What Is Diacetyl in Beer (And How to Test for It at Home)
If you’ve ever taken a sip of beer that reminded you of buttered popcorn, toffee, or butterscotch, chances are you’ve come across diacetyl. While those flavours might be welcome in a cinema snack, they’re usually regarded as an off-flavour in beer. Understanding what diacetyl is, how it develops, and how to test for it at home is a valuable skill for any brewer who wants clean, consistent results.
What Is Diacetyl?
Diacetyl is a natural by-product of fermentation, produced by yeast as they metabolise sugars and amino acids. It belongs to a group of compounds known as vicinal diketones (VDKs), which can impart buttery, toffee-like flavours and aromas to beer.
In small amounts, diacetyl can be acceptable (or even desirable) in certain traditional styles such as some English ales. However, in most modern beers — particularly lagers and hop-forward styles — it is considered a flaw because it masks clean malt and hop character.
Why Does Diacetyl Happen?
Diacetyl is formed early in fermentation but is normally “cleaned up” later, as healthy yeast reabsorb it and convert it into flavourless compounds. If diacetyl is noticeable in your finished beer, it may be due to:
- Poor yeast health (under-pitching, inadequate oxygen, or stressed yeast)
- Cutting fermentation short (not giving yeast enough time to reabsorb diacetyl)
- Low fermentation temperatures that slow yeast activity
- Bacterial contamination (lactic acid bacteria such as Pediococcus and Lactobacillus can produce significant amounts of diacetyl)
How to Test for Diacetyl at Home
Commercial breweries use laboratory equipment to measure diacetyl, but homebrewers can still check for it with a straightforward forced diacetyl test. Here’s how it works:
The Forced Diacetyl Test (VDK Test)
- Draw Two Samples: Take two small beer samples (around 100ml each) into clean, covered glasses or jars.
- Heat One Sample: Gently heat one of the samples to about 60°C by placing it in a hot water bath for 15–20 minutes. Do not boil.
- Cool Both Samples: Bring the heated sample back down to room temperature.
- Compare: Smell and taste both samples side by side. If the heated sample has a stronger buttery, toffee, or “cinema popcorn” character than the unheated one, your beer still contains diacetyl that the yeast has not yet reabsorbed.
What to Do if You Detect Diacetyl
- Extend Fermentation: Leave the beer on the yeast for a few more days to allow them to complete the clean-up.
- Raise the Temperature: Carry out a diacetyl rest — increase fermentation temperature by 2–3°C towards the end of fermentation to encourage yeast activity.
- Review Your Process: Make sure you’re pitching enough healthy yeast, fermenting at an appropriate temperature, and maintaining good sanitation throughout.
Final Thoughts
Diacetyl can creep into even the most carefully made beers, but with a simple forced diacetyl test you can catch it before packaging. Giving your yeast enough time — and the right conditions — to finish the job is the key to brewing clean, professional-quality beer at home.