Islay whisky making

Distilling on Islay Ageing

The ageing process begins when spirit is filled into a cask.

Casks are traditionally stored in dunnage-style warehouses – earthen-floored buildings with thick walls that keep a relatively constant temperature throughout the year – placed on their sides and stacked no more than three high. While there are still many dunnage warehouses in use, most whisky is now stored in racked – similar to dunnage but in tall racks – and palletised warehouses – all casks stored on their ends on stacked pallets. Each style of warehouse has pros and cons, with ease of accessibility and efficient use of space balanced against air circulation and character of maturation. As such, most distillers use a combination of warehousing.

As the temperature in the warehouse changes, pores in the wood expand and contract, and spirit will flow in and out of the staves. As it moves in and out of the oak, it will both extract compounds and leave others behind. Some casks are also charred on the inside, giving a layer of carbon that further acts as a filter, extracting future compounds from the maturing spirit. If the cask has been filled before, there will also be residue from the previous occupant, which will interact with the spirit.

Wood is not the only influence during maturation, and flavour compounds will be created from reactions within the spirit, and between the spirit and the air. The spirit will also evaporate and as oak is porous, the vapour will escape the cask. This lost spirit is referred to as ‘the angels’ share’ in Scotland, and gives warehouses a rarefied and distinctive atmosphere. Due to this evaporation, over time the level of liquid in a cask will drop, further changing the flavour as water, alcohol and other compounds change in concentration.

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Islay sits in the Gulf Stream, with high humidity and a relatively constant temperature all year round, while many of the warehouses on Islay also lie on the shoreline and are traditional dunnage warehouses. This all lends itself to slower ageing, as the expansion and contraction of the wood is theoretically reduced.

While much whisky is matured on site at the distillery where it is made, the vast majority of spirit in Scotland is tankered away, filled into casks and stored in large warehouse facilities on the mainland. Only a couple of distilleries on Islay mature the majority of their spirit on the island. While the environment in which a cask is stored makes a difference to the character of the final whisky, arguments still rage as to the actual impact. This is hotly debated on Islay, where the whisky often shows coastal characteristics and the whisky that is matured on the island is stored close to the sea. 

Once the spirit has spent at least three years and a day in cask, it is legally whisky and can be bottled. However, development of flavour and character doesn’t stop, and most casks destined for single malt whisky spend many more years maturing. Through regular sampling, casks are selected when their flavour meets the need of the owner, at which point they are bottled and released into the world.

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