What does the presence of diacetyl indicate in beer?

Diacetyl is a type of compound that is considered a fault in beer, often imparting a buttery flavor and aroma, which is generally undesirable. The presence of diacetyl typically results from improper fermentation processes, particularly in relation to how yeast is managed during fermentation.

Effective yeast management usually includes ensuring that yeast has adequate time and proper conditions to clean up byproducts of fermentation, such as diacetyl. If the yeast is not allowed to reabsorb these byproducts or if the fermentation temperature is not properly controlled, diacetyl can remain in the finished beer, indicating that the fermentation process was not executed properly.

While the options related to contamination or oxidation might suggest issues with beer quality, they do not directly correlate with diacetyl production. Diacetyl is specifically tied to fermentation conditions and yeast performance, making the concept of improper fermentation the most relevant in this context.