The information technology professionals who keep our computers, network connections, and mobile phones up and running don’t receive nearly enough credit for the vast majority of the time when things work as intended. Instead, they hear from us only when there’s a problem. The situation isn’t all that different for brewers. We might never stop to think, “Wow, they really dialed in the temperature on that English yeast,” or “It must have taken a great deal of trial and error to achieve this harmonious balance of oak and esters.” But when things go wrong, we notice.
Fortunately, there’s a whole vocabulary to describe what we perceive. What follows is a quick reference to some of the most common off-flavors found in beer, as indicated on the Beer Judge Certification Program (BJCP) score sheets used in competitions. Whether you read about one of these off-flavors in our Beer Reviews, discover one of these off-flavors in the beer you’re drinking, or you receive feedback from a judge on your homebrew, you can use these descriptions to understand what might have gone wrong in the beer. For homebrewers, you’ll discover tips to help keep those flaws out of your next batch.
How the BJCP describes it: Green apple–like aroma and flavor.
What it is: Acetaldehyde (CH3CHO) is found in such diverse foodstuffs as ripe fruit and coffee. It’s an intermediate compound that pops up when yeast converts glucose to ethanol, but it all but disappears in most healthy fermentations.
How to prevent it: The goal is to encourage a strong, healthy fermentation, which means practicing good sanitation, pitching a sufficient population of healthy yeast cells into well-oxygenated wort, and keeping your beer on the yeast until fermentation is complete (take a hydrometer reading to make sure). Also, certain yeast strains, especially American lager strains, produce more acetaldehyde than others, so if this off-flavor is a repeat offender, consider using a different strain.
How the BJCP describes it: Puckering, lingering harshness, and/or dryness in the finish/aftertaste; harsh graininess; huskiness.
What it is: Astringency is that palate sensation you might associate with immature red wine or black tea. Sometimes called tannic, astringency is usually associated with over-extraction from grain husks, which happen to be rich in tannins.
How to prevent it: Extract brewers who use specialty grains should limit the steep to about half an hour, keep the temperature below 170°F (77°C), and use fewer than two quarts of water for each pound of grain (four liters per kilogram). All-grain brewers should pay attention to acidity and include acidulated malt or lactic acid to keep the mash pH to within 5.2–5.6. Make sure the grain bed doesn’t exceed 170°F (77°C) during the sparge, and stop sparging if the specific gravity of the runoff drops below 1.010.
How the BJCP describes it: Artificial butter, butterscotch, or toffee aroma and flavor. Sometimes perceived as a slickness on the tongue.
What it is: Diacetyl is produced to some degree by every yeast strain, but it usually disappears as yeast cells wrap up fermentation and absorb their by-products. Diacetyl is acceptable in some Irish and Scottish styles, but it’s generally considered a flaw. The low and slow nature of lager fermentation doesn’t promote reabsorption of diacetyl, so it’s a common off-flavor in lager styles.
How to prevent it: The easiest remedy for diacetyl is to simply raise the temperature toward the end of fermentation, boosting yeast metabolism and promoting diacetyl reabsorption. Lager brewers call this a diacetyl rest, and it’s nothing more than a 10–25°F (6–14°C) increase in temperature as fermentation slows. Bacteria can also produce diacetyl, so make sure your sanitation practices are good as well.
How the BJCP describes it: At low levels a sweet, cooked, or canned corn-like aroma and flavor.
What it is: Dimethyl sulfide (DMS) derives from another compound called S-methylmethionine (SMM). SMM occurs naturally during the germination (malting) of barley, but it’s usually volatilized away during kilning. Very pale malts such as Pilsner, however, don’t enjoy the benefit of extended kilning, so they retain more SMM, which becomes DMS in the mash tun and boil kettle.
How to prevent it: Light lagers that combine Pilsner malt with a subdued cold fermentation are virtual incubators for DMS. Pilsner malt can have up to eight times as much SMM as standard pale malt, so substitute good old 2-row when appropriate. Extending your boil to 90 minutes, boiling vigorously, cooling rapidly, and never, ever covering a kettle of hot wort will also help reduce DMS.
How the BJCP describes it: Aroma and/or flavor of any ester (fruits, fruit flavorings, or roses).
