This will be the first in a series of blogs exploring off flavours in beer. Diacetyl is one of the most common, appearing across both commercial and homebrewed beers. Whilst it’s a desired flavour in a few styles such as Belgian ales, Trappist styles and even some classic English styles (Fullers purposely allow diacetyl into London Pride), for most – and in particular lagers, it is an undesirable off flavour and can be a sign of poor brewing practices.


Tastes / Smells Like:

Butter, Butterscotch - it’s actually used to produced artificial butter flavours


Potential Causes:

Diacetyl is one of over 500 different compounds produced when yeast ferments wort, diacetyl is a naturally occurring byproduct of fermentation that the yeast will produce no matter what. However, just as the yeast produces diacetyl, it should also get rid of it. Some of the reasons it might not include;


-       Old, weak or mutated yeast with insufficient nutrition

-       Poor aeration of wort when yeast is pitched

-       High early fermentation temperatures, during the yeasts rapid growth phase

-       High flocculating yeast

-       Bacterial infection caused by poor cleaning and sterilisation processes

-       Transferring beer off the yeast too early

-       Weak or short boils


How best to avoid it:

If you’ve been experiencing a diacetyl off flavour in your beers, the below are common practices employed by both commercial and home brewers to help minimise the risk.


-       Time: Yeast needs time to get rid of diacetyl, so don’t transfer off the yeast too early. Wherever possible, have patience and allow the beer remain in contact with the yeast for up to a week after it hits final gravity.


-       When brewing lagers complete a diacetyl rest; this involves warming up your fermentation by between 6-10°C for a two day period when the beer is 2-5 s.g. points away from target gravity.


-       Store packaged beer cold, beer that contains alpha acetolactate when exposed to heat will develop higher levels of diacetyl as the alpha acetolactate degrades.


-       Krausening involves adding a small volume of actively fermenting wort to beer that has just finished fermenting. Not only will this help complete fermentation it will clean up any diacetyl in the beer.


-       Using fresh, highly viable, medium flocculation yeast will give it the best chance to absorb diacetyl. Some yeast strains produce a lot of diacetyl, while others produce less, if you’re unsure on the strain to use for your specific style please contact us and we’ll be happy to advise.


-       Allow liquid yeast to begin initial growth with the use of a yeast starter and add a yeast nutrient such as this.


-       Supply sufficient oxygen for yeast growth through a thorough aeration before pitching your yeast, but be cautious of over oxygenating and minimise introducing this after pitching yeast.


-       Sanitize well, bottle carefully, leave some yeast in suspension and minimise oxygen pick-up as much as possible. These steps should minimise the risk of any lactic acid bacteria entering your beer, which can lead to sourness and diacetyl flavours as the beer matures.


If you have any questions about anything above please pop them into the comments below, and let us know any other off flavours you’d like us to cover in this series.