Oh That Smell! Beer Off-Aromas
1. Rub-a-dub, the beers in the tub
Soapy flavors may lead you to think you dropped a bar of Ivory in your bottling bucket. More than likely the flavor is coming from fatty acids left over in your cold-break trub (the precipitation of proteins after boiled wort is rapidly chilled). They combine with a warm fermentation to produce some unpleasant soap-like flavors.
Avoidance tactics: Cool wort quickly to avoid contamination. Keep most of the hot and cold break out of your fermenter. A neat trick: Rack your wort from a first, temporary bucket, carboy, or kettle into its primary fermenter after several hours of settling to remove it from cold break and dead yeast. It also keeps yeast from the fatty acids.
2. Popcorn, anyone?
If your fermentation aroma is reminiscent of movie-theater butter, chances are you’ve got diacetyl in the making. The finished beer will have varying degrees of a buttery, sometimes rancid butter or butterscotch, aroma and flavor. In small amounts diacetyl lends body fullness, which is a desirable roundness acceptable in some stouts and pilsners but found to be a fault in most other styles. Certainly, copious amounts are not welcome at all. Guinness Stout, Pilsner Urquell, and Redhook ESB all exhibit diacetyl at levels above the threshold where your senses can perceive it.
Diacetyl is usually created chemically in beer from byproducts of yeast fermentation. Yeast typically convert it to another compound that can’t usually be detected at such a low threshold. The degree of conversion depends on several factors, including contact time with yeast, temperature, type of yeast, and yeast viability.
Yeasts aren’t the only organisms that can create diacetyl. The bacteria strains Zymomonas, Pediococcus, and Lactobacillus can create abnormally high amounts of this compound. Usually, bacteria contaminations are also accompanied by a sour, acidic smell and taste and a stable haze throughout the beer. If your beer reeks of diacetyl but you did not ferment it above the optimum temperature, crash cool it, or remove it from the yeast too soon after fermentation, chances are you need to review your sanitation procedures. Do not, for instance, use your mouth to start a siphon. Lactobacillus thrives in your mouth.
Avoidance tactics: In general to reduce diacetyl, choose a strain of yeast that is recommended as a clean fermenter, keep the fermentation temperature within the range recommended for the yeast, and allow the yeast a diacetyl rest. That is, keep the beer in contact with the yeast for one to three days past fermentation at the high end of the temperature range. If you use a large amount of adjuncts, such as simple sugar syrups, rice, and corn, that are deficient in FAN (free amino nitrogen), add yeast nutrient with amino acids, because this will decrease diacetyl. Kraeusening your beer will also help. This is especially true for lagers, because lager yeast ferments and reabsorbs diacetyl at a slower rate due to temperature.
3. Rotten eggs
Burnt match, rubbery, cooked/ rotten vegetables. Sulfur compounds, mainly dimethyl sulfide (DMS), are responsible for these types of odors. DMS is found in malt and, to a lesser degree, in hops, but it can also be created by yeast and wort-spoiling bacteria. The naturally occurring amount of DMS is usually scrubbed out by an adequate wort boiling between 30 and 90 minutes and may be blown off as CO2 bubbles knock it out of solution during fermentation. In general, though, six-row malting barley has more DMS-precursor than two-row. Also, more highly kilned malts (Vienna, Munich) will make beer that has less DMS.
Avoidance tactics: Preventing DMS is easy. Keep the lid off your boil kettle to allow all volatiles — compounds that turn to gas during the boil — to escape. Clean work surfaces and all equipment that comes in contact with the wort, so as not to introduce bacteria. Do not pitch unhealthy yeast. Also, don’t leave the beer in contact with the primary fermenter’s sediment of trub and dormant yeast more than one week past initial fermentation. Also, properly aging lagers reduces sulphur compounds in some lager yeasts. Finally, choose the appropriate malt for each style of beer. Light lagers allow for a certain amount of sulfur in their aroma, hence the use of lightly kilned malt, whereas pale ales are usually without DMS.
4. Cotton mouth
Does the beer leave a feeling like bitter grape skins in your mouth? It’s not quite the clean bitterness of an IPA but a mouth-coating sensation that is distracting and unpleasant.
Astringency can come from hops, grains, and bacteria. When used liberally without the balance of malt, hops can leave a grassy astringency. Grains, if extracted at high pH, will impart tannins from the grain husk to the final beer. To a lesser degree bacteria can leave an astringency accompanied by more prevalent rancid, acidic, or vinegar-like flavors and smells.
Avoidance tactics: Usually, astringency can be minimized by keeping the recipe in balance (bitterness of hops vs. sweetness of grains), keeping the ratio of water to grain low when mashing and sparging, and watching your mash runoff pH (keeping it below pH 6 to minimize phenolic, tannic extraction).
5. I can’t hear you; I’ve got a banana in my beer
Fruity, estery, sometimes even solvent-like aromas are often the tip-off that you’ve got a lot of esters in your beer. Ethyl acetate (fruity, solvent-like, or nail-polish remover), isoamyl acetate (banana), and ethyl hexanoate (apple) are among the main contributors to fruitiness. While fruitiness is a component of all beers — it is a main competitor to alcohol production — it can be unwelcome in some styles. Lagers in particular should normally have below-threshold amounts of esters. On the other hand doppelbocks and other high-gravity lagers are notable exceptions. As the gravity increases, so do esters.
