The Bitterness Problem

It used to seem so simple—know your hops and when you add them to the boil, and you’ll know how bitter your beer will be. Now, thanks to IPA’s evolution and lots of new research, bitterness is getting … complicated. Here are key takeaways to help you dial it in.

Joe Stange Sep 10, 2023 - 19 min read

The Bitterness Problem Primary Image

Photo: Matt Graves

IPAs have gotten soft. Whether that’s a good thing depends on your taste, or—if you’re running a brewery—whether that softness sells.

At the same time, IPAs have gotten harsh in seemingly new ways. Granted, when you have a really great hazy IPA—bright, juicy, smooth, without the cloy—you get what it’s all about. But how many great ones are there, really, compared to those that kick your tongue in the end with some astringent, rubbery hop bite?

Sorry for painting with a broad brush; among 10,000-plus breweries in North America, there are always exceptions, and here at the magazine we’re lucky to taste some of the very best. Yet it shouldn’t take a ream of quantitative data to convince you that bitterness has changed. All we need to do is observe the ongoing popularity of hazy IPAs and to see that even many so-called West Coast–style IPAs are soft in profile, lacking crispness and sharp edges—many taste like hazies without the haze. We can see it in recipes we’ve published over the past nine years, and we hear it from brewers on the podcast—fewer IBUs, little to no hops in the kettle, more hops in the whirlpool, and many more in the tank.

It’s no wonder that pilsners and cold IPAs are both brewer-driven trends. We’re looking for reliable, consistent bitterness anywhere we can get it.


For me to get up on the soapbox and plead with you to bring real bitterness back would be an honest thing to do, but it’s not the best use of this space or your precious time. Instead, it’s enough to recognize that food and drink trends are cyclical—and when things come back around, we have the know-how to do them better than before, with an awareness of past mistakes and deeper technical knowledge about how to achieve our goals.

With that in mind, let’s look at some of the recent science on bitterness in the context of modern IPA. Along the way, we’ll ponder some practical ways to harness it as one of beer’s truly addictive qualities—while still, I think, pleasing the crowd.

How We Taste Bitterness

It’s a fascinating evolutionary quirk that we can both taste and enjoy bitterness. It’s long been thought that our ability to detect it was a defense against poison, but the evidence for that is mixed—plenty of bitter things are healthy. Consider the argument put forward by Yvan De Baets, founder-brewer at the Brasserie de la Senne in Brussels and one of the finest bitter-beer brewers in the business. He likes to say that our enjoyment of bitterness sets us apart from our animal selves—it’s a sign of culture and civilization.

As people who taste and evaluate a lot of beer, we’re used to thinking about bitterness in a nonlinear way. Besides noticing how much bitterness, we tend to taste and describe different kinds and qualities of bitterness—soft, sharp, round, smooth, crisp, drying, resinous, quick, lasting, clean, firm, etc.


Oddly, how our taste buds detect bitterness may be much simpler than that. Our bitterness receptors appear to be pretty straightforward, according to Randy Mosher, author of Tasting Beer, contributor to this magazine, and a noted wizard on anything sensory-related. “Despite having 25 bitter receptors, there’s no evidence that we perceive different kinds of bitterness per se,” he says. “It appears that they all get dumped into the same perceptual bucket.”

Mosher is writing a book about the physiology and psychology of how we taste, so he is deep in the weeds on this topic. (Look for Your Tasting Brain: How It Works and How to Use It—unless the publisher changes the title—sometime in 2024.) He explains that humans and other mammals tend to respond to all bitter chemicals in the same way—on our tongues, they all create the same type of bitterness.

Yet we perceive differences anyway. Factors include mouthfeel and the solubility of bittering compounds, which don’t all react with our saliva the same way. What we perceive as different kinds of bitterness has a lot to do with everything else we sense—body, sweetness, carbonation, alcohol, and more. One example: A light lager of about 20 IBUs can seem more bitter than a thick, sweet stout that’s 40 IBUs on paper. There’s just a lot more going on in there.

Likewise, compare a lean West Coast IPA and a fluffy hop-juice bomb with screwdriver-like residual sweetness. Even if they had the same IBUs—calculated or real—we wouldn’t perceive that bitterness the same. “Multimodality matters,” Mosher says. “Bitterness, like acidity, is often closely coupled with mouthfeel characteristics, and those do affect the qualities of bitterness.”


So, it’s not only hops that affect bitterness—it’s the context of the whole beer and how we taste it. Meanwhile, both that context and the compounds in hops themselves are a lot more complex than most of us realize.

The Trouble with IBUs

We all know that the International Bittering Unit is an imperfect tool. Yet it’s still the best tool we have for predicting or measuring bitterness.

