Banana, Band-aid, skunk, manure, fake butter… wine can sometimes exude the strangest of aromas. The following is a key to understanding these odd aromas and an explanation of why we experience these smells in wine.
Fake butter (often mislabeled as “popcorn”) comes from excess diacetyl in a wine. Diacetyl is a product formed during the wine’s first fermentation (when yeasts turn the grape’s sugars into alcohol). It is a product of the secondary fermentation as well: when malic acid (a strong acid) is converted into the softer lactic acid, giving the wine fuller body and, sometimes, a buttery flavor.
Banana aromas, like the butter flavors mentioned above, are a by-product of malolactic fermentation. Is the wine “bad” if you smell banana? Probably not, if the aroma is in small, barely traceable amounts.
Band-aid, Barnyard, Manure
These aromas are indicative of Brettanomyces, (lovingly dubbed “Brett” by the wine world). “Brett” is actually a yeast which will spoil the wine, but in small amounts, some wine drinkers actually enjoy it. Others absolutely despise it.
This odd aroma is the result of grapes being picked when they are underripe. It occurs most often in Sauvignon Blanc.
Matchstick aromas come from sulfur dioxide, which winemakers use in the vineyards to protect wines from mold and in the wine cellar to preserve wines. Even if winemakers choose not to add sulfur dioxide, it will always be found in slight amounts in wine: it is a byproduct of fermentation and is always present in wines. (See “Sulfites in Wine” for more on this fascinating subject.)
The presence of mold aromas are a result of bacteria that has spoiled the wine, wine being made from moldy grapes, or wines being fermented in unclean barrels.
Like mold, musty aromas can come from unclean barrels. It can also be a result of the wine being corked (see “Corked Wine” for more on this subject).
The smell of rotten egg in a wine comes from hydrogen sulfide, which is the result of the overuse of sulfur dioxide (see “Matchstick” above). The presence of hydrogen sulfide is worsened when the wine has low levels of nitrogen, which somehow works to combat the presence of too much hydrogen sulfide. Varietals like Riesling and Chardonnay, which naturally have low levels of nitrogen, are most susceptible to this aroma.
Hydrogen sulfide and other sulfur products in a wine can combine to create larger compounds which give off this offensive aroma.
Along with the aroma of cabbage, this interesting aroma is a product of fermentation, like the aromas of butter and banana.
If you smell wet cardboard, the wine is corked. See more on these aromas and what causes cork taint in “Cork Production and Cork Taint”.