For all the glamour around it, distilling is surprisingly simple. It can be likened to boiling a kettle and collecting the steam.
The art of distilling is an ancient one. The true origins are lost to the mists of time, but there are a few key pieces of evidence that have been discovered. A perfectly preserved terra-cotta pot still was discovered in Taxila, what is now north-west Pakistan, and dated to approximately 500 BC. A similar type of clay pot still is still used to create Ancestral style Mezcal in Mexico. We believe that a similar method of distillation was being practiced in China around the same time, or maybe even before, and have some evidence that they were using copper to craft stills by AD 25. In Europe, through the writings of Aristotle (Meteorology 340BC approx.), we can infer that distillation was practiced in ancient Greece. But, he only mentions using the principle for the purification of saltwater. A later work of greek writing, dating from the 4th Century AD is from the alchemist Zosimos of Panoplis, who describes the art of distillation and explains the equipment used. He credits the design of the still to an earlier alchemist called Maria Judaea. Distillation became widespread in medieval Europe, stemming from the Islamic world and spreading through Spain and Italy. It is believed that distillation was brought to the British Isles by monks who had travelled to Spain for pilgrimage or to Salerno, Italy, for the medical university. It is believed that this knowledge was then spread to Scotland and the rest of the British Isles by itinerant priests and healers.
A typical distilling run in Scotland compromises of the following operations. To begin, the first still, known as the wash still, is filled with the wash, and brought to the boil, the vapours are condensed and collected.. The collected liquid is known as low wines. These go into a holding tank, ready to fill the second still. Remaining liquid and solids in the wash still is known as pot ale and is removed to be made into animal feed. The second still is called the spirits still and is usually smaller than the wash still. The running tends to be similar, but the liquid is cut into three fractions. The middle fraction is separated, and used to fill casks. The first and third fraction are recycled back into the spirits still to be used in the next run. The spirit has to go through something called a spirits safe. The spirits safe allows the distiller to monitor and control the flow of new make spirit, as well as recording the volume, temperature, and density of spirit produced. The spirits safe also allows the Master Distiller to move the spout between receivers to make cuts. The spirits safe used to be locked and the only key held by a HMRC representative on site, but the key is now held by the Master Distiller, as Revenue have other ways to monitor spirit production.
Distillation is the process of separating the constituent parts of a liquid mixture by heating to produce vapour, and then condensing and collecting said vapour. Liquids will evaporate naturally at room temperature.
When distilling whisky, the wash will contain approximately 92% water, 8% alcohol, 0.1% volatile (distillable) congeners, non volatile dissolved material, and a small amount of suspended solids. This will be rectified into a clear, colourless spirit with a mix of water between 5.2- 35%, alcohol between 65- 94.8% and about 0.5% congeners.
Pure alcohol will boil at 78.5°C, whereas pure water boils at 100°C. However, if you held a still at 78.5°C you wouldn’t end up collecting 100% alcohol due to the water bonding to the ethanol. So, stills have to be run at a temperature between 78.5 and 100°C. Because the ethanol is more volatile, it will evaporate faster than the water and the collection will contain a higher concentration of alcohol. This principle also applies to the congeners that are present in the wash, and these also exist on a spectrum of volatility, with some being more volatile and having a lower boiling point than ethanol e.g. methanol, which will be separated into group 1 fractions and some less volatile, having a higher boiling point e.g butanol, which separate into group 3 fractions. Congeners that have a higher boiling point than water, and therefore do not evaporate within the still are known as group 4 fractions. The binding of water to ethanol means that under standard conditions you cannot exceed 97.25% ABV when distilling. This is because at this point it has achieved a state known as an azeotrope. Reaching this point is next to impossible with a pot still, because they do not have enough reflux, and the energy input to the amount of liquid collected is deeply uneconomical.
The typical pot still has 4 key parts to it: The pot itself; which tends to look like an onion and holds the wash or low wines. The head; which is the top of the still. The swan neck; which is where the still bends, and the Lyne arm; which connects the still proper to the condenser. Pot stills come in a variety of shapes, sizes, and styles. Changes in the design of the still can have a profound impact on the character of the spirit produced. This is because different aspects of still design have an impact on the reflux that occurs within the still. Reflux is the evaporation and condensation of liquid that returns condensate into the original mixture. Reflux happens within a still because molecules transfer energy when they collide with the air, liquids, or internal surfaces which can be enough of an energy loss to take them from being a gas to being a liquid again. This means that less volatile particles are condensing and re-mixing which helps to concentrate and purify the vapours, giving a higher collected alcohol strength. In whiskey production, high reflux isn’t always desirable. A lot of flavour can be lost in a high reflux distilling environment. To minimise their reflux, some distilleries run short stills very fast, which will minimise the temperature difference between the bottom and top of the still. Alternatively, if a distillery wants a purer, more neutral, spirit they utilise tall stills, attach dephlegmators to the Lyne arm or water jackets on the head of the still to increase temperature different between the top of the still and the boiling liquid. Other factors that can influence reflux are; angle of lyne arm, gauge of different parts of the still and temperature of heat source.
