Fermentation and distillation: where whisky's character begins
How fermentation creates character, what stills do to the spirit, and why distillery choices define everything downstream.
- Fermentation turns sugar into alcohol and creates many of whisky's fruity flavours.
- Distillation concentrates the alcohol and decides how light or heavy the spirit is.
- Pot stills make richer, heavier spirit. Column stills make lighter, cleaner spirit.
- The distiller chooses which part of the run to keep. This is called making the cut.
- These choices are made before the whisky ever sees a cask, so they define its basic character.
Fermentation: making the wash
The wort is cooled to around 20 degrees Celsius and pumped into large vessels called washbacks. Yeast is added, and it feeds on the sugar, producing alcohol and carbon dioxide. The liquid bubbles and foams, and the temperature rises. After two to four days the sugar has been used up and the result is a weak, beer-like liquid called wash, at around 8% ABV.
It is not simply a step on the way to alcohol. The yeast makes hundreds of flavour compounds, many of them fruity, such as esters. Later in fermentation, bacteria can add further complexity.
What changes the wash
Distillation: concentrating the alcohol
Alcohol boils at a lower temperature than water. When the wash is heated in a still, the alcohol and the flavour compounds that go with it turn into vapour first. The vapour rises, is cooled in a condenser and runs off as liquid that is much stronger than the wash.
What goes into the vapour, and what is left behind, depends on how the still is built and run. That is why two distilleries using the same wash can make very different spirits.
Pot stills
Pot stills are large copper vessels with a bulb-shaped base, a neck and a pipe called the lyne arm leading to the condenser. They are used one batch at a time, which is why they are called batch stills.
Scotch single malt is usually distilled twice. The first still is the wash still. It turns a wash of about 8% ABV into a liquid called low wines, at roughly 20 to 25%. The low wines then go into the spirit still, which makes a much stronger spirit of around 70%. Some distilleries distil three times for a lighter spirit, including Irish distillers and Auchentoshan in the Lowlands.
How still shape changes the spirit
Spirit that is in more contact with copper, or that has to travel further, comes out lighter.
Making the cut
In the second distillation, the spirit that comes off the still changes as the run goes on. The distiller divides it into three parts:
- Foreshots (the heads). The first part. It is sharp and harsh and is set aside.
- The heart. The middle part. It is the part that is kept and put into casks.
- The feints (the tails). The last part. It is heavy and oily and is also set aside.
The feints and foreshots are normally added to the next batch of low wines. Where the distiller starts and stops collecting the heart is called the cut points. A wide cut gives a heavier, more characterful spirit, and a narrow cut gives a lighter, cleaner one. Most of it is done by watching the strength of the spirit through a glass box called the spirit safe.
Column stills
A column still, also called a continuous or Coffey still, is a tall tower with a series of plates inside. Wash is fed in continuously, and the vapour passes up through the plates, getting stronger and purer at each one. It runs without stopping and can produce spirit at a much higher strength than a pot still, so the result is light, clean and neutral.
Column stills are used for grain whisky, for much of the whisky in blends, and for most American whiskey. Bourbon is distilled to no more than 80% ABV (160 proof) so that it keeps flavour from the grain. Often a second pot-style still called a doubler is used after the column. See single grain and bourbon.
Why stills are made of copper
Copper reacts with sulphur compounds in the vapour and removes them. Without copper, the spirit would smell of rotten eggs or cabbage. The contact with copper also helps to make the spirit sweeter and cleaner.
What comes off the still
The result of distillation is a clear, strong spirit called new make. It is usually diluted a little before it goes into casks. It already has a lot of the distillery's character, whether fruity, cereal, oily or smoky, but it is raw and sharp. The next step is maturation, which turns it into whisky. See maturation explained and how whisky is made.
How fermentation time changes flavour
Most of the sugar is turned into alcohol within the first day or two, and the liquid reaches about 8% ABV. This fermented liquid is called the wash, and it is essentially a rough, malty beer without hops.
