by Kevin Wise

If you are new to craft beer, or know someone who is, you have undoubtedly witnessed “bitter beer face.” In 1995, Keystone Light made sure you knew what bitter beer face looked like in a popular television advertisement. “Never a bad taste, never a bitter face” was their slogan, stating in fine print that this information was “based on IBU bitterness rating points.” Indubitably, Keystone Light was not bitter. But where does bitterness in today’s burgeoning selection of craft beers actually come from? And what are these IBU rating points the commercial made mention to in their ad?

In order to address the roots of bitterness in beer, let’s start by dissecting a hop plant. The hop plant, officially known as Humulus lupulus, is the brewing ingredient that imparts bitterness in beer. When the mature cones of the female hop plant are cut open, tiny yellow sacs can be seen. These sacs, also known as lupulin glands, contain a yellow powdery substance that looks like pollen when extracted from hop cones. Lupulin is packed with resins and oils that, when brewed, provide hop bitterness flavors to beer. There are many different oils, but a particular fraction of these oils, named alpha acids, are of prime importance in delivering bitterness to beer.

Lupulin from different hop strains contains different percentages of alpha acids, ranging from about 3 percent—17 percent. In the absence of brewing, the alpha acids themselves are not very soluble in water and thus do not actually impart any substantial bitterness. But during the brewing process, a certain percentage of the insoluble alpha acids are converted into more soluble counterparts known as iso-alpha acids. The heat added to the hops during the boiling phase of the brewing process causes the alpha acids to chemically change their shape ever so slightly. This small transformation in chemical structure from alpha acids to iso-alpha acids drastically affects the perception of bitterness.

So how do these iso-alpha acids correlate to bitterness? A bitterness measurement was developed, called IBU, or International Bitterness Unit. In practical terms, one IBU equals one milligram per liter or one ppm (part per million) of iso-alpha acids in beer. Beers can range from one to about 100 IBU, with an upper limit of about 110 IBU being the theoretical maximum. Although some beers claim IBUs above 110, discerning any difference in bitterness is most likely beyond the threshold of human perception. IBUs can be precisely measured in the laboratory using advanced methods such as chromatography and spectrophotometry, but brewers have devised other shorthand equations that are used in lieu of purchasing expensive equipment.

Mainstream light lagers are somewhere in the 5-10 IBU range. Below is a list of some popular Buffalo beers and their IBU levels. Beers were chosen to show different styles for purposes of comparison. Notice how Imperial Pale Ales (or IPAs) have higher IBUs than beer in other styles. For a given strain of hops, brewers have access to extensive hop profile information and can decide which hop will best suit their bitterness profile needs. Brewers may select hops that contain a higher percentage of alpha acids if they wish to create a beer with higher perceived bitterness.

Aviator Red: 28 IBU

Czech Style Pilsner: 30 IBU

Lake Effect (APA): 33 IBU

That IPA: 57 IBU

Ebenezer IPA: 57 IBU

No Lux (Black IPA): 65 IBU

Resurgence IPA: 80 IBU

Hayburner IPA: 84 IBU

However, quantifying the IBUs of a beer may not be meaningful at all. Many other factors can affect perceived bitterness of a beer. The IBU was designed to be measured by brewers to achieve consistency in brewing, not as a standard for comparison across beers. Beers high in roasted malts may balance or mask the perceived bitterness from iso-alpha acids. Other factors such as carbonation, alcohol content, food pairing, and other essential oils found in hops may affect perception of beer bitterness. Psychological phenomena such as beer aroma or appearance and expectation also play a role in bitterness detection.

But do some people actually crave hops? Russian River Brewing co-founder Vinnie Cilurzo introduced the notion of a “lupulin threshold shift.” This “lupulin shift” is partially defined as: “when a once extraordinarily hoppy beer now seems pedestrian.” Perhaps the palate adapts to hops over time, or maybe consistently drinking beers with higher and higher IBUs somehow tempers or desensitizes the palate? However, someone who exclusively enjoys Keystone Light will most certainly perceive a difference in bitterness when they switch from their beer to a Hayburner IPA.

For further discussion of alpha acid chemistry and other beer science reviews,