Human beings have been using casks for over 2,500 years now, starting with primarily daily commodities such as salt, pickled fish and gunpowder, and increasingly now for maturing wines and spirits.

It is not precisely known though how the latter began. It could probably be the traders or men on ships bound for the colonies who found that storing the wines and spirits in these casks enhanced the character of the stored liquid.

In the beverage industry every single element – the base ingredients, process and material – contributes towards building the character of the beverage. In regard to aged spirits, such as whisky, it is observed that ~60% of the character is acquired by the spirit during maturation.

No wonder then that casks are given such high priority. Though there are ~600 species of oak, it is mainly European oak (Quercus robur) and American oak (Quercus alba) that are used for ageing.

Europe Vs. America

European oak is a much faster growing variety and leads to a wide grain. The wide grain contributes with a well-developed complexity and character for the aged spirit.

The tannins in European oak are higher, which lends astringency to the spirits during early stages; but over time lignin compounds break down slowly, releasing the spicy, toffee and candied fruit notes.

On the other hand, American oak is a slower growing species and has a tighter grain, which results in sweet and delicate flavour compounds.

American oak is high in lactones, delivering sweet notes such as vanilla, coconut and butterscotch. It is estimated that up to 97% of new casks used to mature Scotch whisky are currently made from American oak.

The type of oak, previous fill and the number of fills is considered while selecting the casks to age a new spirit. While the warehouse managers monitor the casks’ well-being and wonder about the share being discreetly sipped by angels, the master distillers await eagerly the days to pass to experience the results of their efforts and hand over the baton to master blenders to create their signature on the final blends.

Cask production

We need to wait almost 100 years before we can consider harvesting an oak tree. The timber is then sawn into smaller pieces, the defects sorted out, and the best quality wood quality is preserved for production of staves, which make up each cask.

Less than 50% of the timber ends up in staves. The staves are further air-dried for months, or dried in electric kilns to acquire the required moisture level and to allow the chemical components to build up.

Suitable staves are then assembled in a cooperage – a place where casks are built and repaired. The craftsmen who perform these tasks are called coopers.

At the cooperage, staves are assembled as per the cask size: 30–35 staves for an American Standard Barrel (ASB) and 40- 45 staves for a Hogshead.

The ends of the staves are bent by applying force using steam, clamps and machines. Iron hoops are placed later in order to lock them into a cask shape.

In order to activate the chemical components of oak the inner surface of wood, which gets in contact with the liquid, is exposed to heat. This is done either by toasting or charring. This makes the surface more porous, enabling the liquid to explore the inner areas of the wood.

Toasting & charring

Toasting is done at 100-200 degrees Celsius for 15-45 minutes, based on the requirement. Toasting is classified from light to heavy and activates vanillins (vanilla) and eugenols (spicy characteristics such as cloves and cinnamon).

Charring is done at 250 degrees Celsius for 15-45 seconds. The charring levels are classified from light to thick – the latter is also called the “alligator char”, where the inner surface cracks and look like the skin of an alligator.

Charring activates the lactones (heavy and sweet notes) and hemicellulose (brown sugar, caramel and toffee notes). Water is sprayed to halt the fire and in both the cases the charcoal formed due to the wood being exposed to heat will also help in absorbing the unpleasant sulphur compounds during ageing.

The cask is then fixed with permanent iron hoops at different levels; the head and bottom lid are fixed; a bung hole is drilled and checked for leaks before it is certified as ready for use.