Whisky Myths Debunked #8 – Diageo’s Cling Film Casks

by Oliver Klimek on June 17, 2012

In 2008 there was a slightly disturbing news about Diageo trying to use cling film wrapped around whisky casks to stop evaporation. Not very surprisigly this immediately prompted many not-so-friendly responses, but after the storm had calmed down there were no new reports about further developments.

Meeting high level Diageo representants at the Malt Manaics 15th anniversary reunion in Scotland made it possible to shed some light on this issue as the topic of the angels’ share popped up a few times during our visit.

At our tour of Dalwhinnie we were told that by optimizing casks and warehouse conditions Diageo had managed to lower the overall angels’ share, but it would not be advisable to let it fall below 1%. Then the spirit would suffer from maturation problems because of the lack of interaction with air.

In a discussion the next day I specifically addressed the cling film issue and got the response that this actually was an experiment to find out how much whisky gets trapped in the cask wood during maturation by removing evaporation as a factor of spirit loss. Well, the 2008 report certainly reads different, but I have to admit that I did not find an original statement by Diageo but only articles reporting about this.

So beating evaporation either was wishful thinking on Diageo’s part based on the experiment, or it actually was their original intention but they discovered that this would create quality problems, so they tried to make the best of it by using it for the purpose they stated now.

Anyway, I am positively sure that we will not see shrink-wrapped casks for the maturation of Scotch in the future.

Anorak Appendix

The faint of heart may wish to skip this, this is going deep into anorak territory.

I am not sure if the cling film experiment will give you a correct answer to the question how much whisky is trapped inside the cask wood after maturation. Wrapping cling film around a cask changes the boundary conditions for the forces governing evaporation and soaking wood.

Evaporation of whisky in a cask essentially means that there is a dynamic equilibrum between molecules moving outside the cask and molecules from the liquid taking their position. Sounds easy but in fact this is a very complex thermodynamical problem.

Neglecting the minor components of whisky we have two substances with different physical properties (water and ethanol), two states of matter (liquid and vapour) and two driving forces behind the movement of molecules (capillary motion pulling the liquid into the dry wood pores and diffusion governed by differences of concentration between cask interior, wood pores and outside). To make it even more complicated, there is a continously sinking filling level affecting the balance of vapour and liquid and the wood structure changing with increased soaking because of cellulose fibres absorbing water and swelling. Calculating spirit loss over time would most probably require solving an extremely nasty differential equation, and even then there would be uncertainties because the exact stucture of the wood is next to impossible to integrate into the calculation.

But even if we can’t calculate it exactly, on a molecular basis it is just water and ethanol molecules finding their way out of the cask driven only by natural molecular movement. Assuming the normal case where the cask wood is not entirely soaked, evaporation in the lower part of the cask filled with whisky happens in the outer areas of the soaked inner layer of the wood, in the empty cask top it takes place on the surface of the liquid. In both cases the molecules have to cross the dry pores of the wood to reach the outer surface of the cask.

Molecules are blind and brainless. When they evaporate they don’t know if there is any cling film wrapped around the cask, they just start moving. When they hit an obstacle (i.e. another molecule) they either stick to it, bump off or make a chemical reaction depending on their kinetic energy and the inter-molecular forces. Water and ethanol molecules can’t react with cling film, they stick or bounce. Some may find their way back where they came from, but some will also be absorbed by the wood. Remember that those molecules would have been lost without cling film. So over time, a cask wrapped in cling film will accumulate more liquid than a cask without cling film. The question is just how big the difference is.

How about just weighing a freshly emptied cask, then heating it gently to let all whisky evaporate from the wood pores without changing the wood by toasting and then weighing the cask again? This should give you a reasonably accurate measurement of how much whisky gets trapped inside during maturation.

A Little Note To Diageo

I am happy to do your job here to set things straight. It tickles my scientific fancy and this probably made me think more about what is going on in a cask than if that story had not come up.

But next time, it’s your turn. When people keep misunderstanding me, it may be that they are malevolent. But I may also have a communication problem. Or it may be a vicious circle of both.

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