Changes in beer bitterness level during the beer production process

Abstract

Beer has been enjoyed by consumers for years. Today, hops are inextricably associated with this beverage. Although they have been the subject of research for decades, knowledge of their bittering components and interactions during the beer production process is still incomplete. Current literature clearly indicates that the bitterness experienced in beer comes from a much wider range of compounds than just iso-α-acids. Although compounds that can be classified into β-acids, humulinones, hulupones, hard resins, and polyphenols are characterized by lower levels of bitterness and are present in hops in lower quantities than α-acids, they might determine, together with them, the final level of bitterness in beer. Unlike α-acids, the influence of compounds from these groups, their transformations, changes in their content during the beer production process and factors that affect their final concentration in beer have not yet been thoroughly studied. In case of α-acids, it is known that factors, such as chemical composition of wort, its extract and pH, amount of hops added and α-acids’ content, boiling time, and temperature at which hops were added influence the level of bitterness. This phenomenon is further complicated when dry hopping is used. Due to the presence of humulinones, polyphenols, and α-acids, a relatively simple spectrophotometric determination of IBU can give erroneous results. IBU determination, especially in dry-hopped beers, should be coupled with HPLC analysis, taking into account appropriate bitterness coefficients.

Full text

Vol.:(0123456789)
1 3
European Food Research and Technology (2023) 249:13–22
https://doi.org/10.1007/s00217-022-04154-0
REVIEW ARTICLE
Changes inbeer bitterness level duringthebeer production process
KrystianKlimczak1· MonikaCioch‑Skoneczny1
Received: 21 August 2022 / Revised: 3 October 2022 / Accepted: 8 October 2022 / Published online: 20 October 2022
© The Author(s) 2022
Abstract
Beer has been enjoyed by consumers for years. Today, hops are inextricably associated with this beverage. Although they
have been the subject of research for decades, knowledge of their bittering components and interactions during the beer pro-
duction process is still incomplete. Current literature clearly indicates that the bitterness experienced in beer comes from a
much wider range of compounds than just iso-α-acids. Although compounds that can be classified into β-acids, humulinones,
hulupones, hard resins, and polyphenols are characterized by lower levels of bitterness and are present in hops in lower quanti-
ties than α-acids, they might determine, together with them, the final level of bitterness in beer. Unlike α-acids, the influence
of compounds from these groups, their transformations, changes in their content during the beer production process and
factors that affect their final concentration in beer have not yet been thoroughly studied. In case of α-acids, it is known that
factors, such as chemical composition of wort, its extract and pH, amount of hops added and α-acids’ content, boiling time,
and temperature at which hops were added influence the level of bitterness. This phenomenon is further complicated when
dry hopping is used. Due to the presence of humulinones, polyphenols, and α-acids, a relatively simple spectrophotometric
determination of IBU can give erroneous results. IBU determination, especially in dry-hopped beers, should be coupled with
HPLC analysis, taking into account appropriate bitterness coefficients.
Keywords Beer· Hops· Bitterness· Hop resins
Introduction
Beer has been continuously appreciated by the consumers
for years. Traditionally, beer is made of three constituents:
water, malt, and hop. The last ingredient, hop, has been
used for hundreds of years in the beer production process.
It imparts a characteristic flavor and aroma to the beer and
improves the microbiological stability of the finished prod-
uct. As a result, nowadays, it is inherently associated with
this beverage. The complex chemical composition of raw
hop has been a subject of research for decades. Nevertheless,
the knowledge about this raw material—its complex interac-
tions and transformations—occurring during the beer pro-
duction process is still incomplete. It is widely known that
the characteristic beer bitterness is largely due to isomerized
α-acids formed by the action of high temperature during
boiling of wort with hop. However, this raw material also
has a wide range of other compounds that can impart a bit-
ter taste. Available publications indicate that hop fractions
so far not recognized as having an impact on beer flavor
can significantly alter it. Complex processes occurring at
various stages of beer production are no less important
than the chemical composition of this raw material. Beside
α-acid isomerization during wort boiling, processes such
as humulinones extraction or iso-α-acid loss during dry
hopping can significantly alter product taste. Awareness of
these phenomena can help brewers produce beers with the
intended sensory characteristics.
The aim of the paper is to present the current state of
knowledge on bitter substances found in hop cones and to
describe changes in their concentration in wort during the
production process, together with the known factors influ-
encing these transformations.
Contribution for the Special Issue: The chemistry behind malt and
beer production – from raw material to product quality.
* Monika Cioch-Skoneczny
monika.cioch@urk.edu.pl
1 Department ofFermentation Technology andMicrobiology,
University ofAgriculture inKraków, ul. Balicka 122,
30-149Kraków, Poland
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

14 European Food Research and Technology (2023) 249:13–22
1 3
Bitter substances inhop cones
Among dozens of flavorings used in brewing over the cen-
turies, only hops (Humulus lupulus L.) have become the
priority ingredient of this beverage. Around 97% of the
world’s crop of this perennial plant is intended for brew-
ing purposes. It is a dioecious plant (although it should
be noted that there are monoecious plants in wild North
American populations). The inflorescences of this plant,
commonly known as cones, contain yellow glandular hairs,
also known as hop glands or lupulin, which are only found
in female plants. It is the presence of these glands that
determines the well-established position of hops in the
brewing industry [1]. The typical chemical composition
of a dried hop cone is shown in Table1. From the brew-
ing point of view, the most important components of hops
are resins and essential oils produced by the mentioned
glands. Hop resins usually constitute quantitatively the
second fraction in a dry hop cone. Due to their high con-
tent of α-acids, they impart the bitterness to the beer. The
compounds in this fraction also improve foam stability
and microbiological stability of beer. Until now, it was
believed that the substances included in hop essential oil
fraction only provide aroma; however, there are reports,
suggesting that they can increase the perceived level of
bitterness and change its character—making it sharper.
In terms of sensory attributes, polyphenol content is
another important factor, as it can clearly affect beer qual-
ity parameters, impart antioxidant properties, and can
alter the perceived level of bitterness. Mclaughlin etal.
[2] conducted a study involving the addition of a specific
quantities of polyphenols and/or iso-α-acids to beer. Beers
to which polyphenols were added were characterized by
higher level of bitterness, which was described as linger-
ing. Additionally, those beers received higher scores in
the categories of “metallicity”, “spiciness”, and “medical
taste” [2–5].
Depending upon the variety and growing conditions, the
total hop resin content can be in the range of 15–30% of the
dry hop cone weight. This fraction is defined as compounds
soluble in diethyl ether or cold methanol. The resins can be
divided into soft resins, which are soluble in hexane, and
hard resins, which are not (Fig.1). The extracted soft resins
take the form of a thick, viscous liquid resembling in appear-
ance of honey, while the hard resins are a dark dusty solid.
