iso-α-acids (isohumulones)
compound class / compound-iso-alpha-acids
イソα酸
Iso-α-acids (isohumulones): a compound formed by the isomerization of hop α-acids during wort boiling. A classical major bitter compound in beers made with kettle-hopping. Exists as a pair of stereoisomers, cis and trans.
On this page (11 sections)
Reading the bitterness
Iso-α-acids (isohumulones) are compounds formed by the isomerization of hop α-acids during the wort-boiling process, and are the classical major bitter constituent in beers made with kettle-hopping. The molecule contains a pair of stereoisomers, cis and trans, and this encyclopedia treats the trans form as a separate entry.
In a three-alternative forced-choice test (ASTM Method 1432) using lager beer base material with no hop addition (n=14 volunteers), the group detection threshold for iso-α-acids was reported as 7.1 mg/L (range 4.5–11.2 mg/L). The individual threshold was read from a sigmoid curve at a 66% correct-response rate; this value represents the detection threshold in a hop-free matrix, not the bitterness intensity of finished hopped beer itself.
Under the same experimental conditions, the group detection threshold for tetrahydro-iso-α-acids was 2.7 mg/L (range 0.7–10.0 mg/L), a lower value than iso-α-acids' 7.1 mg/L. However, the ranges of the two distributions overlap substantially, and since the abstract does not report a statistical comparison test, the difference in point estimates should not be read as clear separation.
In sensory and instrumental analysis of more than 120 commercial highly hopped beers, the bitterness intensity rated in sensory evaluation showed a non-linear relationship with BU (Bitterness Units), and the concentrations of isohumulones, humulinones, and ethanol were proposed as the major explanatory variables for bitterness prediction. This approach, distinct from single-compound threshold studies, supports that iso-α-acids are a major component influencing the bitterness intensity of actual beer populations.
The longer the hop addition boils in the wort, the higher the iso-α-acid concentration in the resulting beer tends to be. Under a 90-minute boil, α-acid concentration became nearly zero, and it was reported that nearly all of the α-acids transferred into the wort had been converted. Under the same conditions, humulinone concentration changed less than iso-α-acid concentration, indicating that iso-α-acid formation requires a higher temperature than humulinone formation. With aged, more heavily oxidized hops, the amount of α-acids added is itself reduced, so less iso-α-acid is formed, but the proportion of the added α-acids that was actually converted did not correlate with the degree of hop deterioration. It has also been reported that, given sufficient boiling time, the unwelcome aftertaste from aged hops is softened by iso-α-acids.
Iso-α-acids in beer are a collective name for 6 compounds — the cis and trans isomers of isohumulone, isocohumulone, and isoadhumulone — and in fresh, conventionally hopped ale, roughly 30% is described as the trans isomer and 70% as the cis isomer. In sensory evaluation by a trained panel, the bitterness of humulinones arising from oxidation was rated at about 66% (±13%) of that of iso-α-acids. With dry hopping, it has also been reported that batches with 24 hours or more of hop contact showed a smaller change in the trans/cis ratio after 30 days of storage at room temperature, and less oxidative degradation, than batches with shorter contact time.
Sources for this overview
Checked for this overview: Confirmed the definitional relationship of existing as a cis/trans stereoisomer pair.
The Human Bitterness Detection Threshold of Iso-α-Acids and Tetrahydro-Iso-α-Acids in Lager Beer
Checked for this overview: Confirmed group detection threshold of 7.1 mg/L and test conditions. / Confirmed threshold comparison with the tetrahydro form and the overlap of distributions.
Evaluation of Nonvolatile Chemistry Affecting Sensory Bitterness Intensity of Highly Hopped Beers
Checked for this overview: Confirmed that iso-α-acids are a major explanatory variable in regression analysis of more than 120 beer brands.
The Impact of Hop Freshness on Kettle-Hopped Beers
Checked for this overview: Used the tendency for iso-α-acid concentration to rise with longer boiling time / Used the point that a 90-minute boil brings α-acid concentration to nearly zero as conversion nears completion / Used the comparison that iso-α-acid formation requires a higher temperature than humulinone formation / Used the point that, with aged hops, the amount formed decreases but the conversion proportion itself does not correlate with HSI / Used the conclusion that, given sufficient boiling time, the aftertaste from aged hops is softened by iso-α-acids
Impact of Dry Hopping on Beer Flavor Stability
Checked for this overview: Used the composition of iso-α-acids as 6 compounds (3 types × cis/trans) / Used the approximate trans:cis ratio (30%:70%) in fresh beer / Used the tendency for dry hopping of 24 hours or more to suppress the change in trans/cis ratio during storage
Effects of Dry-Hopping on Beer Chemistry and Sensory Properties - A Review
Checked for this overview: Used the figure rating humulinone bitterness at 66% (±13%) of iso-α-acid bitterness
Other names: イソアルファ酸 (ja)
Follow the connections
tetrahydro-iso-α-acids →has a lower human detection…→ iso-α-acids (isohumulones)
iso-α-acids (isohumulones) →contributes to→ bitterness →can be measured separately …→ astringency
trans-iso-α-acids →is an isomeric subset of→ iso-α-acids (isohumulones)
Measured values (1)
| Kind | Compound | Target | Value | Conditions | Source |
|---|---|---|---|---|---|
| human detection threshold | iso-α-acids (isohumulones) | — | 7.1 mg/L | Hop-free lager beer base material (n=14, ASTM 1432) | Kolpin KM, 2009 |
Values are the reported values of each source (with units and conditions). Thresholds and EC50 depend on the panel and method; they are not single true values.
