Fermentation is a captivating biological process, playing a crucial role in the creation of all spirits.
This article seeks to demystify fermentation for those looking to distil from scratch. It will help you better understand some of the complexities distillers need to navigate. We also have more category specific fermentation articles for Whisky, Rum and Brandy that detail each of those.
Here, we’ll explain why the story of yeast turning simple sugars into the much-celebrated ethanol is far from the full picture. There’s a supporting cast of other organisms involved in fermentation, as well as by-products such as heat, carbon dioxide and flavours.
All of these contribute to the final product’s unique character.
By the end of this article, you will be equipped with sufficient insights on how the fermentation process works. You’ll also understand some of the factors you’ll need to consider to go about doing it in a distillery environment.
Fermentation basics
Fermentation is the first crucial step in the distilling process. Depending on the source of the sugar solution involved, there are various terms you’ll find such as wort (whisky production), must (for grapes and Brandy), and sometimes just unfermented liquor. Irrespective of the names – all of these solutions contain different concentrations of fermentable carbohydrate.
The type of sugar being fermented varies as it is based on the source.
Cereal-based liquors mainly contain glucose, maltose, maltotriose, and dextrins. In contrast, grape and molasses fermentations have glucose, fructose, and sucrose. Agave primarily contains fructose. This matters as the optimum conditions for each vary.
For any fermentation to be efficient, the four major elements required are:
- A carbohydrate source (A sugar-rich liquid, the type of which is determined by the distiller’s choice of raw materials and what they intend to make)
- Healthy and viable yeast (Dosed in a specific amount that converts the sugar solution into alcohol and other flavour compounds)
- A clean fermentation vessel
- Temperature control
Choosing a cultured yeast
Yeast is not one homogenous entity. There are different strains. And with each different strain, there’s a different set of optimum conditions and set of flavours to monitor.
Some of the yeast strains used in distilling are handed down through generations. Many however, are purchased from commercial producers – much in the same way as bakers do.
The more specialist the yeast producer, the more variety and flavour specific the type of yeast they have on offer.
These yeasts, dried and typically sold in vacuum-sealed packets, are not just tailored to produce specific types of spirits. They are often made to suit styles within them too.
A more “neutral” yeast that favours ABV creation over flavour compounds may be suitable to make vodka. However, when it comes to whisky or rum, there are numerous trade offs depending on what style you are making – heavy, light, fruity, floral etc. The flavours yeast brings can range widely.
Most producers will test several strains to find the profile they want before settling into consistent production. (see our Yeast article for more about it).
Nature vs nurture is true for yeast too!
The yeast strain is just the base – how you handle them matters.
Changes in temperature, pH levels, oxygen, and nutrient levels will all impact flavour (and are factors in the distillers power to control). Let’s add further complication into the equation too… Fermentation can also be ‘soured’ intentionally with competing bacteria for additional flavours. For example, though dunder, muck or backset.
Much of the value of a commercial yeast strain is in the flavour consistency it allows within specific conditions. Because of that, there are dozens of examples of producers who use dual strains (or more) to reach their optimum balance. Four Roses for example, use five, while Scotch Whisky makers Holyrood use different combinations depending on the style of New Make they are looking to create.
Beyond flavour – the metric distilleries are heavily focused on is yield. What yield does a yeast strain typically generate?
This is commonly measured in litres of alcohol per tonne of raw material. Just as they do for flavour control, distillers also monitor various factors like temperature, pH, specific gravity, and alcohol profiles throughout each of the phases of fermentation with yield in mind.
While most will swear by the efficacy and brilliance of their specific strain (or strains) – all will agree that getting the selection right is vital for any distillery.
The stages of all fermentation
While it’s a constantly evolving process, you can break down four clear stages of fermentation.
Phase 1: Lag phase
In this initial stage there’s minimal activity. Once added, the yeast is preparing to start growth and cell division processes.
- Yeast is introduced to the sugar-rich liquor using a process known as ‘yeasting the mash’ or ‘pitching the yeast’.