Literature sources indicate that the bittering properties of
hops are almost exclusively caused by the substances con-
tained in the soft resins fraction, especially α-acids. How-
ever, recent publications indicate that bitterness in beer is a
much more complex phenomenon, and other, hitherto over-
looked compounds can modify its level [3, 4].
A-acids are compounds that are poorly soluble in water.
In a study by Baker etal. [7], content of these compounds
in hops was in the range of 1.3–12.6 wt%, depending upon
variety. However, there are varieties with higher α-acid con-
tent. The maximum concentration of these compounds in
finished beer is around 14ppm, but according to Fritsch and
Shellhammer [8], α-acids even at concentration of 28ppm
do not impart any perceptible bitterness to beer. When wort
is boiled with hops, α-acids are isomerized to the corre-
sponding epimers. It is believed that temperature in excess
of 80°C is required to initiate the reaction. The spacial con-
figuration of the resulting compounds is dependent on the
boiling time and temperature. Available literature data indi-
cate that cis-iso-humulones are more resistant to high tem-
peratures and aging due to their more stable thermodynamic
configuration. Additionally, cis-iso-α-acids undergo lower
losses during fermentation and storage and are characterized
by more intense bitterness than trans-iso-humulones. How-
ever, they have inferior foam stabilizing abilities to trans iso-
mers. For a standard beer wort, the ratio of cis:trans isomers
is typically ~ 2,5:1. An important indicator concerning the
issue of α-acids is their utilization rate. This term is defined
as the ratio of iso-α-acids obtained, typically in the finished
beer, to the quantity of α-acids that have been added (usu-
ally in hops, but they can be added in other forms, such as
hop extract). The efficiency of the isomerization reaction
is dependent on the number of factors. Various literature
sources place typical utilization at less than 30%, or in a
range of 40–65%. Verzele and Keukeleire [9] report a maxi-
mum utilization during wort boiling to be 60%, while values
of 25–30% are found in finished beers, where the content of
these compounds decreased due to losses. When using fresh
hops, iso-α-acids can be responsible for about 70% of the
bitterness of beer, but this value strongly depends on the type
of beer. Among these compounds, there are mainly 3 acids:
humulone, cohumulone, and adhumulone, which occur in
the highest amounts. Other compounds in this group are
found only in trace amounts, and for practical reasons, they
are usually not mentioned [3, 4, 7–13].
Table 1 Typical composition of dried hop cone [4]
Compound group Typical content [%]
Resins 15–30
Essential oils 0.5–3
Proteins 15
Monosaccharides 2
Polyphenols 4
Pectins 2
Amino acids 0.1
Waxes and steroids trace-25
Ash 8
Moisture 10
Cellulose and other 43
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

15European Food Research and Technology (2023) 249:13–22
1 3
The β fraction found in soft resins can be divided into
β-acids and uncharacterized soft resins. Β-acids are charac-
terized by low solubility in water and do not isomerize dur-
ing boiling. In a study by Baker etal. [7], content of β-acids
in the tested hop varieties was in the range of 1.0–6.8 wt%,
depending upon variety. Other authors present even broader
ranges. The conditions present in the wort (e.g., relatively
low pH) cause that only trace amounts of these compounds
are transferred into the finished beer. Approximately 70–85%
of these compounds remain in hop after boiling; that is why,
initially, it was believed that they have no significant effect
on beer flavor. However, β-acids show the ability to be oxi-
dized to compounds showing greater solubility in water,
which may have a noticeable effect on the sensory charac-
teristics of the beverage. After separation of α- and β-acids,
a hitherto uncharacterized fraction of soft resins remains,
among which α- and β-fractions are distinguished. So far, the
properties of its constituent compounds and potential effects
on beer parameters have not been studied. It is believed that
compounds in this fraction are intermediate decomposition
forms of α- and β-acids, which will eventually become hard
resins [4, 6, 7, 14, 15].
Hard resins are defined as the fraction insoluble in hex-
ane. To date, this is the least understood fraction of hop
resins. However, current literature indicates that compounds
in this group can impart bitterness to beer. Hard resins usu-
ally account for 2–3% of dry hop cone weight. In a study
conducted by Almaguer etal. [16], the authors showed that
it is possible to obtain beer with desired sensory characteris-
tics using only hard resins. It is believed that a considerable
fraction of hard resins are oxidized soft resins. During hop
aging, their soft resin content decreases in favor of hard res-
ins. However, it should be noted that hard resins are already
detected in the earliest stages of hop cone development,
hence primary resins, and those formed by the decomposi-
tion of other compounds are distinguished. Almaguer etal.
[4] have divided hard resins into five groups: α, β, δ, ε, and
uncharacterized soft resins. The α fraction represents a small
part of the hard resins, and as of today, its brewing properties
have not been studied. Among the β-fraction, xanthohumol
is quantitatively the most important component of the group,
while the other compounds have not been thoroughly char-
acterized. Xanthohumol is a strong antioxidant and exhibits
a wide range of antimicrobial and antiparasitic properties.
Because of its effects, it is the subject of dozens of stud-
ies on its use in, among others, the treatment of metabolic
disorders and related diseases. However, it is found in trace
amounts in finished beer, because it is isomerized to isox-
anthohumol. Both xanthohumol and isoxanthohumol can
impart bitterness to beer. However, according to Intelmann
etal. (2009), the bitterness level of these compounds is much
lower than that of iso-α-acids. The δ-fraction is the only one
that is soluble in water, but most likely contributes little to
the bitterness in beer. The ε-fraction is found in the highest
Fig. 1 Hop resins’ composition [4, 6]
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

16 European Food Research and Technology (2023) 249:13–22
1 3
amounts and can account for up to 80% of the total hard
resin content. In a study by Dresel etal. [17], the authors
rated the bitterness of 5% alcohol aqueous solutions enriched
with hard or soft resins, in the amounts in which they are
found naturally in beer, on a 5-point scale (0—bitterness not
perceptible, 5—very intense). Soft resins were rated 4.25
(± 0.25), ε-resins 0.51 (± 0.08), and total hard resins 0.5
(± 0.25), while the bitterness was nearly undetectable in the
δ fraction – 0.08 (± 0.04). These results indicate that most
of the bitterness of the hard resins seems to come from the
water-insoluble ε-fraction.
Humulinones and hulupones are the last distinct fraction
found in hop resins. They are intermediate oxidation prod-
ucts of α- and β-acids, respectively. Therefore, they should
be classified as hard resins. However, due to their solubility
in organic acids, they can also be classified as soft resins.
