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.