Fermentation is about so much more than producing alcohol. It’s crucial for the development of flavour compounds and is generally considered to be just as important a stage of whisky making as the process of distillation and maturation that follow it.

But what is whisky fermentation? What happens during this stage of the production process? And how does fermentation affect flavour?

Let’s dive in and find out.

WHAT IS FERMENTATION?

In the world of whisky, fermentation is simply the conversion of sugars into alcohol.

This process is a crucial step between mashing and distilling and sets the stage for the development of flavours and aromas that define a whisky’s profile.

THE PROCESS OF FERMENTATION

Once mashing is complete, yeast is added to the sugary liquid, known as wort, and the fermentation process is started.

The mixture takes time to get used to its new surroundings and will sit for 12 hours or so in the washback. Gradually, as the yeast starts to reproduce, the wort starts to bubble and froth, and in the process, creates alcohol and carbon dioxide, thereby releasing a wide range of flavour compounds in the process.

After fermentation is complete, the mixture, now referred to as wash, is pumped from the washback and into the wash still to start the process of distillation.

YEAST

Yeast, the unsung hero of fermentation, is a single-celled organism that loves sugar!

Different yeast strains produce varying flavours and aromas, which is why distilleries often cultivate their own yeast strains or use specialised ones to achieve the desired outcome. Here at the Borders Distillery, we use Pinnacle-M yeast.

CARBON DIOXIDE

Carbon dioxide is a natural by-product of the fermentation process and plays a crucial role in the overall transformation of raw ingredients into alcohol.

It contributes to the development of the desired flavours and aromas by interacting with the compounds and helps create a suitable environment for the yeast to thrive.

During the fermentation, the carbon dioxide rises to the top creating a kind of invisible blanket, which causes the mixture to bubble and foam, often violently. If you were to take a breath over a washback during fermentation, you would feel a very sharp sensation in the top of your nose similar to consuming every fizzy drink you’ve ever drunk, all at once. Not pleasant.

Carbon dioxide has to be captured and disposed of very carefully. Most distilleries have closed-fume cupboards or carbon dioxide extractors to safely collect it.

WASHBACKS

The washback is a large cylindrical container, usually around 12ft across by 20ft deep that can hold up to 30,000 litres on average. Washbacks have the job of providing a controlled environment for the yeast to thrive and for fermentation to begin.

They usually come in three different types:

Wooden

Often found in older distilleries, wooden washbacks are normally made of pine, oak or larch and have the advantage of efficient heat insulation, making it easier to maintain a constant temperature. The disadvantage is that they are harder to clean and need to be fully replaced every 40 years or so.

Cast Iron

During the 18^th and 19^th centuries distilleries came to see the drawbacks of wooden washbacks and searched for an efficient alternative. At the time, cast iron was thought to be the modern answer to wood and had the advantage that the washback could be sealed tightly.

Today, they are increasingly rare to find as most distilleries replaced them with stainless steel at the earliest opportunity.

Stainless Steel

The vast majority of washbacks are made from stainless steel. Stainless steel is far easier to clean than wood and can be sealed, preventing bacteria and pollutants from getting inside.

TEMPERATURE

Temperature control during fermentation is vitally important.

Distilleries aim to maintain specific temperature ranges to encourage yeast activity and control the rate of fermentation, which is normally 20°C to 32°C.

Higher temperatures tend to produce more robust, fruity flavours, while lower temperatures can lead to a cleaner, crisper taste.

THE MAGIC OF TIME

Fermentation duration is a critical factor in the production of whisky.

While it can vary from one distillery to another, fermentation time usually lasts from 40 to 100+ hours. Here at the Borders Distillery, we tend to ferment our mash for around 80 hours.

FLAVOUR

During fermentation, yeast produces a range of chemical compounds, including alcohols, esters, and fatty acids. These compounds contribute to the whisky’s aroma and flavour, creating complex notes of fruit, spice, and earth.

Longer fermentation times allow for the development of fruitier flavours and aromas. Distilleries that aim for fruity, ester-rich profiles often opt for extended fermentation periods.

THE LONG & SHORT OF IT

And talking of fermentation, we have just released the second instalment in our Workshop Series – The Long & Short of It.

For this edition, our distillers have quite literally ‘played with time’, combining whisky with a short fermentation of 55 hours and an extra-long fermentation of 150 hours, and to say that we’re excited about it would be an understatement!

On the palate, the short fermentation reveals fresh bursts of gooseberry skin, citrus zest, and green apple which plays perfectly with the notes of fig pudding, butterscotch, and sultanas from the long fermentation.

FINAL THOUGHTS

Fermentation is the heart and soul of whisky production. It’s where the magic begins, where simple ingredients transform it into a symphony of flavours and aromas.

So, the next time you savour a dram of whisky, take a moment to appreciate the art, science and nature of fermentation that has made it possible.

And why not book a tour of The Borders Distillery to see the fermentation process in action for yourself?