Since it is difficult to clearly classify them into one group or
other, they are not listed in the diagram. The typical content
of hulupones and humulinones in dry hop cone is usually
less than 0.5%. Their content increases with storage time,
and they are not detected in fresh hops. In a study by Algaz-
zali and Shellhammer [18], the authors estimated the bitter-
ness of humulinones and hulupones to 66 (± 13%) and 84%
(± 10%) of iso-α-acids, respectively. The values found by the
authors are higher than previously believed, which indicate
that these compounds may have a much more important role
than previously thought. Eventually, hulupones are oxidized
to unbitter hulupic acid [3, 4, 6, 16–22].
To quantify the scale of bitterness in beer, the Interna-
tional Bitterness Units (IBU) scale was developed in the
1950–60s. At that time, brewers used baled hops that were
generally stored under conditions that can be described as
unsuitable from today’s perspective. Due to inadequate
storage conditions, these hops lost 40–80% of their original
α-acid content by the time they were used for brewing. This
led to the need for quick and simple method to quantify
the bittering potential of beer. The determination of IBU
involves the extraction of bitter substances from beer with
acidified isooctane, followed by the determination of absorb-
ance at λ = 275nm. Theoretically, for beers containing only
iso-α-acids, 1 IBU ≈ 1mg iso-α-acids/L. Unfortunately,
other compounds that show absorbance at the wavelength
used, such as α-acids, polyphenols, humulinones, and xan-
thohumol, also enter the solvent phase. Because they exhibit
different levels of absorbance, as well as different levels of
bitterness than iso-α-acids, the actual perceived level of bit-
terness is usually lower than the measured IBU value. This
problem becomes particularly apparent in dry-hopped beers.
The shortcomings of this method can be avoided with HPLC
by determining each component contributing to beers’ bit-
terness separately. However, even with this analysis, factors,
such as content of alcohol, polyphenols, and essential oils,
together with pH, carbonation level, and water chemistry,
among others, alter the bitterness perceived by the con-
sumer. So far, no internationally accepted method has been
established that fully captures the actual level of bitterness
experienced [21, 23].
Bitterness level changes duringthebeer
production process
Although hops can be added at any stage of beer production,
they are mainly added during boiling and during or after
fermentation (in a process known as dry hopping). Hopping
during boiling is the traditional method of adding hop, which
imparts a desirable level of bitterness. Normally, hops are
boiled with wort for 60–90min. Shorter times (less than
15min, or even after the boiling is over—while the wort is
cooling) are used when hopping for aroma, to retain as much
of the highly volatile hop essential oils as possible. Another
hopping method, used primarily in India Pale Ale beers, is
dry hopping. The main purpose of this method is not the
modification of the bitterness levels, but the extraction of
volatiles to give the beer eligible, heady scent. Changes in
bitterness levels can also occur during beer storage. The
following sections describe the changes in levels of bitter
compounds that take place during boiling, fermentation, dry
hopping, and storage.
Boiling
As mentioned earlier, isomerization of α-acids to iso-α-
acids occurs when boiling the wort with hops. Thanks to
the isomerization reaction, the unbitter α-acids are trans-
formed to bitter iso-α-acids. The limiting factor for the
efficiency of this reaction is the relatively low solubility of
these compounds under the conditions found in the wort
(approximately 60mg/L at pH 5.5, 100°C). Factors affect-
ing the utilization of α-acids include boiling time, wort tem-
perature, sugar content, water chemistry, quantity of hops
added, and pH level. Significant losses of α-acids occur due
to inadequate boiling times, aerobic conversions, adsorption
on solids, precipitation with breakthrough, and losses dur-
ing fermentation. During the boiling stage, approximately
25–30% of the bittering substances are lost with the spent
hops, and another 25–40% in the precipitated breakthrough.
Gänz etal. [24] found that the protein substances responsible
for the loss of these compounds in the breakthrough are not
free amino acids, but uncoagulated proteins. Those that have
already precipitated do not react with iso-α-acids. What is
interesting, Jaskula etal. [25] report that finishing mashing
at 95°C instead of the common 78°C allows for higher uti-
lization (~ 58% instead of ~ 42% in the authors’ results), and
lower concentration of trans isomers. The authors contribute
the increased utilization to reduced losses of α-acids in the
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

17European Food Research and Technology (2023) 249:13–22
1 3
trub, due to higher protein coagulation at the mashing stage.
Those findings are consistent with the data by Gänz etal.
[24]. In the study by Jaskula etal. [26], the concentration of
polyphenols in the wort had no effect on the loss of bittering
substances [9, 11, 12, 24, 25].
In a study by Malowicki and Shellhammer [27], the
authors examined the effects of the temperature on the
isomerization rate of the α-acids. They evaluated the temper-
ature range of 90–130°C in a model acetate buffer, pH 5.2,
containing on average 64.4ppm of α-acids. At the standard
boiling temperature of 100°C, 60% isomerization occurred
in 90min. By the time of 60min which is often used in
home brewing, the degree of isomerization was only about
30%. The authors note that in the 100–130°C temperature
range, isomerization occurs more than 2× (229% on average)
faster for every 10°C increase. At 130°C, 60% isomeriza-
tion takes place in only 8min. At the same time, the iso-α-
acid degradation products levels increase along with boiling
temperature. In the temperature range of 100–130°C in time
required for 60% isomerization, authors found 10.8–14.0%
of degradation products. Extending the boiling time beyond
the point at which 60% utilization was achieved resulted only
in a decrease in the concentration of iso-α-acids, in favor of
their degradation products. According to Kappler etal. [11],
degradation products can cause an unpleasant, pungent bit-
terness. In the author’s study, a temperature of 90°C resulted
in a more than 2 × reduction in the isomerization rate, com-
pared to 100°C. Similar results were obtained by Jaskula
etal. [12]. However, it should be noted that isomerization,
although slow, still takes place at 80°C; hence, keeping the
wort after boiling at this temperature should be avoided to
prevent the formation of excessive bitterness, or accumula-
tion of degradation products. Hopping for aroma in whirl-
pool should also be conducted with rapid cooling of the wort
[11, 12, 27].
Elevated wort pH can accelerate isomerization of
α-acids and reduce their losses. Although the rate of
isomerization is not dependent on the pH of the environ-
ment (within the range expected in beer wort), α-acids
are relatively poorly soluble at low pH levels. Elevated
pH increases their solubility which allows for faster reac-
tion pace and higher isomerization rate, by increasing the
substrate concentration. In wort with a pH of 8–10, a 90%
utilization rate can be achieved. The bitterness obtained in
wort with increased pH is described as “unpleasant”. In a
study by Jaskula etal. [26], the utilization obtained at pH
6.0 and 7.0 (59.7 ± 1.0 and 81.3 ± 1.3%, respectively) was
significantly higher than that at pH 5.2 and 5.5 (37.5 ± 0.7
and 43.4 ± 0.9%, respectively). Kappler etal. [11] exam-
ined the recovery rate of preisomerized α-acids after boil-
ing them for 60min in pH 4–8 buffer. At pH 4.0, only 58%
of the original iso-α-acid was found after boil. At pH 5.0,
80% was detected, and at pH 8.0 95%. Reduced utilization
and retention of α-acids may be an important factor in the
production of sour beers [11, 26].
