For those aspiring to become distillers or contemplating setting up a new whisky distillery, the bulk of the journey involves a deep understanding of the distillation process and equipment you will need.
Only once you have the equipment can you you focus on the crucial areas to hone your skills. Except, it’s also a catch 22 situation. You’ll only know what equipment you really need, once you’ve understood the skills used to operate them!
We have covered Grain Intake, Storage & Milling, Mashing, Wort Separation, Yeast, and Whisky Fermentation elsewhere to get to this stage. Now, it’s the turn distillation!
Here’s a handy introduction to help you scope out what’s involved once you begin using stills. The aim is demystify some of the design elements and how equipment relates to process and vis versa.
The two stages of whisky distillation in pot stills
Pot still distillation is one of the oldest methods used for whisky production. It is often considered the traditional type of distillation, especially for Single Malts.
A pot still system typically consists of a copper pot where the wash (the end result of fermentation) is heated and a condensing system where the vapour is condensed back to liquid form.
The process is rooted in the principles of boiling points and vapour-liquid equilibrium. As different compounds in the wash have different boiling points, by heating the liquid slowly and cooling the vapour carefully you can separate them. But you’ll likely already know all about the principles of distillation so we won’t expand on this further!
For Whisky, the distillation process is divided into two primary stages: the “wash distillation” and the “spirit distillation”. These two stages are crucial for achieving the desired alcohol content and flavour profile of the distillate.
Stage 1: Wash distillation (first distillation)
- Heating the wash. In this stage, the fermented wash, containing about 8-10% alcohol by volume (abv), is heated in the copper pot. The choice of copper is essential as copper’s catalytic properties also plays a role in enhancing the flavour by removing some impurities (such as undesirable sulphur compounds).
- Volatile components. As the wash is heated, volatile components like alcohol and congeners (flavour compounds) evaporate. Water, being less volatile, mostly stays behind.
- Collection of “low wines”. The vapour rises through the pot still and goes into the condensation system, where it is turned back into liquid. This liquid, referred to as “low wines,” has an alcohol concentration of about 20-25% abv.
- Discarding the “pot ale”. What’s left in the pot still after this first distillation is called “pot ale”. It’s a mix of water, yeast and barley residues, soluble protein and carbohydrate, and variable copper levels. This is usually discarded or used as animal feed.
Pot distillation is a very simple process that’s centuries old. That said, it’s not without small details that can make a difference to the quality.
Details for those learning about or starting to distil whisky
Typically, the fist distillation is done at reasonable pace, partly due to the volume of liquid that needs to be distilled being high, but also due to the fact that almost all the distillate created (called low-wines) will be used. There’s less of a need for highly precise flavour based cuts.
Slowing this process down by using cold jackets higher up the neck of the still, or thumpers / dephlegmators to increase reflux can increase the %ABV, while reducing the overall fluid being distilled over. For those who are re-using smaller hybrid stills, this can be a good idea, but as most don’t – it’s a false economy due to the added time it takes.
This makes more sense when you look at the quantities involved.
Let’s assume the starting ABV is around 8-10% ABV (based on a typical barley ferment). Based on the pure alcohol measurement – this means only 12-14% of the overall total of the liquid being charged into the still is going to be of interest to you once it’s finished the two distillation runs.
Assuming you has an efficient modern still, for every 100 units of incoming wash about 60 units become pot ale, the residues after the first distillation. The rest , only 40 units, goes into the second distillation.
That’s already a volume reduction of over half the fluid. It also explains why the wash still is always bigger than the spirit still when you see them in distilleries.
In some historic Scotch distilleries, the quantity of pot ale remaining after the first distillation is even higher. Over two-thirds of the charge…
What is pot ale and what is done with it?
Pot ale is characterised by its pale yellow, golden hue and has a distinct aroma of burnt malt cereal and yeast. The primary constituent is water (as the distillation has extracted the alcohol).
In addition to water, pot ale contains inactivated yeast and proteins originating from barley, forming a kind of ‘slurry’. Depending on the wort separation and fermentation, it’s possible there are still some solid remnants present, such as fragments of barley husks.
While distillation is an efficient way to separate alcohol and water, it’s not 100% efficient. The alcohol content in pot ale is typically around a 0.2 – 0.5% ABV. For most, the process of distilling this residual alcohol would consume an excessive amount of energy, making it economically non-viable.
Instead, the pot ale is run through evaporation units and condensed into pot ale syrup, which is sold to farmers to feed cows.
By running it through an evaporation process, it reduces bulk waste from over 20 tonnes of water to around 1 ton of solids. From a distiller’s perspective, that makes it far more practical to move around and dispose of. It also greatly reduces the costs involved in transport.
Stage 2: Spirit distillation (second distillation)
Distilling Low Wines. The “low wines” from the first distillation, now around 25%ABV are put back into the pot still for a second round. This further concentrates the alcohol and flavour compounds.
Fractionation. As the vapour rises through the pot still during this second distillation, it is often passed through a “spirit safe” allowing the distiller to accurately separate the distillate into different fractions: the “heads,” the “heart,” and the “tails.”
