The extent to which single malt Scotch whisky is a creature of its environment is the subject of lengthy, on-going debate. Many people even doubt the existence of 'whisky terroir,' noting that malting barley is usually not grown close to the distillery where it is processed and the influence of the prevailing micro-climate is of minimal consequence. It is often argued that differences between single malts have most to do with variations in production practices and equipment.
While the jury may be out in terms of actual spirit production, when it comes to maturation of that spirit, however, the environment certainly plays a part, and Scotch whisky matures in a notably different way to its Kentucky counterpart, for example.
During maturation, the porous oak casks 'breathe,' and many changes occur to the spirit due to the dialogue between liquid, wood and external atmosphere.
Pungent sulphur compounds diffuse out from the cask, while air diffuses in, promoting a series of chemical reactions, with a proportion of the higher alcohols being transformed into esters and other complex compounds which have a beneficial effect on the character of the maturing spirit.
The amount of bulk loss varies according to temperature and humidity levels, as does the rate of maturation. In the prevailing cool and relatively damp Scottish climate a reduction in both strength and volume occurs as time passes, with an average evaporation loss of around two per cent per annum - often referred to as the 'angels' share.' In the USA, by contrast, temperatures are higher, with larger seasonal variations, leading to a greater loss of water from casks than in Scotland, and the strength of maturing whiskey may rise as a result. Maturation also tends to be more rapid.
Maturation rates may vary quite significantly depending upon the type of warehouse in which the casks are stored. Traditionally, Scotch whisky was matured in 'dunnage' warehouses - relatively small and built of stone or brick, with slate roofs and earth floors.
Most distillers continue to favour such warehouses for optimum maturation of whisky intended to be bottled as single malt.
Despite this, practical considerations now dictate that the bulk of whisky is matured in warehouses which are usually much larger and have concrete floors, being 'racked' to hold up to 12 casks between floor and roof, or 'palletised' to accommodate barrels stored upright on wooden pallets. These are much more cost-effective and convenient for handling than dunnage warehouses, where casks are stacked no more than three high on wooden rails.
One distillery which even goes so far as to build new warehouses in dunnage style, however, is Benromach at Forres on Speyside. Distillery Manager Keith Cruickshank explains that the Benromach take on dunnage warehouses includes the use of modern insulation materials.
"This means that there is rarely little more than one degree of temperature variation from day to day," he notes, "so we have a relatively constant temperature and humidity regime.
Dunnage-style warehouses are the best for consistency." In 'racked' or 'palletised' warehouses there are temperature differences between casks stored close to the ground and those located near to the roof. Maturation occurs most rapidly in the warmest part of a racked or palletised warehouse, which is invariably close to the roof.
There is also less circulation of air when casks are packed tightly on pallets, and Whyte & Mackay Master Blender Richard Paterson observes that casks stored in the centre of a palletised warehouse might ideally require as much as six months to a year longer than those in other locations in order to mature fully, as there is less evaporation from them.
"I think that in damp warehouses, whisky becomes more mature, softer, and the rough edges are broken down," he maintains. "In dry warehouses, the whisky can remain a bit sharper and hotter, and retain a degree of immaturity for longer." Whatever the style of warehousing in which maturation takes place, many commentators argue that the location of the warehouses has an influence on the character of the whisky we drink.
Due to the scale on which large companies principally concerned with blended Scotch whisky now operate, it is not feasible for more than a small percentage of their malt whiskies to be matured at the distilleries where they are produced. Centralisation of maturation operations - often in Central Scotland - has increasingly become the order of the day.
When the malt whiskies in question are used for blending, it would take a remarkable nose indeed to detect differences in the finished blend due to where the component malts have been matured, but is this also the case with single malt bottlings?
At Bowmore distillery on Islay Head Warehouseman Willie MacNeill - universally known as 'Ginger Willie' - has no doubts on the subject. "You don't get the same whisky when it's not matured on the island," he declares.
"There is heavy sea air and no pollution.
We have galvanised hoops on casks here because the old mild steel ones rusted, the salt air affected them. Those sorts of conditions are bound to have a little effect on the whisky.
"Number 1 warehouse was the first warehouse to be built when the distillery was established. It is partly below sea level and you get very little temperature change in it. During the winter, when it got down to minus eight degrees outside, it was appreciably warmer in the number 1 warehouse. To me, the ideal warehouse has thick, old walls and is slightly below sea level.
"You get damp, salt air. Evaporation is slower than in the big, modern ones.
You don't lose as much spirit through evaporation as you do in the middle of the mainland." As with so many aspects of Scotch whisky, there is an array of variables to consider in relation to maturation, but as the process drives up the 75 per cent of a whisky's flavour, these variables are clearly of great significance.