Common flavor descriptors: Buttered Popcorn, Buttermilk, Yogurt
Possible causes: Diacetyl is produced by all yeast during fermentation, but is usually reabsorbed by the yeast cells. Non-reabsorption or over production is caused by too low temperatures during fermentation, mutated yeast, or racking too soon. It can also be formed by bacterial contamination.
Avoid by: increasing temperature at end of fermentation (diacetyl rest), allowing fermentation to completely finish, allowing beer to condition in the fermenter before racking, maintaining solid yeast health, practicing good sanitation.
Note: low levels acceptable or even desirable in some styles