Sugar content is another important factor. The prevailing
claim in the literature is that high sugar content limits the
utilization rate. Different results were obtained by Jaskula
etal. [26], where isomerization remained on similar levels
in the 10.0–15.2° P extract range. Only in the wort with an
extract of 22°P did the degree of isomerization decreased
by 11% compared to the 12° P sample. However, non-over-
lapping results are presented by Kappler etal. [11]. In the
authors’ study, the extract content affected the preisomerized
α-acids concentration. The authors detected that the recov-
ery of preisomerized α-acids decreased linearly from 90%
at 10° P, to only 52% at 18° P. The results of Jaskula etal.
[26] are also contradicted by the results obtained by Justus
[28], where the utilization at an extract of ~ 20° P was more
than twice as low as that obtained at ~ 12° P. These reports
suggests that sugar content does affect the utilization degree
[11, 26].
As the hop content increases, the utilization of hop
acids decreases. In a study conducted by Irwin etal. [29],
the authors observed the highest utilization rate (~ 30%) of
α-acids at their concentration of ~ 20mg/L in wort. Utili-
zation decreased with increasing dose of α-acids, reaching
value below 20% at their dosage of ~ 90mg/L. The authors
note that this decrease is not linear. According to Justus [28],
the α-acid content of hops also plays a significant role in
the utilization. In the study, higher doses of hops with low
α-acid content (Tettnang—1.9%) resulted in reduced utiliza-
tion—30.6%, compared to the analogous in terms of α-acid
amount dose of bittering hops (Polaris—17.6%)—45.3%
[28, 29].
The last factor that could affect the degree and rate of
isomerization is the chemical composition of the water used
to produce the beer. The ions found in the greatest quantities
in drinking water, and thus also in brewing water are Ca2+,
Mg2+, Na+, K+, SO42−, Cl−, and carbonate ions. Literature
indicates that divalent metal ions Ca2+ and Mg2+, and mono-
valent Na+ and K+ can accelerate the rate and degree of
α-acid utilization. This is confirmed by the study by Jaskula
etal. [26], where addition of each of these ions at doses
of 5mg/L resulted in increased utilization rate by 9–23%.
Metal ions that are considered as generally undesirable
can significantly increase utilization rates. In the author’s
study, 5mg/L Fe3+ allowed for over 80% utilization rate.
The authors used reverse osmosis water for brewing. At the
same time, Justus [28] reports that increased Ca2+ and Mg2+
content can increase breakthrough formation, resulting in
turn in reduced utilization. Punčochářová etal. [30] found
no statistically significant differences in the content of iso-α-
acids in beers brewed with hard and soft water. Due to these
ambiguous reports, it is difficult to determine unequivocal
role of this ions on beers’ bitterness [3, 26, 28, 30].
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

18 European Food Research and Technology (2023) 249:13–22
1 3
An interesting issue from a beer storage perspective is the
cis:trans isomer ratio. Intelmann etal. [31] report that the
typical ratio in the wort is 2.5:1. Jaskula etal. [12] report
that with shorter than standard boiling times as well as
lower wort temperature, the trans:cis ratio increases, i.e.,
more trans isomers are formed. This relationship was most
pronounced when hop was added for 10min to the wort at
80°C, which resulted in nearly 3 × greater trans:cis ratio
than one obtained at 100°C in time of 90min. It should be
noted that at such low temperatures, isomerization occurs
very slowly, and the ratio shifts toward the cis isomers with
increasing boiling time. Nevertheless, this is another reason
to combine whirlpool hopping with intensive cooling. Kap-
pler etal. [11] found that the pH and sugar content of the
wort does not affect the radio of isomers formed [3, 11, 12,
31].
The transformations of β-acids during wort boiling are a
much less-understood issue than those of α-acids. As men-
tioned earlier, these compounds have very low solubility
under the conditions found in wort. In a study by Haseleu
etal. [14], the authors subjected an isolated colupulone to
conditions similar to wort boiling. Among its decomposition
products, cohulupone and 5 other compounds were detected,
where all of them were characterized by a low sensory
threshold and a lingering bitterness. In continuation of this
study, Haselau etal. [14] studied transformations of isolated
colupulone and lupulone by boiling them in a model wort
of pH 5.8. The sensory threshold of obtained compounds
depending on specific compound was determined to be in
the range of 7.9–90.30μmol/L. Cohulupone produced a bit-
terness sensation similar to that of iso-α-acids, while tricy-
clic derivatives gave a sensation of long-lasting, pungent
bitterness. The lowest sensory threshold of 7.9μmol/L was
determined for cohulupone, which is lower than the sensory
threshold for iso-α-acid trans-isocohumulone (19.0μmol/L).
The authors also analyzed 12 commercially available beers
for the presence of these compounds. Tested beers were
characterized by a significant difference in the concentra-
tion of these compounds, but in all of the samples, their
content was well below their sensory threshold. The sample
with the highest cohulupone content, described as a “bitter
beer”, contained only 3421 ± 10.3nmol/L of this compound.
These publications suggests that β-acid transformation prod-
ucts formed during wort boiling probably do not contribute
significantly to bitterness sensation. However, they may
exhibit synergistic effects with other bitter substances, which
determine the final bitterness of beer [14, 32].
Available literature indicates that compounds present in
the hard resin fraction may affect perceived bitterness to a
greater extent than β-acids. A study by Dresel etal. [17]
showed that the hard resin’s bitterness is derived mainly
from the ε-fraction. In a study, the authors conducted a fer-
mentation trial. When ε-resin extract was added after boil,
the bitterness in the beer (27 IBU) was described as medici-
nal or herbal. When the extract was added at the beginning
of boil, the beer (27 IBU) had a pleasant, harmonic bitter-
ness. Almaguer etal. [16] conducted brewing trials in which
hops were replaced with extracted hard resins derived from
Hallertauer Taurus (bittering hops) and Hallertauer Perle
(aroma hops). The resin extract was added at the beginning
of the boil (90min), or 10min before flameout. Beers in
which the resins were added 10min before the end of the
boil showed a very mild level of bitterness. Those in which
the resins were added for 90min showed a mild bitterness
that was described as harmonic in both cases, but the beer
with bittering hop resins scored better. Additionally, the
bitterness was more stable during storage in beers dosed
with bittering hop resins. The authors note that hop resins
extracted from bittering hops gave overall better beer than
those extracted from aroma hops. The obtained results indi-
cate that boiling time changes the perception bestowed to
beer by the hard resins [16, 17].
