IBU (International Bitterness Units, also written BU) is a number derived from the absorbance of beer measured at a wavelength of 275 nanometers. What it measures is a specific chemical absorbance, not the perception of bitterness itself as it happens on the tongue. It has long been used as a proxy that roughly tracks the concentration of iso-α-acids produced from hops during boiling, but a study of more than 120 commercial beer brands found that the number and the bitterness intensity people rated did not correspond linearly. Moreover, in processes such as dry hopping — adding hops after fermentation — a reversal has even been reported in which IBU rises while the iso-α-acids themselves decrease. IBU is a way of putting a number, by a fixed method, on compounds related to bitterness; it is not a direct score of the quality or feel of that bitterness.
That IBU measures absorbance arising from hop bitter acids in beer has been confirmed across multiple studies.
One study covering more than 120 commercial beer brands reported that the relationship between IBU values and rated bitterness intensity was nonlinear.
Multiple studies have confirmed that while IBU rises with dry hopping, iso-α-acid concentration actually falls in the majority of the commercial beers examined.
How far the change in IBU after bottling explains the amount of harsh bitterness compounds that form during storage has not yet been measured.
What IBU measures — hop-derived bitter acids
The starting point of beer bitterness is the hop. During wort boiling, hop α-acids isomerize into iso-α-acids, which, in the classic understanding, are the principal compound responsible for beer bitterness. A study measured the human detection threshold of iso-α-acids itself in a hop-free lager base, and found a group-level figure of approximately 7.1 mg/L.
Scope: The principal hop-derived bitter substance in beer. Its human detection threshold has been measured in a hop-free lager base
Note: An isomerization-product class within the hop bitter acid family (compound-hop-bitter-acids). / 2026-08-31: calibration-downgraded from established to supported because the supporting evidence has no full-text-checked source. May be re-upgraded once full text is checked.
A study that measured the human bitterness detection threshold of iso-α-acids itself, directly demonstrating its identity as a bitter stimulus (group mean 7.1 mg/L).
In sensory plus instrumental analysis of more than 120 commercial beer brands, sensory bitterness intensity showed a nonlinear relationship with BU, and a bitterness prediction based on isohumulone (iso-α-acid), humulinone, and ethanol concentrations was proposed. This supports, through a different approach (a large cross-brand regression) than the threshold study (Kolpin 2009), that iso-α-acids are the main explanatory variable for bitterness intensity across real beers.
Limitations: Abstract checked only. This is a contribution estimate from a regression model, not a sensory test of isolated iso-α-acids. The brand set is skewed toward highly hopped beers.
What IBU captures as a spectrophotometric measurement is the absorbance resulting from these hop bitter acids dissolved into the beer. In a study analyzing commercial beers, the concentration of hop bitter acids varied greatly from brand to brand.
Scope: Commercial lager beers chemically analyzed in the cited study
Note: Concentration and product profile vary. / 2026-08-31: calibration-downgraded from established to supported because the supporting evidence has no full-text-checked source. May be re-upgraded once full text is checked.
Hop bitter acids in commercial beers were analyzed, and concentrations varied.
Limitations: Composition specific to each product.
On the mechanistic side of why these compounds taste bitter, hop-derived compounds including iso-α-acid stereoisomers, α-acids, and 8-prenylnaringenin have been reported, through structural bioinformatics, to activate several types of human bitter taste receptors.
Scope: Hop-derived bitter compounds including iso-α-acid stereoisomers, α-acids, and 8-prenylnaringenin
Note: Recorded from a structural bioinformatics paper that restates the attribution to the 3 receptors as established prior knowledge. The underlying receptor-assay source (Intelmann et al. 2009, Chemosensory Perception) could not be obtained in abstract or full text and is listed as metadata_only in sources.csv. Per-receptor entities (receptor-tas2r1 / tas2r14 / tas2r40) have deliberately not been created until the assay-level source can be read.
The abstract states that the bitterness of hop-derived compounds was found to be mediated by 3 bitter taste receptors, TAS2R1, TAS2R14, and TAS2R40, and reports a structural bioinformatics characterization of the binding modes of trans-/cis-isocohumulone, -isohumulone, -isoadhumulone, cohumulone, humulone, adhumulone, and 8-prenylnaringenin at the orthosteric binding sites of these receptors. A conserved asparagine in transmembrane helix 3 is essential for recognizing hop-derived compounds, and the surrounding binding-site residues determine ligand specificity.
