Have you ever brewed a beer that leaves you with a taste that is slightly reminiscent of the buttered popcorn you get at the movie theater? Well, if that butter flavour has kicked you in the tastebuds, something has gone wrong in your brewing process!
In a world where brewers are subject to the potential more than 20 different off flavours in beer, diacetyl is one I am VERY sensitive to, as are many others and can leave your brewing reputation in shambles if not corrected.
First things first...what is diacetyl?
Also known as butanedione with the chemical formula (CH3CO)2, diacetyl is a natural byproduct of fermentation and is actually used in the production of artificial butter. Although diacetyl is typically undesirable (with the exception of a few styles that can tolerate it), it cannot be completely eliminated and therefore it's important to understand the best ways of controlling it...
1) Yeast Metabolism - the yeast used for brewing beer forms a precursor called Alpha Acetolactate (AAL), which is actually tasteless but as the beer ages, AAL is converted into diacetyl. This conversion is accelerated by the addition of heat which means if your fermentation temperature is too high, AAL (the precursor to diacetyl) is produced in abundance.
Leave the beer on the yeast for a week or so after hitting your final gravity, because although the yeast cannot metabolize the AAL, it can and will absorb the AAL, breaking it down into flavourless compounds meaning your beer will not express that buttery flavour!
Also, ensure you have pitched enough yeast (over-pitching is fine...under-pitching is deleterious). Some have engaged in a process called "Krausening" where a new yeast starter is made using the fully fermented beer and pitched to ensure AAL is fully absorbed.
Note: Lagers need more contact time with the yeast since they are fermented at lower temperatures and AAL is produced more slowly.
2) Bacterial Infection: Most commonly found in keg lines but can also be evident when home brew is infected during the brewing process.
Cure: Good sanitation practices! Ensure everything that touches your beer during the process is CLEAN and SANITIZED. Same can be said for your keg lines - keep them clean!
3) Poor Aeration: Yeast need oxygen to reproduce! No oxygen, no growth, less effective fermentation = diacetyl.
Make sure to shake, stir, splash or do anything you know will work to get air/oxygen into your wort before adding your yeast. Since the oxygen is boiled out of the wort, this step is essential or it will equate to underpitching! Don't forget though, once fermentation starts, you don't want any more oxygen...that would equate to more diacetyl :)
I hope this helps you a little more with your brewing success. Remember we are always here to answer your questions.