This week’s off-flavor is instantly recognizable to anyone who has ever pulled a green, hairy, past-its-prime Camembert from the fridge and sniffed a sample. It’s the smell of rotten wood, of decomposing leaves, of old books, and of your grandma’s attic. It’s the wet towel you threw in the trunk a couple of weeks ago and rediscovered this morning.
Musty aromas and flavors can, indeed, be caused by mold and mildew, and it’s usually pretty clear if your beer suffers from such an issue: That telltale fuzzy film floating atop the fluid requires no explication. Some sources claim that mold can be skimmed from the surface if such contamination is discovered early, but you do so at your own risk. Mold is best avoided in the first place, which means sanitation.
Of course, if you brew beer from malts or malt extracts that have developed mold prior to use, then this flavor is likely to hang on throughout brewing and fermentation, and you’re more or less stuck with it. Avoid this by using fresh malts and malt extracts. If you need to store opened cans or jugs of liquid malt extract after opening, keep them in the fridge, and pour a small amount of vodka on top to prevent mold growth.
Another possible culprit is the interior of your temperature-controlled fermentation chamber. Especially if you live in a humid climate, moisture can collect on the walls of your fermentation fridge or freezer, and this character can carry over into your fermenting or lagering beer. So periodically, clean those climate-controlled coolers!
Musty flavors can also be the result of oxidation. Though small amounts of oxygen can help certain cellared and sour styles along, this is a delicate balance and not a flavor that should be present in IPA, German lager, brown ale, and other everyday beer styles. Musty, however, does have its place in one beer style: Bière de Garde. According to the Beer Judge Certification Program (BJCP) 2008 style guide,
"Commercial versions will often have a musty, woodsy, cellar-like [aroma] character that is difficult to achieve in homebrew…The “cellar” character in commercial examples is unlikely to be duplicated in homebrews as it comes from indigenous yeasts and molds. Commercial versions often have a “corked,” dry, astringent character that is often incorrectly identified as “cellar-like.” Homebrews therefore are usually cleaner.
It’s nearly impossible to describe this flavor without trying it yourself, so you're best bet is to go sample some bières de garde to see what this means.
So, how can you avoid musty-tasting homebrew?
- Sanitize, sanitize, sanitize. When in doubt, sanitize again.
- Don’t store wort or beer in moldy areas.
- Avoid aeration after fermentation has started. Oxygen should only touch wort that will soon be inoculated with yeast.
These simple precautions will keep your homebrew free of that musty smell. As for the cheese in the back of your fridge, well, that one’s up to you.