Of course, the reality of it – certainly in today’s commercial industry – is that whisky production is more than just sticking the spirit into oak casks for a few years and waiting until it’s ready. Today’s producers forge and shape the final character and flavour of their whisky by deliberately and strategically manipulating several different parameters of maturation. These parameters include:
Time – that is, how long the spirit spends in the wood. We’ll look at this in more detail in a moment.
Oak species – for example, American oak, Spanish oak (aka European oak), French oak, and so on. The different oaks have different porosity, permeability, cellular structure, wood sugars, and chemical make-ups that all impact how the spirit, wood, and oxygen interact with one another to shape the development and character of the resulting whisky.
Freshness of the cask – that is, is the cask being used for the first time, or is it a refill cask that’s previously already matured whisky for a stint? (The terminology will be 1st fill, 2nd fill, 3rd fill, etc, or simply just refill). Clearly, a 1st fill cask will have a faster and more pronounced influence on the spirit than a 3rd fill cask. However, this needs to be carefully monitored – we don’t want the wood influence to dominate or overpower the spirit to the point that it’s too dry and woody.
Cask provenance and history – the vast majority of casks used for maturing whisky (particularly Scotch whisky) will have been seasoned by maturing some other liquid previously, e.g. bourbon, sherry, etc. Obviously, the cask’s previous filling soaks into the wood, changing its nature and also absorbing the more “up front” woody flavours. The previous filling might also leave behind a residual flavour footprint. Hence, a cask that previously held bourbon will impart very different flavours to the spirit than a cask that previously held, say, chardonnay or port. The use of new “virgin oak” was traditionally shunned in the Scotch industry but, more recently, is finding favour when used as a finishing cask, or for short maturation periods with newer distilleries looking to bring youthful spirit to market sooner.
Cask size – the influence of the wood is obviously a function of how much of it there is, relative to the amount of spirit. In the case of smaller casks, the ratio of the contact surface area of wood relative to the volume of spirit contained is larger, and so smaller casks will influence the spirit at a relatively faster rate. Larger casks such as 500 litre sherry butts might therefore need longer time periods to mature their whisky than, say, a 250 litre sherry hogshead.
Finishing the whisky in a different cask to the one it was originally filled into – also sometimes referred to as “ace-ing”, the classic example is whisky that might spend, say, 8-10 years in an ex-bourbon cask, then get finished in an ex-wine cask for 6-12 months. The finishing cask will add a new nuance or texture to the whisky, ideally adding also to its complexity. A variation on finishing is wholesale re-racking – a not uncommon practice whereby whisky sitting in tired, old, casks is rejuvenated by transferring it into new casks to freshen and liven up the spirit. Both Bruichladdich and Benriach were keen exponents of this when those brands were re-activated in the 2000’s with stock that had previously been filled into poor quality or tired casks; Tamnavulin undertook a similar program in 2017 with stock that was originally ear-marked for blending but instead re-purposed for bottling as a single malt, and thus needed to be “polished up” by re-racking into 1st fill ex-bourbon casks.