The fermentation process is the stage where the foundation of all whisky character is established. Yes, distillation furthers this and ageing brings even more layers into the mix – but you can’t make great whisky without making a good wash first.
The difference between a good ferment and a bad one isn’t just ABV, time and money either. It’s about FLAVOUR!
Understanding the dynamics of fermentation – the interplay between yeast, sugars, and environmental factors – will allow you to understand what’s happening in the fullest sense.
In this article, we delve into everything you need to know about the fermentation process in the context of whisky making…
From Mash to Wort
Before fermentation begins, distillers must prepare a quality mash, which is a mixture of water and grains (usually barley, rye, corn, or wheat).
We have a separate article about Mashing here. But in summary – grains are milled to facilitate the extraction of fermentable sugars, these are then mixed in with water of varying degrees of temperature.
So many elements of good fermentations start at mashing phase, as without the optimal enzyme activity and starch conversion, there’s not much that can be done to fix this thereafter.
Once the mashing phase is done, distillers filter the (bulk of) grains from the extremely sugary water, which they call wort.
Yeast is then added to the wort, in order to convert fermentable sugars into alcohol and the fermentation process can begin.
Fermentation process
Fermentation is initiated when yeast is introduced into the wort, instigating a series of complex biochemical reactions. While it’s one continuous process, each sub-stage of fermentation has a name / term, and each comes with nuances that are worth understanding.
Below is the fermentation process, but for more on the elements involved, we’ve written more about Yeast & Bacteria and the role they play separately.
Lag Phase
Once a distiller has added the yeast, it’s really anticlimactic. But although it looks like nothing is happening – there’s actually a lot going on. The yeast is acclimating to the new environment, absorbing nutrients and undergoing a period of adaptation. This phase can last several hours.
Yeast acclimatisation. The yeast undergoes a period of adjustment, getting accustomed to the environment and absorbing the necessary nutrients from the mash.
Cellular respiration. Yeast cells respire aerobically, using oxygen present in the liquid (and headroom of the tank), producing essential cellular components necessary for fermentation.
Preparation for reproduction. Yeast cells prepare for a period of rapid reproduction by increasing their mass and ensuring the synthesis of vital molecules.
Exponential phase
After a while, things start to happen. The yeast starts consuming the sugars, resulting in a burst of activity where they reproduce rapidly, and the production of ethanol and carbon dioxide commences.
Yeast reproduction. Yeast cells reproduce asexually through a process known as budding, multiplying exponentially and dominating the environment.
Ethanol production. The yeast consumes sugars to produce ethanol and carbon dioxide through anaerobic respiration, a key step in alcohol creation.
Heat generation. This phase generates significant heat, necessitating careful temperature control to avoid hindering the yeast activity.
Stationary phase
As the sugar levels deplete, the yeast’s growth stagnates. But that doesn’t mean it’s the end of the ferment. It is during this period that crucial flavour compounds, such as higher alcohols and esters, are formed, defining the whisky’s eventual flavour profile.
Decreased sugar levels. As the sugar content decreases, the yeast’s growth plateaus, marking the transition into a slower, steady state of activity.
Congener formation. Flavour and aroma compounds, known as congeners, are produced during this phase, which plays a crucial role in defining the whisky’s character.
Alcohol tolerance. Yeast cells are gradually reaching their alcohol tolerance limit, beyond which they cannot survive.
Decline phase
Eventually, the yeast succumbs to the high alcohol environment created by its metabolic activity, signalling the end of fermentation.
Nutrient depletion. A decrease in essential nutrients leads to a reduction in yeast activity, with cells entering a state of dormancy.
Cell death. Many yeast cells die during this phase, unable to withstand the high concentration of alcohol and diminishing nutrient supply.
Autolysis. One of the reasons distillers don’t let completed ferments sit is that yeast cells start to lyse. This can potentially release undesirable flavours into the wash.
Optimal conditions
During fermentation, distillers must maintain a steady temperature between 20-30°C is crucial. Too high temperatures can kill the yeast, while too low temperatures will slow their activity.
Temperature fluctuation isn’t just from the environment – it rises from the process itself as mentioned above.
The fermentation duration can span from 48 to 96 hours, depending on the desired product profile. Longer fermentation times generally yield richer flavours and a higher concentration of congeners.
Does it matter if you have wooden washbacks, or stainless steel tanks?
We’ve all seen images of Scotch Whisky distilleries and their glorious historic fermentation halls.
Wooden washbacks (the vessels in which fermentation takes place), typically made from Oregon pine not only look impressive, but they also ooze tradition and alter the entire look of a tun room. Stainless steel washbacks just don’t have the same visual effect – although, we think they are glorious in a different way…
When it comes to performance, there is quite a big difference between them however. Wooden washbacks have an insulating quality, protecting the fermentation from the cold, which sometimes comes in handy in winter. That said, stainless steel tanks allow for cooling jackets – great for manually adjusting the temperature on the go.
The most important difference between the two is hygiene. Stainless steel is more easily cleaned and requires less maintenance. Meanwhile wood harbours certain bacteria that have their own profound influence on flavour creation. That can be a positive influence, or a really negative one depending on what’s growing and how it’s managed.
They both have advantage and drawbacks. In the end it is not so much a question of which type of fermentation vessel is better; it is a matter of which a distiller prefers.
Monitoring alcohol levels during the fermentation process
All distillers want maximum flavour, but they also want two other elements too – consistency and good alcohol yield.
Through the fermentation process, alcohol levels will steadily increase, reaching a final Alcohol by Volume (ABV) of 7-10% for most whisky ferments. Consistent monitoring is vital to achieve the desired ABV. For obvious reasons – as it has a substantial effect on downstream processes like distillation.
Alcohol concentration. Regular monitoring helps in tracking the increase in alcohol concentration, aiming to achieve a final ABV within the 8-12% range.
Flavour development. The level of alcohol can influence the flavour profile, making monitoring a vital part of crafting a quality whisky.
Preparation for distillation.The alcohol content at this stage can impact the efficiency and outcomes of the distillation process.
Once fermentation is complete, the wash – now a mixture of water, alcohol, and small particles fermented grains – is ready for distillation.
It is essential to harvest at the right time to prevent unwanted flavours from developing due to yeast autolysis (as mentioned above) or bacteria entering into the mix. This is why ferments are so carefully timed around a distillery’s distilling schedule.
A laborious process for some, an invisible world of possibility for others
The fermentation process in whisky production is an intricate stage, where the foundation for the whisky’s flavour is laid. This process involves a complex interplay between yeast, sugars, and various environmental factors.
Each stage of fermentation contributes uniquely to the whisky’s profile, from the initial yeast acclimatisation to the final alcohol yield and congener formation.
The need for mastery around the fermentation process underscores the skill and dedication necessary to produce high-quality whisky. While the lion’s share of attention is around distilling and maturation – fermentation is where distinctive flavours start. For those who care to see beyond just another row of fermentation tanks – it’s where the innovative spirit of the distilling industry can truly be seen too…