Fermentation
During fermentation, there is a significant decrease in the
perceived bitterness. This is caused mainly by the decreas-
ing concentration of iso-α-acids due to a number of factors,
among which are: their adsorption on the surface of yeast
cells and on surfaces in contact with the beer, their precipita-
tion with sediment, and on the surface of fermentation foam.
Additionally, the decrease in pH levels during the fermenta-
tion reduces the perceived level of bitterness.
In a study conducted by Justus [28], the average IBU
decrease among 14 brewed beers was 33.7%. The authors
note that in beers that owe a substantial portion of their bit-
terness to whirlpool hopping, the decrease in bitterness is
greater. This may be due to the formation of higher pro-
portion of trans isomers during this type of hopping. It is
known that trans isomers are more prone to losses during
fermentation than cis isomers. The use of poorly flocculating
yeasts as well as higher wort extract increases the decrease
in bitterness. In the authors’ study, the greatest decrease in
IBU occurred during the first 2days of fermentation. After
first 2–3days, decreases in IBU were no longer noticeable.
In the study by Popescu etal. [33], the average decrease in
EBU during fermentation was 27.5–36.19% [28, 33, 34].
Significant changes in IBU levels as well as perceived
bitterness occur as a result of dry hopping. The primary
purpose of this process is to extract hop oils to impart an
aroma to the beer, which is difficult to obtain when hops
are added during the boiling. However, current literature
clearly indicates that there is a significant change and a split
between perceived bitterness and IBU levels thanks to this
process. The dry hopping usually results in a decrease in
iso-α-acids’ levels. Simultaneously, extraction of α-acids,
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

19European Food Research and Technology (2023) 249:13–22
1 3
polyphenols, and humulinones from the hops takes place. It
is possible that compounds found in hard resins, especially
those from the ε- and δ-fractions, may be extracted, affect-
ing the bitterness levels, but so far this issue has not been
studied. Due to the lack of available literature, the exact fate
and typical content of hulupones is also unknown. Β-acids
are most likely not extracted during this process due to their
poor solubility. Due to the aforementioned different levels of
bitterness of individual hop bitter substances, and their dif-
ferent absorbance levels at λ = 275nm, the usual IBU analy-
sis may give results far from the actual perceived bitterness.
Maye and Smith [35] investigated changes in bitter com-
pounds levels due to dry hopping. Beers with 51ppm of
iso-α-acids, 0ppm α-acids, and 0ppm humulinones were
dry-hopped with Cascade hops at 1lb./bbl. (approximately
3.87g/L). After 3days of hopping at 16°C, HPLC analysis
was performed. The iso-α-acid content dropped to 32ppm,
and 13ppm α-acids and 13ppm of humulinones were
detected in the beer. The IBU determined in the traditional
way before dry hopping was 40, and it increased to 49. The
authors also conducted an analysis in which the IBU was
calculated taking into account the bitterness ratios of the
different fractions. In this case, the IBU before hopping
was 51, and it dropped to 40.5 after hopping. Comparable
results were obtained by Foster etal. [36]. In the authors’
study, 4–5% of α-acids and 50–60% of polyphenols pre-
sent in hops dissolved in beer. As mentioned, α-acids do
not have a bitter taste. There is also no literature regarding
whether they express some synergistic effects with other
compounds in beer, resulting in modifying bitterness levels.
However, they show a little absorbance at the wavelength
used in the IBU determination, which might interfere with
this measurement. Titus etal. [37] investigated the changes
in polyphenol concentration during first 1–94h of dry hop-
ping. Surprisingly, the concentration of polyphenols did
not increase gradually with prolonged hopping time, but
reached a maximum after 3h. The polyphenol content of the
beer after 24h of hopping was at its lowest observed level
(about 24% less than in the sample after 3h), after which
it gradually increased without exceeding the concentration
obtained after 3h. According to Lafontaine and Shellham-
mer [38], on average, 75% of humulinones found in hops are
extracted into beer at 13.3–15.0°C in 24h. With increasing
hop amount, the extraction rate decreases to be only 47%
at a dosage of 16g/L. Maye etal. [39] report that in beers
with IBU below 25, dry hopping can increase the level of
perceived bitterness. In beers above 40 IBU, dry hopping
usually decreases this level, unless high hop doses (above
11.6g/L) are used. The reason for the decrease in perceived
bitterness is the decrease in the iso-α-acids’ content, which
is not sufficiently compensated by the extraction of other
bitter substances. Parkin and Shellhammer [40], based on
their results, suggest that the increase in bitterness with
dry hopping is mainly due to humulinones, which have a
7–10× greater effect on bitterness level than polyphenols.
A 7mg/L increase in humulinone levels increases per-
ceived bitterness analogously to a 15 IBU increase. For a
100mg/L increase in polyphenols, the increase is only 2.2
IBU. Hahn etal. [41] determined the typical content of poly-
phenols and humulinones, among 121 commercially avail-
able intensively hopped beers. Polyphenol content ranged
from 135 to 697mg/L, with a mean content of 329mg/L.
The average content of humulinones was 17mg/L. Thus,
these compounds may largely account for the perceived
bitterness among these beers. Ferreira etal. [21] suggest
that humulinones may account for up to 28% of the bitter-
ness in dry-hopped beers. Because hopping during the boil
typically uses smaller amounts of hops than dry hopping,
and these compounds are found in hops in small amounts,
humulinones are only found in significant amounts in dry-
hopped beers. The authors determined the losses of these
compounds during the production process to be 22% during
wort boiling, 14% during fermentation, and 32% during bot-
tle refermentation [21, 35–37, 40–42].
During dry hopping, the pH of beer increases. In a study
by Maye etal. [39], the authors dry-hopped a beer with Cas-
cade hops at 0–6 lbs./bbl. The pH increase was nearly linear
at 0.1 pH units for every 1 lbs./bbl. (approximately 3.87g of
hops/L). Lafontaine and Shellhammer [38] observed a pH
increase of ~ 0.14 at a similar hop dose. The increase in pH
due to this process may increase the solubility of α-acids.
Additionally, higher pH values result in intensified bitter-
ness sensing. Maye etal. [39] determined the increase in
bitterness with a pH increase of 0.1 to be comparable to an
increase in IBU of 2–3 units [38, 39].
