Buttery or Butterscotch (Diacetyl) Aroma or Flavor
Cause:
Sometimes a desireable trait, diacetyl is produced by the yeast and also by bacteria. It is present in the fermentation process and normaly decreases over time.
Corrective Action:
Do not stop the fermentation process short. Leave the beer ferment for 7 to 10 days. Higher fermentation temperatures decrease the level of diacetyl. Always watch your sanitation procedures throughout the process.
Tip:
To identify exactly what diacetyl smells like, take a whiff of some imitation butter extract.