Heads: The first fraction to come off, containing higher amounts of undesirable compounds (like methanol). This is either discarded or returned for another round in the next batch going through the wash still.
Heart. The middle fraction is the most desirable distillate and what will become whisky after aging.
Tails: The last fraction, contains heavier compounds that could add undesired flavours if included in large amounts. This is usually returned for another round in the next batch going through the wash still.
The second distillation not only concentrates the alcohol but also enables complex chemical interactions. Esterification, for example, can occur, combining acids and alcohols to produce esters, which contribute to the whisky’s aroma and flavour.
As for the numbers and the waste?
Let’s continue the math for that same 100 units of wash (60-65 of which have already been discarded as pot ale). Only 40 went into the second distillation.
After the second distillation, around 2-5 are unwanted heads, about 15 units are captured in the hearts and go on to be matured into whisky. A further 5-10 in tails that are recycled and sent back to the next round of distillation.
This leaves us with possibly as little as 10 units or as much as 20 left in the still that wasn’t distilled over. While the principle is the same – it’s liquid that didn’t distil over, it’s different to pot ale as it is made up of different compounds. It is called lees.
Spent lees is an oily, opaque liquid, with a slightly oily aroma. As the stills have done their job separating the alcohol out from 25% to around 70% abv, the alcoholic concentration within the spent lees left over in the pot is typically less than 1% ABV.
Two stages – almost total extraction
In summary, the two stages of whisky distillation in pot stills are a harmonious blend of art and science. They are deeply rooted in tradition, but underpinned by chemical principles.
They raise the percentage of alcohol from a starting 8-10%ABV to an end quantity of around 70%. All the while the concentrate rich, complex flavours that are appreciated worldwide. Almost all the useable alcohol is extracted and the two waste products (or by-products) have less than 1%ABV.
Because the amount of fluid you start with is large and the amount of distillate it results in is small, it can seem like a huge feat of reduction. However, the process is remarkably efficient at separating the alcohol, and remarkably good at giving you options to capture specific flavours.
The distillate from kept in the hearts cut, also known as New Make Spirit, is now moved onto maturation, where it’s cut with water to optimal strength, usually around 63-65% and placed in oak casks.
Design of pot stills: Influence on quality and whisky distillation
The design elements of a whisky still, from the shape to the size and all the extras, greatly influence the quality of the final product as well as the efficiency of the distillation process.
Core components of a pot still
Pot: The pot is the bulbous bottom part where the initial charge is placed and heated.
Swan neck: This is the curved pipe leading from the pot to the Lyne arm.
Lyne arm: This is the horizontal or angled pipe that carries vapours from the swan neck to the condenser.
Condenser: This component cools down the vapours back into liquid form.
The science of reflux and condensate
Reflux refers to the process where some of the rising vapours condense back into liquid form and fall back into the pot, to be re-distilled. The design of a still impacts on the amount of reflux and therefore, the type of spirit being produced.
Temperature gradients also play a role. The temperature in the still is not uniform; it’s hotter at the bottom and cooler at the top as the pot is the part being heated. The taller the still, the greater that differential is and the greater the energy needed for the heat to rise and the vapour to climb. Tall stills encourage a lot of reflux and therefore a lighter spirit.
Heating and cooling
The way a sill is heated influences the speed at which vapours rise.
A slow rise means more time for vapours to condense back into liquid and fall back, increasing the amount of reflux. This might help with water and alcohol separation but it will also impact flavour (positive or negatively depending on your intention). From a design perspective, picking the right heating mechanism to respond to your desired distilling tempo is key.
The type of condenser used, such as a shell and tube or worm tub, also affects the rate of condensation and thus the amount of reflux.
There are operational considerations too. The design of the pot still can make it easier or more challenging to clean and maintain. Furthermore the size and shape of the pot still influences how easily the distillation process can be scaled up for larger production volumes.
It’s worth being mindful of the wear and tear of copper. Different components degrade at various rates. Modular designs with bolted flanged segments facilitate easier repairs. Finally, while it’s easy to overlook at planning phase, keep in mind how anti-vacuum, pressure relief, and overfill protection mechanisms are integrated into the day to day process process.
The role of the lyne arm and its angle
A lot has been said about the angle of Lyne arms in whisky circles, namely as an industry in-joke for the level of detail enthusiasts will speculate over. Many of us are proud nerds! From a design perspective though, joke or not, it’s likely that it has an impact on the amount of reflux a still produces.
A Lyne arm angled upwards promotes lighter spirits as more reflux occurs. A downward-angled Lyne arm will result in less reflux and thus a heavier, oilier spirit. It might be marginal at the point it occurs compared to the huge surface area of stills, but it’s a detail that can further augment a House Style should a new distillery founder want to optimise every possible detail in achieving it.
Pot still recap
So far, we’ve introduced you to the traditional pot still distillation method and explained its basic components: the copper pot and the condensing system. We’ve highlighted the importance of the two primary stages of whisky distillation: “wash distillation” and “spirit distillation”.
Lastly, we’ve sought to demystify the design elements and the interplay between pot still equipment and process, as well as some of the numbers involved.
