25th November 2018
Whisky Production: Maturation and Bottling
After distillation the spirit is not yet whisky. It must first be matured in oak casks for at least 3 years before bottling.
Why is whisky stored in wooden casks?
Whisky was originally consumed straight after distillation, without the mandatory wood maturation. Nobody knows for sure how this practice of oak maturation began. It has been suggested that because whisky was traditionally a seasonal product, it would be stored in containers for consumption throughout the year.
What these early whisky drinkers will have noticed is that after some time in contact with wood the spirit became much mellower. This is attributable to the reaction of the spirit with the wood and is one of the major contributing factors to today’s mandatory maturation period. Compounds within the wood, such as lactone, eugenol, and vanillin (responsible for the typical vanilla flavours), are pulled into the maturing spirit, contributing a variety of flavours and aromas. Additionally, the sharp aromas within the new make spirit are lost over time, resulting in a smoother whisky.
It is not only flavour that the wood contributes. The new make spirit, as it comes off the stills, is almost completely clear in colour. Whisky’s typical, golden colour is a direct result of the tannins from the wooden casks.
What the cask was previously filled with can also have a large impact on the final product. Due to laws governing its production, new barrels must be used for aging bourbon whiskey. This leaves an abundance of barrels that can’t be reused for bourbon maturation. Since there is no such law demanding Scotch whisky be matured in new barrels, many of the bourbon casks end up in Scotland. These casks then lend flavours of vanilla and caramel to the maturing Scotch whisky, typical characteristics of bourbon whiskey. Ex-sherry butts are also a popular choice, although the decline in the sherry industry over the past years makes them rarer and therefore more expensive. Sherry casks generally lend sweet, fruity flavours to the whisky.
Once filled, the whisky barrels are transferred to the warehouse for storage. Traditionally, the whisky was stored in stone-built dunnage warehouses. Here the barrels were stored on their sides and stacked up on wooden racks. With industry expansion in recent years and the resulting need for higher inventories, it is becoming more common for large, multi-storey warehouses to be used and for the barrels to be stored upright on pallets, essentially maximising storage space. Each warehouse style has both advantages and disadvantages, balancing the efficient use of space against extraction from the wood.
Temperature changes in the warehouse cause the pores in the wood to expand and contract, allowing the spirit to flow in and out of the staves. This leads to the extraction of flavour compounds as mentioned above. Additionally, many of the casks are charred on the inside, providing a layer of carbon that acts as a filter, cleansing the spirit of unwanted compounds. As oak is porous, the whisky is also able to breath. Over time, some of the alcohol will evaporate from the cask, known as the angels’ share, leading to a change in flavours as the concentrations of alcohol, water and other compounds changes.
Once the spirit has spent at least three years maturing, it is legally allowed to be called whisky and can be bottled. The flavour development doesn’t stop after only 3 years however, and the majority of whiskies will be matured for much longer, with periods of 10 or 20 years not being uncommon.
Even single malt whisky is blended whisky
As mentioned at the beginning of this series about whisky production (“What is Whisky Made From?”), a single malt does not have to come from a single barrel. The bottled whisky is commonly a mix of several casks from the distillery, allowing the manufacturers to achieve a higher level of consistency in their product despite small variations between batches and casks. It is the role of the blender to design these recipes and manage the inventories in such a way to ensure brand consistency in the future. Of course, distilleries also release single cask expressions, which as the name suggests do come from a single cask and therefore do not require blending.
Filtration and Bottling
Once the whisky is emptied from the cask it is likely that fragments of wood will be present in the liquid. Additionally, the whisky may have a cloudy haze due to the oils present. These qualities are generally not accepted by the consumer, who expects a bright, brilliant liquid without floating particles. For this reason the whisky will be filtered before bottling.
Prior to filtration it is also common that the whisky is reduced in strength. Depending on how long the whisky has been maturing, the strength could be anywhere between 40% and 70%, as a result the whisky is mixed with water to bring it down to a reasonable drinking strength, a minimum of 40% but commonly also 43% or 46%. Of course, it is also possible to purchase cask strength whiskies, which are not diluted at all. Additionally, it is common for the producer to add caramel colouring prior to filtration in order to ensure that the colour is consistent over multiple batches. After filtration the whisky is stored in a whisky vat ready for bottling.
Due to the difference in size between individual distilleries, there is no one size fits all approach when it comes to bottling. In the industry everything is observable from hand-filling of bottles, through semi-automatic bottling lines where the bottles are hand fed onto the line at one end and retrieved at the other, all the way to fully-automatic systems where even this step is processed by machines.
It is important to note that whisky maturation is dependent on the wooden casks. Once the whisky has been bottled, maturation no longer occurs and the whisky will not improve in flavour over time (in fact, once opened oxidation can quickly degrade the quality of the whisky). That is to say that a 20 year old single malt kept on a shelf for 10 years (unopened or otherwise) is not a 30 year old whisky but a 20 year old whisky that has been kept on a shelf for 10 years.
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- produced entirely in Scotland;
- that has been distilled at an alcoholic strength by volume of less than 94.8 per cent so that the distillate has an aroma and taste derived from the raw materials used in its production;
- that has been matured entirely in an excise warehouse or a permitted place in Scotland in oak casks of a capacity not exceeding 700 litres for a period of not less than three years;
- to which no substance has been added except water and/or plain caramel colouring; and
- that is bottled at a minimum alcoholic strength by volume of 40%.