If a wine smells like rotten onions, it could be a sign that what wine flaw is present?

D.

Ok, unless you’re a winemaker, this might have been a tough one. Rotten onion aromas are a sign of mercaptan compounds. These horrible-smelling compounds can be created after fermentation when hydrogen sulfide and other basic sulfur compounds create compounds that smell like rotting onions or spoiled garlic. Brettanomyces usually smells of manure, like a barnyard. Oxidation has a scent reminiscent of rancid nuts. And TCA can be detected by its distinctive odor of a dirty wet dog sitting on wet cardboard in a damp cellar.