In order for grape juice to turn into wine, yeasts metabolize sugar to create alcohol. After that’s complete, ‘malo’ gets underway. This ‘secondary fermentation’ involves tart malic acid (found in green apples) being converted into softer lactic acid (found in milk). The result is rounder, creamier wines, which develop buttery flavors due to a malo byproduct called diacetyl. As it happens, diacetyl is the very same compound that’s added to microwave popcorn to provide its signature “AMC concession line” aroma.