Beer aging
The biggest problem during beer storage are the quality
changes that take place. One such change is the degrada-
tion of bittering compounds which progresses with increas-
ing storage time in unsuitable conditions. In the case of
beers that have not been dry-hopped, these changes mainly
include loss of iso-α-acids. The key factors that determine
the changes occurring during storage are temperature and
time. In a study by Walters etal. [43], the authors examined
the content of iso-α-acids in beer with its original content
of 15.3mg/L, after periods of storage at different tempera-
tures. When the beer was stored for 156days at 40°C, their
content fell to 4.5mg/L (71% loss). When stored at 0 and
25°C for 220days, there was no change in the concen-
tration of these compounds. The authors noted that trans-
iso-α-acids, especially at 40°C, degrade to a much greater
extent than cis-iso-α-acids. Their minimum concentration
was already reached after ~ 120days of storage. When stored
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

20 European Food Research and Technology (2023) 249:13–22
1 3
at 25°C, the concentration of cis isomers remained constant,
while trans isomers degraded slowly. Their loss was only
stopped when beer was stored at 0°C. Comparable results
were obtained by Intelmann [31] and Schmidt etal. [13].
Schmidt etal. [13] note that the variety of hops used had
no effects on the loss of trans isomers. Intelmann etal. [44]
report that compounds derived from the transformation of
trans-iso-α-acids may be responsible for the formation of
an unpleasant, lingering bitterness. In the authors’ results,
beers with higher pH showed higher stability of these com-
pounds. From the literature data, it appears that when beers
hopped only for bitterness are stored at low temperatures,
loss of iso-α-acids should not be a major problem. The situ-
ation is different in case of beers rich in trans isomers. As
for now, the only known way to obtain such beer is to apply
a whirlpool hopping at 80°C. As a large number of trans
isomers form in these conditions, these beers may change
their sensory characteristics to a greater extent, in case of
improper storage [13, 31, 43, 44].
Since dry-hopped beers contain a different bittering
compounds profile (mainly due to the additional signifi-
cant content of humulinones and polyphenols), they may
be more susceptible to improper storage conditions. In a
study by Kemp etal. [45], the authors examined the changes
that occur when dry-hopped beers are stored for 3, 6, and
10months at 0, 3, and 20°C. When stored at 20°C, the
first 3months brought the greatest decreases in levels of
humulinones, α-acids, and iso-α-acids. Degradation of
these compounds was minimal in subsequent months. After
10months of storage, average losses were 22%. When stored
at 3°C, the authors describe the losses as “minimal”. Inter-
estingly, after 10months of storage under these conditions,
the levels of α-acids and humulinones increased, at the
expense of iso-α-acids loss. In sensory terms, beers stored
at 20°C were considered as less bitter than those stored at
3°C. Perceived bitterness correlated well with the levels
of bitter substances. Ferreira and Collin [46] analyzed Bel-
gian dry-hopped beers after 2years of storage at 20°C. The
average loss of iso-α-acids under these conditions was 25%,
while the average losses of humulones and humulinones
were as high as 91% and 73%, respectively. Comparing the
results of the authors cited shows a large loss in humulinone
content between 10 and 24months of storage. It is possible
that these compounds undergo rapid degradation during this
period, or that a yet unknown factor is responsible for their
degradation. In a study by Jaskula etal. [12], beers dosed
with hop polyphenols extract were characterized by a higher
stability of iso-α-acids, which may be related to the antioxi-
dant properties of these compounds. Šibalić etal. [47] report
that different hop varieties have quantitatively different com-
position of polyphenol compounds, which may translate into
different antioxidant capacity of the beer, and, as a result, its
stability during storage [45–48].
Summary
Current literature indicates that the perceived bitterness in
beer may come from a much wider variety of compounds
than just iso-α-acids. Although compounds belonging to
β-acids, humulinones, hulupones, hard resins, and poly-
phenols are characterized by lower levels of bitterness, and
are found in hops in lower amounts than α-acids, they may
determine, along with them, the final level of bitterness in
beer. The influence of compounds from these groups, their
transformations, and changes in their content during the beer
production process along with the factors affecting their final
concentration in beer have not yet been thoroughly studied.
This phenomenon is further complicated when dry hopping
is applied. It also follows that a relatively simple spectro-
photometric determination of IBU values, which also detects
others compounds with a different bitterness factor, may give
erroneous results. Hence, it is closer to the truth to couple
IBU analysis with HPLC analysis, taking into account the
respective bitterness coefficients. From the moment the wort
is obtained, unavoidable changes take place resulting in a
deterioration of bitter compounds. Due to the degradation of
trans-iso-α-acids and humulinones at elevated temperatures,
especially beers hopped during whirlpool and dry-hopped
should be kept at low temperatures to avoid quality losses.
Author contributions KK and MC-S: original draft preparation; KK
and MC-S: review and editing; KK: visualization. All authors have read
and agreed to the published version of the manuscript.
Funding Not applicable.
Data availability The authors confirm that the data supporting the find-
ings of this study are available within the article.
Declarations
Conflict of interest The authors declare no competing interests.
Compliance with ethics requirements This article does not contain
any studies with human participants or animals performed by any of
the authors.
Open Access This article is licensed under a Creative Commons Attri-
bution 4.0 International License, which permits use, sharing, adapta-
tion, distribution and reproduction in any medium or format, as long
as you give appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons licence, and indicate if changes
were made. The images or other third party material in this article are
included in the article's Creative Commons licence, unless indicated
otherwise in a credit line to the material. If material is not included in
the article's Creative Commons licence and your intended use is not
permitted by statutory regulation or exceeds the permitted use, you will
need to obtain permission directly from the copyright holder. To view a
copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

21European Food Research and Technology (2023) 249:13–22
1 3
References
1. Szczepaniak O, Dziedzinski M, Kobus-Cisowska J, etal (2019)
Chmiel (Humulus lupulus L.) jako surowiec o właściwościach
prozdrowotnych: aktualny stan wiedzy. Tech Rol Ogrod
Leśna.3:9–12
2. McLaughlin IR, Lederer C, Shellhammer TH (2008) Bitterness-
modifying properties of hop polyphenols extracted from spent hop
material. J Am Soc Brew Chem 66:174–183
3. Preedy VR (2011) Beer in health and disease prevention. Aca-
demic Press
4. Almaguer C, Schönberger C, Gastl M etal (2014) Humulus
lupulus—a story that begs to be told. A review. J Inst Brew
120:289–314. https:// doi. org/ 10. 1002/ jib. 160
5. Dietz C, Cook D, Huismann M etal (2020) The multisensory
perception of hop essential oil: a review. J Inst Brew 126:320–
342. https:// doi. org/ 10. 1002/ jib. 622
6. Hao J, Speers RA, Fan H etal (2020) A review of cyclic and
oxidative bitter derivatives of alpha, iso-alpha and beta-hop
acids. J Am Soc Brew Chem 78:89–102. https:// doi. org/ 10.
