Diacetyl is a natural by product of fermentation. It also known to have a butter or butterscotch taste note. I love bourbons that have this note - think ND OGD. Scientists now suspect a link between diacetyl and Alzheimer's.

I'm not for sure the butterscotch note I get on some bourbons is from diacetyl and I'm not really worried, but thought this was interesting.