Limitations: The attribution to the 3 receptors is presented as established prior knowledge rather than a new measurement in this paper, and the abstract reports no cell-based receptor assay. The underlying primary assay literature (Intelmann et al. 2009, Chemosensory Perception, DOI 10.1007/s12078-009-9049-1) could not be read in this survey and is registered as metadata_only. The docking results are computational predictions, not measured activation.
Plasmids encoding 25 human TAS2Rs were transiently expressed in HEK293T cells and stimulated with 15 hop-derived compounds including α-acids, β-acids, trans/cis-iso-α-acids, isoxanthohumol, xanthohumol, and 8-prenylnaringenin. Depending on chemical structure, these activated hTAS2R1, hTAS2R14, and hTAS2R40.
Limits: The threshold concentrations and EC50 values obtained from the receptor assay were far lower than the values determined in human psychophysical experiments. However, the rank order of potency across compounds was similar between the two experiments.
Limitations: Abstract checked only. This limitation guards against treating the in vitro threshold as a human perceptual threshold.
What IBU does not measure — the gap with perceived bitterness
A study examining more than 120 strongly hopped commercial beer brands found that the relationship between IBU values and rated bitterness intensity was not linear but showed a nonlinear response. This means that the same difference in IBU does not correspond to a constant difference in felt bitterness.
beer shows a nonlinear relationship between: Bitterness Units (BU) and rated bitterness intensitysupported
Scope: More than 120 highly hopped commercial beer brands
Note: Model performance outside highly hopped beers has yet to be verified.
In strongly hopped beers, Bitterness Units (BU) predicted rated bitterness with a nonlinear response.
Limitations: Scope of application is strongly hopped commercial beers.
Limits: The relationship between Bitterness Units (BU) and rated intensity modeled in highly hopped beers is further constrained by the finding that dry hopping raises IBU through the extraction of humulinones and polyphenols while removing iso-α-acids in the majority of the commercial beers surveyed. Therefore, even at the same nominal IBU, the composition behind the IBU input to this kind of model differs between dry-hopped and kettle-hopped beers.
Limitations: This is a cross-study inference; the source itself neither tests nor comments on the Hahn 2018 model. Abstract-level extraction; the magnitude of IBU or concentrations was not obtained.
A review by an independent group states in writing, "It is important to note that bitterness is not a direct equivalent to IBU. In some cases, the value of IBU can increase disproportionately to the perceived bitterness level," making explicit the non-equivalence and non-proportionality between IBU and perceived bitterness. In the dry-hopping context, humulinones and polyphenols contaminate the IBU measurement, so a single linear correspondence from BU to perceived intensity does not hold — an overall judgment pointing the same direction as Hahn 2018's nonlinear finding.
Limitations: A review; it does not give a quantitative function (the shape of the nonlinearity) for the discrepancy. The description is mainly in the dry-hopped beer context.
Context: In 16 conventionally hopped commercial lager brands, the relationship between the rank-rating bitterness intensity from 14 trained assessors and the HPLC-quantified concentration of 6 bitter compounds (iso-α-acids and their derivatives) was well modeled by PLS regression, with a correlation of 0.92. This shows a strong correspondence between compound concentration and perceived bitterness in the concentration range typical of conventional lagers, and functions as a scope limitation showing that the Hahn 2018 nonlinearity emerges in the highly hopped range. It also reports that time-intensity profiles showed large qualitative and quantitative individual differences among assessors.
Limitations: Abstract checked only (via a single retrieval route: a publisher-deposited abstract on OpenAlex). The 0.92 correlation is the PLS model's goodness of fit, not a test of linearity itself. Individual compound concentrations, not BU (IBU), are used as the explanatory variable.
