Beer is a high art that can reach into peoples’ souls, but its physical reality is purely chemical.
Few of us think about it that way. It’s usually easier and more useful to talk about beer at a conceptual level, using various strategies such as semantic structures and categorization schemes. Our main organizing tool is the language of beer styles, which we’ve based on historical patterns even as they reflect current practice and marketing language. Wine people have something similar they call prototypes—idealized exemplars of grapes or blends from particular locations. These are less fluid and more limited than beer styles.
At perception’s most primal level, we all rely on personal memory associations triggered by the chemicals that make up odors. Accumulated throughout our lifetimes, these are useful for helping to recall an identity—but to communicate effectively with others, we must translate them into a commonly understood language. For experts, that generally means using industry-specific vocabulary.
For beer’s positive aroma qualities, we usually describe them with the same culinary language we would use to describe a dish of food. There’s nothing wrong with that, but it does rely on a shared understanding of terms and references that vary by culture. For more problematic odors, meanwhile, we’re more likely to call out their chemical names.
So far, so good. Conceptual frameworks and general vocabulary are great until you want to get under the hood to fix a problem or tinker with the mechanics of a particular beer. If you don’t understand the chemistry, solutions are either guesswork or based in tried-and-true experience. Chemistry tells us the source of certain qualities, what leads to the loss of flavors, and ultimately how to manage the complex processes of brewing and fermentation to achieve the desired results.
To me, it’s just good diligence to try to learn everything there is to know about our favorite beverage. It’s undeniable that our bodies are responding to chemical reality before our brains make meaning of it. Chemistry behaves predictably—it’s often quite complicated, but it doesn’t have to be scary.
Having recently embarked on an effort to dig into beer’s chemistry, I’ve found that this has proven to be another useful tool in my tasting kit. We all struggle to name odors; chemistry offers another way in. Is it fruity? Must be esters. Citrusy? Terpenes. Something cheesy or goat-like? Organic acids.
It isn’t exactly a flavor wheel, but chemical families mostly mirror beer’s main odor categories. You can even learn to think of them as aroma “primaries.”
About Aroma Chemistry
Almost any organic chemical with the right volatility can be smelled—and there is an uncountably large number of them. We have the ability to smell them all, including ones that might be invented in the future. Our sense of smell is just about unlimited.
Every molecule has certain physicochemical properties. There are thousands of these, some of which are pretty hard to understand. However, we can narrow these properties down to just a few that are most important.
First are the chemical families, typified by so-called functional groups somewhere in their structure—these make them esters, aldehydes, fatty acids, and more. Each family has a range of possible characters, although sometimes there are outliers.
Second is molecular length (or weight), based on the number of carbon backbone atoms. Chemists often abbreviate these carbon lengths as C4, C6, and so on. Aroma qualities vary by length in mostly predictable ways. When would-be flavorists are learning their molecules, they receive homework that consists of a handful of one family at certain lengths. So, that is the basic structure of their categorization.
There are other meaningful properties. For example: Whether a molecule is water-soluble or not obviously affects how it behaves in a beverage, and it even provides clues to its pleasantness: Water-insoluble ones are generally most pleasant. We’re quite sensitive to water-soluble molecules containing sulfur, chorine, or nitrogen; many of these are quite unpleasant.
Some Chemical Families and Their Qualities, Oversimplified
Hydrocarbons are the most basic types of organic chemicals, composed only of carbon and hydrogen. Except for terpenes, they play no significant role in beer aroma.
Fatty (organic) acids are a broad family of chemicals produced by plants and microbes. They are typically perceptible in beers as off-flavors—acetic acid (vinegar) and various “goaty” odors (caproic, aka hexanoic acid, etcetera). They are essential in all fermented and distilled beverages as precursors for the formation of esters: The mix of organic acids going in largely determines the estery aromas coming out.
Alcohols contain an OH group tagged onto the carbon structure. It’s pretty obvious that fermentation creates alcohols, a self-defense strategy by yeast to kill competing bacteria. Alcohols are minor players in beer aroma, ranging from harsh and alcoholic to floral and fruity. However, the esters created when alcohols combine with fatty acids are central to fermentation aroma.