Avoidance tactics: If your ester levels are unpleasantly high, there are several practical remedies. Obviously, choose a strain of yeast that ferments with low levels of esters. Refer to published data or ask a supplier for recommendations. To minimize any yeast’s ester production, ferment cool, aerate the wort well before pitching an adequate amount of yeast, lower the original gravity, and use a yeast nutrient that has biotin powder.
6. Day-old (liquid) bread
Hints of cardboard, paper, sherry-like, and sometimes rotten vegetables typically indicate degrees of oxidation in your beer. Oxidation is accompanied by a reduction in the malt and hop flavor in your beer. The problem: Low levels of aldehydes (one type of flavor chemical in beer) that once were reduced or fresh have oxidized.
If your beer is exhibiting early signs of staling, your yeast can to some degree reverse oxidized aldehydes. (Remember, yeast are great scavengers!) This may be your only defense once oxidation sets in. Once oxidation has progressed to the more pronounced sherry-like or vegetable-like flavors, there isn’t anything you can do to reverse it.
Avoidance tactics: Oxygen has many opportunities in the brewing process to stale your brew. Transfer your homebrew from vessel to vessel with the least amount of splashing possible. At bottling time minimize headspace in the bottle to three-quarters of an inch. Just prior to capping, some homebrewers even tap the sides of their bottles to release CO2 out of the beer and “push” oxygen out of the bottle, capping quickly thereafter. It is true that yeast will absorb some of the oxygen in the bottle but certainly not all of it, as they are no longer in their vigorous growth cycle. This often leaves plenty of oxygen left to cause staling reactions.
For all-grain brewers there is at least one more area to fight oxygen ingress. Although this is a subject of debate in some brewing circles, hot-side aeration could be staling your beer. Minimize splashing, frothing, or otherwise violently stirring your mash during the mashing and sparging process. While it is certainly beneficial to thoroughly mix your grains, try not to let the surface of the mash bed get disturbed.
7. Band-Aid
Chlorophenolics. It’s a scary word and a powerful compound that can be tasted when it’s present in only a few parts per billion! Phenols found in malt (and to a lesser degree in hops) can combine with free chlorine in water to form chlorophenolics.
Avoidance tactics: Watch your use of chlorine-based cleaners/sanitizers, and find out if your area’s water is chlorinated. Ask also what steps you can perform to remove such chlorine. Certain chlorines can be boiled off. Others may require charcoal filtering.
Wild yeast may also be the cause of Band-Aid type flavors, especially in the summer when they are more prevalent in the air stream. Keep the cooling wort covered, particularly after your wort boil, and use a 0.5-micron or tighter sterile air filter when oxygenating your wort.
8. Heavy metal
Does your beer taste like metal? Tinny, slightly astringent? Usually this indicates an excess of free metals in the water supply. Bloodlike, metallic tastes can also come from residual iodophor in the beer. Some people say iodophor can taste “shrimp-like.”
Avoidance tactics: Soak your copper wort chiller in an acidic solution of vinegar prior to throwing it into the wort. This will remove most of the oxidized copper before your wort does. After it has soaked in vinegar, rinse it well with warm tap water. Then it is ready for the wort as usual.
To additionally cut down on metals, check with the municipal water department to make sure your brewing water is low in metals, reduce trub carryover from the brew kettle to the primary fermenter, and be careful of the hops you choose for dry hopping. Willamette hops, for example, can leave an unpleasant metallic flavor when used for dry hopping.
Also, if you’re using a German-style tin minikeg, make sure the keg isn’t ruptured on the inner plastic coating. These items were originally meant for one-time use, so you need to be extremely careful when cleaning them. If only your minikegged beer is tainted, you’ll need to buy a new keg. Rinse the new keg after emptying it, and soak it in a sanitizing solution. No need for abrasives, as they will ruin the liner.
9. Fusel oil
Hot, spicy, nail polish remover. And how come there’s no head retention? You’ve got fusel alcohols. Although these aromas and flavors are characteristic of a strong ale, they should be minimized or eliminated if possible. Fusel alcohols are very aromatic, being the product of a hot fermentation or a high starting gravity.
Avoidance tactics: Your best weapon is the right yeast. Find one that is neutral and temperature tolerant, such as an American ale. This is a good choice if you want to minimize fusel alcohol in a big (high starting gravity) beer. Otherwise, keep the fermentation on the cool side and make sure you follow general guidelines for yeast pitching quantities and wort aeration. Over time fusel alcohols can combine with acids to become esters, which tend to be more mellow on the palate. This is one of the reasons barleywines and the like age gracefully.
10. An apple a day
Despite your plans to make homebrew, you’ve got something more reminiscent of hard cider. There are two possible diagnoses here. First, you used some type of refined sugar in an excessive way. Second, your beer is still very young and is left with lots of acetaldehyde.
Avoidance tactics: Using sugar, especially table sugar, is a tricky proposition that can often leave a cidery flavor if the sugar is used without regard for balance. There is no hard rule or percentage, but you may want to back off or replace the sugar with malt if your beer tastes like cider or has an undesirable cidery quality.
Acetaldehyde is another story. It is most often identified with the bruised-apple/green-apple aroma. This compound is low in most commercial beers, but Budweiser contains enough so that you can taste it. Aging the beer in the presence of yeast should reduce acetaldehyde. In the future keep beer at a stable temperature and choose a different yeast strain if the problem seems to be yeast dependent.
Detective work
Taking good notes, noting temperatures, mistakes, insights, and substitutions is a good preparation for sensory troubleshooting. Once the beer is ready to be tasted, retrace your steps if it is problematic. Combining what your senses are telling you with what your notes have to say will usually lead you to a cause for your problem.
