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Cool and Collected
With the cask contributing up to 70 per cent of a malt’s flavour, oak management has become an essential element of every distillery manifesto. But it’s not just the provenance of the oak that matters, as the ageing warehouse in which casks reside also contributes to the quality and character of the contents.
Three types of aging warehouse are used in Scotland, dunnage (also known as ‘traditional’); racked (or ‘commercial’; and palletised, with distilleries typically utilising at least two of these three options. The influence which ageing warehouses can have on maturation has been monitored more closely since the 1980s, when the neccesary instrumentation was developed.
Each type of warehouse provides a similar (but variable) environment for a varied inventory, that always spans a range of ages.
This practicality prevents the loss of an entire year’s production in the event of a fire or other calamity.
Dunnage warehouses are widely hailed as the ultimate accommodation for single malts. The most traditional type of warehouse, being low-rise with thick brick or stone walls and a slate roof, it is certainly the most aesthetic.
But dunnage warehouses also provide the best air circulation, helping to keep temperatures more stable, while earthern floors help release moisture which promotes humidity.
Barrels and/or casks are usually stacked a maximum of three high on top of each other. Anymore and the weight bearing down on underlying barrels would become excessive.
Racked warehouses, a concept dating from the 1950s, are constructed on a far larger scale than dunnage, using brick, cement blocks or a steel clad structure.
Barrels sit on tall racks fitted with galvanised steel rails, stacked up to 8 or even 12 barrels high.
Some distillers claim dunnage warehouses yield superior malts compared to racked, though if they always roll their best casks into dunnage warehouses this could be a self-fulfilling belief.
Comparing the two types of warehouse actually involves various considerations.
Running costs are an initial factor, with dunnage warehouses more expensive to operate than racked, not simply because of the usual three-barrels-high limit.
Moving casks in a dunnage warehouse is essentially a hands-on task, whereas fork lift trucks can easily negotiate racked warehouses. This means a job requiring an hour in a racked warehouse could take three to four hours in a dunnage.
Access to the warehouse also raises the question of temperature.
Racked warehouses have larger doors, typically left open for the duration of the job in question, creating temperature spikes. In contrast, dunnage warehouses have far smaller doors, that also tend to remain open for much shorter periods.
Moreover, thin walls and tin roofs do of course transmit varying temperatures more readily than thick stone walls and a slate roof, and while temperature fluctuations can be daily in a racked warehouse they tend to be seasonal in a dunnage.
Nevertheless, average annual temperatures are similar within both types of warehouse.
Barrels at the bottom of a racked warehouse actually experience similar conditions to a dunnage, though ascending the racks shows a graduation of micro-climates, with the greatest temperature differentials at the top of the warehouse.
Exactly how this affects maturation (as opposed to the influence of individual casks) is difficult to determine. However, some believe that greater temperature differentials can affect the character, rather than the quality, of the whisky.
The extent to which weather conditions influence the interior of the warehouse is another consideration.
As the volume of whisky within a warehouse acts as an ‘insulator,’ it takes a vast temperature differential outside the warehouse to affect the interior.
In Speyside, for example, the external temperature can vary from minus 25C to 25C throughout the year.
However, this sees a fluctuation of between 4-10C within a warehouse.
As the temperature rises, the rate of evaporation is likely to follow suit, which is typically 2-2.5 per cent of a cask’s total contents per annum.
As this includes alcohol and water, there is a simultaneous decline in alcoholic strength and volume.
Meanwhile, the evaporation rate slows down as alcoholic strength declines. This means a malt barrelled at 63.5% abv may reach 58% abv after 12 years, 56% abv after 18 years, 54-55% abv after 25 years, and 46-50% abv after 50 years.
The evaporation rate may increase marginally in a racked warehouse, as a consequence of temperature fluctuations, caused for example by the sun beating down on a tin roof.
However, the influence of a faster or slower evaporation rate on the ageing process is still not fully understood.
Humidity levels are another factor, with higher humidity seeing more water entering the cask, while lower levels of humidity mean correspondingly less water ingress. Exactly how this affects maturation is unclear, though some distillers believe higher humidity promotes a slower (and more preferable) rate of maturation.
Dunnage warehouses tend to have more stable humidity levels than racked. However, some racked warehouses restrict the amount of concrete to pathways between racks, retaining earthern sections under the racks to promote humidity.
The location of a warehouse within a distillery site can also foster an individual microclimate (with each warehouse also being a macrocosm with a series of microcosms). This individuality can be enjoyed by the dram with bottlings such as Glenmorangie Cellar 13, a name which refers to the dunnage warehouse in which the malt was aged.
Located close to the sea (Dornoch Firth), relatively higher humidity in this warehouse promotes a higher level of oxidation. This in turn yields more pronounced mint, vanilla and floral notes, alongside familiar fruitiness including pear drops, lemon, baked apples and apricot.
The beauty of the warehouse debate, for those of us who thrive on detail, is that it continues to raise intriguing questions.
Does longer maturation, and less active oak, such as a second or third fill, heighten the influence of the warehouse?
And how significant (or not) is the use of heat reflective paint on a tin roof, or the varying number of skylights in a racked warehouse, compared to minimal or even an absence of fenestration in a dunnage?
Similarly, using the terms dunnage and racked can require an additional definition. Some dunnage built in the 1960s have thin concrete walls and floors, while some racked warehouses are insulated which promotes a more stable temperature.
Trying to quantify the significance of such details may be impossible, but that doesn’t compromise our love of speculation (nor do we care if this level of obsession prompts someone to say “get a life.” This is our life).
So, what’s the verdict on dunnage versus racked? While the ageing warehouse is hardly considered a ‘primary’ influence on the resulting character of a whisky, some distillers say whisky matures differently in each type of warehouse.
Others say both can produce the same quality whisky, and that the determining factor is actually the quality of the wood.
Another aspect of the debate are palletised warehouses, essentially the preserve of grain whisky, though which may also be used to age malts.
A 1980s concept, palletised warehouses are typically a brick structure with a concrete floor, while each pallet usually holds six barrels.
As pallets are closely stacked, there is less air flow and a wider temperature differential closer to the top of the warehouse, which means six or seven pallets high. As this system maximises space, palletised warehouses are the least expensive to run.
As barrels invariably stand upright on a pallet this creates a different head space within the cask.
Losses through evaporation tend to be slightly higher as the barrel ‘end’ isn’t in direct contact with the spirit, as in this position it’s the barrel end, rather than staves, that is exposed.
Published in Whisky Magazine Issue 36 December 2003 ·Page 64-66
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