Understanding Whisky Production Unit 6: Maturation & Blending
Chapter 15: Casks, Warehousing, and Blending
Barrels, hogsheads, butts, dunnage versus racked warehouses, the angel's share, cask disgorging, vatting, and marry-down consistency across batches.
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A warehouse is far more than a storage shed; it is a giant, slow-breathing respiratory system. Traditional dunnage warehouses, with earth floors and thick stone walls, maintain cool, humid microclimates where alcohol evaporates slightly faster than water. Modern palletised or racked warehouses, by contrast, experience greater temperature swings that accelerate spirit expansion and contraction through the stave wood.
Every year, roughly two percent of cask volume is lost to the atmosphere through evaporation, known poetically as the angel’s share. Once mature, casks are disgorged and vatted together to ensure brand consistency, before decisions are made on filtration, proof dilution, and the addition of permitted spirit caramel for colour uniformity.
Warehousing and Blending
Each distiller ensures that warehouse capacity meets the requirements of the product they are marketing. Generally, the larger the whisky business, the greater the overall warehouse capacity. This capacity holds maturing inventory specified by the blender(s) for sales forecasts developed many years earlier. The distiller also lays down (add new cask stocks to the warehouse inventory) new inventory to meet sales predictions for current and future years. This is quite a complicated algorithm.
Distillers with many different products in market will lay down stock based on:
- The type and make of new make spirit, (grain, malt, rye, bourbon, peated or non-peated malt etc.).
- The type of cask it will be matured in.
- The treatment of the cask it will be matured in.
- The spirit age required (either by legislation or by product labelling).
- The market’s predicted volume of future sales.
Conversely, distillers with one or two similar products in market will have a much simpler task when laying down stock. However, they too will be constrained by the future sales predictions.
The blender plays a key role in meeting the challenges of laying down maturing inventory. The blender’s role is pivotal when recovering from positions of over- or under-optimum stock positions. The blenders will have a detailed overview of all maturing stocks. This ensures that maturing inventory meets both current and future brand requirements.
Most, if not all, distillers will have a wood policy that meets the blender’s requirements in ensuring that the correct new make product is filled into the correct type and treatment of cask. This is vitally important in making sure that ongoing consistent product quality requirements are met. It also significantly affects the logistics of cask supply for future fillings.
Some disgorged casks will be refilled immediately, while others may go for repair, rejuvenation or other treatment. Meanwhile, new wood stocks will be introduced. Some industries make the choice of cask very simple - the US bourbon industry is heavily regulated. Legislation states that bourbon must only use a brand-new oak barrel for all fillings - more on this later.
The Cask
Most whisky distillers mature their inventory in oak casks. These casks will differ in shape and capacity. They will generally be made of either American or European oak.
Scotch whisky regulations (The Scotch Whisky Regulations, 2009 No 2890) state that whisky must be matured for a minimum of three years in an oak cask, not exceeding 700 L in capacity.
Irish whisky regulations (The Irish Whiskey Act 1980) state that whisky must be matured in wood for a minimum of three years.
Canadian regulations (Government of Canada: laws-lois.justice.gc.ca) state their products should be matured in small wood for no less than three years.
The American bourbon industry (American regulations 27CFR 5.22 Code of Federal Regulations) generally has no minimum age requirement, although straight bourbon has an age requirement of two years and all bourbon must be matured in new oak barrels.
Maturation must take place in the country of production. It is important you fully understand the regulations relating to your own products. These regulations are readily available on the internet.
The following diagram shows the basic features of a typical cask:
Figure: Diagram of a basic cask
The terminology used above relates to any shape or size of cask. These terminologies will be regularly heard in any cooperage conversation where casks are repaired, rejuvenated, or built.
Watertight, barrel shaped wooden containers that could be rolled and stacked have been recorded in history dating back to 300-400 years BC. Therefore, their use for holding liquids is not a new phenomenon.
With the exception of the bourbon industry, new casks are very rarely used in whisky maturation. The majority of casks used for whisky maturation have previously been used in the bourbon, wine, brandy, port and sherry businesses. Many of the casks will also have been previously used for whisky maturation. They can be used for several successive fillings before the inner surfaces become exhausted, making them unsuitable for efficient maturation. These casks will go to the cooperage for repair and rejuvenation. Individual businesses will outline all of this in their wood policy.
Cask Specifications
The following diagram gives specifications for the main casks used in whisky maturation (this is not a fully inclusive list):
As shown above, there are several types of cask. Generally, the volume of liquid they hold, and their shape, differentiates them.
The Sherry Cask Reality
There is a charming story marketing departments love to tell: noble solera barrels from ancient Andalusian bodegas, tended by silent monks for sixty years before being loaded onto ships bound for Scotland.
Unfortunately, reality is far more pragmatic. Today’s “sherry casks” are basically purpose-built interns with convincing accents. Historically, transport casks of Oloroso and Amontillado arrived in Britain by ship, and when emptied, the barrels were sold to Scottish distillers for a song. That accidental tradition ended abruptly in 1986, when a European Union directive (supported by Spain’s Consejo Regulador) decreed that any wine bearing the name “Sherry” had to be bottled within the Jerez triangle.
