What defines a dunnage warehouse?

The realities of cask maturation

At first glance, warehousing may not seem like the most interesting aspect of a whisky’s creation. There is generally little activity involved in whisky maturation, yet it is crucial to the final product and often features heavily in both product branding and distillery tours. How many times have you heard references being made to the unique local climate, the brisk sea air or the cool damp earth where the casks rest in ancient buildings right beside the stillhouse? Comparatively, was any mention made of the huge number of casks stored at other sites, in completely different surroundings? The degree of influence that these factors can have on the quality of a whisky has been the subject of much debate and research over the years.

Historically, whisky was mostly matured on-site in what is known as a dunnage warehouse. Fairly simple in design, this involves an earth floor over which casks are racked in long rows on wooden rails, usually just two or three high depending on their size. Such warehouses had thick stone walls, shuttered windows, little in the way of ventilation and a heavy padlock on the door to keep out uninvited guests. The lack of climate control wasn’t too much of an issue, as for all the rain it gets Scotland is a fairly temperate climate and the internal temperature of the warehouse wouldn't actually fluctuate too much throughout the year.

Immediately you can imagine the difficulties in dealing with such a system with regards to managing inventory, especially after years of innovation and technological advances have resulted in distilleries producing millions of litres per year. Gone are the days when distilleries could keep the majority of their stock maturing on distillery premises, especially when so many distilleries are a part of global operations run by the likes of Diageo and Pernod Ricard, contributing to blends that are distributed internationally. Over the course of many decades there is a lot of effort involved in maintaining records of each cask’s contents and ensuring they can all be accessed with a minimum of complication when it is their time to be bottled. As a result, we now see an increasing number of modern warehouses featuring a more sophisticated metal racking system that can allow a dozen rows of casks to be stored one atop the other, with specialised lifting equipment making lighter (and faster) work of moving the whisky. Diageo, for example, store millions of maturing casks in a central facility near Stirling, where rows of racked warehouses contain everything from decades-old rare single malts to huge volumes of young grain whisky, brought from distilleries across the country.

Tradition in an age of modernisation

Dunnage warehouses, in comparison, are not the most efficient means of maturing whisky. While it is certainly possible for casks to be stored on multiple floors, this introduces further complications when it is time to remove casks for bottling and replace them with newly filled casks. Racked warehouses can hold a far greater number of casks on the same surface area, and in recent years when faced with the need to construct new warehouses this has been the more common choice.

Taking this efficiency a step further, many casks are now kept in a palletised warehouse, which as the name suggests involves casks being stored on wooden pallets rather than rolled onto racks. The issue here is that the casks must be kept upright on their ends rather than resting on their sides, but in doing so the whole pallet can be transported without ever having to remove individual casks, brought to specialised equipment to be emptied and re-filled. This makes cask handling much faster as well as allowing for even greater numbers to be packed into each warehouse. So far it is primarily grain whisky, with its generally faster turnaround in terms of maturation, that has experienced the move over to this format.

There is of course an argument to be made that dunnage warehouses remain the best option in terms of product quality. Their stable environmental conditions produce reliable, repeatable results, as does the traditional method of keeping the casks on their side. Keeping casks upright alters the surface area to volume ratio for the maturing spirit, changing the process of maturation and as well as evaporation losses. Similarly, racking casks twelve-high can result in those at the top experiencing a slightly different climate to those at ground level, which though minimal can have an impact over many years of maturation.

The costs of compromise

Humidity and temperature are both important considerations, not only because the microclimate of the warehouse will play a role in the chemical reactions taking place between the spirit, wood and air over the years but also in the infamous evaporation of spirit over time, often referred to as the angels’ share. Towards the ceiling, higher temperatures may well effect a greater overall loss, but it is also worth noting that because the ethanol and water in the cask have different volatility, humidity will change their relative rates of evaporation. This is why warehouses in hot, dry countries can see their whisky rapidly becoming stronger as it matures, as opposed to the gradual drop in %abv experienced in Scotland.

Dunnage warehouses minimise this loss and encourage a more gradual maturation, at least as we find them in Scotland, thanks to the relatively cool temperatures and humid conditions at ground level encouraged by natural earth floors. Modern palletised and racked warehouses, on the other hand, can experience more rapid maturation, inconsistent microclimates and variable losses throughout the storage space. Some of the larger brands such as MacAllan are investing significant resources in experimenting with sophisticated new warehouses with the means to control these conditions more effectively without sacrificing storage capacity, at a cost that will prove prohibitive for many distilleries.

As always, the word of the day is compromise. The degree to which all of these factors actually translate into ‘quality’ of spirit is very hard to say, though there have been increasing efforts in scientific investigation which have identified some patterns, such as the changes in temperature and humidity throughout a warehouse altering the prevalence of certain characteristics in the spirit, like perceived grassiness, etc. Of course, with so many elements factoring into the product at this stage that it is very difficult to quantify such contributions, particularly when the result is subject to such, well, subjectivity. There comes a point when brand owners must consider the significant savings that can be made in storing ten thousand casks per warehouse instead of one thousand, which in turn will result in more affordable bottles for consumers.

The fundamental question is one of product. In crafting a blend that will be sold worldwide in vast quantities, the realities of working in such volumes dictate the need for a more efficient solution that will no doubt see any inconsistencies ironed out amidst the vast scale of production. If, on the other hand, you're considering that lone Dalmore ex-sherry puncheon that hasn't been touched for thirty years? Best hope it's spent that time hidden away in a cool, calm dunnage, quietly settling down into a dram quite unlike any other.