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Flavor Fever: Deconstructing the Heady Aromas of IPA

Randy Mosher dissects the intricate workings of IPAs—from the malt to the hop compounds that make them special and compelling—so we can approach our brews and our sensory vocabulary with deeper thought.

Randy Mosher Nov 9, 2021 - 12 min read

Flavor Fever: Deconstructing the Heady Aromas of  IPA Primary Image

Photo: Matt Graves

IPAs currently account for about half of all craft beers sold in the United States. Their bold flavors and shapeshifting personalities, plus the fun of terroir and varietal characteristics, make them endlessly fascinating. Like most beers, they’re mostly malt, hops, water, and yeast—but hops are obviously the starring attraction.

Yet no matter how hoppy the beer, more than 98 percent of the dry ingredients are typically malt or other grains. It’s crucial to have that malty counterbalance to the bitter, herbal character of hops. Together, they’re perfect—as we’ve generally agreed for the past thousand years or so.

First, Don’t Overlook the Malt

To understand IPA today requires a review of the past four decades of its evolution. Early craft ales—IPAs included—employed plenty of crystal or caramel malt, transplanted from early homebrews, in which it compensated for the diminished aroma of malt extracts. Flavors range from a light kettle-corn caramelization to toffee and burnt-sugar notes. Those used in IPAs stick to the lighter half of the spectrum and typically show caramel or toffee notes, which can make the beer a bit heavy on the palate. Taste Sierra Nevada Pale Ale and you’ll get the picture.

Over the past 20 years, there’s been less of this, allowing the delicate flavors of hops to shine through. Many modern IPAs use no crystal malt at all. One problem is that it ages rather poorly, picking up a characteristic leathery-oxidized flavor from compounds such as 2-isobutyl quinoline. This effect seems to be strongest in mid-colored malts at about 40° Lovibond.

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The historic base of IPA remains pale-ale malt. At 2 to 4°L, it makes a gorgeous pale amber beer. Extra color comes with a bolder aroma: A brisk nuttiness results from kilning under low-moisture conditions, bolstering its refreshingly dry qualities. Today’s brewers increasingly blend the pale-ale malt with paler lager malt, or they avoid it entirely in the newer hazy/juicy style, where smooth and fat are the goals. Other malts, however, have useful flavor notes: Spun sugar from crystal 10 and confectionary caramel from Vienna malt or caramel 20 are the most useful.

Adjuncts were not abundant in IPAs until the recent wave of hazies, where grains such as flaked oats, wheat, and rye can make up as much as half of the fermentables. They mostly add body and creamy texture, the exception being rye’s sugar-and-spice notes. Because of its gluey nature, it maxes out at 30 percent of the grist.

The Range & Vocabulary of Hop Aroma

The original IPA hops were landrace varieties, spontaneously developed and carefully nurtured for centuries. The most famous of these was East Kent Goldings. Contemporary brewing texts always make a point of saying that the finest hops are mandatory for IPAs. In fact, the famous “October” beer that was IPA’s forerunner took that one step further, using recently harvested hops—which, as we all know, taste freshest.

While East Kent Goldings still makes marvelous beer, it’s an understatement tto say that a lot has happened in the hop world in the past 200 years. Accelerating over the past few decades, we now face a kaleidoscope of new varieties, which taste of everything from strawberries (Barbe Rouge) to white wine (Nelson Sauvin, Hallertau Blanc) to tropical punch (Galaxy and many New Zealand varieties). The choices can be overwhelming. Unless you just want to go by what’s popular, you’ll need to really dig in to begin to understand the personality of each variety.

Hop aromas have always been difficult to get a grip on. There is, for example, a longstanding term in hop parlance—“spicy”—that has no relationship to any identifiable spice, but is often used as a code word for hoppiness itself. Saaz hops, which often get that label, represent this best. As long as we all know what we’re talking about, it’s an okay word, but it highlights the inadequacy of descriptive language, even among experts.

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One reason hops are challenging to describe is the vastness of their aromatic composition. One of the most aromatically complex of all herbs, hops contain hundreds of identifiable odorous chemicals at varying levels, depending on variety, location/terroir, growing conditions, age, storage conditions, and more. Brewing and fermentation specifics create further changes. Unlike malt—whose flavor spectrum can be perfectly tracked by terms for various cooked, baked, and roasted foods—hop aromas don’t track anything so familiar. As a result, we’re literally at a loss for words. While we can learn the names of a few of these odor chemicals, most have only a passing similarity to things we know well.

The majority of key hop-aroma compounds are terpenoids. Think about the word “turpentine,” from which it is derived, and you’ll get the picture. These have sharp, oily, resiny aromas spanning a range from piney through floral of all sorts to delicate citrus. They’re abundant in the plant world; we pick and choose from that as best we can for our vocabulary.