Overnight, Scotland’s supply of genuine ex-bodega casks evaporated. Rather than abandon the style, distillers began commissioning brand-new casks, constructed in Spanish cooperages (frequently from American white oak rather than European oak) and “seasoning” them for 12 to 24 months with young fortified wine made specifically for the job. Once seasoned, the wine is quietly demoted to vinegar, and the cask is shipped north to whisper tales of Spanish sunshine to Scottish spirit. It is an industrial fiction, but a highly effective one that yields magnificent flavour.
This inspection regime includes a quick review for any broken or damaged staves (the narrow strips of wood that form the sides of the barrel). This inspection regime should also include the requirement to look for “inserts” within the cask which accelerate maturation. This relates specifically to casks for whisky maturation - these must be removed to conform with the Scotch Whisky regulations.
It also involves a nosing to ensure the cask has no off-odours such as sour or musty. Once this final check has been completed, the cask goes to the filling store where it will be filled with the required volume of spirit. This may be done at small manual filling stores in adjoining warehouse space on the distillery site. However, more commonly, the spirit will be tankered off-site.
Many large distillers now have centralised filling and warehouse activities. This eliminates a significant number of empty and filled cask movements. The high strength new make whisky will be diluted with good clean water (usually demineralised water) to the maturation strength which is typically between 60.0% ABV and 68.0% ABV. Like most things in the whisky industry, not one size fits all. The distiller can use some discretion in choosing the filling strength where cask supply is limited.
This process may be fully automatic, manual or a mixture of both. The key principle is not to overfill, in order to avoid alcohol loss. The filling hole is blocked with a suitable fitting bung. This filling process will take place either on the bilge for traditional or racked warehouse, or on the end for palletised stock - more on this later.
Each individual cask is clearly marked with the spirit’s make, filling strength, volume of whisky and date of filling. It may also have a specific mark to identify the cask’s history, but this will depend on the individual distiller’s own requirements. Historically, information on strength, volume and date of filling will have been stencilled onto the end wood. More modern filling and warehouse operations now use barcoding for cask identification.
The Warehouse
Most people, when they imagine whisky maturing, picture something deeply romantic: a traditional dunnage warehouse with thick granite walls, a damp earthen floor, and casks stacked three tiers high, while a bearded warehouseman hums a tune last heard when broadswords were considered accounting tools.
The truth is that while dunnages are magnificent, they are also hopelessly impractical. You cannot drive a forklift into a dunnage; every cask must be rolled and stacked by hand in the gloom. Today, roughly sixty percent of Scotch matures in racked warehouses (steel-framed structures eight to twelve tiers high), forty percent in palletised warehouses (vast aircraft-hangar-sized complexes where casks stand upright on pallets), and only a romantic four percent sleep in traditional stone dunnages.
In a palletised warehouse, tens of thousands of barrels sit beneath insulated steel roofs doing their chemistry homework, monitored by barcoded tracking software and forklifts that hum quietly along the aisles.
The Angels’ Share and Celestial Theft
The Angels’ Share: a phrase so poetic you almost forget it describes a serious accounting problem.
From a thermodynamic standpoint, nothing ascends to heaven. Casks are porous wooden membranes; volatile molecules simply escape through microscopic pores in the staves into the surrounding air. In Scotland, where the climate is cool and damp, alcohol evaporates faster than water, causing the spirit’s alcoholic strength to slowly drift downwards over time. In the baking summer heat of Kentucky or India, the reverse occurs: water molecules evaporate more readily, causing proof inside the barrel to rise.
In Scotland, the angels demand a tithe of one to two percent per annum. With over twenty million casks resting in Scottish warehouses, that represents more than one hundred million litres of alcohol vaporising into the sky each year: enough spirit to fill forty Olympic swimming pools.
Not all of this airborne alcohol reaches heaven. A significant portion feeds a local resident: Baudoinia compniacensis, a dark, sooty fungus that thrives exclusively on airborne ethanol vapours. It coats the exterior walls, roof slates, and nearby trees around maturation warehouses in velvety charcoal black. It is harmless, but serves as the biological equivalent of that one friend in every pub who popped in for a quick half-pint three weeks ago and is now permanently part of the furniture.
The Blender
This is a very important role within the whisky production process. The blender controls all things related to the quality of the company’s products and the maturing inventory. This person has full responsibility for the wood and filling policies. They will carry out regular assessments of the maturing inventory to ensure it is going to meet the original planned quality standards.
Blending is defined as the choosing and mixing of spirit (in this case whisky) of different character, to produce the desired finished product quality.
The blender’s challenge is to take all their skills and knowledge of wood treatment and mix the different types of whisky to make a new product of high quality in an ever-changing maturing inventory. Meanwhile, they must maintain the desired product quality and consistency.
