How influential this is depends on various factors, including the volume and rate of air flow, as well as the temperature and humidity levels. These are, in turn, determined by the individual design of a warehouse, as well as the location and degree of exposure to the climate.
An initial division to note is between the traditional dunnage compared to a modern racked warehouse. A dunnage is a low building with thick stone or brick walls and a slate roof, where casks are stacked two or three high. Racked warehouses are larger-scale brick or metal-clad constructions, with casks typically stacked eight to 12 high.
Air enters racked warehouses through vents at ground level, then ascends and exits through vents in the roof. The standard choice used to be vents with adjustable louvres, while more recently it is a fixed mesh.
“Lower level vents are typically a rectangular mesh, 50cm long by 20cm tall, fitted at 5m intervals. There are fewer vents at the top of a warehouse, but they are larger, a 50cm by 50cm panel, designed so that air can pass through, but not rain,” says David Simpson, director at Caledonia Casks.
Windows are absent from racked warehouses, but are a principal entry point for air into dunnage warehouses. This can mean one window at each end, though numbers and dimensions are hardly standard, even at the same site.
“The number and position of windows in a dunnage can be a case of aesthetics as much as air flow, with windows on either side of a door, for example, to give symmetry,” says David.
Bruichladdich’s Port Charlotte dunnage warehouses have permanently open windows fitted with a grill, while the original late 19th-century wooden shutters can be closed, but remain open.
But even closed windows can provide air flow, as some are fitted with slatted sections. It’s not all about windows either; many dunnage have vents.
Stuart Urquhart, operations director at Gordon & MacPhail, says, “Every one of Gordon & MacPhail’s dunnage warehouses has vents, either a grill or mesh. The ground level vents are 10cm off the ground, with corresponding vents in the roofs.”
Entrance doors also provide opportunities for air flow, whether single or double doors of varying dimensions, while large roller shutters are exclusive to racked warehouses. In addition to size, the frequency and duration of doors being open matters, when moving casks or taking samples.
“We do a lot of vatting in warehouse No. 12 so the door can be open for several hours a day, and is influenced far more by air flow than our other warehouses where we open doors then close them behind us,” says Adam Hannett, head distiller at Bruichladdich.
The rate of air flow varies depending on the climate, whether windy or still, while a rainy or dry day affects humidity levels. Beyond climatic fluctuations, location also plays a role.
“How exposed a warehouse is to the elements plays a part. For example, Glendronach’s small dunnage warehouses are nestled in the valley, between other distillery buildings. The larger racked warehouses, however, are more exposed, and with greater height as well as cask movements, temperature fluctuations can be greater,” says Rachel Barrie, master blender at Glendronach, The BenRiach and Glenglassaugh.
“Traditional dunnage warehouses tend to maintain a fairly steady 5–10°C all year. Racked warehousing, however, shows greater temperature fluctuations – typically 5–15°C – with higher temperatures at the top.”
With Glendronach 17km from the sea, and Glenglassaugh right on the coast, atmospheric differences influence the maturation.
“Racked warehousing at Glenglassaugh is further from the sea than the dunnage, where air is likely to be more saturated. The coastal air at Glenglassaugh is laden with sea spray, which can in turn influence the complexity of the spirit,” says Rachel.
Tracking air movement through warehouses would be fascinating – perhaps infrared cameras could reveal the routes.
A vital function for incoming air is to disperse ‘saturated air’ i.e. saturated with alcohol vapours which have evaporated from casks. Alcohol molecules are heavier than air, which means that after exiting a cask they have a downward trajectory.
“We’ve measured the concentration of alcohol vapours at various heights. It’s essentially from the ground up to a metre, with a very low level at three and six metres,” says Michael Henry, master blender at Loch Lomond Group.
Seasonal consideration is that higher temperatures in summer promote a greater rate of evaporation, and so the concentration of alcohol vapours in the air is greater than during the winter.
Rachel explains, “A dunnage warehouse with less air flow and cask movement means saturated air is likely to be replaced more slowly. Saturated air appears to develop greater complexity from the oak. Dunnage warehouses are ideal for the slow, complex reactions possible during long maturation in sherry casks, even for 60 years or more.”
The temperature of air flowing into warehouses is another factor, with Scotland’s ambient temperature averaging 2–8°C in winter, and 14–20°C in summer.
As the temperature rises in the spring, warmer air begins to enter the warehouses, which still contain colder ‘winter air’. Colder air is more dense and has more ‘weight’ than warmer air, which means that warmer air is unable to disperse colder air. Consequently, warmer air travels 1–2m into the warehouse, and starts to rise.
In autumn colder air begins to enter the warehouses, which still contain warmer ‘summer air’. As colder, denser air has more weight and momentum, it is able to disperse warmer air which consequently rises. Colder air also has the potential to travel deeper into the warehouse than warmer air.
This means that casks closer to vents, windows and doors experience greater air flow, and so a broader range of temperatures than casks positioned more centrally.
One consideration is that the rate of reactions taking place in a cask accelerates as the temperature rises, and slows as it decreases.
Some air also enters and leaves casks as part of an annual ‘cycle’. From winter to summer the temperature rises, causing liquid to physically expand within the cask. The headspace (‘empty space’ in the cask above the surface of the liquid) also expands through pores and joins in the cask, pushing air within the headspace out from the cask. As the temperature drops the liquid and headspace contract, drawing fresh air into the cask.
The vital element within air is oxygen, which dissolves in the spirit and instigates oxidation. This complex, not fully understood series of reactions modifies flavour compounds and creates new notes, including fruitiness.
Air flow also influences evaporation rates, which average two per cent of the cask’s contents per annum.
“Casks that experience greater air flow have higher evaporation rates, resulting in the volume of liquid decreasing and the headspace increasing more rapidly. This in turn increases the ratio of wood to liquid,” says Brendan McCarron, Glenmorangie’s head of maturing whisky stocks.
Although air flow has such a comprehensive influence, it is rarely discussed. But that’s changing.
Gregg Glass, whisky maker and blender at Whyte & Mackay, says, “I go through all the warehouses at Fettercairn once or twice a month, assessing different temperature ranges, humidity levels and cask characteristics to build up a profile for each warehouse. Whisky lovers have a great desire for more connection, and increasingly that’s down to the influence of individual warehouses.”