Diacetyl and VDK

I must confess it did bring a smile to my face to see Cloudwater brewery post something scientifically nonsensical on their blog soon after announcing their head brewer was leaving and not being replaced. Maybe, just maybe, there is a need to technical brewers after all!

The offending passage is where they call for their publican customers to:

"know to dispense cask when it’s conditioned out past any rise and fall of diacetyl producing VDK"

Ignoring the dubious point that it's the customer's job to make sure their beer doesn't taste of diacetyl, saying "diacetyl producing VDK" doesn't make any bleedin' sense. Diacetyl doesn't produce VDK, it is a VDK. It's like saying "ethanol producing alcohol" or even "brie producing cheese".

VDK stands for vicinal diketones. "Vicinal" means "neighbouring" and "diketone" says the molecules have two ketone groups. Diacetyl is more formally known as 2,3 butanedione. To break that down the 2 and 3 say which carbon atoms have the functional groups and carbons 2 and 3 are obviously neighbouring. The "but" bit of "butane" says the molecule has four carbon atoms and the "ane" says it is saturated with hydrogen, the "di" says there are two functional groups and the "one" at the end says they are ketones. So, from reading the name it is possible to see that the molecule has this structure:

CH3COCOCH3

Diacetyl is a particularly flavour active compound, i.e. it doesn't take much of it to have a strong taste. A touch of it brings something special to Timothy Taylor's Landlord and Pilsner Urquell, but too much of it, particularly in pilsners, is overpowering and an off flavour.

Aside from infection it gets into beer as it's made by yeast during fermentation. In the most active part of fermentation it's made faster than it's broken down so it accumulates. If given enough time (the diacetyl rest or warm conditioning) the yeast will break it down towards the end of fermentation as growth slows.

It is however, not the only VDK made during fermentation. 2,3 pentanedione is also made. A chemically similar compound, the structure of which I'm sure you can now work out. It is much less flavour active than diacetyl, but because of its chemical similarity some laboratory analyses can't separate the two so will give a result as "total VDK". And this is presumably where the confusion comes from, as diacetyl will be noticed as a flavour fault in beer, but the lab result may come back listing VDK.