Calculating the IBUs of a recipe works reasonably well—assuming most of a beer’s bitterness comes from kettle additions. When we’re calculating IBUs, we’re estimating the amount of isomerized alpha acids (iso-alpha acids) and nothing else. That isomerization is induced by heat—namely, by boiling-hot wort. The combination of those compounds and that process is what creates the most stable and predictable bitterness in beer.

When we calculate the IBUs of a recipe, what we’re counting is straightforward: One IBU equals one part per million of iso-alpha acids. Let’s take a moment to appreciate how simple and clean that is—because as soon as we start looking more closely at just about everything else related to bitterness in beer, things get messy.


For a start, calculated IBUs are one thing while actual hop utilization is another. The hop utilization is how many of those iso-alpha acids actually end up in the beer—those are the most stable part of your real IBUs. A lot of things affect hop utilization, including boil time, type of hop, wort gravity, wort pH, and more. Yet the biggest blow to that bitterness happens during fermentation, when perhaps a third of the IBUs that you’ve put into the wort simply drop out—the lower pH causes the alpha acids to become less soluble in the beer.

So, calculated IBUs are not real IBUs, which are best analyzed with a pricey spectrophotometer or even pricier high-performance liquid chromatography (HPLC). These kinds of analyses include other compounds that contribute to bitterness, not just iso-alpha acids. Those other compounds include humulinones, which are alpha acids that have oxidized over time, as well as polyphenols. Heavily dry-hopped beers are going to be high in those, which is one reason why many brewers don’t bother calculating IBUs for hazy IPAs anymore.

Unfortunately, those kinds of high-tech analyses are out of reach for homebrewers and most smaller breweries. There may be lower-cost alternatives—a 2019 study out of Brazil identified a cheaper and reasonably accurate method using UV light and a digital camera. The democratization of tech seems likely to catch up with this problem sooner or later.

Meanwhile, we still have the most important tools we need: our tongues and our opinions. Brewing at home and tasting the results, many times I’ve thought, “more hops next time,” or “fewer hops next time.” Never once have I thought, “Wish I had a proper spectrophotometer analysis.” On the hands-on level, we like what we like, and we dial it in.


Alpha, Beta, and ABCs

There are five kinds of alpha acids, but most of the bitterness we taste comes from two: humulone and cohumulone. There are also a few types of beta acids, often ignored yet important to how we perceive bitterness—especially over the long term. That’s because of something called hulupones, created as beta acids oxidize over time. If your ratio of beta-to-alpha acids is high enough, the bitterness in your beer will diminish more slowly thanks to those hulupones.

There is more to be said about beta acids—and other compounds—when it comes to the perceived quality of bitterness. As Stan Hieronymus has written here for us, some hops contain higher amounts of auxiliary bitterness compounds, or ABCs. Essentially, ABCs are all the bitter stuff in hops that are not alpha acids. German researchers found that these compounds “mask a harshness and lingering character of the bitterness and make a positive contribution to the quality and harmony of the bitterness in beer.”

The hops highest in ABCs are more associated with lagers than IPAs. These have relatively high beta-to-alpha acid ratios, such as Hallertauer Mittelfrüh and Saaz. However, they also include a few newer varieties with relatively exotic aromas, such as Adeena (herbal, floral, lemon), Contessa (pear, floral, lemongrass), and Lórien (lime, floral). It’s worth experimenting with these so-called “lager hops” to see whether they have something to offer IPA. Indie Hops, which developed Lórien, says that brewers have had success with it in West Coast–style IPAs, citing its hop-oil content of 2 ml/100 grams and aroma notes of “tangy mixed citrus with fresh melon, wildflowers, and cinnamon spice.”

Put simply, iso-alpha acids are associated with a harsher bitterness, while the hulupones derived from beta acids are associated with a smoother, more pleasant bitterness.


It’s also worth thinking about how beers that pack a high-alpha punch tend to leave a lingering, resinous bitterness on the tongue. Why do some of those compounds stick to our tongues while others don’t? Once again, Mosher explains that these bitter compounds vary in the degree to which they’re soluble in our spit, and that affects how long the bitterness seems to last.

This is interesting, too: According to a recent Hopsteiner study, the solubility of some otherwise hydrophobic bittering compounds appears to increase with higher ABV, suggesting a more intense (or efficient) bitterness may be possible in higher-strength beers.

De Baets at de la Senne watches both the alpha- and beta-acid content, as he packs a substantial amount of bitterness into highly drinkable beers, from svelte Taras Boulba (4.5 percent ABV, 50 IBUs) up to the Brusseleir dark IPA (8 percent ABV, 70 IBUs). He’s also an advocate for hop quality and prefers German varieties.

“You want a fine bitterness in your beer,” he says. “To achieve that, you really need good hops. Some high-alpha varieties are fine, but their bitterness can be harsh at times. I will always prefer an early crop for those, as when they stay too long on the plant, their alphas are higher, but there is a risk of getting a harsh, unpleasant, and unrefined bitterness from them. A good relationship with a skillful grower helps a lot here. Of course, I like the European varieties a lot, and I think you get the finest bitterness with the so-called dual hops, like German Perle for instance.”


