The goal of this research was to investigate the bitterness intensity of known hop acid oxidation products, humulinones and hulupones. This was carried out by first creating suitable extracts of humulinones and hulupones of high purity for sensory testing. Using previously established oxidation methods and the addition of preparative liquid chromatography, high purity extracts of humulinones and hulupones were prepared; 93% and 92% purity respectively as measured by High Pressure Liquid Chromatography (HPLC).
The isolated humulinone and hulupone extracts were then spiked into unhopped lager, which was prepared at the Oregon State University pilot brewery. The beer samples were spiked in concentration intervals, ranging from 8 mg/L to 40 mg/L for the humulinones and hulupones. Additionally, a set of beers were prepared with commercially available iso-α-acid extract in the concentration range of 6mg/L to 30mg/L. The humulinone, hulupone, and iso-α-acid beer samples were evaluated by a trained panel in Quantitative Descriptive Analysis (QDA) sensory testing.
The trained QDA panel on beer bitterness found humulinones to be 66% as bitter as iso-α-acid (+/-13%), and hulupones to be 84% as bitter as iso-α-acids (+/-10%). The sensory results were substantially higher than previous estimates of 35% for humulinones and 50% for hulupones (26, 29). Additionally, the trained panel was able to detect the bitterness intensity of humulinones at a concentration as low as 8 mg/L, which is the first estimation of the bitterness detection threshold of humulinones. The conclusions of this work demonstrated that the bitterness intensity of hulupones and humulinones is greater than what was previously suspected and that their influence on beer flavor should not be ignored.