Iso-co-humulone Does Not Necessarily Contribute to the Roughness of Beer Bitterness
- 1Brewing Research and Development Laboratory
- 2Asahi Breweries, Ltd.
Abstract
[Conclusion] Iso-co-humulone does not necessarily contribute to the roughness of beer bitterness. The long-repeated rule that a high cohumulone proportion produces rough, harsh, or lingering bitterness should not be accepted as a general law of brewing and should not be used by itself to judge hop quality or to predict the sensory quality of beer. [Background] The cohumulone–roughness hypothesis has been reproduced for decades in brewing literature, referenceworks, trainingmaterials, andhop-selectionpractice, despite limiteddirect evidence and timeintensity findings inconsistentwith a simple universal rule. This persistence is consequential: brewers, particularly those entering the field, may reject otherwise suitable hops or misdiagnose rough bitterness solely from a cohumulone specification. [Methods] Fractions containing iso-co-humulone, iso-normal-humulone, or iso-ad-humulone (each combining its cis and trans forms) were added separately to unhopped beer at 25 BU. Lingering bitter aftertaste was evaluated blind by six trained panelists on a nine-point anchored scale and by a lipid-membrane taste-sensing system. In a separate 3,000-L brewing comparison, the iso-co-humulone proportion was increased from 28.2% to 39.8%; eight trained panelists performed blind paired comparisons of lingering aftertaste and bitterness crispness (kire). [Results]Mean lingering-bitterness scores were 4.9, 6.2, and 6.9 for iso-co-humulone, iso-normal-humulone, and iso-ad-humulone, respectively. Corresponding taste-sensor aftertaste responses were 0.24, 2.89, and 2.73. In the brewing comparison, aftertaste choices were divided 4:4, whereas seven of eight panelists selected the higheriso-co-humulone beer as having greater crispness (one-sided exact p = 0.035; two-sided p = 0.070). [Interpretation] Roughness was not measured as a separate multidimensional attribute, and the brewing comparison was not independently replicated. Nevertheless, the convergent model-beer, sensor, and brewing observations contradict the necessary-persistence premise of the traditional roughness rule and do not justify treating a high iso-co-humulone proportion as automatic evidence of rough beer bitterness. A cohumulone percentage on a hop specification is not a sensory verdict.
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BibTeX
@misc{kishimoto2026isocohumulone,
title = {{Iso-co-humulone Does Not Necessarily Contribute to the Roughness of Beer Bitterness}},
author = {Toru KISHIMOTO},
howpublished = {ChemRxiv},
year = {2026},
month = sep,
doi = {10.26434/chemrxiv.15008934/v1},
url = {https://chemrxiv.org/doi/full/10.26434/chemrxiv.15008934/v1},
note = {CC BY},
}
RIS
TY - UNPB
TI - Iso-co-humulone Does Not Necessarily Contribute to the Roughness of Beer Bitterness
AU - Toru KISHIMOTO
PY - 2026
DA - 2026/09/16
PB - ChemRxiv
DO - 10.26434/chemrxiv.15008934/v1
UR - https://chemrxiv.org/doi/full/10.26434/chemrxiv.15008934/v1
AB - [Conclusion] Iso-co-humulone does not necessarily contribute to the roughness of beer bitterness. The long-repeated rule that a high cohumulone proportion produces rough, harsh, or lingering bitterness should not be accepted as a general law of brewing and should not be used by itself to judge hop quality or to predict the sensory quality of beer. [Background] The cohumulone–roughness hypothesis has been reproduced for decades in brewing literature, referenceworks, trainingmaterials, andhop-selectionpractice, despite limiteddirect evidence and timeintensity findings inconsistentwith a simple universal rule. This persistence is consequential: brewers, particularly those entering the field, may reject otherwise suitable hops or misdiagnose rough bitterness solely from a cohumulone specification. [Methods] Fractions containing iso-co-humulone, iso-normal-humulone, or iso-ad-humulone (each combining its cis and trans forms) were added separately to unhopped beer at 25 BU. Lingering bitter aftertaste was evaluated blind by six trained panelists on a nine-point anchored scale and by a lipid-membrane taste-sensing system. In a separate 3,000-L brewing comparison, the iso-co-humulone proportion was increased from 28.2% to 39.8%; eight trained panelists performed blind paired comparisons of lingering aftertaste and bitterness crispness (kire). [Results]Mean lingering-bitterness scores were 4.9, 6.2, and 6.9 for iso-co-humulone, iso-normal-humulone, and iso-ad-humulone, respectively. Corresponding taste-sensor aftertaste responses were 0.24, 2.89, and 2.73. In the brewing comparison, aftertaste choices were divided 4:4, whereas seven of eight panelists selected the higheriso-co-humulone beer as having greater crispness (one-sided exact p = 0.035; two-sided p = 0.070). [Interpretation] Roughness was not measured as a separate multidimensional attribute, and the brewing comparison was not independently replicated. Nevertheless, the convergent model-beer, sensor, and brewing observations contradict the necessary-persistence premise of the traditional roughness rule and do not justify treating a high iso-co-humulone proportion as automatic evidence of rough beer bitterness. A cohumulone percentage on a hop specification is not a sensory verdict.
ER -