Pot stills have a variety of heat sources, these are divided into direct and indirect heating. Traditionally these stills would have been wood fired, a practice a small number of new craft distilleries are trying to revive. Then, when whiskey making industrialised, stills became coal fired. This practice has pretty much died out, but I do know Yoichi in Japan still coal-fire their stills. Wood and Coal firing require a lot of experience to operate, and constant monitoring to ensure an even heat on the still. It also creates increased costs, to manage workplace safety and materials. Some stills are now direct fired using gas flames, which are less labour intensive but still run the risk of creating hot spots and scorching. Direct firing is interesting for flavour creation because the increased Maillard reaction and the associated flavours added to the final spirit has to be weighed against the risk of creating hot spots and scorching the wash, which can ruin the entire spirit run.
The most common way of heating stills, currently, is with a steam jacket. This provides even heat over the greatest surface area, as well as allowing the distiller to control the temperature precisely. A steam jacket also minimises the chance of scorching, so the likelihood of generating off flavours is minimised. Some stills are heated with a steam coil, this is a coiled pipe within the still and offers many of the benefits of the steam jacket, but can be prone to scorching. The rarest form of indirect heat is the external heat exchanger, where the liquid in the still is pumped out, heated and pumped back in, significantly increasing the surface area to volume ratio for heat transfer.
Once the spirit has run through the still it has to be collected. It is incredibly difficult to collect vapour, so this has to be turned back into a liquid. This is done using something called a condenser. Condensers come in two main forms. The first is called a worm tub and is more traditional. It is made from a coiled copper pipe (the worm) and a container of cold water, usually made of wood (the tub). The second type of condenser is the shell and tube condenser, and these are more common to see nowadays within the industry. A shell and tube condenser is made from a bundle of small pipes (tubes) contained within a jacket (shell). The cold water is pumped through the small pipes and the spirit vapours enter the cavity within the jacket. The shell and tube condenser provides a greater amount of copper contact compared to the traditional worm tub, as well as more space efficient heat transfer.
There is another style of still that is used when making grain whiskey. It is known as a column still and has a few important differences to pot stills. The popularisation of using column stills to make whiskey originates with a design by Irish exciseman Aeneas Coffey. A column still is formed of a series of chambers in a tall tower, separated by plates, upon which vapours can rise to the next level and liquids can move down to the previous level. The temperature gradient created within the column still means that there is precise separation of the different fractions on different plates, allowing distillers to collect a very pure and high strength spirit. These rectification plates have many different designs, but the most common types are: The bubble plate and the sieve plate. The rectification plate works by forcing vapours to rise through boiling liquid on each level which massively increases reflux. These plates must have a method of allowing vapour to rise to the next level, and have a downcomer to let liquid flow down to the previous level. A column still is usually steam heated, either with external heat exchangers, or steam injection. A major advantage of the column still is that it can be run continuously, as long as equilibrium is maintained. This is why the 7 grain distilleries in Scotland produce more litres of whisky a year than the 140 malt distilleries combined, and at a fraction of the cost. Equilibrium is maintained in a number of ways. Firstly, The volume of liquid added to the still has to equal the volume being taken off. Secondly, the temperature gradient must be consistent. And finally the mash bill of the wash has to be consistent, so the conjoiners collecting on each plate remain consistent.
Sulphides are a natural and unavoidable byproduct of fermentation. The issue is sulphur is concentrated by distilling. If it’s allowed to reach too high a concentration, it can ruin the collected spirit. Sulphur in spirit can present as a savoury or meaty aroma. However, in high levels it tastes and smells of rotten eggs. Luckily, sulphur binds strongly with copper to form copper sulphate, which is one of the main reasons copper is still used in distilling. Copper has other benefits to distillers: Firstly, it has excellent heat conductivity, allowing even and responsive heating. The lattice structure of copper atoms leaves delocalised electrons which readily transfer energy. Copper reacts quickly with oxygen to form a film of copper oxide, which is relatively inert, meaning that copper stills can last a very long time if properly maintained. A further benefit of using copper in pot still construction is its antibacterial qualities. Copper is known to be a “contact killer” of bacteria, yeast and viruses. Copper vessels have been used since ancient Egypt to sterilise water, so before water-purification was understood, copper stills were useful in making washes safe for human consumption. These antimicrobial qualities are still useful, even if we have more modern methods to ensure the potability of the finished product. Finally, Copper is still used because it is visually appealing. The imposing quietude of the gleaming stills lined up in a stillroom is a singular experience; and the heat and noise of a still running full tilt is magnificent.
Spirit collected off a still is separated into 3 groups: heads, hearts, and tails. They are also called foreshots, middles, and feints. The hearts contain potable spirit. the heads and tails contain volatile fractions that are either detrimental to flavour or potentially dangerous to health. An amount of group 1 and group 3 fractions are usually included in the heart cut, because in low levels they can add aroma, taste, and texture to the final spirit. The heads and tails still contain ethanol, so are usually recycled back into the first distillation.
In Scotland, whisky is usually double distilled, meaning it goes through a wash still and then a spirit still. In Ireland, triple distillation is more common, where a second spirit still is added to the process. There are examples of triple distillation in Scotland and double distillation in Ireland. So, this difference isn’t enshrined in law, just common practice. Adding in the third distillation ends with a higher strength spirit containing fewer congeners, which tends to be perceived as smoother.
Spirit coming off of a whisky still cannot yet be called whisky. It is referred to either as new make spirit or British plain spirit. It must rest in casks for a minimum of 3 years before we can call it whisky.
