WSET® 2 Reading, Chapter 19: The Distillation Process (Class Notes)

Distillation was originally used several centuries ago to produce medicines. Once people discovered the pleasurable effects of drinking distilled products, the use of distillation to produce strong alcoholic drinks became widespread. For the production of spirits, the alcoholic liquid goes through a process of distillation, the aim of which is to increase the alcohol content of the liquid.

In order to produce a spirit by distillation, it is necessary to start b using fermentation to make an alcoholic liquid. The aim of distillation is to separate the alcohol from the alcoholic liquid, most of which is water. When increasing the alcohol content of a liquid using distillation, advantage can be taken of the fact that ethanol (drinkable alcohol) boils at a lower temperature (78.3°C) than water (100°C). By heating an alcoholic liquid, the alcohol can be boiled off, and then collected, cooled, and condensed back into a higher-strength alcoholic liquid. The water, which constitutes the largest proportion of fermented alcoholic liquids, is mostly left behind, along with any solids, colour compounds, sugars, and most acids.

The equipment used for distillation is called a ‘still’. There are many different kinds of still, but they can be divided into two broad categories: pot stills and column stills.


The pot still is the oldest, simplest kind of still. Pot distillation is a batch process. This means that once a distillation is completed, the still must be cleaned and refilled before the next distillation can begin.

The still consists of a pot-shaped vessel, usually made of copper, which contains the base alcoholic liquid. The alcoholic liquid is heated and this causes the alcohol to evaporate, turning it into a vapour. These vapours rise up the neck of the still, which extends above the pot like a chimney. Vapours in the neck then flow into a condenser, which uses cold water to condense them back into a liquid. This new liquid contains a higher level of alcohol than the original liquid. However, pot stills ca only raise the level of alcohol in a liquid by a relatively small amount, so two or more successive distillations are needed to obtain a spirit of sufficient strength from an alcoholic liquid.

During the second distillation, the distiller only keeps part of the liquid that is collected from the condenser. The most volatile components, which boil off first, are called the heads. The heart (or spirit) follows second and contains the highest proportion of ethanol, and the lowest proportion of undesirable impurities. This is the part that is used to make the spirit. The least volatile components, known as the tails, boil off last. Heads and tails are not used in the final spirit because they contain concentrated levels of undesirable components. Instead, they are returned to the pot to be redistilled with the next batch because they still contain some desirable ethanol. Even with multiple distillations, the spirits produced are far from pure and retain a lot of character and flavour.


A column still is so-called because of its arrangement in a tall, vertically structured column. The design is quite complex, and there are many variations.

However, all column stills are internally divided into a number of levels called ‘plates.’ These plates have holes to allow both the alcoholic liquid and vapour to easily move up and down the still. The alcoholic liquid is heated and enters the still as a vapour. Once operation is underway, the alcoholic vapour passes upwards through the still. At each level some vapours will condense, forming a liquid layer on every plate. The rising vapours are forced through this layer of liquid causing it to boil, which , in turn, forces vapours upwards through the plate above. These mini-distillations take place on each plate, with the process continuing all the way up the still. With each distillation the concentration of alcohol increases. Therefore with a sufficient number of plates, a column still can produce a spirit that is almost pure ethanol. Such spirits are very smooth and light in character compared with those produced in a pot still. However, it is important to note that not all column stills are used to make such pure spirits.

Unlike a pot still, a column still can be run continuously and efficiently meaning it is able to produce a constant flow of new spirit. This is why these stills are also known as ‘continuous stills.’


There are various generalisations we can make when looking at the influences of distillation strength on the flavour of the spirit. As a general rule, spirits that are distilled to a lower alcoholic strength, like those produced using a pot still, contain more impurities and more flavour character, including that of the base material (barley, corn, apples, grapes, cherries, sugarcane, agave). However, these impurities also make the spirit harsher, so they generally need to be matured in oak or charcoal-filtered to soften them. Conversely, spirits distilled to a higher alcoholic strength, like those produced using a column still, are lighter in flavour and character. When reduced (watered down) to a standard bottling strength of around 40% abv, these spirits are relatively smooth, so they can be bottled and consumed without a period of maturation.

It is important to remember that it is the distillation strength, rather than the still type, that affects the flavour of the spirit the most.


Spirits can either be aged or unaged. Unaged spirits are stored in stainless steel tanks until they are bottled, normally shortly after distillation. Aged spirits are stored in oak vessels, typically barrels. The oak changes the colour and flavour of a spirit. Oak can soften out the harsh alcohol sometimes found in spirits that have just been distilled. It can also add oak flavours such as vanilla and sweet spice, and it allows oxygen to dissolve in the spirit over time to add an extra level of complexity.

Because sugar is non-volatile, all spirits are bone-dry when they come off the still. Any sweetness in the final bottled product is either added (e.g. with dark rums), or comes from the breakdown of oak into sugars during ageing (e.g. in Bourbon). The tannins and most acids that appear in fermented alcoholic drinks are also non-volatile, so they do not appear in the spirit. However, tannins can be absorbed from oak barrels during maturation.

All spirits are water-white when they come off the still. Any colour in the final  spirit comes either from oak ageing or added colourings, such as caramel. Most aged spirits have their colour adjusted with caramel for consistency. Some short-aged spirits have all of their colour removed by filtering the spirit through charcoal—some white rums are treated this way.

Nearly all spirits have their alcohol level reduced before they are bottled and released for sale. De-mineralised water (used because it is neutral) is added slowly and mixed with the spirit to reduce it to an alcoholic strength suitable for bottling, typically around 40% abv.


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