De Baets says that you can get a very refined bitterness from low-alpha Noble hops—but use too much, and you risk getting vegetal notes from all the hop matter. So, there is another balancing act. He also recommends controlling pH, targeting 5.2 during the boil.

Finally, he raises an unorthodox consideration for managing bitterness: tank geometry. “You know my love for ‘flat’ tanks,” he says, referring to his relatively wide, squat fermentors. “They actually will also help get a finer bitterness than the tall, narrow ones. In those, the harsher bitter substances will be thoroughly mixed with the beer by the strong CO2 convection within the tank and have a negative impact on the quality of bitterness.”

Bitterness or “Bitterness” from the Whirlpool?

From tasting so many IPAs and knowing how enough of them are made, I’ve developed a strong impression that kettle IBUs are not the same as harder-won whirlpool IBUs.

To explain: Imagine you have two beers that are identical, except that one gets its calculated 40 IBUs from a kettle addition, while the other gets its 40 only from the whirlpool—accounting, of course, for less isomerization at a lower temperature. My suspicion is that the kettle-hopped beer would taste noticeably more bitter than the whirlpool-hopped beer—assuming the latter is conditioned properly, so that polyphenols and other unpleasantly harsh hop-stuff is allowed to drop out. It would be an interesting experiment—not only to taste them side by side, but also to subject them to proper spectrophotometer analysis. If that study has been done, I haven’t found it.


Moving away from boil additions to whirlpool additions, and relying on the latter for bitterness, is a major factor in why—to return to my opening line—IPAs have gotten soft. To that point, a recent Japanese study found that iso-alpha acids are much stabler in the beer throughout fermentation, whereas the non-isomerized alpha acids mostly disappear. This matters to the whirlpool because the lower temperatures are isomerizing a much smaller percentage of the alpha acids—a function of both heat and time. Isomerization is not a binary switch; in fact, it’s not even linear.

As hopping moves from the kettle through the whirlpool and well into the cold side, the isomerization of alpha acids—our traditional way to predict bitterness—matters less and less. Instead, other compounds become more important contributors.

Polyphenols and Dry Hops

Scientists have known since the 1950s that hop compounds besides alpha acids contribute to beer’s bitterness. Brewers, meanwhile, have long known that dry hopping affects bitterness—and not always by increasing it.

Often, the dry-hopped version of a beer tastes a tad softer and less crisp, de-accentuating the bitterness compared to the same beer that didn’t soak up hops in the tank. On the other hand, some IPA dry-hop quantities in recent years have been eye-popping—and that probably increases the bitterness because of these other compounds. Whether that ends up being a pleasant kind of bitterness depends not just on hop quality but also on conditioning and polishing—i.e., on the skills of the brewer.


Above, I mentioned how calculated IBUs don’t account for humulinones and polyphenols. Several recent studies, chiefly from Oregon State University, have found that those compounds—and not only iso-alpha acids—are responsible for some of the bitterness in beer, especially in dry-hopped beers. Iso-alpha acids remain the most important predictor of bitterness, but when IPA brewers eschew kettle hops for substantial whirlpool and dry-hop charges, it makes sense that these other compounds would become more important.

While those compounds contribute to bitterness, it’s not always the pleasant kind. Anyone who’s tried more than a few questionable hazy IPAs is probably familiar with “hop bite” or “hop burn,” a kind of astringency known to anyone who thought it would be fun to chew on a hop pellet.

There are ways to get that unpleasantly bitter stuff out of solution while preserving that beautiful aroma. Cold and time are part of the solution; coarse filtering is another option, as is centrifuging, which can pull out some of the more volatile, unpleasant hop compounds while leaving more desirable ones, such as linalool and other aromatic terpenes.

At Sapwood Cellars in Columbia, Maryland, they don’t have a centrifuge; they rely on Biofine after dry hopping. Cofounder Scott Janish—also the author of The New IPA—finds that their hazies are still hazy. “This is not to clear it up—it doesn’t clear it up,” he says. “This is more trying to speed up the time a beer peaks.”

They typically add the Biofine a few days before packaging. “For us it seems to help a little bit in terms of the smoothness of a beer, peaking a little sooner, and just making a more drinkable beer—because that’s really what we’re after.”

When it comes to bitterness, there is more to dry hopping than polyphenols. Some research has found that the aromatic oils absorbed in the process lead to an increased perception of bitterness—another reminder that our brains are weird. Finally, there also is evidence that dry hopping removes iso-alpha acids from solution, taking away some of the primary driver of bitterness.

So, once again, it’s complicated. Ultimately, there’s no substitute for trial and error in the brewhouse backed up by sensory evaluation. Brew, but verify.