What it is: An ester is a member of a complex group of organic compounds that includes such diverse members as isoamyl acetate (the major aromatic compound in bananas) and butyric acid (the major aromatic compound in vomit). A beer that is described as “fruity” usually contains significant ester levels.
How to prevent it: Esters are essential to some styles (think of the banana notes in a German Weißbier) and very much out of place in others (think of a clean German Pilsner), so the need to suppress them depends upon your intentions. Ester production depends mostly on yeast strain, fermentation temperature, and pitch rate, so if you don’t want esters, the easiest cure is to switch to a clean-fermenting yeast strain, pitch a healthy population of yeast into well-oxygenated wort, and maintain a cool fermentation.
How the BJCP describes it: Aroma/flavor of fresh-cut grass or green leaves.
What it is: Some tasters perceive certain hops varieties such as Fuggle, Mosaic, and Tradition as having a grass-like profile. Malt and hops can also become grassy tasting if they’ve become stale or haven’t been stored properly. Grassy notes can also come from including large amounts of vegetal material in the kettle or fermentor.
How to prevent it: Use a high alpha-acid hops for bittering to reduce the amount of vegetation in the kettle and avoid dry hopping for more than a week, as extended contact time with plant matter might make your beer taste like a plant. If you find that certain hops varieties come across as grassy on your palate, switch to more pleasant cultivars.
How the BJCP describes it: Similar to the aroma of a skunk.
What it is: Lightstruck beer is what happens when hops-derived compounds react with ultraviolet light to create an aroma that is similar to the secretions of Pepé Le Pew. The archetypal lightstruck beer is a European pale lager in a green bottle because green glass admits a good deal of ultraviolet light (clear bottles are, of course, worse).
How to avoid it: To avoid lightstruck beer, don’t expose your beer to light! Really, it’s that easy. Cover those carboys with a blanket (or store them in a closet), and package your homebrew in brown bottles or stainless kegs.
How the BJCP describes it: Tinny, coiny, copper, iron, or blood-like flavor.
What it is: Metallic flavors usually come from—surprise!—metal. That metal may come from the mash tun, the boil kettle, or even your water itself. Sometimes stale malt can come across as metallic as well.
How to prevent it: To keep the metallic aroma and flavor out of your beer, consider switching to stainless-steel brewing equipment. Stainless may be expensive, but it’s durable and inert. Brewers who use porcelain enamel–coated steel pots should check for chips and cracks, which can expose wort to the underlying metal. Those with well water may want to have their water tested for iron and switch to bottled water if it comes back high. And finally, always buy fresh malt from a reputable supplier and store it in a cool, dry area.
How the BJCP describes it: Stale, musty, or moldy aromas/flavors.
What it is: Must needs little introduction. It’s rotting wood, decomposing leaves, your grandma’s attic, and the wet towel you threw in the trunk a couple of weeks ago and rediscovered this morning. Mold and mildew can cause musty flavors, and once these unwelcome fungi have established themselves, there’s little one can do apart from skimming and hoping, or dumping and weeping. Oxidation can also lead a beer down the musty path.
How to prevent it: As with many off-flavors, prevention starts with good sanitation. Then pay careful attention to avoid oxidation after fermentation has begun. And, obviously, don’t store wort or beer in moldy areas. Disinfect your temperature-controlled fermentation chamber periodically, especially if you live in a humid climate.
How the BJCP describes it: Any one or a combination of stale, winy/vinous, cardboard, papery, or sherry-like aromas and flavors.
What it is: Oxygen exposure degrades alcohols and aromatics into compounds that tasters usually describe as stale and papery. Very well made, high gravity styles can occasionally improve with age and oxidation, but as with wine, this is an exception, not the rule.
How to prevent it: The remedy is simple: Avoid introducing oxygen after primary fermentation. But that which is simply stated isn’t necessarily simply executed. Homebrewers are all but guaranteed to pick up some oxygen along the way, but oxygen exposure can be reduced by avoiding splashing during racking, bottling, and other activities that involve moving beer from vessel A to vessel B.
How the BJCP describes it: Spicy (clove, pepper), smoky, plastic, plastic adhesive strip, and/or medicinal (chlorophenolic).