Limits: From an analysis of 34 commercial lager brands, 10 with differing compositions were selected for sensory evaluation. Beers high in polyphenols and hop acids were perceived as "harsh" and "progressive"; conventionally hopped beers as "sharp" and "instant"; and those using tetrahydro-iso-α-acids as "diminishing," "rounded," and "acidic" in bitterness — "The hopping strategy adopted by brewers impacts on the nature, temporal profile and intensity of bitterness perception in beer." This constrains the idea of a single BU-to-intensity function, in the direction that, even at the same level of bitter-compound content, intensity, character, and time course vary with what was used to make it bitter.
Limitations: Abstract checked only. A cross-study inference, not a direct test of the Hahn 2018 model. Individual intensity values and compound concentrations are not in the abstract.
The correlation coefficients between lager beer sensory bitterness intensity (rank rating) and the concentration of each acid were: cohumulone 0.54, humulone 0.74, adhumulone 0.67, tetrahydrocohumulone -0.23, tetrahydrohumulone -0.01, and tetrahydroadhumulone 0.07 — limited for any single acid, while the multivariate (PLS) model reached 0.92.
Limitations: Full text checked. See the paper for sample size and panel composition.
In regressions of Bitterness Units (BU) against the concentration of each bitter compound class, R² was 0.5911 for iso-α-acids alone, 0.7569 for α-acids, 0.8739 for α-acids and their conversion products, and 0.8904 for the total.
Limitations: The main text is behind a paywall and was not checked; only the Supporting Information was verified. This shows that BU is not a linear indicator of any single compound class, but this data does not address the relationship to sensory bitterness.
When the scope is narrowed to dry-hopped beers, the gap becomes even clearer. Multiple independent studies have pointed out that the IBU number does not reliably track either the perceived bitterness intensity or the iso-α-acid concentration itself in dry-hopped beers.
bitterness units (BU / IBU) does not reliably track Perceived bitterness intensity or iso-α-acid concentration in dry-hopped beersupported
Scope: Dry-hopped and highly hopped beers. Pilot-scale experiments and commercial-product surveys across 3 independent studies
In the majority of the commercial beers surveyed, dry hopping raised IBU while total iso-α-acid concentration fell. In other words, the measured change in IBU did not track the compound class that IBU is conventionally said to measure.
Limitations: Abstract-level extraction; the abstract does not give the magnitude of the IBU increase or iso-α-acid decrease, or how many of the 15 brands showed a decrease.
In discussing beer bitterness, the paper states, 'The BU method is most frequently used when the bitterness of beer is in question. However, BU values do not always correlate with the actual perceived bitterness,' and also, 'The BUs were correlated with the levels of humulinones.' In the data, measured BU rose then fell along with HSI (at HSI 0.3-0.7: Aurora 22, 29, 39, 35, 34 BU; Styrian Wolf 21, 25, 45, 42, 35 BU), while panel-rated bitterness quality peaked at HSI 0.3 (on a 1-5 scale: Celeia 4.9 +/- 0.21, Aurora 4.9 +/- 0.21, Styrian Wolf 5.0 +/- 0.16) and then declined. For Celeia, while iso-α-acids fell from 0.64 +/- 0.05 mg/L at HSI 0.3 to below the detection limit at HSI 0.6 and 0.7, BU stayed within a range of 21-28.
Limitations: Bitterness quality and BU are separate measures, and were not statistically correlated with each other in the text read. The discrepancy is an inference drawn from reading 2 tables side by side, not a correlation result stated by the paper itself. 10 assessors, 1 brewing trial per cell. The iso-α-acid concentrations in these beers are very low in absolute terms, consistent with dry-hop-only bittering, so they should not be directly compared with kettle-hopped beers.
Intensely dry-hopped styles contained the highest hop phenol concentrations, up to 159 mg/L, matching or exceeding the humulinones and iso-α-acids in the same beer. The authors state that while humulinones are a known contributor to the soft bitterness of hazy styles, the role of hop phenols remains poorly understood, and conclude that hop phenols may be playing a larger role than previously thought in the bitterness and mouthfeel of modern dry-hopped beers.
Limitations: Abstract-level extraction of a compositional study. The abstract reports no sensory panel results, and the suggestion regarding bitterness and mouthfeel is the authors' proposal, not a measured perceptual result. The number of beers is not stated in the abstract.