1080/ 03610 470. 2020. 17126 41
7. Baker GA, Danenhower TM, Force LJ etal (2008) HPLC analy-
sis of α- and β-acids in hops. J Chem Educ 85:954. https:// doi.
org/ 10. 1021/ ed085 p954
8. Fritsch A, Shellhammer TH (2007) Alpha-acids do not contrib-
ute bitterness to lager beer. J Am Soc Brew Chem 65:26–28.
https:// doi. org/ 10. 1094/ ASBCJ- 2007- 0111- 03
9. Verzele M, De Keukeleire D (2013) Chemistry and analysis of
hop and beer bitter acids. Elsevier
10. Bastgen N, Becher T, Drusch S, Titze J (2021) Usability and
technological opportunities for a higher isomerization rate of
α-acids: a review. J Am Soc Brew Chem 79:17–25. https:// doi.
org/ 10. 1080/ 03610 470. 2020. 18408 93
11. Kappler S, Krahl M, Geissinger C etal (2010) Degradation
of Iso-α-acids during wort boiling. J Inst Brew 116:332–338.
https:// doi. org/ 10. 1002/j. 2050- 0416. 2010. tb007 83.x
12. Jaskula B, Kafarski P, Aerts G, De Cooman L (2008) A kinetic
study on the isomerization of hop α-acids. J Agric Food Chem
56:6408–6415. https:// doi. org/ 10. 1021/ jf800 4965
13. Schmidt C, Biendl M, Lagemann A etal (2014) Influence of
different hop products on the cis/trans ratio of Iso-α-acids in
beer and changes in key aroma and bitter taste molecules during
beer ageing. J Am Soc Brew Chem 72:116–125. https:// doi. org/
10. 1094/ ASBCJ- 2014- 0326- 01
14. Haseleu G, Intelmann D, Hofmann T (2009) Structure determi-
nation and sensory evaluation of novel bitter compounds formed
from β-acids of hop (Humulus lupulus L.) upon wort boiling.
Food Chem 116:71–81. https:// doi. org/ 10. 1016/j. foodc hem.
2009. 02. 008
15. Skomra U, Koziara-Ciupa M (2020) Stability of the hop bitter
acids during long-term storage of cones with different maturity
degree. Polish J Agron 40:16–24. https:// doi. org/ 10. 26114/ pja.
iung. 406. 2020. 40. 03
16. Almaguer C, Gastl M, Arendt EK, Becker T (2015) compara-
tive study of the contribution of hop (Humulus lupulus L.) hard
resins extracted from different hop varieties to beer quality
parameters. J Am Soc Brew Chem 73:115–123. https:// doi. org/
10. 1094/ ASBCJ- 2015- 0327- 01
17. Dresel M, Dunkel A, Hofmann T (2015) Sensomics analysis
of key bitter compounds in the hard resin of hops (Humulus
lupulus L.) and their contribution to the bitter profile of pilsner-
type beer. J Agric Food Chem 63:3402–3418. https:// doi. org/
10. 1021/ acs. jafc. 5b002 39
18. Algazzali V, Shellhammer T (2016) Bitterness intensity of oxi-
dized hop acids: humulinones and hulupones. J Am Soc Brew
Chem 74:36–43. https:// doi. org/ 10. 1094/ ASBCJ- 2016- 1130- 01
19. Liu M, Hansen PE, Wang G etal (2015) Pharmacological profile
of xanthohumol, a prenylated flavonoid from hops (Humulus
lupulus). Molecules 20:754–779
20. Intelmann D, Batram C, Kuhn C etal (2009) Three TAS2R
bitter taste receptors mediate the psychophysical responses to
bitter compounds of hops (Humulus lupulus L.) and beer. Che-
mosens Percept 2:118–132
21. Silva Ferreira C, Thibault de Chanvalon E, Bodart E, Collin S
(2018) Why humulinones are key bitter constituents only after
dry hopping: comparison with other belgian styles. J Am Soc
Brew Chem 76:236–246. https:// doi. org/ 10. 1080/ 03610 470.
2018. 15039 25
22. Mackegård IE (2021) A hopeful study of hop. Master thesis.
Swedish University of Agricultural Sciences
23. Oliver G, Colicchio T (2011) The Oxford companion to beer.
Oxford University Press, New York
24. Gänz N, Becher T, Drusch S, Titze J (2022) Interaction of pro-
teins and amino acids with iso-α-acids during wort preparation
in the brewhouse. Eur Food Res Technol 248:741–750
25. Jaskula B, Spiewak M, De Cock J etal (2009) Impact of mash-
ing-off temperature and alternative kettle-hopping regimes on
hop α-acids utilization upon wort boiling. J Am Soc Brew Chem
67:23–32. https:// doi. org/ 10. 1094/ ASBCJ- 2008- 1203- 01
26. Jaskula B, Aerts G, De Cooman L (2010) Potential impact of
medium characteristics on the isomerisation of hop α-acids in
wort and buffer model systems. Food Chem 123:1219–1226.
https:// doi. org/ 10. 1016/j. foodc hem. 2010. 05. 090
27. Malowicki MG, Shellhammer TH (2005) Isomerization and
degradation kinetics of hop (Humulus lupulus) acids in a model
wort-boiling system. J Agric Food Chem 53:4434–4439. https://
doi. org/ 10. 1021/ jf048 1296
28. Justus A (2018) Tracking IBU through the brewing process: the
quest for consistency. MBAA Tech Q 55:67–74
29. Irwin AJ, Murray CR, Thompson DJ (1985) An investigation
of the relationships between hopping rate, time of boil, and
individual alpha-acid utilization. J Am Soc Brew Chem 43:145–
152. https:// doi. org/ 10. 1094/ ASBCJ- 43- 0145
30. Punčochářová L, Pořízka J, Diviš P, Štursa V (2019) Study of
the influence of brewing water on selected analytes in beer.
