Roll out the barrel

The first account of storing alcohol in barrels comes from the 5^th Century BC courtesy of the Greek historian Herodotus who wrote about the use of palm-wood in the shipping of Armenian wine over to Mesopotamia. However, it seems likely that the move from clay amphorae to what we would understand as a watertight wooden container probably occurred several hundred years before that. Not only was wood a stronger material than brittle fired clay, it could also be formed into barrel-like shapes which allowed it to be more readily moved and stored.

The expansion of the Roman empire resulted in the meeting (and conquering) of many cultures across Europe. During this expansion, the Romans discovered many technologies they did not possess – the crafting of wooden barrels for storing liquids was one of these. The Celts are recognised as inventors of the wooden barrel, but it was through the Gaul’s that Roman culture would adopt them. Carrying amphorae large distances over land was simply not practical and as the empire expanded, the Romans planted new vineyards for the locally garrisoned soldiers along their route. Once they reached Gaul, they discovered that the wine from the region was of a particular high quality, and they started to import it back home (but not before a period of ripping up vineyards to try to protect their domestic producers!). This wine made its way back to Rome in the wooden barrels which the Gaul’s had crafted to store and transport their beer in. By the middle-ages, wooden barrels has become Europe’s main storage and shipping containers – for both liquid and dry goods. The huge growth in both the need to store goods as well as to export them led to the Worshipful Company of Coopers being founded and receiving its Royal Charter in 1501.

We’ve already highlighted in our article about oak that the interaction between liquid and wood results in some 200 different substances being extracted or transformed due to the contact between the spirit and the cask itself. Having looked at the wood which is used to make casks, it’s now time to look at how the size and type of cask influences maturation.


By Gerard Prins - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=14985572

Whilst the length of time that whisky is left in the cask will influence its flavour, so too will the cask itself. Not all casks are as 'active' as each other. A first-fill cask will impart more oak-driven flavours than a cask which has been used two, three or four times. Likewise, the smaller the cask, the more liquid is in contact with the wood inside. A higher surface area to liquid ratio = more interaction of the spirit with the wood, therefore increased maturation characteristics.

A wide variety of casks are used in the modern day whisky industry. The most common is the ex-bourbon cask. Under the laws laid down for creating bourbon in the United States, casks filled with fresh distillate must be brand new oak and can only be used just the once. Therefore, the bourbon industry is constantly both producing casks and then possessing casks that can’t legally be used again for the production of bourbon. This potentially would be a problem, were it not for a lovely symbiotic relationship in that whisky producers in other countries actively desire casks which have been ‘pre-seasoned’ with additional flavours. Likewise, taking casks which have held so-called predecessor liquids also ensures that some of the more astringent and tannic wood elements have already been removed before reusing the cask again. The used bourbon casks are shipped (often dismantled to be reassembled by a local cooper) over from the USA and then refilled with new distillate. 
 

 
Not only does the use of second-hand casks help remove some of the most bitter elements from the wood, it also, allows the new distillate to take on interestingly flavours and characteristics from whatever predecessor liquid was held prior. Oloroso sherry – a fortified wine which originated in the south west of Spain is commonly used by the whisky industry. The sherry is matured in an oak cask like whisky itself with ages varying from ‘no age’ through to around 30 years for a particular matured example. Oloroso is a dry sherry with dark nutty flavours that often tastes of raisins, figs, warm spices and sometimes furniture polish. When whisky is matured in a cask which has previously held Oloroso it will take on these flavours over time, adding to its depth and complexity. The whisky industry uses a wide variety of sherries, some sweeter flor-type sherries, some darker, some with increased bitterness. It also uses these casks for short term maturation periods – finishes – where the whisky is re-racked into a fresh cask for an additional 'punch' of maturation after a period of time in previous (often ex-bourbon) cask. This allows the whisky to take on some of the sherry flavours, but it does not produce as intensive colour and flavourings as if the distillate was filled into a totally fresh sherry cask for it's entire life.

A wide variety of casks are presently used for both full-term maturation and finishing, each changing both the colour and flavour characteristics of the whisky over time. Here is a quick run down of some cask types and what influences on flavour you might expect:
 

Casks are stored either in traditional ‘dunnage’ warehouses, where they’re stacked three high, or, in some cases on modern pallets. All warehouses will be cool and to some degree a touch damp to allow the air to circulate. This is particularly important as wooden casks are porous, this allows the spirit to evaporate which is essential for the maturation process. This evaporation is called the ‘Angel’s Share’ – it is usually around 2% each year for Scottish distilleries, but in hotter climbs can easily be as high as 10%+.  When filling a cask, the porosity of the wood also takes around 3% of the distillate in initial absorption. This is known as ‘in-drink’ or the ‘Devil’s Cut’.