Parallel measurement of the BU method and the HPLC method (direct quantification of iso-α-acids) in 34 commercial lager brands found a tendency for BU to read higher than HPLC in 60% of samples (BU 8-36 mg/L vs. HPLC total 8-41 mg/L). Furthermore, CATA sensory evaluation of 10 brands with differing chemical profiles found that 9 of 13 terms (harsh, progressive, sharp, instant, diminishing, rounded, acidic, metallic, vegetative, etc.) differed significantly among brands (p<0.05), with perceived "quality" differences that BU values alone cannot explain splitting systematically by type of hop product (conventional hopping / dry hopping / tetrahydro products). As a 4th dataset independent of the 3 existing claim lineages (Parkin/Rutnik/Calvert), this shows the BU-HPLC mismatch and BU-independent perceptual quality differences within a single dataset.
Limitations: The "overestimation" by BU was observed in only about 60% of samples; the rest were comparable or BU was in fact lower than HPLC (e.g., brand AA was BU 34 mg/L but HPLC 41 mg/L). The sensory side is qualitative CATA description (frequency data), not absolute intensity, and no correlation coefficient on an intensity scale is reported.
What IBU does not measure — the dry-hopping reversal
Dry hopping — steeping hops in beer after fermentation or during cold conditioning — involves no boiling, so no isomerization of α-acids occurs. Iso-α-acids therefore do not, in principle, increase. Yet the IBU of dry-hopped beer rises anyway. A study combining an experiment that dry-hopped a hop-free ale in stages with a survey of 15 commercial beer brands sampled before and after dry hopping attributed the IBU rise not to iso-α-acids but to the newly extracted humulinones (oxidized α-acids) and polyphenols, and found that total iso-α-acid concentration actually fell in the majority of the commercial beers examined.
Scope: Pilot-scale dry hopping of a hop-free ale (0-16 g/L, 0-72 h), and a survey of 15 Pacific Northwest commercial beer brands sampled before and after dry hopping
Note: The 2 halves move in opposite directions: the conventional IBU proxy rises while the compound class it is supposed to stand in for falls. Central counter-evidence against treating IBU as a reading of iso-α-acids.
A trained sensory panel quantified the increase in bitterness when a hop-free ale was dry-hopped at 0-16 g/L over 0-72 h, alongside IBU and hop acid/polyphenol measurements. The increase in beer bitterness (IBU) from dry hopping was attributed to the extraction of humulinones and, in some cases, polyphenols. A parallel survey of 15 commercial beer brands sampled before and after dry hopping found a decrease in total iso-α-acid concentration in the majority of samples tested, showing that the dry-hopping process can remove iso-α-acids from beer.
Limitations: Abstract-level extraction. The number of panelists, magnitude of IBU, and iso-α-acid concentration are not given in the abstract, nor are sensory results corresponding to the commercial beer survey reported. The abstract as a whole still regards iso-α-acids as the main contributor to beer bitterness; this is a limitation on the IBU proxy, not a replacement of iso-α-acids.
A review by an independent group (the Kraków University of Agriculture) synthesizes reports from multiple labs, stating, "A decrease in iso-α-acids, and a concomitant increase in α-acids, humulinones, and polyphenols, which are extracted from hops during the process, are well documented in the literature." It cites adsorption onto the hop material (citing demonstrations by Oladokun et al.) as the mechanism for the iso-α-acid decrease, names humulinones as "the main factors responsible for the increase in perceived bitterness in beers after the dry-hopping process," and gives their relative bitterness as 66% (±13%) of iso-α-acids. This shows that the two-directional movement reported by Parkin 2017 — IBU rising while iso-α-acids fall with dry hopping — is reproduced across the literature as a whole.
Limitations: A review, not new primary data. Many of the cited primary sources are from the Shellhammer/OSU lineage (the same lab as Parkin), so this does not substitute for a fully independent primary replication, but it does show convergence of reports across multiple countries and labs.
Context: Brands inferred to be dry-hopped (inferred from humulinone detection: O, Q, AA, T, V) showed high polyphenol and humulinone concentrations, e.g., TPC>250 mg/L and up to 3 mg/L, and were described sensorially as "harsh" and "progressive" in bitterness. It states of humulinones, "a recent report by Hopsteiner suggested that humulinones are approximately 65% as bitter as iso-α-acids" (citing Steiner, 2015).
