What undesirable character does diacetyl create in beer?

Diacetyl is a compound that can be produced during the fermentation process, particularly from yeast metabolism. It is well-known for imparting a buttery flavor to beer, which is considered undesirable. This buttery characteristic can overpower the intended flavors of the beer and detracts from the overall drinking experience.

When brewers aim to create a clean and crisp profile in their beers, the presence of diacetyl is often seen as a flaw, especially in styles that should remain neutral or highlight hop or malt characteristics. Proper fermentation management, including allowing yeast to clean up diacetyl before kegging or bottling, is crucial in preventing this off-flavor.

The other options do not accurately describe diacetyl's impact. Fruity aroma is generally associated with esters produced by yeast, harsh bitterness relates to hops, and excessive sweetness may result from unfermented sugars or a lack of proper fermentation, but none of these are linked with diacetyl which is distinctively characterized by its buttery flavor.