The following schematic shows an example of the type of end-to-end process in which the blender will play a key role:
The diagram above is a dynamic model. Sales forecast and orders will change from market-to-market and year-to-year. There are also some fixed elements within it. Once the distilling business has reviewed its sales forecast, they will look at their current maturing inventory. Using this, they must decide on the volumes of new make product to be distilled and laid down in order to meet future sales demands over a potentially wide range of products and age categories. Once this decision is made, it locks in production volume by distillery, as well as the raw materials required to produce these volumes. It will also lock in the cask supply based on the actual sales orders and wood to be refilled after disgorge, along with any new wood required.
Once produced, the new make will be added to the maturing stock. The mature stock needed to meet current sales activity will then be removed from the warehouse. This above diagram displays the breadth of these key players’ involvement.
The blending process’ main aim is to produce a consistent product that meets customer requirements. Blending requires a very highly developed skill set. It relies on one’s ability to nose and taste liquids in order to differentiate their character and quality.
The blender must also be fully aware of current and pending legislation across the world. In particular, they must be familiar with legislation relating to manufacturing methods and potential labelling requirements.
The Blending Process
We have just taken a close look at the blender(s) and where their role fits within the whisky industry. Let us now take a quick look at the blending process itself.
In this process, sales forecasts are received, and the blending teams review the quality of the maturing stock.
- They must identify all the casks required to make a particular blend or part-blend.
- Next, a manual or computer driven process selects the requisite casks and produces cask “dropping” lists that will be used by warehouse operators to drop (safely remove the casks) the required casks.
- This process might involve various types of warehouse operations in a number of different locations.
- Once all the casks have been collected from storage, they are consolidated or pulled together and quickly re-assessed for quality. This may involve moving casks by road or other forms of transport to a central disgorge facility.
- Once the quality test has been passed (nosing for any substantive off-odours), the casks are emptied into a stainless-steel trough. Liquid from palletised stock is pumped from the casks while they remain on the pallet. In both instances, any course solids or particulate material will be removed by filtration, as these bulks of part-blended or full-blend whisky are produced from the individual casks selected.
- The cask ledger books / computer database will be updated, and the casks will be removed from the inventory.
- The casks are then returned to the filling store for refill. Alternatively, if required, they are sent for repair and rejuvenation.
All of this activity will have been organised and fulfilled some time ahead of the planned bottling plan. Generally, a significant number of tanks are dedicated to holding blends or part-blends either in centralised locations or at the bottling plants themselves. These bulks of part- or full-blends are checked for actual volume (L) and strength (% ABV) as part of routine Revenue and Excise or other local legislative requirements.
The following is a diagram of casks being disgorged:
The figure above clearly shows the mechanics of the process. From cask selection through to disgorge it appears a very straightforward process. However, this process becomes significantly more complicated when there are many different products in the market. There are further stages of filtration, colouring, spirits reduction, bottling strength and accounting. These all take place post-blending. They are normally undertaken in the bottling plant. These stages may or may not be required, depending on the particular product.
Pre- and Post-maturation Treatment
Some whisky industries have very strict legislation that controls what can be done to the distilled product, both pre- and post-maturation. This accompanies legislation on the method of production.
Scotch regulation whisky is very specific. According to The Scotch Whisky Regulations, 2009 No 2890:
- Maturation must take place in an oak cask.
- The cask’s capacity must not exceed 700 L.
- Maturation must last a minimum of three years.
- The whisky must retain the colour, aroma and taste derived from the raw materials used, and the method of its production and maturation.
- No other substance can be added except for water or caramel.
The final product blended from the pre-selected casks will often vary in colour from blend to blend. Despite this, it will have the same quality and nosing character. Some distillers take great pride in this fact, stating that their product is of natural colour. Other distillers add caramel to the blend, to deliver a consistent product colour. This is achieved by adding predetermined quantities of caramel, based on actual blend colour and the standard colour required.
This is very specific, but there are no restrictions on how many times the liquid is filled or removed from casks of different qualities and flavours. Some whisky businesses finish individual products by preparing an original blend, or single product, then refilling this liquid into other casks of different quality to provide additional flavour enhancement. This process is called finishing. The resulting product will have identifiable Scotch whisky characteristics, as defined in the regulations.
Historically, the Scotch whisky industry used a process called marrying. This was where the blended liquid was allowed to mature or rest for a specific time, generally in large wooden or stainless-steel vats. This allowed all the different liquids in the make-up of the blend the time to interact. Generally, the majority of large whisky producers do not follow this traditional process.
A pre-maturation treatment is used for producing nearly all Tennessee whiskeys. The distilled product is filtered through maple charcoal chips, before filling into cask for maturation. Most Tennessee whiskeys use variants of this process, which is termed the Lincoln County Process. Lincoln County is where Tennessee whiskey was first produced.
In the absence of specific country or legal requirements, you can add additional flavouring via a number of different techniques. These include using oak chips, oak staves and other wood types and treatments.
Key Takeaways
Warehousing environment governs evaporation rates and cask-liquid dynamics. Blending is the art of assembling disparate casks into a harmonised, repeatable profile that balances wood extractives against the distillery’s core spirit character.
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