Question: is diacetyl in beer bad for you?
Answer: When beer goes bad: Diacetyl. ... Diacetyl is an easy thing to prevent or control unless you've got a yeast infection. An excess of diacetyl can be the result of a contaminated yeast which isn't up to the job of converting the buttery flavour into a flavourless one, so the yeast needs to be fit and healthy.