Aldehydes are a rather shape-shifting group, generally green and grassy when the molecules are short, getting kind of “soapy” at about C10—think cilantro or cumin—but they can also have orangey/citrus qualities. Created by oxidation of fatty acids, (E)-2-nonenal (usually abbreviated as T2N) adds a famously cardboard-like quality to stale lager. On a more positive note, some aldehydes created by Maillard reactions during kilning contribute malty and caramel notes.
Terpenes proper are hydrocarbons. They are volatile and poorly soluble in beer, so they mostly dissipate in brewing, although dry hopping can contribute some. Terpenoids is a broader term that includes similarly structured chemicals having different functional groups, making them alcohols or other types. This transformation often occurs during fermentation. Terpenes are not generally produced by fermentation; in beer, they come almost exclusively from hops, which contain hundreds of different terpenoids. Their aroma character ranges from evergreen to floral to citrusy and others. It is believed that terpenoids, like esters, act in pools, making them hard to pick out individually. (See “Diving into Beer’s Aroma Pools,” beerandbrewing.com.)
Esters form when an alcohol and a fatty acid combine, lending beer much of its fermentation character. There are two main types to know: acetate esters and branched-chain esters. The first group forms in fermentation when ethanol enters the mix—these are generally bright and fruity. Some, like isoamyl (banana) and ethyl hexanoate (apples, anise), can sometimes be perceived individually, but research on wines has shown that ester aromas generally meld together into a “fruity” pool regardless which esters are present. Ethyl acetate is always most abundant, but it has a generically vinous aroma character that’s hard to pick out unless it rises to a high enough concentration to trigger a “solventy” trigeminal sensation. Branched-chain esters form as beer ages, lending spiritous-vinous characters.
Ketones, or rather diketones, are best known in beer as buttery diacetyl (2,3-butanedione), a by-product of yeast usually present because of incomplete conditioning.
Lactones are ring-shaped variations of esters. They’re rare in beer, but they may show themselves as coconut aromas in oak-aged beer, wines, and spirits.
Volatile phenols are ring-shaped molecules that can easily join together into polymers. Sometimes of immense size, these are important to texture in wine. Volatile phenols are small enough to smell; in beer, they’re mostly associated with the strains of yeast used in Bavarian weissbier and many Belgian ales. Volatile phenols can also come from the breakdown of wood, giving oak-aged beers their characteristic vanilla and sometimes cinnamon notes.
Heterocyclic Maillard compounds are ring-shaped molecules containing oxygen, nitrogen, and/or sulfur. The chemistry is immensely complex, but they’re responsible for the full range of nutty, toasty, and roasty aromas produced in all but the palest malts by kilning and roasting.
Sulfur is an element that forms many different types of chemicals to which we are extraordinarily sensitive. It finds its way into beer via sulfur-containing amino acids such as cysteine; yeast and bacteria may then metabolize it into compounds such as sulfides and sulfites. The best known of these is DMS (dimethyl sulfide), which has a kind of creamed-corn aroma, but other sulfides can be more cabbage-like. Sulfite is basically sulfur dioxide, smelling sharply of burnt match. Other sulfur molecules can take many configurations, but as a group they’re known as thiols. Even in minute quantities, these can be important players in beer aroma. A small molecule called ethanethiol can add garbage/sewer odors because of bacterial contamination. Some thiols are important in grapefruit, passion fruit, and other pleasantly fruity aromas derived from modern hop varieties. They’re also important potentiators of other fruity aromas in beer. (See “The Complex Case of Thiols,” beerandbrewing.com.)
Amines are nitrogen-containing chemicals that don’t occur in beer. Good thing, because they are odors of death and decomposition—think smelly fish.
Chlorine/bromine compounds are among the most potent ones we know, smellable sometimes at or below one part per trillion. They have musty or medicinal qualities, and they’re always due to contamination in beer, either from mold or incompletely rinsed bar-glass sanitizer.
I’m leaving out many other chemical groups because they play little or no role in beer aroma—even as problems. Whatever their identity, odorous chemicals are the creations of specific metabolic pathways in plants and microbes. Knowing these is another way brewers can control their end products. The rabbit hole goes deep, but it’s something to explore as a brewer.
My suggestion: Keep these in mind as you taste some beers—narrowing odors down to their families is a helpful step in identifying them. You may even end up appreciating your beer in a whole new language.