The Moving Parts of Hop Aroma

While many terpenoids are present in hoppy beers, a few are particularly prominent and worth getting to know. Here are a few, with aroma descriptors from GoodScents Company, an online aroma chemical resource:

  • Geraniol: sweet, floral, fruity, rose, waxy, citrus
  • Linalool: citrus, floral, sweet, bois de rose (rosewood), woody, green, blueberry
  • Beta-citronellol: floral, leather, waxy, rose bud, citrus
  • Nerol: sweet, natural, neroli (orange blossom), citrus, magnolia

To this I would add a few notes. Geraniol is prominent in all the “C-hops” (Cascade, etc.), and in my mind it corresponds best to marigolds or geraniums. Linalool can be found in orange, lavender, and prominently in coriander seed. Beta-citronellol smells of lemongrass to me.

Many of these terpenoids are so closely related that they can change into one another under certain conditions, especially during fermentation. Nerol and linalool are isomers of each other, as are geraniol and alpha-terpineol. Since isomers are molecules containing the same number and type of atoms, all that is required is rearrangement. This is the phenomenon called “biotransformation” that is well-known in the double dry-hopped hazy IPAs, where actively fermenting yeast transforms the less desirable pungent floral notes of geraniol into the pleasing citrusy floral linalool and sometimes even into beta-citronellol. Interestingly, the latter is present in beer but absent from hops, so a transformative pathway is its only route into beer. All of these have been found to undergo chemical transformations in acidic media (such as beer) over time—one reason, perhaps, for the loss of flavor as hoppy beer ages.

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Many terpenoids form complexes called glycosides—small aromatic molecules bound to heavier sugar molecules. This makes them nonvolatile and therefore unsmellable. As much as 21 to 36 percent of linalool, for example, is bound; some may be released by thermal or enzymatic means during fermentation, but much remains in the beer. Crucially, the enzymes in our saliva and from bacteria in our mouths can break these bonds and liberate fresh aroma compounds. This is one of many good reasons to pay attention to the retronasal smell—the odors passing up from your gullet and exiting through your nose.

Some other chemical groups contribute to hop aromas. Aldehydes such as (Z)-4-hexenal contribute to the “green” aroma of fresh hops. Some esters contribute subtle fruity notes; many are closely related to geraniol. A couple of carotenoids are important: Beta-ionone brings violet or iris-root floral notes; beta damascenone is associated with overaged hoppy beers, presenting a foxy Concord-grape or applesauce odor; carboxylic acids such as cheesy-smelling isovaleric acid usually result from improper storage.

One important group of aroma-active hop compounds contains sulfur. It is smellable in vanishingly small amounts because our exquisite sensitivity to sulfides and thiols is due to copper and zinc complexes in some of our olfactory receptors. Hop companies have had to purchase much more sensitive analytical equipment just to study them. Despite low parts-per-billion concentrations, they play an outsize role, especially in the newer fruity and tropically tinged varieties. With long, unwieldy names, they are usually just abbreviated. Perhaps the best know is 4-MMP (4-mercapto-4-methylpentan-2-one), with a fruity black-currant character. Others are responsible for sauvignon-blanc/passion-fruit and latex/guava notes. Hop plants create thiols with oniony off-flavors in response to stressful growing conditions. “Onion bagel” is an apt description for these toasty notes.

Aside from the challenge we have of learning dozens of different hop compounds, an odor is never the simple sum of its parts. Odorants may positively stimulate sensory receptors, but they can also inhibit them. They also compete with each other for receptor binding spots, and the most competitive odorants are not necessarily the ones stimulating the strongest sensation. Many terpenoids are well-known masking agents. So, the pattern ultimately presented to our nose for decoding is anything but logical. This is a problem—but clever brewers can tinker and find magical combinations that are much more than the sum of their parts.

Communicating What Our Nose Knows

So, that’s the biochemistry of hop aromas. Now let’s look at it from the viewpoint of our nose and vocabulary that we can actually use. For help, we can look at tools such as aroma wheels and spider diagrams, which can serve as vocabulary prompts. In Figure 1 is one I put together based on descriptors that should be familiar to most beer enthusiasts.

However, there are a million ways to slice and dice the experience. Besides functioning as a flavor wheel, these spider charts can be used for visual notetaking by placing a dot for each attribute, with higher values placed farther from the center. Connecting the dots gives a recognizable pattern, which is why they now are often included with information about hop varieties provided by hop companies.

You can’t make IPA without yeast, but its effects on the beer are far outshone by the depth and complexity of hops. The effects of yeast are subtle; more push and pull, affecting the total pattern without revealing itself too much. Certainly, there are classic yeasts associated with British ales that possess specific characteristics. Hazy-juicy IPA brewers appreciate yeasts that enhance the creaminess of the beers and engage in biotransformation.

It’s easy to focus on the conspicuous aspects of IPAs: their bold flavors and bright aromas. But a true revelation, for brewers and enthusiasts alike, requires peering beneath the surface into their beckoning depths.

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