The Distillation
Distilling with a Scottish Pot Still
This article is about the operation of the stills. Of course, there are differences in operation between the stills of individual distilleries. Some heat up quickly and then burn slowly and others heat up and burn quickly. It is not only the geometric shape of a still that determines the taste of the raw whisky, it is above all the distillation procedure that has a significant effect on the taste or the aromas present in the raw whisky.
The Principle of Distillation
The principle of distillation was already known to the Egyptians. Substances can be separated from each other by heating them to different evaporation temperatures. The substances that evaporate first at low temperatures can be collected and thus separated from the rest. But the Egyptians only knew distillation for perfume production. It was not until the Middle Ages that Celtic monks discovered the production of whisky - the water of life.
Due to alcoholic fermentation, the wash (beer) contains around 8 to 10% alcohol (ethanol = ethyl alcohol). The alcohol content depends on the yeast used and the duration of fermentation.
If the wash is heated, the substances that boil earlier than the water evaporate from the beer as the temperature rises. The mixture of water and alcohol results in a new boiling temperature. An alcohol-water mixture with an alcohol content of 8-10% results in a boiling temperature of about 93°C. By further heating after this temperature, the mixture starts to boil. During boiling, the alcohol content of the mixture decreases because the alcohol evaporates faster than the water. The alcohol content in the steam is higher and the alcohol content in the remaining bubble is lower.
The following diagram shows the basic structure of a Scotch malt whisky distillery with two stills and the distillation process. Distilleries with triple distillation and three stills expand analogously by one stage. Many larger distilleries have four, six or even more stills, but they are operated in parallel pairs rather than one after the other.
The interconnection of the stills can be even more complex if, for example, the first distillate from several wash stills or several production runs of a single wash still is directed to a single spirit still. A 3:2 or 4:3 ratio of wash to spirit stills is also common. It is also possible to triple distil on two stills by feeding the end product of the second distillation in the Spirit Still, as a whole, to the empty Spirit Still again.
The diagram shows a simple distillery with one wash and one spirit still.
The Wash Still
The task of the wash still is quickly described. It is used for the first distillation of the wash or, more commonly, the beer. The capacity of the still and the fermentation tank (wash back) are usually matched to each other. 15,000 to 30,000 litres are the rule.
When superheated steam is introduced into the heating cylinders of the wash still, the wash begins to heat up. The heat movement (convection) inside the still causes the wash to turn over. The wash rises in the warm areas of the heating cylinders and then sinks back down in the cooler areas.
After about half an hour, the time has come. Above the heating cylinders, the liquid first begins to boil and light substances (especially aroma-bearing esters) rise into the air space above the liquid level.
The constant supply of gaseous substances leads to a slight overpressure in the still and the gases rise into the neck of the still. But their journey does not last far. The wall of the still is still too cold and the evaporated substances settle on the wall of the neck in small droplets. Over time, more and more droplets collect on the wall and join together to form larger drops, which flow back into the Wash Still's boiler following the force of gravity.
As the energy increases, the entire wash begins to boil at a temperature of about 78 degrees. The whole still heats up and the surface of the liquid becomes agitated. Due to the high heat supply, powerful bubbles form and the liquid foams up and splashes into the upper neck of the wash still.
Only in the Wash Stills are there glass windows in the neck behind which you can observe the bubbling wash. If the wash is boiled too high, liquid can enter the condenser through the lyne arm. This would not be so bad in itself if the wash did not still contain solid components of the barley grains, which clog the thin tubes of the condensers.
Great attention is therefore required of the stillman. In distilleries that do not want to take enough time for this monitoring of the boiling, soap is therefore added to the wash, which dissolves the surface tension of the wash and effectively prevents boiling over. Since the soap becomes liquid at 50 - 60 degrees and boils only at well over 100 degrees, there is no danger of soap molecules spilling over into the distillate.
The first distillation in the wash still usually takes about 4 to 7 hours. The temperature of the wash still is around 78 degrees. This is the temperature at which ethanol evaporates. The entire heat supply to the pot still via superheated steam is used up for the evaporation of the alcohol.
Low Wines
At the end of distillation, all the alcohol has evaporated down to about 1% vol. and has been collected in the Low Wines Receiver - a collection container.
This Low Wines Receiver contains not only pure alcohol. All substances with a lower boiling point than alcohol are contained in it. Substances with a slightly higher boiling point were also torn out of the molecular bond by the strongly bubbling liquid and pulled over into the Low Wines with the light alcohol molecules. These molecules also include a large amount of water, which forms an azeotrope with the alcohol. After the first distillation, the low wines have a typical alcohol content of 20 to 25 % vol.