Potravin Slovak J Food Sci 13:507–514
31. Intelmann D, Haseleu G, Hofmann T (2009) LC-MS/MS quanti-
tation of hop-derived bitter compounds in beer using the ECHO
technique. J Agric Food Chem 57:1172–1182. https:// doi. org/
10. 1021/ jf803 040g
32. Haseleu G, Intelmann D, Hofmann T (2009) Identification and
RP-HPLC-ESI-MS/MS quantitation of bitter-tasting β-acid
transformation products in beer. J Agric Food Chem 57:7480–
7489. https:// doi. org/ 10. 1021/ jf901 759y
33. Popescu V, Soceanu A, Dobrinas S, Stanciu G (2013) A study of
beer bitterness loss during the various stages of the Romanian
beer production process. J Inst Brew 119:111–115. https:// doi.
org/ 10. 1002/ jib. 82
34. Jaskula B, Goiris K, Van Opstaele F etal (2009) Hopping tech-
nology in relation to α-acids isomerization yield, final utiliza-
tion, and stability of beer bitterness. J Am Soc Brew Chem
67:44–57. https:// doi. org/ 10. 1094/ ASBCJ- 2009- 0106- 01
35. Maye JP, Smith R (2016) Dry hopping and its effects on the
international bitterness unit test and beer bitterness. Tech Q
Master Brew Assoc Am 53:134–136
36. Forster A, Gahr A (2013) On the fate of certain hop substances
during dry hopping. Brew Sci 66:94–103
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

22 European Food Research and Technology (2023) 249:13–22
1 3
37. Titus BM, Lerno LA, Beaver JW etal (2021) Impact of dry
hopping on beer flavor stability. Foods 10:1264
38. Lafontaine SR, Shellhammer TH (2018) Impact of static dry-
hopping rate on the sensory and analytical profiles of beer. J Inst
Brew 124:434–442. https:// doi. org/ 10. 1002/ jib. 517
39. Maye JP, Smith R, Leker J (2018) Dry Hopping and Its Effect on
Beer Bitterness, the IBU Test, and pH. BrauW Int 2018:25–29
40. Parkin E, Shellhammer T (2017) Toward understanding the bit-
terness of dry-hopped beer. J Am Soc Brew Chem 75:363–368.
https:// doi. org/ 10. 1094/ ASBCJ- 2017- 4311- 01
41. Hahn CD, Lafontaine SR, Pereira CB, Shellhammer TH (2018)
Evaluation of nonvolatile chemistry affecting sensory bitterness
intensity of highly hopped beers. J Agric Food Chem 66:3505–
3513. https:// doi. org/ 10. 1021/ acs. jafc. 7b057 84
42. Egts H, Durben DJ, Dixson JA, Zehfus MH (2012) A Multi-
component UV analysis of α- and β-acids in hops. J Chem Educ
89:117–120. https:// doi. org/ 10. 1021/ ed101 0536
43. Walters MT, Heasman AP, Hughes PS (1997) Comparison of
(+)—catechin and ferulic acid as natural antioxidants and their
impact on beer flavor stability. part 1: forced-aging. J Am Soc
Brew Chem 55:83–89. https:// doi. org/ 10. 1094/ ASBCJ- 55- 0083
44. Intelmann D, Haseleu G, Dunkel A etal (2011) Comprehensive
sensomics analysis of hop-derived bitter compounds during
storage of beer. J Agric Food Chem 59:1939–1953. https:// doi.
org/ 10. 1021/ jf104 392y
45. Kemp O, Hofmann S, Braumann I etal (2021) Changes in key
hop-derived compounds and their impact on perceived dry-hop
flavour in beers after storage at cold and ambient temperature. J
Inst Brew 127:367–384. https:// doi. org/ 10. 1002/ jib. 667
46. Ferreira CS, Collin S (2020) Fate of bitter compounds through
dry-hopped beer aging. why cis-humulinones should be as feared
as trans-isohumulones? J Am Soc Brew Chem 78:103–113.
https:// doi. org/ 10. 1080/ 03610 470. 2019. 17050 37
47. Šibalić D, Planinić M, Jurić A etal (2021) Analysis of phenolic
compounds in beer: from raw materials to the final product. Chem
Pap 75:67–76
48. Jaskula-Goiris B, Goiris K, Syryn E etal (2014) The use of hop
polyphenols during brewing to improve flavor quality and stability
of pilsner beer. J Am Soc Brew Chem 72:175–183. https:// doi. org/
10. 1094/ ASBCJ- 2014- 0616- 01
Publisher's Note Springer Nature remains neutral with regard to
jurisdictional claims in published maps and institutional affiliations.
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

1.
2.
3.
4.
5.
6.
Terms and Conditions
Springer Nature journal content, brought to you courtesy of Springer Nature Customer Service Center GmbH (“Springer Nature”).
Springer Nature supports a reasonable amount of sharing of research papers by authors, subscribers and authorised users (“Users”), for small-
scale personal, non-commercial use provided that all copyright, trade and service marks and other proprietary notices are maintained. By
accessing, sharing, receiving or otherwise using the Springer Nature journal content you agree to these terms of use (“Terms”). For these
purposes, Springer Nature considers academic use (by researchers and students) to be non-commercial.
These Terms are supplementary and will apply in addition to any applicable website terms and conditions, a relevant site licence or a personal
subscription. These Terms will prevail over any conflict or ambiguity with regards to the relevant terms, a site licence or a personal subscription
(to the extent of the conflict or ambiguity only). For Creative Commons-licensed articles, the terms of the Creative Commons license used will
apply.
We collect and use personal data to provide access to the Springer Nature journal content. We may also use these personal data internally within
ResearchGate and Springer Nature and as agreed share it, in an anonymised way, for purposes of tracking, analysis and reporting. We will not
otherwise disclose your personal data outside the ResearchGate or the Springer Nature group of companies unless we have your permission as
detailed in the Privacy Policy.
While Users may use the Springer Nature journal content for small scale, personal non-commercial use, it is important to note that Users may
not:
use such content for the purpose of providing other users with access on a regular or large scale basis or as a means to circumvent access
control;
use such content where to do so would be considered a criminal or statutory offence in any jurisdiction, or gives rise to civil liability, or is
otherwise unlawful;
falsely or misleadingly imply or suggest endorsement, approval , sponsorship, or association unless explicitly agreed to by Springer Nature in
writing;
use bots or other automated methods to access the content or redirect messages
override any security feature or exclusionary protocol; or
share the content in order to create substitute for Springer Nature products or services or a systematic database of Springer Nature journal
content.
In line with the restriction against commercial use, Springer Nature does not permit the creation of a product or service that creates revenue,
royalties, rent or income from our content or its inclusion as part of a paid for service or for other commercial gain. Springer Nature journal
content cannot be used for inter-library loans and librarians may not upload Springer Nature journal content on a large scale into their, or any
other, institutional repository.
These terms of use are reviewed regularly and may be amended at any time. Springer Nature is not obligated to publish any information or
content on this website and may remove it or features or functionality at our sole discretion, at any time with or without notice. Springer Nature
may revoke this licence to you at any time and remove access to any copies of the Springer Nature journal content which have been saved.
To the fullest extent permitted by law, Springer Nature makes no warranties, representations or guarantees to Users, either express or implied
with respect to the Springer nature journal content and all parties disclaim and waive any implied warranties or warranties imposed by law,
including merchantability or fitness for any particular purpose.
Please note that these rights do not automatically extend to content, data or other material published by Springer Nature that may be licensed
from third parties.
If you would like to use or distribute our Springer Nature journal content to a wider audience or on a regular basis or in any other manner not
expressly permitted by these Terms, please contact Springer Nature at
onlineservice@springernature.com