Most discussions about hops naturally include bitterness. As well as hop oils, lupulin glands within the cone contain hard and soft resins and polyphenols. The soft resins include alpha acids and beta acids, both of which contribute to bitterness, with isomerized alpha acids (iso-alpha acids), converted during wort boiling, the primary source.
In fact, there are multiple alpha acids, and brewers consider the percentage of particular humu-lones and resulting isomers when choosing hops and formulating recipes. Consumers simply equate hop bitterness—as opposed to overall bitterness, which may be influenced by other factors, such as roasted malt—with International Bitterness Units (IBU), although they only approximate the amount of iso-alpha acids in a beer and the impression they leave.
Various research indicates genetics create differences in bitterness perception just as they do in aroma perception. “Just like some people are colorblind, some people are taste-blind and simply can’t taste bitter things that others can,” said John Hayes from Penn State’s College of Agricultural Sciences. “It turns out that different bitter foods act through different receptors, and people can be high or low responders for one but not another.”
Experiments in Germany have determined that hop-derived substances activate three specific bitter receptors (out of 25 that have been identified), and they react differently to the various isomers. Those German food scientists found bitterness perception does not increase linearly and does not continue to increase at all above a certain level of intensity—about 50 milligrams per liter iso-alpha acids (broadly about 50 IBU). Bitterness, of course, is only part of taste, and flavor is a combination of what is tasted in the mouth and aroma. University of Pennsylvania psychologist Paul Rozin established a larger importance of smell in perception of flavor in the 1980s, finding there are differences between orthonasal (breathing in) and retronasal (breathing out) perceptions of odors. Some of this is thought to be because of the way odors are first absorbed in the olfactory epithelium, differing based on the direction of the airflow across the epithelium, which is inside the nasal cavity. Retronasal smells activate parts of the brain associated with signals from the mouth, which helps to explain why we perceive flavor as occurring in the mouth even when the largest component is provided by what we smell.
That’s one reason why a drinker might describe a beer as smelling bitter even though bitterness is a taste sensation perceived mostly on the tongue. This psychological interplay between aroma and taste that creates flavor has obvious implications for the overall hop impression of any beer, providing another explanation for reports such as these:
An experiment conducted by 35 members of the Rock Bottom Breweries group found no apparent relationship between measured bitterness and hop flavor or hop aroma, but a significant correlation between perceived bitterness and hop flavor or hop aroma. It appears that when drinkers smelled or tasted “more hops,” they tasted additional bitterness even if the level of iso-alpha acids was the same.
In a study in Belgium, scientists used hop oil fractions to create beers with different aroma profiles. Drinkers rated beers dosed with spicy hop essence higher in intensity of bitterness than beers without the essence even though they had equal bitterness units. In contrast, dosing beers with floral hop essence resulted in lower intensity scores. The spicy hop essence also enhanced mouthfeel.