Pot Ale
At the end of the distillation process, the pot ale (also called spent wash) remains in the wash still with a residual alcohol content of about 1 % vol. But not only alcohol remains in the liquid. Valuable proteins and minerals from the grain are also found in the pot ale. This is why the pot ale is thickened after draining and sold as high-quality animal feed.
Only a few distilleries have such evaporation plants, as they are associated with high investments. The town of Rothes in Speyside operates such a plant jointly for all distilleries in the vicinity. In order not to lose the residual heat energy in the pot ale - after all, it is still around 70 degrees - during tanker transport, the pot ale is usually drained via a heat exchanger that feeds the heat energy back into an upcoming distillation process.
The large stills have walls that are only a few millimetres thick. They are therefore very sensitive to overpressure and underpressure. The greatest accident to be expected of a still is collapse due to negative pressure. When distillation stops, the pot ale is drained and the still cools down, negative pressure is created inside. If this negative pressure becomes too strong, the still will collapse with a loud bang. As this has happened more than once in the past, an automatic overpressure and underpressure safety valve is now fitted to every still to ensure pressure equalisation with the environment. For filling and emptying the stills, there is another venting valve, which is nowadays usually operated automatically with the pumps.
Distillation Balance 1
The distillation balance of wash still distillation is compiled in the following table.
| Wash | Low Wines | Spent Wash | |
|---|---|---|---|
| Total in litres | 30.000 | 11.252 | 18.748 |
| Alcohol content in % vol | 10% | 25% | 1% |
| Alcohol in litres | 3.000 | 2.813 | 187 |
The figures in the distillation balance show that the reduction in the amount of water from the Wash to the Low Wines - significantly reduces the second distillation amount for the Spirit Still.
In summary, the first distillation serves exclusively to reduce the amount of liquid to 1/3 and to retain solid components of the grains that were still in the wash.
Usually this first, heavy distillation lasts only four hours. The influence on the taste of the final whisky is therefore small. The extreme heating of the wash and the strongly boiling liquid ensure that, in addition to the alcohol and a larger amount of water, many heavy fusel oils also pass into the low wines.
The Spirit Still
The second distillation in the smaller Spirit Stills is much more careful and slower. Typically, the second distillation takes about 8 hours. As this is twice as long as the first distillation, the result of two wash distillations is often collected together in the low-wines receiver and fed into the Spirit Still as a whole.
The Spirit Still has the greater influence on the taste of the raw whisky. The second distillation takes place much more slowly so that the alcohol and aromatic substances separate better from the water. For this reason, the Spirit Stills do not need windows to observe the boiling low wine.
The distillation process itself is the same as in the wash still. The low wines are heated and before the evaporation temperature of ethanol (78 degrees) is reached, the earlier evaporating and slightly volatile foreshots rise first. These foreshots are mainly light compounds such as ethyl acetate and volatile esters, which have a pungent aroma.
The Spirit Safe
The aggressive foreshots are undesirable in the raw whisky and are therefore not fed into the collecting vessel by switching the distillation flow in the Spirit Safe. The principle picture from above is shown again here for an exact tracing of the task of the Spirit Safe.
The Spirit Safe has a long history and a special task. With distillation, the British customs authorities require all stills and pipelines to be sealed. The stillman therefore has no way of tasting the raw whisky. So how does he determine when the foreshots have run through and the good-tasting heart of the distillation has begun?
Experience teaches the stillman that the first clue is the time of heating until the middle cut appears. First, there is a long period of heating until the first distillate becomes visible in the Spirit Safe. After all, thousands of litres have to be heated to over 70 degrees. The foreshots then run for around 20 minutes. In order to neatly determine the switchover point for catching the core, the Spirit Safe contains several glass containers in which the distillate can be caught and instruments begin to float. If you catch the distillate and measure its density with the buoyancy of a spindle, you can determine the alcohol content via a table. While the foreshots are running, the alcohol content of the distillate drops from around 85 to 75 % vol.
But this is not the only instrument that has to be observed. The density of the liquid depends very much on its temperature. Therefore, the temperature of the liquid is also measured and the density corrected with it. Using the density and the temperature, the alcohol content of the distillate can then be read from a table hanging next to the Spirit Safe.
What happens to the foreshots? They are not poured away but returned to the Low Wines Receiver. But the foreshots do not accumulate through this constant backflow. This is where the true magic of distillation comes into play. Through catalytic reactions with the copper of the Spirit Still, the aggressive foreshots are transformed into pleasant aromatic substances. This is a constantly repeating process and so the amount of foreshots in the Spirit Still remains constant.
