What Are VDKs in Beer And Why They Matter

When people describe a beer as “buttery” or “butterscotch-like,” it’s usually not a compliment, it’s a sign that VDKs (Vicinal Diketones) are present. These compounds can sneak in during fermentation and, if not handled properly, stick around in the final beer.

One VDK in particular — diacetyl — is one of the most common off-flavors brewers want to avoid. The good news? It’s preventable and measurable.

Let’s break it down.

What Are Vicinal Diketones?

VDKs are a group of compounds formed during fermentation, mostly as a byproduct of yeast metabolism. The two most relevant for brewers are:

Diacetyl (2,3-butanedione): buttery, popcorn, butterscotch aroma
2,3-pentanedione: more mild, honey or toffee-like

Both compounds are produced early in fermentation and should be reabsorbed and broken down by healthy yeast during a proper diacetyl rest.

If the yeast is stressed or not viable anymore, removed too early, or fermentation is rushed, these flavors can stay in the beer.

How Sensitive Are We to Diacetyl?

Matrix	Flavor Threshold (approx.)
Water (reference)	~10 µg/L
Lager Beer	20–30 µg/L
Ale Beer	40–50 µg/L
Hazy/Estery Styles	50–100 µg/L (masked)

Key takeaway: Even low levels of diacetyl can be obvious in clean, crisp beers like pilsners or lagers. In heavily hopped or ester-driven styles, it’s harder to spot but still important to control.

VDK Expectations in Beer Styles

Beer Style	VDK Sensory Expectation
Lagers / Pilsner	Very low, any diacetyl is often seen as a fault
British Ales	Up to 50 µg/L may be acceptable
Stouts / Porters	Can mask mild levels
IPAs / NEIPAs	May hide VDKs, but excess can dull hop brightness
Sours & Mixed Ferments	Variable, some styles tolerate mild buttery notes

How to Prevent VDK Problems

Don’t rush fermentation. Let yeast finish its full cycle, including the cleanup phase.
Do a diacetyl rest. Especially for lagers, raise temperature slightly at the end of fermentation to help yeast clean up.
Keep yeast healthy. Pitch enough, aerate properly, and control temperature.
Avoid early filtration or cold crashing. Removing yeast too soon can trap diacetyl in your beer.

Why You Should Lab-Test VDKs (Not Just Taste)

Sensory testing is useful but diacetyl is tricky:

Some people are very sensitive; others can’t detect it at all.
Other flavors like hops, esters, or roast can mask VDKs.
2,3-pentanedione has a higher threshold and milder flavor — it often goes unnoticed unless measured.

That’s why professional breweries and competitions rely on lab testing using EBC/MEBAK methods, like:

EBC Method 9.24.1: Headspace GC or colorimetric detection at 340 nm
Results in µg/L, for both diacetyl and 2,3-pentanedione

At testmybeer.com, we measure VDK and provide clear feedback so you don’t have to guess if your beer is clean.

How to Understand Your VDK Test Result

VDK Concentration (Diacetyl)	What It Means
< 20 µg/L	Excellent for lagers and clean styles
20–40 µg/L	Acceptable in some ales, watch for sensitivity
40–60 µg/L	Noticeable, borderline for most beers
> 60 µg/L	Likely to impact flavor, consider process changes

Final Thoughts

Diacetyl and other VDKs are some of the most common and fixable off-flavors in brewing. With a careful fermentation and a proper rest, most beers will naturally clean up these compounds.

But if you’re unsure whether your beer is buttery or clean, lab testing gives you a clear answer. Especially for breweries releasing lagers, pilsners, or export products, knowing your VDK levels is essential.

Want to be sure your beer is truly clean?

Send us a sample and let testmybeer.com help you catch unwanted diacetyl before your customers do.