Got Wood?

In parts 1, 2, & 3 of our blog series we looked at the history of distilling, the shapes of stills, and the importance of copper, but today we’ll be shifting focus to look at the humble oak barrel and its significance in the production of whisky.

Barrels are a curious invention.  When empty they can be bounced upright.  When filled with 200kg-250kg of spirit they can be stacked a dozen high, and individually they can be easily manoeuvred by hand, by a single person (unless you come upon some stairs).  All the while facilitating the rich microcosm of reactions and interactions that turn harsh new make spirit into delicious whisky, rum, or brandy.  We’ll get into why alcohol is aged almost exclusively in oak barrels as opposed to any other kind of wood a bit later on but for now, we’ll simply say oak + alcohol = good.

Barrels are kind of a big deal

The process of crafting a barrel suitable for ageing spirit or ‘coopering’ is not as simple as slapping some posts together.  Wet coopering – crafting barrels with such high precision that they are watertight has been a highly respected profession for centuries and one that was once an essential component of global trade and commerce.  Everything from the way the tree is split into planks to how long the planks are left to dry before being crafted into a barrel can have a significant impact on both the flavour and quality of the spirit that is aged within (as well as how much of it leaks out during ageing).

In fact, a lack of properly seasoned oak is commonly regarded as one of the main factors in the defeat of the Spanish Armada by the British in 1588.  After capturing a fleet of galleons carrying some  1700 tons of barrel staves destined for Spain, Sir Francis Drake set fire to them, forcing the Spanish Armada to set sail for England with their victuals stored in barrels made from unseasoned, green oak.  The overstressed green barrels promptly split and the provisions stored within either went rotten or simply leaked away, leaving most of the Spanish sailors malnourished or otherwise glued to the latrine.

Porque no los dos?

So we’ve established that barrels are pretty important but why oak? Why not pine? Or red gum?  To answer this we need to look a little more closely at the ageing process specifically, evaporation, extraction, and micro-oxygenation.

The staves of a barrel need to act like a semi-permeable membrane, balancing permeability and porosity to allow air in while minimising the amount of spirit and water that gets out.  The dryer the outside air, the more water that is drawn out, as the surrounding air becomes more humid more alcohol is evaporated.  This is the infamous angels share and although its effects make distillery accountants very sad, the process is a necessary evil.

While sitting in the cask, new make spirit can make a very effective solvent, leaching the woods own naturally occurring flavour compounds including lignin, vanillin, & tannins into the spirit, as well as whatever other flavours may have been left behind from the casks’ previous use.  This process is a relatively quick one and balancing tannins with the body of the spirit is essential for the production of palatable whisky.

The most important factor, however, is micro-oxygenation, a process that takes years and allows the development of flavourful aldehydes, fatty acids, and eventually fruity esters, transforming youthful new make spirit into smoother more mellow and flavorful whisky.

Whereas other woods may have the right amount of porosity or the right balance of tannins, or sweet vanillin’s and other sugars to add to the spirit, only oak has the trifecta.  Not too many tannins, good flavours, and a tight grain structure that’s just permeable enough to let air in while minimising the leakage of spirit out.