If fermentation is stopped at around 48 hours, the result tends to be cleaner and more cereal-like. If it runs on for 75 to 100 hours or more, a second phase begins in which naturally occurring bacteria produce further compounds. These often add fruity, creamy and sometimes sour notes that survive distillation. Some distilleries are known for long fermentations because of the fruitier spirit they get.
Wood or steel washbacks
Washbacks are made from wood, often Douglas fir or larch, or from stainless steel. Stainless steel is easy to clean and keeps temperatures stable. Wood is harder to sterilise and can harbour natural bacteria in its pores, which some distillers believe contributes complexity. In practice, the difference is subtle, and both kinds make excellent whisky.
The numbers in a double distillation
In a typical Scotch malt distillery, the numbers run roughly like this:
- Fermentation produces a wash of around 8% ABV.
- The first distillation in the wash still produces low wines of roughly 20 to 25% ABV.
- The second distillation in the spirit still concentrates the heart of the run to around 70% ABV.
- Reduction. Water is added to bring the new make down to about 63.5% ABV before it is filled into casks, a strength that is standard for Scotch.
Exact figures vary from distillery to distillery and from batch to batch, so treat these as rough guidance.
Reflux, lyne arms and condensers
Three parts of the still have a large effect on style, even though they get less attention than the shape of the pot:
- Reflux. When vapour rises and meets cooler copper, heavier compounds condense and fall back into the pot for another go. More reflux means a lighter, cleaner spirit. Tall stills, narrow necks and boil balls all increase reflux.
- The lyne arm. This is the pipe that carries vapour from the top of the still to the condenser. An upward-angled lyne arm encourages more reflux and a lighter spirit, while a downward-angled one lets more through and gives a heavier spirit.
- The condenser. Modern shell-and-tube condensers cool the vapour quickly and give more contact with copper. Traditional worm tubs, long copper coils sitting in cold water outdoors, cool more slowly and produce less copper contact, which is often said to leave a heavier, meatier character. Talisker, Dalwhinnie, Mortlach and Cragganmore are among those that still use them.
Double, triple and beyond
Most Scotch malt is distilled twice. Triple distillation, standard in Irish whiskey and used at Auchentoshan in Scotland, runs the spirit through a third still. It produces a lighter, smoother spirit. Springbank takes a different route, distilling some of its spirit two and a half times.
Each extra distillation gives the distiller more control, but it also strips out character, so the choice is a deliberate decision about style. See single pot still for how Irish distillers combine malted and unmalted barley in pot stills.
How American whiskey is distilled
Bourbon distillers generally use a column still as the first still, followed by a second pass in a pot-like "doubler" or "thumper". The fermented mash, which includes the grain and is known as the beer, is fed straight into the column, so the distillery does not need to filter the solids out first. The result is a stronger, heavier spirit than most Scotch, and a lower proof is used when filling barrels. Straight bourbon cannot be distilled above 160 US proof (80% ABV), and must go into the barrel at no more than 125 proof (62.5% ABV).
Still terms in plain English
Fermentation and distillation: quick answers
Why is whisky distilled twice?
The first distillation concentrates the alcohol from about 8% to about 20 to 25%. The second concentrates it further and lets the distiller choose which part to keep. Two runs give a good balance of flavour and strength.
What is the difference between a pot still and a column still?
A pot still works in batches and makes a richer, heavier spirit. A column still runs continuously and makes a lighter, cleaner spirit at a higher strength.
What are the cuts in distillation?
The cuts are the points at which the distiller starts and stops collecting the heart of the run. The first part (foreshots) and last part (feints) are set aside.
Why do some distilleries distil three times?
A third distillation makes a lighter, smoother spirit at a higher strength. Irish distillers traditionally triple distil, and Auchentoshan does the same in Scotland.
What is a worm tub?
A worm tub is an old-style condenser made of a long copper coil sitting in a tub of cold water. It tends to make a heavier, more savoury spirit than a modern condenser.
Maturation explained
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