Structured claims — open a section to read each claim's confidence, evidence and limits
Composition and content (3)
Scope: Iso-alpha-acids exist as a mixture of cis/trans isomers
Note: A definitional relationship in the naming. / 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.
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compound-trans-iso-alpha-acids —is_isomeric_subset_of→ compound-iso-alpha-acidsScope: A general description of the compound composition of iso-α-acids present in beer
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compound-iso-alpha-acids —has_composition→ "6 compounds combining the cis and trans isomers of isohumulone, isocohumulone, and isoadhumulone"Scope: A general description of many fresh, conventionally hopped ales (based on a review of prior literature)
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compound-iso-alpha-acids —has_composition→ "In typical fresh, conventionally hopped ale, roughly 30% of iso-α-acids are the trans isomer and 70% are the cis isomer"Sensory (5)
Scope: ASTM method 1432 three-alternative forced-choice test in a hop-free lager beer matrix, 14 volunteers
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compound-iso-alpha-acids —has_human_detection_threshold_of→ "Group-level detection threshold of 7.1 mg/L in a hop-free lager beer matrix"Scope: ASTM method 1432 three-alternative forced-choice testing in an unhopped lager beer matrix, with 14 volunteers per compound
Note: In the abstract, the tetra distribution (0.7-10.0) is wider than that of iso (4.5-11.2), so the ordering of the point estimates should not be read as a clear separation between the 2 panelist distributions. The abstract does not report a statistical comparison of the 2 thresholds.
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compound-tetrahydro-iso-alpha-acids —has_lower_human_detection_threshold_than→ compound-iso-alpha-acidsScope: 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.
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compound-iso-alpha-acids —contributes_to→ sensation-bitterScope: A sensory panel evaluation conducted by Algazzali and Shellhammer (9 trained panelists, 0–15 point scale). The original source is a presentation at the 2015 ASBC Annual Meeting, and may not be a peer-reviewed journal article
Note: Secondhand information read in the body of the review by Klimczak et al. (2023). The original source (Algazzali & Shellhammer, 2015 ASBC Annual Meeting proceedings) itself has not been checked. The figures were copied as given in the review.
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compound-humulinones —differs_from→ compound-iso-alpha-acidsScope: The conclusion section of the overall kettle-hopping experiment in Rutnik 2023 (the authors' discussion integrating chemical analysis and sensory evaluation)
Note: This is not an experiment that isolated and verified the masking effect on its own, but a statement from the authors' conclusion section combining chemical analysis and sensory evaluation. It is also noted that aroma quality and intensity decline as HSI rises, even with a longer boiling time.
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compound-iso-alpha-acids —reduces→ "The perception of an unwelcome aftertaste from oxidation products derived from aged hops (under boiling conditions of 45 minutes or more)"Mechanism and receptors (1)
Scope: The kettle-hopping experiment in Rutnik 2023 (a comparison at the same HSI with different boiling times)
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compound-humulinones —differs_from→ compound-iso-alpha-acidsHow processing and cooking change it (4)
Scope: A comparison experiment (kettle hopping) using 3 Slovenian hop varieties (Celeia, Aurora, Styrian Wolf), hop storage index (HSI) 0.3–0.7, and boiling times of 5, 45, and 90 minutes
Note: A variety × boiling-time comparison within the same HSI condition. It is also noted that bitterness (BU) tended to be higher with longer boiling time.
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process-boil-hopping —increases→ compound-iso-alpha-acidsScope: Rutnik 2023, 3 Slovenian hop varieties, HSI 0.3–0.7, the 90-minute boiling condition
Note: The isomerization rate (conversion proportion) differed by hop variety; it is reported that Celeia converted at a higher rate than Aurora or Styrian Wolf.
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compound-hop-bitter-acids —transforms_in_solution_into→ compound-iso-alpha-acidsScope: An experiment at homebrew scale. Dry-hop contact time was varied from 0 to 96 hours, and trans/cis ratio was compared before and after 30 days of storage at room temperature
Note: The authors state that it was 「概ね(generally)」 (in translation: "generally") lower, and it is also noted that the difference between the 1–12 hour conditions and the 72-hour condition was not statistically significant. The trans/cis ratio is a measure used in prior research as an indicator of beer staling.
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compound-iso-alpha-acids —is_selectively_degraded_during→ process-beer-storage