- Different distillers around the world use varying quantities of yeast. The rate they add the yeast affects the fermentation duration – higher doses equal faster fermentation. The result means that fermentation can last between 48-72h, up to a few weeks.
- It’s crucial to add yeast when the liquor is at the right temperature to prevent microbial contamination and optimum efficiency going forward. More on this in our articles on Whisky Specific Fermentation and Wort Separation.
- Yeast cells require nutrients like amino acids, minerals, and vitamins for growth and fermentation. While these nutrients are often provided by the raw materials themselves, in some circumstances, these can also be added as supplements.
Phase 2: Growth phase (aerobic fermentation)
This second stage is vital for the formation of flavour compounds resulting from yeast growth (such as higher alcohols like amyl alcohol).
Beginning a few hours after inoculation, yeast cells start multiplying rapidly and start to produce alcohol.
This aerobic phase occurs in the presence of (limited) oxygen and sees a drop in specific gravity as yeast consumes fermentable carbohydrates, releasing carbon dioxide and heat.
As the oxygen depletes and a carbon dioxide layer forms just above the top of the ferment, yeast growth slows down and eventually stops.
Phase 3: Fermentation phase (anaerobic fermentation)
This dynamic stage starts once all oxygen is used up. It is typified by a notable increase in heat, as carbohydrates transform into alcohol and carbon dioxide.
Key for flavour development, this phase witnesses the formation of esters and aldehydes.
High alcohol levels inhibit many bacteria, while the predominant yeast population also outcompetes wild variants. Once distillers / brewers have reached this far, they tend to rest assured that the liquor will remain mostly uncontaminated.
Phase 4: Post-fermentation phase
This phase starts when all fermentable carbohydrates have been consumed, marking the end of fermentation.
No more alcohol is produced, and the fermented liquor is ready for distillation. Typically, there is a notable plateau in the temperature rise once a ferment has reached this phase.
Delay in transferring to the distillation process could risk negative microbial contamination (see below).
Other micro-organisms play a significant role in fermentation
While yeast primarily drives fermentation, other organisms including certain bacteria, can also influence the process. This can play out in different ways – either enhancing flavour profiles or introducing unwanted characteristics.
Some bacterial contamination is almost inevitable in all ferments.The reasons are simple – raw material and yeast supplies can bring bacteria into the ferment. Moreover, the fermentation temperature range of 20-35°C and the presence of sugar provides an ideal environment for bacterial growth.
It’s not necessarily a negative thing. Both wild yeast and bacteria can either be beneficial for flavour or detrimental to product quality and fermentation efficiency depending on what, how much and how long.
Bacteria can be categorised into two camps
- Beneficial bacteria: like those added deliberately in ferments or helpful for secondary fermentations.
For example, in sour mash whiskey production, lactic acid bacteria is added to the mash to create a more acidic environment. This can deter harmful bacteria and contribute to a better optimum pH start point for the yeast to begin from.
Another example is in rum production and the inclusion of dunder – the leftover liquid from previous distillations. Dunder is rich in nutrients and beneficial bacteria. This can add depth and richness to the flavour profile of the rum.
- Harmful bacteria are those that produce sulphur compounds, acrolein, diacetyl, and fusel oils. All of these can lead to off-flavours and odours in the final distilled product.
These can include bacteria that produce hydrogen sulphide, which can lead to sulphur compounds that have a rotten egg smell. Acetobacter can convert alcohol into acetic acid and water, leading to a vinegar-like taste. Meanwhile bacteria that produce diacetyl can give a buttery flavour that, in high amounts, is undesirable.
That said, the typical and predominant bacterial contaminants for distillery ferments are lactic acid bacteria. They often lower yield, increase acidity, and introduce off-flavours if left to proliferate – which can happen at the final stage of fermentation if there are delays waiting for transfer to be distilled. These are reasonably easy to manage.
To mitigate these harmful effects, distillers take several steps:
- Hygiene and sanitation. Maintaining a clean and sanitary environment in the distillery is crucial to prevent the growth of harmful bacteria.
- Temperature control. Since bacteria thrive at certain temperatures, controlling the temperature during fermentation can inhibit unwanted bacterial growth.
