Whisky Magazine Issue 119
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We investigate the difference between these two methods of aging
Malt whisky develops up to 70% of its character during the aging process, which takes place in different types of aging warehouse. The traditional type is dunnage, with racked warehouses originating in the 1950s – 60s. Dunnage certainly looks very different to racked warehouses. But with most distilleries using both, the question is whether there are differences in the environment they provide, the key factors being temperature, humidity levels and air circulation.
Dunnage warehouses are low-rise with stone or brick walls, earthen floors and a slate roof, with casks stacked on top of each other a maximum of three high (any more and the weight bearing down would be excessive). Racked warehouses are more high-rise, using brick, cement blocks or a steel clad structure, with concrete floors and tin roofs. Casks are stacked up to 8 –12 high on tall racks fitted with steel rails.
The thinner walls and tin roofs of a racked warehouse transmit temperature changes more readily than the thicker walls and slate roofs of a dunnage. Additionally, racked warehouses can have a larger range of temperatures between the floor and roof (depending on the height). Nevertheless, average annual temperatures are similar in both types of warehouse.
The significance of temperature is that it prompts a vital part of the aging process known as a ‘cycle.' When the temperature rises in spring and summer the spirit expands within the cask and penetrates into the oak staves, which contain vario...