Understanding Whisky Production Unit 6: Maturation & Blending
Chapter 15: Casks, Warehousing, and Blending: Dunnage Storage to the Vatting Room
Barrels, hogsheads, butts, dunnage versus racked warehouses, the angel's share, cask disgorging, vatting, and marry-down consistency.
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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. Other casks not included in this schematic will include the 650 L capacity port pipe and the wine barrique (225-300 L capacity). Other smaller casks such as the quarter cask (50 L capacity) and the firkin ( 40 L capacity) may also be filled primarily by the small craft type distilleries.
As already stated, by definition Scotch, Irish and Canadian whisky must be matured in oak or wooden casks. They must not exceed 700 L in capacity. Bourbon must be matured in a newly charred oak barrel. The American standard barrel (ASB) has the greatest internal surface area to liquid ratio and is the most common cask used in the spirits industry.
Once the casks have been sourced to meet the respective distillers’ filling and wood policy requirements, they will be inspected prior to filling.
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
Maturation aims to improve the aroma and taste of the new make whisky - as you learned in the Maturation in Wood lesson. We may describe harsh and pungent new make products as mellow and smooth after maturation. Maturation takes place in warehouses of various sizes. Their locations vary widely, along with their climatic conditions.
In general, three different types of maturation warehouse are used to mature whisky across the world:
- The traditional dunnage warehouse - primarily used for Scotch and Irish whiskies. They are historically likely to have been built on the distillery’s site. They rarely exceed three or four floors in height.
- The modern racked warehouse
- The modern palletised warehouse
Modern centralised racked and palletised warehouses are the predominant mode of maturation, and were discussed in the Maturation in Wood lesson.
These warehouses will be located in many different parts of the world, with many different local climatic conditions.
- The climatic conditions in Scotland will be very different to those found in Kentucky and Tennessee in America. These in turn will be different to those found in some parts of Canada.
- The temperature range from summer to winter in Scotland is quite narrow, when comparing the range in summer and winter temperatures for some parts of America and Canada. Thus, there are no controls employed in Scotland other than natural venting.
- In more extreme climatic conditions, heating can be used during winter months.
Filling and emptying maturation warehouses in an efficient manner that meets all requirements is not as simple as it might seem. This is particularly true for those large businesses with single malts, blended malts and blended whisky products of varying styles and ages. Smaller businesses with far fewer variables in the product mix will find the maturation process less complicated.
The whisky maturation process relies on the movement of casks to and from the warehouse for filling and disgorging (emptying). Other spirits industries, however, keep the cask in the same static position. It is then filled and emptied from that position.
If a maturation warehouse operation has huge temperature variations across the warehouse, the maturing inventory can be rotated around the building to ensure that all casks are exposed to the same conditions of temperature and humidty. This is a costly and labour intensive exercise to complete. As a result, this type of process is no longer widely used.
During maturation the following key changes occur: The total volume of alcohol in the cask will decrease, and the alcohol strength will change. The spirit will gain some colour from the wood. The flavour of the spirit will improve (if matured in a taint free cask).
In the American whiskey industry, with its warm, dry climate, the water evaporates more quickly than the alcohol. This allows alcohol concentrations to rise by around 2% per annum. The alcohol loss will be equal to, or even greater than, those found in colder, damper Scotland. This is due to the higher ambient temperature.
The loss in cask volume does not occur from any general leakage of a poorly constructed cask. Instead, the water and alcohol penetrate into the permeable wood and eventually passes very slowly through it and out of the cask; otherwise known as the angels’ share. Our noses would instantly tell us this if we entered any of these three types of warehouse!
Both the maturation rate and the rate of alcohol loss very much depend on the climate of the country of maturation. Larger losses of 10% or more can occur in very warm, dry climates. However, the maturation process will also be much quicker in these conditions. In the cooler, damper climates of Ireland and Scotland, smaller losses of around 1-2% per year are feasible. Each distiller and blender will know the optimum conditions for their own range of products.
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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