Whether you’re considering getting into the industry or getting started distilling yourself – understanding these processes and their equipment is a crucial step in your whisky journey.
Let’s move onto the other side of whisky distillation and the often gargantuan operations – Column stills and continuous grain distillation…
Grain whisky distillation & continuous distillation
The Coffey still has been a cornerstone in the continuous distillation of grain whisky, particularly in Scotland. This technology dates back to the 1820s and has set the standard for Irish, American, and Canadian whisk(e)y production as well.
It consists of two primary columns: the stripping column (also known as the analyser) and the rectifying column. Both are designed for consistent, uninterrupted operation and contribute to achieving several key objectives:
- Increasing production levels for greater efficiency (it’s continuous).
- Producing a lighter spirit suitable for various blends (it distills to a high ABV).
- Reducing the cost per litre of alcohol, making it economically viable for certain products to exist.
Components and operation
The Coffey still, an innovative apparatus in the realm of distillation, is characterised by its dual-column structure.
Stripping column
The stripping column begins the process.
Just like it does for a pot still, typically the wash that enters a stripping column has an alcohol concentration of about 8-10% abv. It’s often preheated too, and at a temperature ranging from 30-40°C.
As it’s dropped into the column, it encounters rising vapour and starts to change its state from liquid into vapour. Think of it as being like the separation in the wash still, except that rather than being forced to heat everything from the base, by having steam rising and wash dropping in, both inputs can be run continuously once in equilibrium. You don’t need to stop to refill, you just pump in more.
Once it’s up and running, the vapour at the top of the stripping column is rich in alcohol, ready to be condensed temporarily as it travels over to the next column. The separated water from the wash, now almost devoid of alcohol (termed ‘spent wash’) continues to fall down and is run off lower down and collected.
A huge part of getting the process working is the right amount of steam. You need it to be hot and for there to be enough steam. But it can’t be too hot, in order for alcohol to separate, and for the water to continue cascading down.
Rectifying column
Concentrated alcoholic vapour from the stripping column makes its way to the base of the rectifying column. The rectifying column takes that initial, alcohol-laden vapour from the stripping column and further concentrates it by forcing it to rise through trays (or plates) that create a huge amount of reflux.
Equipped with approximately 40-50 plates or trays, these each one acts as a step where the vapour undergoes continuous enrichment. There’s continuous changing from vapour to liquid and back again, and with each plate, the percentage alcohol gets ever higher.
There’s two advantages of this system. The first is the continuous nature of it. You don’t need to stop to reload the next batch. You just need to maintain the right levels of input (wash and steam), and effectively separate out the component parts along the way.
The second is the level of alcohol you can reach – easily over 90% ABV, should you want to.
Unwanted component separation
Just as with pot still distillation, you have to remove unwanted compounds. In a pot still set up, they are managed by taking effective heads and tails cuts. The issue is that with a continuous still set up – it doesn’t come through in sequence. New input is constantly going into the still, product is constantly coming out…
Therefore, you have to separate them and remove them constantly as well. The way this is done is by understanding their boiling points compared to water and ethanol.
This starts by collecting product near the top, not at the top of the column.
This allows very highly volatile components, (including some alcohol and a very small portion of water) to continue rising and be separated.
In practical terms, the equivalent “heads” are being removed from the very top of the rectifying column, while the “hearts” are being drawn off three or four plates lower.
Other alcohols are concentrated at various points lower down the rectifying column, allowing them to be separated too.
For example fusel oil is a mixture of heavier alcohols and organic compounds that are by-products of the fermentation and distillation process. These alcohols include amyl alcohol, butanol, and other components that can impart off-flavours or odours to the distilled spirit.
Fusel oils will profoundly affect the taste of the spirit being made and are typically removed in the tails cut. To avoid these coming through, column still designs feature a specific component known as the fusel oil decanter system to manage the separation.
This mini column usually is connected to a specific plate where fusel oils predictably accumulate in significant quantity. Once separated, fusel oil can be drawn off for commercial applications, such as in the cosmetics or industrial sectors.
A note on fusel oil separation
Proper separation of fusel oils makes the continuous distillation process more efficient by reducing the need for additional refining steps once the spirit has been collected.
Lastly, depending on the type of whisky being made and the region it’s made in, there may be regulations regarding permissible levels of certain impurities, including fusel oils. In these areas, separation becomes a compliance issue.
And there you have it, two of the most common ways of making whisky. Old school pot stills vs continuous columns. Each involve nuanced proficiency to carry out.
Column stills are obviously more complex in both design and how they operate. There is a lot going on inside the stills, and a lot of areas to monitor whilst distilling. Typically, they are only used by big operations with the technical skill set to be able to run them.
Moreover, they require constant feeding! Mashing and fermentation must keep up with the input demand the still creates, so do the shift patterns of the distilling team…
For those starting up, gaining in-depth knowledge of how each operate is important. Most will choose pot stills for their operation. That said, understanding how columns work helps frame your mindset as to what you want to create and what’s possible. It will help you design a distillery that delivers your ideal whisky each and every time.
