What is VDK testing and why does it matter?

VDK stands for vicinal diketones. The two compounds that matter in brewing are diacetyl and 2,3-pentanedione (also called acetyl propionyl). Both are produced naturally during fermentation as by-products of yeast metabolism.

What they do to flavour

Diacetyl tastes and smells like butter or butterscotch. In some styles and at low levels, that can be acceptable or even desirable. In most beers, it is a fault. 2,3-pentanedione has a similar character but is generally present at lower concentrations.

The problem is not that these compounds exist during fermentation. They always do. The problem is when they are still present at packaging. If a beer is packaged before VDK levels have dropped below the flavour threshold, the finished product carries an off-flavour that cannot be removed after the fact.

Why testing matters

Yeast reabsorbs diacetyl during conditioning. Given enough time and the right conditions, VDK levels fall below the point where they are detectable by taste. But "enough time" varies by strain, temperature, batch size, and fermentation health. Without measuring, you are guessing.

Testing VDK levels at key points during fermentation and conditioning tells you whether the beer is ready to package or whether it needs more time. That prevents two problems: packaging too early and releasing a product with off-flavours, or conditioning too long and tying up tank space unnecessarily.

How VDK testing works with GC

VDK testing is done by headspace gas chromatography. A sample of beer is sealed in a vial and heated. The volatile compounds, including diacetyl and 2,3-pentanedione, move into the headspace above the liquid. That headspace gas is injected into the GC, which separates the compounds and measures them individually.

The Ellutia 200 Series GC with FID handles this analysis. Combined with a headspace autosampler such as the EL2000H or EL2100H, the process can be automated for batch testing. Each run takes less than 20 minutes, and results are available immediately after analysis.

Detection is at low parts-per-billion levels, which is well below the typical flavour threshold for diacetyl in beer (around 10 to 15 ppb in most lager styles).

When to test

Most breweries test at the end of primary fermentation, during conditioning, and before packaging. Some also test after a forced diacetyl test (heating a sample to convert precursors into diacetyl) to check whether there is latent VDK that has not yet converted.

Want to set up VDK testing in your brewery? Get in touch and we can advise on the right configuration.