Humans have known hops for millennia. Despite this long association—or romance, rather—they remain an enduring mystery, especially when trying to make sense of the complex sensations they evoke.
Let’s start with the easy one: bitterness. This requires no special talent to perceive. We experience just a handful of tastes, each with its own mechanisms. Unlike aromas, these tastes don’t blend together to create novel sensations. However, with 26 unique types of receptors, bitterness is by far the most biologically complex taste, essential for us to respond to a wide range of suspicious chemical types. Regardless, the brain tosses responses from all 26 receptors into the same bucket, simply labeled “bitter.”
We all perceive bitterness somewhat differently. Because we have multiple bitterness receptors and may have more than one copy of each, this opens the door to genetic variation, especially because our receptors vary in sensitivity. Most well known is the so-called “supertaster” phenomenon, but there are 93 other known genetic variations in at least 10 types of bitter receptor in humans. In short: Everyone’s a bit different.
Perceptually, there is just a single type of bitterness, but it often feels otherwise. Some bitter percepts may be altered by sensations from other senses, especially astringent mouthfeel from tannins and other polyphenols. This creates a multimodal sensation, as senses merge into a flavor “object” that we can recognize and recall.
Our senses are independent systems, but most of the time they act in concert. Almost as soon as they enter the brain, our chemical senses start blending themselves together. Multimodality, then, is really how we process our world.
Rather than keeping track of innumerable parts and pieces from multiple senses, it’s much more efficient to combine many sensory inputs and tag the entire thing with a single label. In beer, hop bitterness and aroma interact with each other. Geraniol, the rose-smelling aromatic compound found in classic Pacific Northwest hops, makes bitterness seem smoother; however, the terpene hydrocarbons abundant in wet-hopped beers have the opposite effect. It works the other way around, too: Hop-bitter compounds can alter the duration and sensory profile of hop-aroma compounds.
Your Brain on Hops
Hops produce a cornucopia of terpenes and related chemicals called terpenoids, and these make beer unique among alcoholic beverages. There are a lot of them in hops.
Jeff Dailey, sensory manager for the hop company John I. Haas, says there are 600 to 800 of these compounds known to be present in various forms of hops. Terpenes proper are hydrocarbons, but “terpenoid” is a broader term that includes alcohols and other chemical types. They all share certain structural characteristics, but the many possible variations bring differences in aroma, stability, solubility, and other traits. Beyond terpenoids, hops bring green-smelling aldehydes, fruity and rummy esters, spicy phenols, and exotic thiols smelling of grapefruit, passion fruit, or—less pleasantly—cat pee.
You may have heard about terpenes in connection with cannabis, a plant closely related to hops and one that shares many biological pathways with them. Terpenoids, on the other hand, are abundant in citrus, flowers, parsley, herbs, and many spices such as nutmeg, cardamom, and coriander. This makes their vocabulary quite complex and not always top of mind. To further muddy the waters: Despite the abundance of terpenoids, it’s not always obvious—even to experts—which of them are really responsible for a hop’s aroma. Of the hundreds of cataloged terpenoids, not a single one of them has been identified as really, truly smelling like hops.
Hop aroma changes dramatically from harvest through beer production. In the field, they contain a lot of the hydrocarbon terpenes, which tend to have sharp, bright aromas—think raw carrots and turpentine. Highly volatile, many are partially blown off during hop kilning. More compounds are lost during the boil, which is why we typically add hops valued for their aroma late in the boil or in the whirlpool, to help preserve those aromatics.
The chemical compounds you put into a beer, via hops, may end up being something quite different in the glass. Terpenes are quite unstable and readily converted into terpenoids, which smell and behave differently. That’s why most hop sellers use two profiles on their spider charts. In the example below, it’s striking how different the Cascade profile is between raw hops and a cold infusion. Brewing creates further differences, and it does so in ways that are not that easy to predict. This transformation is especially dramatic during fermentation.
Hop aromas in beer are what are known as configural smells. For the same reasons our brains combine sensations to create multisensory “objects,” it also does this with aromas. Each smell enters the body as a stream of inhaled chemicals, generating a complex pattern of olfactory responses from among our 400 or so receptor types. This pattern improves and adjusts as it flows from the receptors through the olfactory bulb.