Limitations: Whether dry hopping was used is an inference not confirmed with the manufacturer. The primary source for the 65% relative bitterness of humulinones is an industry report (Hopsteiner, Steiner 2015), which is close to the "66% (±13%)" cited in the Klimczak 2023 review used elsewhere in this evidence, but both may trace back to the same primary industry report, so this cannot be called a fully independent replication (needs checking).
As an example of a primary study (Maye and Smith) summarized in the review, it is recorded that when Cascade hops were dry hopped at 1 lb./bbl. (equivalent to 386.54 g/hL) for 3 days at 16°C, iso-α-acid concentration fell from 51 mg/L to 32 mg/L, while alpha acid and humulinone concentrations both increased from below the detection limit to 13 mg/L.
Limitations: These are figures from a primary study (Maye and Smith) cited by the review; this entry has not checked the primary paper itself. This is a single example under one dosage, contact time, and temperature condition, and cannot be generalized.
Scope: Dry-hopped beer in a pilot-scale study, a survey of 15 commercial brands, and a hop-freshness trial across 3 varieties
While iso-α-acids were the main contributor to beer bitterness overall, humulinones (oxidized α-acids) and polyphenols were also potentially significant contributors to bitterness, especially in strongly dry-hopped beers, and the IBU increase from dry hopping was attributed to the extraction of humulinones.
Limitations: Abstract-level extraction. The phrase 'potentially significant' is the authors' own hedge and is preserved here as well. The abstract reports no omission/addition test with isolated compounds.
The introduction states, 'For a long time, dry hopping was not considered to contribute to bitterness, since no isomerisation of alpha acids occurs,' and states that humulinones are oxygenation products of α-acids, that their concentration increases during hop storage, and that they carry approximately 66% of the bitterness of iso-α-acids. The conclusion states, 'In dry hopped beers, the main source of bitterness is humulinones, which are increasing and, at a certain HSI, started to drop; this is due to the further oxidation of humulinones to oxidation products.'
Limitations: The figure '66% of the bitterness of iso-α-acids' and its historical framing are cited from prior literature within this paper's introduction, not measured in this study, so it is recorded as context and not promoted to a claim. The description 'main source of bitterness' is the authors' conclusion about dry-hop-only beers and is not generalized to kettle-hopped beers.
The review writes 「静置型(static)のドライホッピングでは、24時間で平均75%のフムリノンが13.3-15.0°Cの工程温度で抽出されるのに十分」 (in translation: "in static dry hopping, 24 hours is sufficient for an average of 75% of humulinones to be extracted at a process temperature of 13.3-15.0°C") (citing Lafontaine and Shellhammer).
Limitations: These are figures from a primary study (Lafontaine and Shellhammer) cited by the review; this entry has not checked the primary paper itself. They cannot be generalized to dynamic (agitated) dry hopping (the authors themselves specify static).
What IBU does not measure — other bitter compounds and the time axis
Hop bitter acids are not the only compounds involved in beer bitterness. Polyphenols have also been detected in analyses of commercial beer, and in intensely dry-hopped styles there are reports that hop-derived phenols reach concentrations matching or exceeding hop bitter acids and humulinones. IBU measurement does not distinguish between these compound classes.
Scope: Commercial lager beers chemically analyzed in the cited study
Note: Polyphenol composition varies with raw materials and process. / 2026-08-31: calibration-downgraded from established to supported because the supporting evidence has no full-text-checked source. May be re-upgraded once full text is checked.
Commercial beers were analyzed for phenolic acids and polyphenols, showing diverse profiles.
Limitations: Composition specific to each product.
Scope: LC-MS/MS profiling of 81 structurally annotated hop-derived phenolic and resin compounds across a wide range of commercial beers
Note: Concentration parity is not by itself sensory contribution: the authors state that the sensory role of hop phenols remains poorly understood, and present the implications for bitterness and mouthfeel as a suggestion.
81 hop-derived compounds were structurally annotated; the most abundant phenols were flavonol and phloroglucinol glycosides, chlorogenic acids, prenylflavonoids, and procyanidins. Intensely dry-hopped styles contained up to 159 mg/L of hop phenols, matching or exceeding humulinones and iso-α-acids. In PCA, the resin profile distinguished beer styles mainly through the ratio of iso-α-acids to oxidized resins, while the phenol profile appeared to reflect the dry-hop dosage and hop product used. The authors conclude that hop phenols, especially flavonol glycosides, transfer during dry hopping at a much higher rate than resins.
