What is the primary reason for the production of diacetyl during fermentation?

The primary reason for the production of diacetyl during fermentation is stress on yeast. Diacetyl, a compound that can impart a buttery flavor to beer, is primarily produced by yeast as a byproduct of their metabolic processes, particularly when yeast is stressed. Factors such as nutrient deficiency, low oxygen levels, or high fermentation temperatures can lead to yeast stress, resulting in increased diacetyl production.

In a healthy fermentation environment, yeast can effectively convert diacetyl to its flavor-neutral counterpart, 2,3-butanediol. However, when faced with stress, their ability to perform this conversion is diminished, causing diacetyl to accumulate in the beer. Understanding the stresses on yeast is crucial for brewers aiming to produce clean, well-fermented beers with minimal off-flavors.