Rotten Eggs! The Diacetyl Rest

Ever opened up a freshly fermented batch of beer and get hit in the face with with the overwhelming smell of vomit inducing rotten eggs? Did you think “Oh shit! My whole batch has gone bad!!” Well you are not alone my friends. I remember the first time I ever had this experience and assumed I must have done something wrong. The entire area I was working in filled with the pungent odor bringing many complaints from those in the immediate area. Well the fact is, there is nothing wrong with your beer. What you are smelling is Diacetyl and completely normal.

Diacetyl MoleculeDiacetyl is a by-product of the fermentation process and can be detected in beer at concentrations as low as 50 ppb (fifty parts per billion). That’s pretty low! So naturally you are wondering how do you get it out the beer.

This is the easiest answer you will ever get. Leave it alone. Let the beer go through what is called a “diacetyl rest”. This simply means to leave your beer in the fermenter for two or three days after fermentation is complete. That’s it. The yeast themselves will clear up the diacetyl they’ve produced if given time to do it. This is of course if you are brewing an ale. For a lager the process is a little more involved and best suited for a future blog.

If you are impatient and want to test your beer to see if Diacetyl rest is complete or not . You can perform what is called a “forcing” test. Pull a few ounces of beer from your fermenter and split it into two covered containers. Refrigerate one container and heat the other to 140°F for an hour, then taste them both. If they taste the same, you’re ready to rack your beer out of the fermenter. If you can taste butter in the heated sample, your yeast is still at work, and you should give it another day or two.

Okay so that is the reason why your beer smells like ass, and that’s how you fix it. 🙂

Thanks for reading!

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