Oak is the most commonly used wood for the maturation of whisky. Its importance to the distiller is often greatly underestimated.

Casks coopered from this wood have the softest tannins least likely to destroy its contents with bitter flavours.

The forest-dwelling Celts were the pioneers of the craft, the Greeks and Romans using the less versatile clay amphora. Yet the term ‘coopering’ is derived from the latin ‘cupa’ for a wooden vessel (from which we get cup), and is a great skill. ‘Wet’ coopers, those capable of making water retentive containers, were the top of their trade, by the time of Pepys a cooper enjoyed significant social standing.

After barley and yeast, oak is the most important component in the making of great whisky, for the cask is not merely a simple container it is also a complex flavour improver. How it does this is important to understanding the relevance of cask selection and wood quality.

There are three profound processes occurring in a maturing whisky cask that influence flavour: evaporation, extraction and micro-oxygenation.

The staves, varying in thickness between 18 to 23mm, act as a Gore-tex-like membrane allowing air and spirit through while retaining the liquid. Smaller alcohol and water molecules pass through the larger oak pores into the surrounding atmosphere via a concentration gradient: the drier the air, more water molecules; the more humid the air, more alcohol. This is the infamous Angel’s Share; depending where you are on the globe it might be alcohol, it might only be water.

Oak’s own naturally occurring flavour compounds – lignin, vanillin and tannin – are leached out of the oak by alcohol. The higher the strength the more readily the compounds are extracted from the wood into the spirit. This happens relatively quickly, in a matter of months, and is the same principle for perfume production. This set of oak-derived basic flavour compounds adds to the spirit’s own complex flavours already present.

However, qualitatively, the most important action of all is the gradual micro-oxygenation of the spirit over years – the actual process of maturation. Much like a venerable wine, microscopic volumes of surrounding air enter through the oak’s pores, as through a bottle’s cork, transforming the existing, youthful, new spirit flavour compounds in to the smoother, mellower flavours associated with age.

Good quality, fresh wood is vitally important to the successful maturation of single malt whisky.

WATERFORD DISTILLERY OAK