Welcome to the intriguing world of whisky fermentation, a vital stage in crafting this beloved spirit.
For new distillers and those keen to learn about whisky production, understanding the fermentation process is key to mastering the art of whisky making.
Fermentation not only transforms basic ingredients into alcohol but also significantly contributes to the whisky’s unique flavour and aroma. This introductory guide delves into the role of different cereals, the importance of starch conversion, and how various factors like yeast types, temperature, and fermentation duration influence the final product.
Whether you are a novice looking to get a grasp on production or expanding your knowledge, this wide angle exploration of whisky fermentation will deepen your appreciation and skills.
Fermenting cereals
Fermentation is a crucial process in the production of various spirits and it’s not just about generating alcohol. From a whisky perspective – cereals like barley, corn and wheat serve as the backbone of this process for several key reasons.
The first is that they are a good starch source.
Cereals are the primary source of starch, which is essential for fermentation. Starch gets converted into fermentable sugars that yeast can then turn into alcohol.
The endosperm of the cereal grain is particularly important because it contains the bulk of the starch that’s essential for fermentation. This starch is processed to produce a sugar-rich liquid called wort, which serves as the foundational ingredient for fermentation. Given that cereals are an expensive raw material, distillers aim to maximise efficiency in production to yield high-quality products.
Have a look at our article on Mashing for more on this.
For those with a mixed mashbill (say that combines Barley, Wheat and Corn) – different cereals have varying starch contents. The selection and ratios used will dictate the potential alcohol yield as well as the optimum mash temperatures, yeast type and fermentation time.
The second is because of cereal’s flavour contribution.
The type and preparation of the cereal, such as peated or non-peated malt (or even roasted) significantly influences the flavour profile of the final distilled product.
Even with the same malting process, regional variations in cereals can introduce subtle differences in flavour profiles. For example, barley grown in Scotland might impart different nuances compared to barley from North America due to soil, climate, and cultivation methods.
Terroir is about more than just raw ingredients – it involves process, people, and more. But it definitely starts with the agricultural origin of the spirit. In the case for whisky, the cereal growing in the field provides a distinct start point for geographically unique flavours.
The third reason is enzymatic activity.
In certain cereals like barley, natural enzymes are present that help in converting starch into fermentable sugars during the mashing process.
Fermentation overview
You can take a deeper dive into how general fermentation works in our other Fermentation article to refresh your memory.
It is also helpful to note some key terms if you are still getting up to speed. Wort refers to the liquid going into the fermentation process. Wash is the term used in whisky/whiskey distilleries for the product at the end of fermentation.
In the context of cereal ferments with the intention of making whisky, a recap of the process looks like this…
The wort, a nutrient-rich liquid derived from mashed cereals, and yeast are introduced into carefully designed vessels to kickstart the fermentation process. As the yeast consumes the sugars present in the wort, it converts them into alcohol and carbon dioxide.
Throughout fermentation, the environment in the fermenter undergoes significant changes. The original gravity of the wort, which gives an insight into the sugar concentration, starts to decrease as fermentation progresses. By the time fermentation concludes, the final gravity indicates the residual sugar content and overall fermentation efficiency.
Alongside alcohol and carbon dioxide, other by-products like residual sugars, acids, and congeners (like esters) are also produced. Each contribute to the flavour and aroma profile of the end product.
The pH of the fermenting wort may also shift. Typically it moves towards the acidic range, especially in cereal fermentations which often conclude with a pH value around 4.
After the completion of fermentation, the result is a fermented liquid rich in alcohol ready to be distilled, and spent (dead) yeast cells.
Fermentation using cereals involves various considerations for distillers, particularly when dealing with the wort that comes out of the mashing stage. The first factor to make a decision over is around residual solids.
The impact of residual solids on ferments and the world of wort
After mashing, the wort may still contain cereal solids (like husks).
On a practical level, when these are carried into fermentation, they can lead to spillages (pressure build-ups can bring on quick releases that can splash surprisingly far!). It’s easily mitigated against though.
However, it also has knock on effects for sanitisation after the ferment. Solid matter is simply harder to remove than liquid alone. Any residual solids (or debris) become hotspots for microbial contamination if the plant is not adequately cleaned.
To make cleaning easier, some distillers choose to remove heavy solids before fermentation, resulting in a wort that contains only soluble and very fine insoluble solids.
Cleaning aside, residual solids and picking whether to carry out an all-in fermentation or having a highly clarified wort often comes down to how a distiller wants to create an optimum environment.
Consistency is key, and understanding what the mash has done, how the yeast will interact next and contamination risks is not a one size fits all situation.
Have a look at our article on Wort Separation for more context on how solids are separated.
Wort clarity and how that influences the process
Depending on if a distiller chose to remove the solids or not, they can produce either a very clear or cloudy wort. This impacts fermentation, and flavour.
