sourdough for craft beer production at a brewery scale. J Ind Microbiol Biotechnol. 42:85-92.
Nagodawithana, T. and K. Steinkraus. 1976. Influence of the rate of ethanol production and
accumulation on the viability of Saccharomyces cerevisiae in “rapid fermentation.” Applied and
Environmental Microbiology. 31(2): 158-162.
Palmer, John. “How to Brew.” N.p., 1999. Web. 15 Mar. 2015.
Saerens, S.M.G., F. Delvaux, K.J. Verstrepen, P. Van Dijck, J.M. Thevelein, and F.R. Delvaux.
2008. Parameters affecting ethyl ester production by Saccharomyces cerevisia during
fermentation. Appl. Environ. Microbiol. 74(2): 454-461.
Smogrovicova, D. and Z. Domeny. 1998. Beer volatile by-product formation at different
fermentation temperature using immobilized yeasts. Process Biochemistry 34: 785-794.
Styger, G., D. Jacobson, and F.F. Bauer. 2011. Identifying genes that impact on aroma profiles
produced by Saccharomyces cerevisiae and the production of higher alcohols. Appl Microbiol
Biotechnol. 91: 713-730.

16. 16
Verbelen, P.J., T.M.L. Dekoninck, S.M.G. Saerens, S.E. Van Mulders, J.M. Thevelein, and F.R.
Delvaux. 2009a. Impact of pitching rate on yeast fermentation performance and beer flavor. Appl
Microbiol Biotechnol. 82: 155-167.
Verbelen, P.J., S.M.G. Saerens, S.E. Van Mulders, F. Delvaux, and F.R. Delvaux. 2009b. The
role of oxygen in yeast metabolism during high cell density brewery fermentations. Appl
Microbiol Biotechnol. 82: 1143-1156.