Understanding Bitterness

Brewers at The Boston Beer Company ran trials to determine how common spices changed perceived bitterness in various beers. They found that there’s more to bitterness than isomerized alpha acids and more to measuring IBU than those isomerized compounds.

Stan Hieronymus Nov 29, 2017 - 9 min read

Understanding Bitterness Primary Image

Frustrating as brewers find it, any effort to measure bitterness analytically complicates it. Perceived bitterness? Quality of bitterness? These are the domain of sensory panels. With the current technology, says hops scientist Val Peacock, machines are not ready to replace tasting panels. “I hope to be proven wrong in the next ten years,” he says. “My goal in life is to prove myself wrong.”

Brewers who are adding increasingly large quantities of hops late in the brewing process, particularly post boil, have made the math more complicated. Intent on maximizing the impact of aroma, they are changing the matrix of bittering compounds in ways scientists had little reason to investigate until now.

IBU, Hops, and Bitterness Aren’t Synonyms

The elephant in the bitterness room is the IBU, originally an acronym for international bitterness unit, but now often a synonym for “brewer used a lot of hops.” It is determined by acidifying and extracting a sample of beer, then taking an absorbance reading at a specific wavelength with ultraviolet light. The procedure must be performed in a lab, and the result is an absolute, a single number that is generally misunderstood. To estimate IBUs, a formula emerged more than fifty years ago as a compromise among brewing scientists on both sides of the Atlantic. (Many small breweries, as well as homebrewers, use formulae or software to calculate IBUs. The greatest weakness of such tools is that utilization is a key component, is often brewing-system specific, and is seldom well calibrated.)

When the formula was developed, most breweries used baled hops, and the hops were not nearly as fresh as pellets—the form most breweries use—are today. Although most of the hops bitterness came from alpha acids isomerized by boiling, a certain percentage resulted from oxidation products. The method adjusts the sum of iso-alpha acids and non-iso hops material by a factor of five-sevenths, based on the assumption that five-sevenths of the bitterness of an average beer in the 1960s resulted from iso-alpha acids and the rest from non-iso hops material.


“IBU will measure whatever is absorbed at that [specified] length,” says Peacock, formerly of Anheuser-Busch and now an independent consultant. “The more complicated the hopping method, the more deviation from measuring pure [iso-alpha acids].” Boston Beer’s trials illustrate how even non-hops matter can alter the results. Brewers made a base beer with hops calculated to contribute 20 IBUs before it was dry spiced for seven days with one pound per barrel (3.8 grams per liter) of coriander, cocoa nibs, dried lemon peel, coffee beans, or cinnamon. The IBUs in the control sample then measured 26, the one spiced with coriander 27, the one with cocoa nibs 24, and the one with cinnamon 80 (yes, eighty).

Nine of the twelve trained panelists rated the cinnamon beer as more bitter than the control, and none found it less bitter. Seven called the coriander beer more bitter, compared to three who called it less; and seven rated the coffee beer (29 IBUs) more bitter, four less. The one with cocoa nibs was the most polarizing. Six found it more bitter, six less.

Those are just a few of the many non-hops ingredients that may change beer bitterness or the perception of bitterness. In addition, genetics play a major role in determining why one beer drinker may perceive bitterness differently than the next. “Just like some people are color blind, some people are taste blind and simply can’t taste bitter things that others can,” says 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.” A person highly sensitive to one bitter compound may be insensitive to another.

The “Other” Alpha Acid

Many brewers are just learning about humulinones, a minor hops acid that can have a major impact on bitterness in heavily dry-hopped beers. Humulinones are formed by the oxidation of alpha acids. Pellets contain a higher concentration of humulinones than baled hops. Dried hops with a higher Hop Storage Index (HSI) have a higher concentration of humulinones. This is variety dependent.


Humulinones are about two-thirds as bitter as isomerized alpha acids, but they are more soluble and will dissolve into beer during dry hopping to increase bitterness. However, that doesn’t necessarily mean heavily dry-hopped beers will taste more bitter or even that they will contain more IBUs. Researchers at Hopsteiner have found that dry hopping reduces the amount of iso-alpha acids that end up in finished beer.

Comparing a low-IBU beer to a high-IBU beer in order to understand the solubility characteristics of humulinones produced this surprising result. Increasing the dry hopping dose from 0 to 0.5, 1.0, and 2.0 pounds per barrel resulted in progressively lower iso-alpha acid concentrations, from 48 to 39, 35, and 30 parts per million, respectively. “This significant loss in bitterness was offset, however, by the large increase in humulinones that dissolved in the beer,” a team headed by Hopsteiner Technical Director John Paul Maye reported in the MBAA Technical Quarterly. For a more recent presentation at the 2017 Craft Brewers Conference, Maye’s team dry hopped a beer with six pounds of Cascade hops pellets per barrel (23 grams/liter). The concentration of iso-alpha acids fell from 42 in a beer that had not been dry hopped to 18 in the heavily dry-hopped beer. The humulinones increased from 5 IBUs to 48.

The bitterness itself was different, as reported in the Technical Quarterly: “Sensory evaluation of a very low IBU beer spiked with 22 ppm of humulinones was compared with the same beer spiked with 14.5 ppm of iso-alpha acids. The bitterness intensity of the two beers appeared to be similar, confirming that humulinones are about 66 percent as bitter as iso-alpha acids. The bitterness profile of the humulinone beer, however, appeared smoother, and there was less lingering on the tongue than with the iso-alpha acid beer. This smooth bitterness makes sense given that humulinones are more polar than iso-alpha acids and should therefore not stick or linger on the tongue as long as iso-alpha acids.”

We’re Not Done

The standard IBU test, using UV Spectrophotometric Analysis, does not work for heavily dry-hopped beers because iso-alpha acids, alpha acids, and humulinones absorb light differently at the wave length used for testing. And because each hops acid has a different level of bitterness intensity, it is not possible to directly correlate IBU to bitterness.

High Performance Liquid Chromatography (HPLC) can separate and accurately measure the concentrations of iso-alph acids, humulinones, and alpha acids in a dry hopped beer—but it is a piece of machinery most breweries can’t afford. Val Peacock does not expect breweries, large or small, to discard IBUs any time soon. “It has some value,” he says. “We have to educate brewers to what it means. They place too much emphasis on it.” Those brewers who can afford to use HPLC will be able to measure various bitterness compounds and compile data to calculate bitterness that more closely correlates with perceived bitterness, but Peacock emphasizes they should include pH when making any adjustment. “People don’t realize how important that is,” he says.

He’s consulted in research at Urban Chestnut Brewing in St. Louis, examining the impact of pH on perception of bitterness. The research concluded that perceived bitterness increases about 2 IBUs for every 0.1 tick up in pH. Hopsteiner researchers found a similar relationship between dry hopping and pH; an addition of a pound of hops per barrel will add 0.14 to pH (or 2–3 IBUs). Dry hopping with six pounds per barrel boosted the pH from a little bit more than 4.4 to more than 4.9.

The end, bitter or not, of what there is to learn clearly is not yet in sight.