Oh That Smell! Beer Off-Aromas

The first sip of a new batch of homebrew is one of the many
joys of brewing. It can be devastating when that experience is tainted with an
unpleasant sensory jolt. “What is that?” your taste buds wonder. There’s something off
about your beer. It doesn’t have to be a complete waste, though. Knowing what
went wrong can help prevent it next time. An ounce of prevention is worth at
least five gallons of cure.

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

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

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.

Issue: June 1998