What is diacetyl, and why is it considered an off-flavor in beer?

Diacetyl is a chemical compound produced primarily by yeast during fermentation, particularly by some yeast strains or in stressful fermentation conditions. It has a distinct buttery flavor and aroma that can be reminiscent of buttered popcorn or butterscotch. This flavor is typically considered undesirable in most beer styles because it can overpower the intended flavors of the beer and create an imbalance.

In many beer styles, brewers aim for clean, crisp, and well-defined flavors. The presence of diacetyl can mask these flavors and is often associated with off-flavors that suggest poor fermentation management or yeast health. While diacetyl may be acceptable in some specific styles, such as certain English ales, in general, it is regarded as a flaw that brewers actively work to avoid by controlling fermentation conditions, allowing proper yeast maturation, and managing fermentation temperatures.