What is diacetyl and how does it present in beer?

What is diacetyl and how does it present in beer?

• A. A sulfurous aroma; contributes metallic notes.
• B. A fruity ester that enhances aroma.
• C. A smoky flavor from peat malt.
• D. A buttery, butterscotch-like off-flavor typical of premature/stressed fermentation; can be downgraded with proper conditioning.

Answer: (D)

Diacetyl is a buttery, butterscotch‑like off-flavor that Yeast produces during fermentation. It shows up as a distinct buttery aroma and can carry into the beer’s flavor if the yeast hasn’t properly reabsorbed it by the end of fermentation. This typically happens when fermentation is premature or the yeast is stressed, such as with high gravity worts or less-than-ideal fermentation conditions. The way to reduce it is through proper conditioning, specifically a diacetyl rest—raising the temperature toward the end of fermentation to encourage the yeast to reabsorb the diacetyl—followed by normal cooling and conditioning to lower the final levels. This is not a sulfurous, fruity ester, or smoky character, which are produced by different compounds and processes.

Diacetyl is a buttery, butterscotch‑like off-flavor that Yeast produces during fermentation. It shows up as a distinct buttery aroma and can carry into the beer’s flavor if the yeast hasn’t properly reabsorbed it by the end of fermentation. This typically happens when fermentation is premature or the yeast is stressed, such as with high gravity worts or less-than-ideal fermentation conditions. The way to reduce it is through proper conditioning, specifically a diacetyl rest—raising the temperature toward the end of fermentation to encourage the yeast to reabsorb the diacetyl—followed by normal cooling and conditioning to lower the final levels. This is not a sulfurous, fruity ester, or smoky character, which are produced by different compounds and processes.