Then, at the output of the olfactory bulb into the brain, something weird and wonderful happens: The response pattern generated by the chemistry is tossed out. It’s lost forever. What replaces it is a new pattern—one of meaning, value, hedonics, context, and more. Like our brains’ multimodal strategy, this economizes on memory and other resources by embodying our multifaceted responses to aroma chemicals into a single percept. However, this also makes these aroma objects challenging to deconstruct.
For me, hop aromas are just about the most impenetrable of all smells.
Something else is also going on: olfactory “buffers.” Well demonstrated in wine, this happens when a group of similar chemicals—esters, in the case of wine—stimulate related olfactory receptors in an overlapping way. This results in a combined aroma note that remains unchanged even when most of its components are removed. In one notable wine study, all but one of 14 fruity esters were replaced, with no noticeable change. Something similar happens with hop terpenoids. Researchers have observed cooperative effects among geraniol (rose/geranium), linalool (lavender/coriander/orange), and beta-citronellol (lemongrass).
This is just one example of several possible olfactory interactions. These may be cooperative, as with the esters. Or they can be destructive, with one chemical squashing perception of another despite displaying little of its own smell. Burnt, barnyard, tar, and musty smells can to this. Although rare, there are true synergies in which a small, even undetectable amount of one chemical can boost or alter the intensity of another, and these can be important in fruitiness. Sulfur-containing thiols are notorious for potentiating other aromas.
Creative interactions are uncommon but striking. Here, two or more aroma compounds come together to create a new aroma unlike any of its components. The “melon” aroma in hops is probably one example. And when you add 3SMP (3-sulfanyl-4-methylpentan-1-ol)—a thiol with a sulfurous/onion odor—to the three terpenoids mentioned above, you get lime-like citrus notes.
What we think of as a generally “hoppy” aroma is itself a configural smell; it doesn’t really resemble any of its components. These configural smells often fold into even more complex mixtures: Beer itself is a configural smell made up of many others derived from hops, malts, and fermentation.
So, the next time you pop a cap off a beer bottle, give it a sniff. That is beer’s configural odor.
In Your Face: Smelling Hops
All of this paints a picture of mind-boggling complexity. It’s a daunting challenge to smell hops in a meaningful way while also navigating how to incorporate them into our beers.
Fortunately, we have a pretty good ability to smell. With some practice, we can develop a workable vocabulary that helps us wrap our heads around this strange flower. Spider diagrams such as the one above can act as a helpful checklist of things to “look” for when you smell. You can even make up a blank one and connect the dots, to make your own profile—or create your own unique spider, reflecting how you perceive the various facets of hops.
We humans excel at categorizing just about anything, and hops are an excellent candidate. One way to organize varieties is more or less along lines of national origin. For a homebrewing book, I once made index cards of several dozen types and laid them all out on the dining table, grouped by their similarities. It was a worthwhile exercise, and I ended up with seven categories representing concepts such as “Noble” or “Brittanic,” with each including hops that fit the description regardless of where they actually originated. Within each group, I ranked them in order of some relevant attribute, which made it easy to find substitutes and try novel varieties.
The longest established method of evaluating hops is called “rubbing.” Most brewers know it well: It involves putting a whole cone or broken-up pellet on the heel of your hand, rubbing until it smears across your skin and heats up a bit. Then cup your hands and smell. It helps to settle yourself in a mind-space with no expectations; just let any random thought pop into your head. Make a note of it and go back for more. It’s messy (and sticky), so have some isopropyl alcohol handy to clean up.
In hop-forward IPAs, combining hop varieties is really where the magic is. For testing blends, there’s a clever hack: Use a cannabis grinder. Once you’ve used it to turn hop pellets to dust, it’s easy to make and evaluate blends by simply mixing specific amounts of different varieties until the mixture is pleasing. You can even sprinkle the mixed dust over a neutral beer and smell. It’s not a perfect prototype of how it’ll smell in your IPA, but it’s a fine start.
With their immense complexity and somewhat alien nature, hops can be really intimidating. The good news is that you don’t have to understand it all to make or enjoy really delicious beers. However, if you want to really dig into it and try to understand everything about them, that journey is just about endless. And for a guy like me, that means the fun will never end.