Limitations: Abstract-level extraction; the number of beers is not stated. This is composition only, with no sensory panel reported, and concentration parity does not establish parity of bitterness contribution — all the more so because the taste thresholds of these phenol classes differ greatly.
Another axis IBU cannot move along is time. Beer does not stand still after bottling: it has been reported that part of the iso-α-acids break down during storage, generating and accumulating cyclized conversion products with a harsh, lingering bitterness. Even with the same IBU at bottling, the quality of a beer's bitterness after storage can differ. This time axis is also addressed from another angle in the research note "Bitterness Has a Clock".
Scope: trans-specific conversion products formed during beer aging. Quantified in stored commercial beer
Note: Explains the change in the quality (not only intensity) of beer bitterness with storage. The threshold range given in the abstract is for the compound set, not per-compound values.
Tricyclocohumol, tricyclocohumene, isotricyclocohumene, tetracyclocohumol, and epitetracyclocohumol were identified as previously unknown trans-specific iso-α-acid conversion products. The abstract states that these compounds show a harsh, lingering bitterness with threshold concentrations ranging from 5 to 70 µmol/L, and their formation during beer aging was confirmed by HPLC-MS/MS.
Limitations: Abstract-level extraction. The 5-70 µmol/L range is given for the compound set as a whole, with no per-compound assignment. The solvent or matrix, number of panelists, and threshold method for the threshold test are not stated in the abstract.
Quantitative LC-MS/MS profiling of 56 hop-derived sensometabolites found that proton-catalyzed cyclization of trans-iso-α-acids is the quantitatively dominant reaction, generating tricyclic and tetracyclic compounds (tricyclocohumol, tricyclocohumene, isotricyclocohumene, tetracyclocohumol, epitetracyclocohumol) with a persistent, harsh bitterness, and that these accumulate in beer as storage time and temperature increase. The authors propose controlling the beer's initial pH and keeping storage temperature low to delay the onset of the harsh bitter aftertaste.
Limitations: Abstract-level extraction. The number of beers, storage temperature, duration, and pH values are not stated in the abstract. The recommendations on pH and temperature are the authors' inference from the chemistry, not a reported sensory storage trial.
Context: An independent group quantified the degradation of trans-iso-α-acids during storage of commercial lager beer by HPLC, derived kinetic parameters (activation energy, rate constant), and concluded that the iso-α-acid loss 'can be used in the beer industry to predict the alteration of the bitterness of beer during storage.' This independently confirms the precursor chemistry (storage-driven trans-iso-α-acid loss) behind the harsh-bitterness conversion products.
Limitations: Does not identify tricyclic or tetracyclic conversion products or conduct a sensory test. The sensory attribution of the harsh, lingering bitterness to those compounds still relies solely on the Munich (Intelmann/Hofmann) group.
Part of the background to the widespread confusion around the IBU number is that hop-forward styles have become mainstream in the market. In data the Brewers Association analyzed itself on the US craft beer market, about 69% of single-can sales were occupied by hop-forward styles such as imperial IPA, IPA, hazy IPA, and imperial hazy IPA, and IPA was reported to be the leading style by volume even in 12-can packs. The fact that strongly hopped styles sit at the center of the market is set down here as background explaining how often readers encounter the IBU number. That makes it all the more necessary to check what the number means.
beer — reported consumption share: About 69% of single-can sales of US craft beer were hop-forward (strongly bitter) styles, broken down as imperial IPA 26%, IPA 22%, hazy IPA 16%, and imperial hazy IPA 5%. In 12-can packs, IPA was the leading style by volume at 21%.contextual
Scope: The US craft beer market, per the Brewers Association's own analysis (based on NIQ data), as of 2025
Note: A self-published analysis article (2026-03-05) by a staff economist of the industry association Brewers Association. Counter-evidence to the generalization that 'mass production strips out bitterness' — bitterness-forward IPA-family styles still sit at the center of the US craft beer market.
