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What’s in a beer?

Part 1 of 3 — The Nose That Knows

Good beer is complex in nature. There are so many different compounds that can be created along the beer making process — some good, some bad. What might be considered a flaw in one type of beer may be the hallmark in another. Evaluating beer is a pursuit that can consume you. (Ironic, I know.) A large part of examining a beer is knowing what is expected and what is not welcome in a particular style of beer. It’s required knowledge for a beer judge. Champaign-Urbana is home to a number of certified beer judges, myself included. (But there are many, many enthusiasts that are as just as passionate, certificate or not.)

In this first of three parts I’ll examine some of the aromas that might be found in beer and discuss what causes them. This will by no means cover all of the perceptible aromas, just a few of the major ones.

This compound is characterized by the smell of green apple, cut grass, fresh pumpkin, or even latex paint. There are two main sources of acetaldehyde. Primarily, this chemical is formed as an intermediate step in the fermentation process. The yeast will naturally take up and convert it to other byproducts in later stages. But if the fermentation is not allowed to continue to completion, there can be plenty left in the solution. In the world of beer production where times is money, sometimes beers get moved through the pipeline before they are done.

The second source of acetaldehyde is a byproduct of bacteria producing acetic acid from ethanol. This is pretty much an infection. The experienced taster can tell the difference between the two sources by discerning the presence of cider or vinegar. Those aromas, and flavors too for that matter, will be present if the source is a bacterial infection. In general, acetaldehyde is considered a flaw. But in a few instances, specifically Budweiser, it has become a notable characteristic.

This one might seem self explanatory, but it can go a bit beyond a large presence of ethanol. There can also be higher order alcohols called fusels present. They can give the impression of “hot” alcohol that has a rough or burning edge like acetone or lacquer thinner. These are generally produced during fermentation by elevated levels of amino acids, warm temperatures, large amounts of yeast growth (i.e. not enough yeast pitched into the wort), or prolonged exposure to high concentrations of ethanol. These can age out over time and are often confused by homebrewers with green, nonmatured beer. 

This is a compound that comes across as butter or butterscotch. It is commonly found in many homebrews and unfortunately a number of commercial beers as well. It is considered a flaw in most ales and all lagers. One of the few exceptions include low levels in some British beers where it is a component of the toffee flavor. Very low levels can actually help round out the malt profile of a beer, but it is a fine line as some people are very sensitive to diacetyl and others are not. 

Dimethyl Sulfide (DMS)
This stuff comes across as sweet cooked or creamed corn. It is common in many German lagers due to the prevalence of its precursors in pilsner malt. A proper helles style beer should certainly give a distinct DMS aroma. This compound is commonly present in most beers, but usually is not perceptible. 

This category of compounds covers a fairly broad range of aromas. They can be identified as many different fruit aromas and even some fragrant flowers. Esters are fermentation byproducts that have a profile dictated by the particular yeast strain. The fermentation environment can influence their production and balance.  

Oxidative aromas
When beer is comes in contact with oxygen, it will change — sometimes for the better, sometimes for the worse. Oxygen exposure can affect beer in a number of ways. First, it can directly combine with compounds in the beer. Many times this will show up as stale, wine-like cardboard or wet paper. Other times, it can come across like a fine sherry. Old ales and barley wines will exhibit some of the later. Alternatively, oxygen exposure can aid in bacterial growth.  Bacteria can throw a very wide range of spoilage aromas, but in the oxidized category they are generally musty or moldy. 

This set of compounds, created by yeast and/or bacteria, is very wide ranging. Belgian and wheat beers in particular showcase the good phenolics in very distinct combinations. On the good side, they can be the clove, banana, and bubble-gum you get in hefeweizens or the peppery ones found in many high-gravity Belgians. However, there is a much broader bad side to these compounds. Phenolics can come across as smoky (think electrical fire, not BBQ), plastics, band-aids, tape-adhesive, or chloroseptic. Yum, I know. Judging beer is not always as fabulous as it may sound. 

This is also called light-struck because it is due to a reaction where hop compounds are broken down when exposed to light. The description is termed “skunky” because it just so happens to be the exact same compound that gives a skunk its foul odor. This is one reason beer is normally found in brown bottles. They help protect the beer from some of the light. However, some beers are found in clear bottles and never seem to get skunked. Those beers are brewed with a specific hop-derived isomer that is extracted chemically. This “tetra-hop” bittering agent is not affected by light and has the added benefit of improving foam stability. However, it is very easy to reproduce the skunked aromas in naturally hopped beers. Just set a green bottle of Heineken in direct sunlight for a few hours, then open and enjoy … well, maybe enjoy is not the right word here.

Sour, like other aromas, can be good or bad. Sourness that comes in the form of vinegar or yogurt is bad. But if we’re talking a leathery, sweaty, horse blanket, then we’re usually talking about something good. This may seem like a contradiction as one might prefer putting yogurt in their mouth, but not licking a sweaty horse. However, the later aroma comes from Brettanomyces, or Brett for short. It is a yeast that gives that distinct barnyard, damp wool, or butcher-shop aroma that is so sought after in sour Belgian beers. However, Brett in almost any other beer is quite out of place. 

This is the rotten egg or, in extreme cases, burnt matches smell. Many lager yeasts will throw in a good amount of sulfur during fermentation but should be diminished in the finished beer. It can, in some beers like the German pilsner, help accentuate the hop crispness. 

These last three aroma categories are based on the building blocks of the beer. They come in a wide variety based on the ingredients used. 

This can be bready or even a little sulfur like. Open a package of dried yeast and take a big whiff or even taste a little and you’ll know this one for good.

Sweet yummy goodness can be portrayed anywhere from light and grainy to caramel sweetness to dark, roasty chalk. 

From American hops, expect a range of citrus fruits including grapefruit, lemons, and tangerines to pine resins. British hops tend to be more delicately floral and woody. Continental hops (Germany and Czech) are generally very peasant and sometimes mildly spicy.

One can spend days and many pints learning which aromas are appropriate in which style of beer. And that is one of the challenges of being a beer judge. For more information on the topic of evaluating beers, check out the Beer Judge Certification Program ( or the Cicerone Certification Program (

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