Diacetyl (2,3-butanedione) is a naturally occurring fermentation compound with an unmistakable buttery, butterscotch, popcorn aroma and flavor. It occurs naturally in beer and other fermented beverages
During fermentation, yeast produces alpha-acetolactate as part of its normal metabolism. Some of that compound escapes the yeast cell and can spontaneously convert into diacetyl. The yeast then takes the diacetyl back up and converts it into compounds such as acetoin, which has a much less intense flavor