bitterness units (BU / IBU)

Bitterness units (BU, or international bitterness units, IBU) have conventionally served as an indication of beer bitterness based on absorbance measured spectrophotometrically at a wavelength of 275 nanometers. They are often used as a proxy for the concentration of hop-derived iso-α-acids, but a study of dry-hopped beer showed that the number does not always track that concentration directly. In more than half of 15 commercial beers examined, IBU rose after dry hopping even though total iso-α-acid concentration fell.

New extraction of humulinones and, in some cases, polyphenols during dry hopping has been offered as a reason why IBU may rise while iso-α-acids fall. Because BU is a measure of absorbance at 275 nanometers, these compounds might raise the reading, but the cited literature did not directly demonstrate overlapping absorbance. A review from the University of Agriculture in Krakow says reports from multiple laboratories have well documented that dry hopping reduces iso-α-acids while increasing α-acids, humulinones, and polyphenols. It identifies humulinones as one major contributor to increased perceived bitterness and puts their relative bitterness at about 66% (±13%) of iso-α-acids.

BU itself need not match the intensity of bitterness when beer is tasted. In a dry-hopping trial using 3 Slovenian hop cultivars, BU first rose and then fell as an indicator of hop storage deterioration increased, whereas panel-rated bitterness quality was highest with fresh hops and then fell continually. The paper did not establish this divergence as a statistical correlation; it emerges from comparing separately reported measurement tables. The paper also says BU corresponded to humulinone amount rather than iso-α-acids. A recent analysis found hop phenolics in heavily dry-hopped beer at concentrations comparable to or higher than humulinones and iso-α-acids in the same beer; its authors suggest their contribution to bitterness and mouthfeel may be greater than previously thought. A single BU number cannot describe the character of beer bitterness.

Iso-α-acids themselves are not uniform either. A 2025 review of mouthfeel states that cis isomers are more bitter than trans isomers, suggesting beers with the same IBU might be perceived differently depending on their isomer ratios. This statement is the review's synthesis of earlier research.