What it is: A phenol combines hydroxyl (OH) with an aromatic hydrocarbon ring. Such compounds exist naturally in many spices and are also produced synthetically for industrial and pharmaceutical applications. Phenolics in beer may be desirable, as with Hefeweizen’s cloves and saison’s pepper, but they can be equally unwelcome, such as when Brettanomyces contamination brings on the barnyard.
How to prevent it: Chlorinated brewing water is enemy number one because chlorine reacts with yeast-derived phenols to create chlorophenols, which the palate equates with licking an adhesive bandage. Switching to bottled water is easy ensurance, as is avoiding chlorine-based sanitizers such as bleach. Also, take a close look at the yeast strains you’re using because some are more phenolic than others.
How the BJCP describes it: Aromas and flavors of higher alcohols (fusel alcohols). Similar to acetone or lacquer-thinner aromas.
What it is: Solvent-like esters are part of the same family as paint thinner, nail polish remover, varnish, and turpentine. Such character is almost always the result of fermenting too hot.
How to prevent it: First things first, make sure that you’re using food-grade plastics that are rated for the temperatures to which you expose them. Then try fermenting at the low end of your yeast strain’s preferred temperature range, starting low and then ramping up toward the end to ensure adequate attenuation.
How the BJCP describes it: Tartness in aroma and flavor. Can be sharp and clean (lactic acid) or vinegar-like (acetic acid).
What it is: The most common cause of sour flavors in your homebrew is contamination (flaw) or inoculation (intentional) with souring bacteria such as Lactobacillus, Pediococcus, and Acetobacter. Brettanomyces can also create sourness, though usually in combination with fruitiness or barnyard funk.
How to prevent it: Surprise! Sanitation is your best friend. An accidentally sour beer every year or two probably just means you were careless with your sanitation regimen, but repeated infections may indicate a systemic infection in your brewery. Sanitize your stainless steel and glass pieces thoroughly and replace all of your plastic pieces with new models.
How the BJCP describes it: The aroma of rotten eggs or burning matches.
What it is: All yeast strains produce hydrogen sulfide during fermentation, but ale fermentations are usually so vigorous that this sulfur off-gasses along with carbon dioxide. Lagers, however, enjoy a generally slower fermentation, and lager yeasts themselves often produce more sulfur than ale strains.
How to prevent it: Lagers need time to mellow, and part of this maturation period entails the release of hydrogen sulfide. Premature packaging may mean trapping sulfur in your beer like a little rotten time capsule. Sulfur can also indicate a nutritional deficiency, so consider adding a bit of yeast nutrient near the end of the boil. Finally, remember that some yeast strains are simply more prolifically sulfuric than others. Try switching strains and see if it helps.
How the BJCP describes it: Cooked, canned, or rotten vegetable aroma and flavor (cabbage, onion, celery, asparagus, etc.).
What it is: Vegetal flavors may come from any combination of DMS, hops volume, and even hops varieties. Some individuals perceive DMS as cooked vegetables, and certain hops varieties such as Summit remind some tasters of garlic or scallions. Large amounts of hops in the kettle or in the fermentor can also lend a veggie-like flavor to your beer.
How to prevent it: See the earlier section on DMS for hints on reducing that particular flaw. As for hops, use a high alpha-acid variety for bittering and avoid leaving dry hops in contact with beer for longer than a week. Finally, take a look at the hops varieties you’ve chosen: You may be sensitive to those allium-like aromatics.
How the BJCP describes it: A bready, sulfury, or yeast-like aroma or flavor.
What it is: Yeast is even more essential to beer than malt, hops, and water. But if your beer tastes yeasty, then there’s either too much residual yeast in your beer, or your yeast has released unpleasant flavors as part of its natural lifecycle.
How to prevent it: First, avoid pouring the entire contents of your bottle-conditioned homebrew. Unless you’re serving Weißbier, it’s almost certainly inappropriate to include the dregs at the bottom of the bottle. Next, make sure your beer has had sufficient time to age, which lets yeast cells absorb fermentation by-products and flocculate out of solution. But don’t let it rest too long: After a period of weeks or months, yeast cells may begin to autolyze, a process by which they cannibalize one another for survival and leave you with residual nastiness.