What is the undesirable effect of high levels of diacetyl in beer?

High levels of diacetyl in beer lead to an unpleasant buttery flavor, which is often described as reminiscent of butterscotch or buttered popcorn. This off-flavor can be particularly problematic, as it detracts from the intended flavor profile of the beer, making it less enjoyable for consumers. Diacetyl is a byproduct of yeast metabolism, particularly during fermentation, and can arise from various causes, including inadequate fermentation time, improper yeast management, or poor conditioning.

The other options do not accurately describe the primary concern related to diacetyl. While increased sweetness, hazy appearance, and excessive bitterness can occur in certain situations or due to different compounds, they are not directly associated with diacetyl. Understanding the specific impact of diacetyl on flavor helps in managing fermentation practices to ensure a clean and appealing final product.