About 69% of single-can sales were hop-forward styles (imperial IPA 26%, IPA 22%, hazy IPA 16%, imperial hazy IPA 5%). In 12-can packs, IPA led by volume at 21%.
Limitations: The source is the Brewers Association's own analysis article and is not connected to the raw NIQ data itself. The definition of the IPA-family group (including imperial IPA and hazy IPA) follows the article's own classification.
Show dataingredient-beer —has_reported_consumption_share_of→ "About 69% of single-can sales of US craft beer were hop-forward (strongly bitter) styles, broken down as imperial IPA 26%, IPA 22%, hazy IPA 16%, and imperial hazy IPA 5%. In 12-can packs, IPA was the leading style by volume at 21%."
claim: clm-h10-brewers-assn-ipa-dominant-share / type: cultural / updated: 2026-09-02
Frequently asked
Does a higher IBU mean a beer is more bitter?
One study, surveying more than 120 strongly hopped commercial beer brands, reported that the relationship between IBU and rated bitterness intensity was not linear but nonlinear. In this range, a difference in the IBU number does not necessarily translate directly into a difference in felt bitterness. We have not been able to locate a source that directly examined the correlation between IBU itself and rated bitterness in conventionally hopped lagers, so linearity there has not been confirmed. Restricted to dry-hopped beer, multiple cases have also been reported in which a rise in IBU does not mean an increase in bitterness compounds.
Does dry hopping actually increase a beer's bitterness?
Dry hopping involves no boiling, so no isomerization occurs, and iso-α-acids do not, in principle, increase. Even so, multiple studies have confirmed that the IBU of dry-hopped beer rises. It has also been reported that, in the majority of the 15 commercial beer brands examined, total iso-α-acid concentration actually fell, and the rise in IBU has been attributed to the extraction of humulinones and polyphenols. An independent review, citing prior literature that gives humulinones approximately 66% of the bitterness of iso-α-acids, frames this discrepancy as a pattern reported across multiple countries and laboratories. Even though the rise in IBU after dry hopping is itself certain, the same number does not tell you what actually increased to make it more bitter.
Is hop bitterness understood down to the receptor level?
That hop-derived compounds, including iso-α-acid stereoisomers and α-acids, activate several human bitter taste receptors has been confirmed in experiments that expressed these receptors in cells. However, it has also been reported that the threshold concentrations obtained from the receptor assay were far lower than the psychophysical thresholds people actually perceive. The report states that the rank order of bitterness intensity across compounds was broadly similar, but the magnitude of a cell-level response cannot simply be read across as human tongue sensitivity. The human detection threshold of iso-α-acids itself has been reported, in an experiment based on a hop-free lager, as a group mean of 7.1 mg/L.
Does the bitterness of beer change after it is bottled?
The source reports that it does change. Multiple studies have confirmed that during storage, part of the iso-α-acids convert into products that were previously unknown, showing a harsh, lingering bitterness. The reported threshold concentrations range from 5 to 70 µmol/L, and these compounds have been detected in stored commercial beer. A separate, independent study derived the kinetics of iso-α-acid degradation during storage and states that this degradation can be used to predict the change in beer's bitterness, though this study itself does not go as far as identifying the conversion products or running a sensory test. Whether the quality of bitterness changes with storage, and how the IBU number moves with storage, need to be confirmed separately.
What is not known
We have not been able to locate a source that directly examined the correlation between IBU itself and rated bitterness intensity in conventionally hopped commercial lagers.
The correspondence between perceived bitterness in dry-hopped beer and the contribution of hop phenols has compositional measurements, but has not yet been verified with a sensory panel.
How high a concentration the storage-generated conversion products reach in actual beer, and how much they contribute to bitterness, has not been measured.
How people perceptually distinguish the gap between IBU and rated bitterness has not yet been sufficiently measured.
The numbers and claims cited here are each based on individual studies. Peer-review status, sample size, and whether a source was checked only by abstract or in full text differ study by study, so the details behind each claim can be checked individually in the Source ledger.
How to cite this page
Aoki, M. (NIGAMI LAB) (2026). What Is IBU? Beer Bitterness Units vs. the Taste. https://nigamilab.com/en/topics/beer-ibu/