Introduction to the Cask Maturation of Whisky

Scotch single malt whisky gets its unique taste from a multitude of production and maturation factors. The following list shows the four most important factors that influence the taste of the whisky.

Different casks in front of the Edradour distillery
The most important sizes of the casks

This article deals with the different cask types, their manufacturing and their rejuvenation after longer use. The photographs were taken at Speyside Cooperage, which rejuvenates used casks for the whisky industry in the heart of Speyside.

The main factory floor at the Speyside Cooperage
Speyside Cooperage - Overview

New make (raw whisky) is colourless and clear. Its aroma also hasn't much in common with what we know as whisky. After distillation the final process of maturation begins, which takes the most time compared to the production. In the UK and the rest of Europe, the legal minimum maturation time is three years. But these three years are of course just the beginning for a single malt whisky. The top products of the malt distilleries reach their prime only after 15 to 25 years. During the slow maturation over many years, the tangy aromas of the young whisky become mellower but at the same time more complex.

The processing of wood on the shop floor
Speyside Cooperage - Wood processing

1. Size of Casks

The casks used in the UK and the US are made from oak wood. Softwood casks are unsuited for maturation, since the wood contains resin, which prevents the cask from breathing. The casks must have a capacity of no more than 700 litres; otherwise the maturation would be too weak. There are three basic cask types:

  • The first one is the sherry butt, the port pipe or puncheon. All three have a capacity of 500-700 litres and are originally used for the maturation of fortified wines. The 700l port pipe is the largest, the 500l puncheon the smallest of the three. While the port pipe is long and slim, the puncheon is shorter and bulkier than the sherry butt. All three are made from European as well as from American oak. The cask sizes are not fixed but rather approximate figures.
  • The second cask type is the bourbon barrel. Bourbon matures in charred American oak casks (American Standard Barrel - ASB). They hold approximately 55 American gallons or 200-210 litres (1 Gal- = 3.78l). Read about the bourbon barrel production at here.
  • The third cask type is the hogshead. There are two sub-types: The sherry hogshead made from old sherry casks, and the ASB-turned-hogshead. The latter is built by reassembling an ASB and adding more staves and new hoops to increase the capacity to 250 litres. Out of four ASBs you can make three hogsheads.
The assembly of the casks at the Speyside cooperage
Speyside Cooperage - Assembly overview

2. Effect on the Spirit

During maturation chemical reactions take place between the alcohol, the aromatic substances and the wood. These reactions increase the amount of esters and aldehydes in the whisky. Beside the sherry or bourbon remainders in the barrel wall, the alcohol also extracts tannins, vanillin and caramel from the wood. The contact of the whisky with the wood of the cask wall is therefore essential. Due to its larger volume (250l), only 75% of the cask wall of a hogshead has contact with the whisky compared to a smaller barrel (158l). With a sherry butt (500l) it is even only 50% of the surface. Distilleries with a high output usually use smaller casks, since the whisky matures faster in these cask types.

The Assembly machine at the Speyside cooperage
Speyside Cooperage - Assembly machine

3. Rejuvenation of the Casks

After a cask has been used for maturation several times, its maturation abilities decrease, since the wood is exhausted. However, to a limited extent these casks can be rejuvenated by removing the old inner charcoal layer and charring the inside again. This procedure releases the vanillin and caramel taste in the wood again. In the past a sweet, boiled liquid made from grapes (Paxarette) was pressed with high pressure into the cask walls in order to give the cask a sherry character again. However, this method has been discredited and hasn't been used for quite some time now.

Translated to the price of the whisky that matured in a bourbon barrel, the barrel amounts to costs of 5 cents per 0.7l bottle of 10-year-old single malt whisky (if the barrel was used three times).

The finial casks at the Speyside cooperage
Speyside Cooperage - Final products

4. Angel's Share

During maturation, the alcohol/water mixture of the whisky warms up and expands in the warm summer months, while in the cold winter months it contracts again, thereby "breathing" the surrounding air through the pores of the oak wood. This becomes especially apparent on Islay, where the sea air affects the taste of the whisky. Usually the whisky loses between 0.5% and 1% of its alcoholic strength per year during maturation. The Scots call this the Angels' Share, since it evaporates irretrievably into the sky. But beside the alcohol also a portion of the water in the whisky evaporates. The loss of liquid is therefore significantly higher than the loss of alcohol (up to 2% per year). Since alcohol has a higher vapour pressure than water at the same temperature, (almost) always more alcohol evaporates than water, and the alcohol content in the cask decreases.

But there are rare exceptions from this rule. Maturation in a dry and hot environment leads to an intensified evaporation of the water content of the whisky. Close to the roof of new, tall warehouses it can get so hot that the alcohol content of the whisky rises to 66%-67% during maturation. The rise of the alcohol content leads to different reactions with the wood and the production of different flavours. Most Scottish distilleries fill their whisky into the cask with an alcohol content of 63.5%, since that's a reasonable compromise between short maturation time and alcohol evaporation.

The testing of the finished cask with air pressure
Speyside Cooperage - Checking the casks