Diacetyl (2,3‑butanedione) is a dialdehyde produced by Saccharomyces yeasts during fermentation, through the oxidation of the α‑acetyl‑methylcarbinol (α‑acetolactate) precursors.

• Perception: taste and smell of butter, popcorn, burnt caramel

• Detection threshold: about 0.1 to 0.15 ppm depending on the palate

• Control:

  • Complete fermentation and extended rest at low temperature (lagering) allow the yeast to reduce diacetyl into 2‑butanone, which is less aromatic.

  • Active yeast and proper oxygenation reduce its excessive formation.

In summary, it is a marker of fermentation cleanliness: too much diacetyl = incomplete fermentation or yeast stress.

In small amounts, it can add body or roundness, but in excess it is generally considered a flaw.
Some styles (e.g. certain English ales) can tolerate a subtle trace, but for most lagers it should be nearly undetectable.