Types of Whisky Maturation Warehouses
The warehouse is a building, mostly attached to a distillery, in which the spirits slowly mature in oak barrels for years. In Scotland, and most of the rest of the world, the minimum period for whiskey to mature is three years in appropriate warehouses. There are three different categories of warehouses; the traditional dunnage warehouses, racked warehouses and the more modern the palletised warehouse.
What is a Dunnage warehouse?
A Dunnage (warehouse) is a traditional type of warehouse for maturing spirits, which is made of stone or brick these have no solid floor, are built rather low and have thick walls. The oak casks stored for whisky maturation within a Dunnage warehouse are stacked directly on top of each other, stacked to a maximum of three layers high. Dunnage warehouses often have only one floor but there are also those with two, three and more storeys in use. However even with this understanding only around 4.5% of all barrels could be stored using Dunnage warehouses in Scotland, and the number is almost certainly less. Major players such as Diageo use the majority of their distillery based dunnage warehouses only partially if at all.
The main advantage of Dunnage warehouses are excellent air circulation and microclimate with a higher level of humidity. Thanks to their thick walls and roofs Dunnage warehouses experience less outside influence than other types warehouses. Unfortunately the relative costs are far higher than in a racked or palletised warehouse, in part due to the far more limited storage capacity which offers a poor ratio of space to stored goods. These costs are further exacerbated by far higher maintenance costs. As equipment for moving the casks do not fit inside dunnage warehouses the barrels always have to be moved individually, and by hand.
What is a racked warehouse?
Racked warehouse are modern warehouses for maturing spirits made of brick, concrete or steel with a solid floor. In racked warehouses barrels can be stacked in long rows between eight and twelve layers high in long, parallel rows. Racked warehouses have been around since the 1950s having been pioneered by Seagram at its Hiram Walker distillery, today most Scottish whisky is matured in racked warehouses. Racked warehouses are built accordingly higher, with thinner walls and tin roofs. The barrels are stored upright rather than lying down, so that the warehouse workers can move them more easily with forklifts.
The primary advantage of racked warehouses are costs, the increased ratio of space to stored goods makes these more cost effective and by allowing increased warehouse floor space for machinery such as forklift trucks casks can be moved more easily. Around 60% of all barrels are currently stored in this way in Scotland. The largest racked warehouse in Kentucky currently have 21 shelving levels spread over 7 floors with a capacity of up to 60,000 barrels.
What is a palletised warehouse?
Pallet storage warehouses, or palletised warehouses are the newest type of warehouse used for maturing spirits. In palletised warehouses the barrels stand upright on special wooden pallets. Depending on the type of barrel, four or six barrels are placed on such a pallet. The barrels are braced together with a steel or plastic strap. The pallets are generally stacked in four to seven layers (though sometimes up to eight) on top of each other. Palletised warehouses have been in use in Scotland since the mid 1980’s and account for nearwly 40% of all barrels currently maturing.
What’s the difference between Dunnage vs rack or Palletised warehouses?
The main between the two types of warehouses is the regulation of temperature. Because racked and palletised warehouses have thinner walls they experience a more varied climate that changes with the seasons. It’s worth differentiating between early and modern Dunnage warehouses however:
Earlyn Dunnage warehouses
Single-story dunnage warehouses that were generally built before 1915 are characterized by their thick outer walls made of natural stone and the open ground. This creates a comparatively constant, cool and humid microclimate in the warehouse. This is similar to that of coastal warehouses. However, these warehouses are only used very rarely outwith the distillery grounds.
Modern Dunnage warehouses
Dunnage Warehouses are still being built today however, these modern dunnage warehouses can only be compared to the more traditional ones to a limited degree. As a rule, these are steel-frame halls clad with sheet metal, like all other modern whiskey warehouses. Only the flat construction and the partly open ground have remained. The storage climate is therefore often different in particular in relation to the temperature fluctuations are significantly greater.
Different climatic zones
Tthe microclimate resulting from the different climatic zones within modern warehouses is also worth a closer look. All warehouses are naturally ventilated with the air supplied through openings close to the floor and ventilation via opening slits in the roofs. This is necessary for fire protection. This natural ventilation creates a temperature stratification in the warehouse halls: cool and slightly damp on the floor, warm to hot and dry in the upper parts of the warehouse. The height of the building is of secondary importance. This kind of air stratification also forms in dunnage warehouses, so it is more important where exactly a barrel is or is standing. At the bottom and (partly) close to the outer wall, it is rather cool. It gets a little warmer and drier in the middle of the warehouse. In the upper areas it can get quite warm, even in Scotland, and it is increasingly drier.
Even within the same warehouse, it is not uncommon for barrels to lose their content faster than others, greater temperature fluctuations translate into a higher angels share. Likewise, in Kentucky, casks in the same warehouse can go lose alcohol while others actually gain abv due to different humidity levels.
Are Dunnage warehouses better for whisky maturation?
No. Dunnage warehouses do not neccesarily produce better whisky than racked or paletised warehouses. While there is some evidence that maturation within one type of warehouse versus another, or even at different places within a warehouse a qualitative comparison is almost impossible for a number of reasons:
Climatic variation across Scotland
Scotland’s temperate climate is relatively uniform in that the average temperatures and humidity levels are fairly consistent, and most of the larger warehouses are to be found in the periphery of Glasgow, Edinburgh and the Fife region. Despite this the very fact that temperature and humidity can fluctuate making it difficult to compare one warehouse to another. Outwith Scotland seasonal changes are often considerably more pronounced
Climatic variation within a site
A relatively full warehouse will experience fewer temperatures and humidity fluctuations compared to a nearly empty one, and microclimate inside a warehouse can change rapidly. Even within a single location different warehouses will be more sheltered or exposed, more or less humid, etc. Furthermore, each warehouse comprises varying microclimates (being cooler nearer the doors for example).
As most whisky is sold as either a blend, or a single malt (itself a blend of whiskies produced at a single distillery) the result on your standard supermarket 10, 12 or 21 year old is going to be impossible to identify. While this is not true for single casks their very nature leads to perhaps the largest obstacle to whisky comparison
A huge amount of the flavour profile of whisky is determined by the properties, and previous content of the cask its matured in. As no two casks are created exactly equal this is one of the largest obstacles to comparing different whiskies, base ingredients or the influence of warehousing. If it was possible to exclude the transformative differences between whiskies created by casks it may be possible to give a more accurate comparison of warehouse types but then we’d not be talking about whisky.
The differences we do know
Maturation takes place differently depending on a number of factors the two most obvious of which are temperature and humidity, higher temperatures mean more chemical interactions; increased rates of extraction, reaction and diffusion. Higher humidity means more alcohol lost versus water, lower humidity can actually see whisky ABD (alcohol by volume) as more water is extracted than alcohol. Changes in temperature at higher altitudes also alter the pressure levels inside a cask increasing the amount of interaction between liquid and wood. In the early days of whisky maturation regular relocations ensured consistent quality, today, the focus is more on blending barrels from different layers.
What limited studies are known do seem to indicate that whiskies matured higher in racked or palletised warehouses become darker, with higher levels of non-volatile elements, more phenolic compounds and esters. By contrast those matured at lower temperatures translate into more pronounced feinty, grassy and cereal notes. Some studies have suggested that grain whisky matured in warmer climates tends to be sweeter but cloudy and less clean compared with the same spirit aged in cooler conditions which are smoother and more pleasant however due to the influence of casks it’s difficult to attribute this to temperature alone.