Butanedione, a.k.a. diacetyl, is a chemical occasionally found in beer. For most styles, it is undesired. For some styles, low levels are acceptable, but typically, high levels of butanedione (diacetyl) indicate something went wrong during the fermentation.
The first two reactions occur inside the yeast cell. They are catalyzed by yeast enzymes. The oxidation of the acetolactic acid to butanedione occurs after the acetolactic acid leaves the cell. Another fate of acetolactic acid is the formation of valine. Malt that contains high levels of valine favors the formation of more butandione.
The butanedione can be reabsorbed by the cell and enzymatically reduced to 2,3-butanediol. This usually occurs after 7-10 days, and diacetyl levels can noticeably decrease.
High levels of butanedione in the finished beer indicate either poor sanitation (which favor bacterial production of butanedione), too much oxygen, poor malt, or poor yeast.
Someday, I am going to find a procedure for the analysis of butandione. I will test all of my beer at various stages. Now all I need is the time....