Once the foreshots have returned to the Low Wines Receiver, the Stillman switches the flow of distillate in the Spirit Safe and directs the still into the Spirit Receiver for later barrel filling. The Spirit Receiver is basically just the tank in which the raw spirit lies until it is filled into the barrels, i.e. the alcohol collection container. For the filling into the barrels, the blessing of a customs officer is necessary, which is why there is this intermediate storage. The spirit receivers have different shapes and materials, sometimes round, sometimes square, sometimes metal, sometimes wood. If the stillman switches the flow of distillate too early, the whisky tastes too aggressive. If he does it too late, there is no harm, because the liquid that is fed into the Low Wines Receiver is produced again during the next spirit distillation. No alcohol is wasted in this way.
Middle Cut
The distillation of the frog must be done slowly and carefully. If the still is heated too high, the backflow of condensing, higher-boiling substances on the still wall to the lyne arm cannot function properly and higher-boiling substances such as fusel oils also get over into the collecting vessel. Fusel oil is a mixture that can contain medium and higher alcohols, fatty acid esters and other substances such as terpenes, aldehydes and carboxylic acids. It is produced during alcoholic fermentation as a by-product of yeast metabolism from amino acids and serves as a flavour and aroma carrier in alcoholic beverages. However, fusel oil is also suspected of being partly responsible for the 'hangover' in the form of headaches resulting from alcohol consumption.
During the collection of the heart (middle cut) over about three hours of distillation, the alcohol content drops from 75 to 60 % vol. Nevertheless, after the changeover, distillation continues at 60 % vol. The fusel oils (faints) that now appear are again fed into the low-wines receiver for copper-catalytic conversion during the next distillation. The distillation of the faints continues for a long time and is only stopped at a residual alcohol content of 1 % vol in the still in order not to lose valuable alcohol with the liquid remaining in the Spirit Still (Spent Lees).
It is rare to have the opportunity to see the Low Wines Receiver from the inside. It contains a milky grey-white alcohol-water mixture on which the thin oily layer of fusel oils (faints) floats.
Distillation Balance 2
This completes a full distillation. The distillation balance for the second distillation in the following table shows the resulting amount of raw whisky. Assuming the example of 30,000 litres of wash at 10% alcohol by volume, the amount of alcohol obtained with distillation of about 2,780 litres corresponds to a yield of 9.27%.
| Low Wines | Raw Whisky | Spent Lees | Faints & Foreshots | |
|---|---|---|---|---|
| Total in litres | 11.252 | 4.117 | 3.376 | 3.759 |
| Alcohol content in % vol | 25% | 67,5 % | 1% | |
| Alcohol in litres | 2.813 | 2.779 | 34 |
Special attention in the balance has to be paid to the faints & foreshots returned for redistillation. They increase both the liquid quantity of the low wines and their alcohol content. As they are a continuous item from one distillation to the next, they are not given any significance in the balance sheet. This initially confusing state of affairs also causes the alcohol content of low wines in the literature and when named in the distillery to fluctuate within a range of 20 to 27 % vol. For our exemplary balance, we simply assumed an average of 25 % vol in the first distillate and 67.5 % vol in the second run. The fact that the volume of the spent lees is 30% of the first distillate is also an example and, like all the values given in the balances, must be regarded more as a guide than as a characteristic value.
New Make Spirit (White Dog) and White Whisky
Once distillation is complete, the raw spirit is obtained. This becomes the finished whisky (or whiskey) through storage in barrels for at least three years (USA: two years). Just as the minimum storage time differs in Great Britain and the USA, so does the name for the raw spirit. In Scotland and the rest of the world, it is called 'new make spirit'. In the USA, the term 'white dog' has become established. It is not known where the term originated, but it was probably already used by the first American settlers. White' certainly because the brandy does not take on a brown colour without being stored in a barrel. Where the 'dog' comes from, however, is not known.
Some American distilleries also sell their 'White Dog', which is not barrel-aged or only barrel-aged for a few days or weeks, under the name 'White Whiskey', 'White Lightning' or 'Legal Moonshine'. The term'moonshine' per se implies something quite illegal. Moonshining is the illegal, domestic production and smuggling of spirits. As this used to take place mostly at night under 'moonshine', it was also given this name. The resulting distillate is called moonshine. In Europe, it is not allowed to sell a distillate under the name 'whisky' without it having been aged in a cask for at least three years. But in the USA, since 2005, many distilleries have seen an opportunity to sell their 'white dog' to make up the revenue gap until they can sell the first whiskey aged for years.
If you want to know more about the geometric shape of pot stills and how they are technically made, visit the following article.
The Master Distiller
The English term 'Master Distiller' could be roughly translated as 'master distiller' or 'master distiller'. In practice, the title of Master Distiller is not firmly defined. Mostly it is the distilling expert, sometimes Master Distiller also means the manager or head of the whole distillery.