Wort derived from cereals contains not just fermentable sugars but also amino acids, proteins, various sizes of carbohydrates (dextrins), minerals, and other compounds. These components nourish the yeast and facilitate effective fermentation. If a distiller choses to clarify their wort (essentially filtering it), they need to also consider what else they are sacrificing at the same time.
It’s easy to see why some distillers believe that retaining certain solids can enrich the flavour profile and that cloudy worts might provide more nutrients, benefiting yeast health. Conversely, there are others who say that it really depends on the mashing techniques used and yeast type. It might also introduce unwanted compounds that could stress the yeast.
There are many examples of those who are adamant about both extremes – untouched and cloudy, or extra clarified. There’s no objective universal advice to be given here other than to understand your options. Look at your mashing process to see how that influences the wort being made (from grist sizing, stirring etc.). Then look at the type of fermentation vessels you have and the sanitation regiment you want to put in place. Often the decision best suited to you becomes instantly visible.
Whichever you pick, to avoid any microbial contamination it’s important to remove all the residual solids from the fermenters at the end of fermentation. Cleaning leads to consistency!
Mashing generally takes place at around 55 to 65°C, optimal for converting starch into fermentable sugars through natural (endogenous) enzymes. Unlike in brewing, distillers’ wort is not boiled after mashing, which makes secondary conversation not only possible during fermentation stages, but key to understand. Let’s delve into that…
Secondary conversion in cereals-based fermentation
In the mashing process, only 75-80% of starch turns into fermentable sugars, which includes glucose, maltose, and maltotriose, accompanied by small and larger dextrin units.
Certain enzymes, resilient to temperature and pH changes, make it through mashing. This is great for distillers and fermentation. Because they remain in the wort, they continue to breaking down these dextrins as fermentation gets underway. This is known as secondary conversion.
The extent of secondary conversion can vary based on the strain of yeast used and the amount pitched.
As fermentation progresses, yeast first consumes glucose and then moves to maltose and maltotriose. By having secondary conversion actively going on, far more of the starch turns into fermentable sugars (raising the total to over 80%) which ultimately, gets consumed by the yeast.
Side note. It’s worth noting that as yeast cannot survive high temperatures, wort is cooled coming out of the mashtun via a heat exchange before it’s added into fermenters and being inoculated. Again, more on that in the Wort Cooling article.
Sampling and analyses
Distillers carry out analysis during fermentation for a number of reasons. Collecting data and regular samples ensures product quality, safety and efficiency, enabling the production of consistent and high-quality spirits.
With this data, they can make informed decisions, tweak the fermentation process if needed, and troubleshoot any issues that might arise.
New distillers learning – what’s helpful to monitor?
By monitoring parameters like original and final gravity, pH and temp, distillers can gauge the efficiency of the mashing process and the overall fermentation performance. Proper analyses can ensure that the process is yielding the desired results.
Turbidity: The cloudiness of the fermented mash can affect the process. Monitoring turbidity helps distillers anticipate potential challenges during both the fermentation and subsequently, distillation. Densitometers can be used to measure this.
Original gravity: Measures specific gravity at fermentation’s start, indicating mashing efficiency. Tools like hydrometers are used to do this .
Final gravity: Monitored at the end of fermentation, this provides an insight into the remaining sugars and fermentation’s completion. A lower final gravity suggests that most of the sugars have been consumed by the yeast, leading to efficient fermentation.
pH level: Monitoring the pH level helps distillers keep track of the acid production by the yeast. A stable pH ensures that the yeast works in an optimal environment, producing alcohol at a consistent rate.
Temperature: Maintaining a consistent temperature is crucial. Yeast is sensitive to temperature fluctuations, and keeping the fermentation environment stable helps achieve uniformity in the final product.
Yeast cell count: This helps distillers monitor the health and quantity of the yeast. A thriving yeast population ensures consistent fermentation.
Volatile acids: These are unwanted by-products and can negatively impact the flavour of the whisky. By monitoring their levels, distillers can take measures to limit their formation.
Esters and congeners: These are chemical compounds that influence the aroma and taste of whisky. Monitoring their formation can help distillers adjust the fermentation process to produce a desirable flavour profile. Sensory evaluation can be a good early indicator here.
Alignment is key
The fermentation process is a critical and complex stage in whisky production, greatly influencing the spirit’s final character.
The careful selection of cereals, precise starch conversion, and the choice of yeast strain all play pivotal roles in shaping the whisky’s profile. Understanding what to monitor and how, can help new distillers fine-tune their craft, creating distinct and desirable flavours.
This wide angle guide only highlights the start point of many of these focus areas. But even in just doing that, it underscores that like all things in the spirits industry, the fermentation process is about a careful alignment of ingredient, process and knowhow that amounts to a sum far greater than the parts.