- pH control. Adjusting the pH of the mash can make the environment less hospitable to harmful bacteria.
- Monitoring and testing. Regularly monitoring the fermentation process and testing for the presence of unwanted compounds can help detect issues early.
- Timely distillation. Ensuring that fermentation is timely followed by distillation can reduce the window during which harmful bacteria can affect the product.
Wild Yeasts – neither positive nor negative
There’s an increasing number of distillers who are emphasising the role that wild yeast plays in building flavours and complexity. Particularly in Agave distillates like Mezcal and Tequila and in American Whiskey production.
It may not be getting the same media attention as barrel finishes or the use of heirloom grains, but it should. What should be clear by this point is that yeast is one the most important components in flavour creation!
Because wild yeast is so specific to its geography, and because each strain performs differently – it’s easy to see why those seeking unique flavours are so enthusiastic about their use.
Let’s look at wild yeast further set against the context of either / or using cultured varieties
While all yeast will develop certain flavours – wild yeasts can be particularly helpful in this area as they create unique combinations and quantities of esters compared to cultured yeasts (that can be used by anyone with a predictable outcome). Therefore letting them thrive in a ferment has some merit.
That said, it’s a double-edged sword. Wild yeast can also compete with cultured yeast during the early stages of fermentation, potentially leading to reduced yield and unwanted flavours. Fundamentally, they are also more unpredictable too, and can bring in unintended consequences.
This trade off is one of the reasons why mainstream spirit producers tend to commit to one or the other, and why they tend to use cultured.
Some categories are different though. Take Clairin, Rhum Agricole and Cachaça for example, who emphasise natural yeasts and fermentation methods. Many artisanal Mezcals and Tequilas use 100 percent wild yeast to ‘spontaneously’ ferment their agave mash too.
Spontaneous, or just erratic?
Spontaneous fermentation is also frequently found in European wineries. This makes it an interesting area to research further for budding brandy makers and those looking for inspiration. That said, the most routinely flagged challenge they encounter is the inconsistent nature of wild yeast.
The second is the tolerance. Wild yeasts typically have a low tolerance to alcohol, often ceasing to function when alcohol levels hit 5 or 6%, which can lead to incomplete fermentation, resulting in wines with excess sugar. Not only is the yield impacted, but as acidity is key for good eau de vie so is the quality (as we covered in the Brandy specific Fermentation article).
The third major challenge is the timeline. For winemakers, the lower quantity of wild yeast on grapes compared to pitching a known amount of commercially inoculated yeast means a moving feast (literally) for when it’ll be ready. Generally, wild yeast takes longer to proliferate, exposing the grapes to potential spoilage, bacteria and oxidation.
The same three challenges are true for Tequila & Mezcal producers working in hot climates, or New World Whisky producers, waiting on their mash to ferment wildly – in all senses of the word.
Whether to inoculate with commercial yeast is a key decision.
We’ll repeat ourselves here, but it’s easy to see why many draw a line between making spirits from cultivated yeasts or from wild yeasts, which impart the essence of local terroir. The reality is that there are many distilleries who use some combination of the two, be it at the start of fermentation or after.
For example – sequential fermentation is a technique where initial spontaneous fermentation is followed by the introduction of Saccharomyces cerevisiae yeast.
This process takes advantage of the slow start of wild yeast, which can enhance the unique aromas and flavours, adding complexity. They build a base layer of flavour and start the alcohol generation, the cultivated variety then builds on that foundation.
During sequential fermentation, there are quite a natural “handover” moments that can become focus control points for distillers to measure and optimise around. For example wild yeast often stops working at low alcohol levels, and by then, you can ensure Saccharomyces cerevisiae has grown in numbers to take over, ensuring complete fermentation.
The decision to pick both and attempt sequential fermentation also depends on the desired pH and acid levels (and the tolerances the base material has). For example, grapes with low pH and high acidity can naturally inhibit spoilage during the initial slow phase of spontaneous fermentation, making using a slow wild yeast a low risk process. The same might not be true with sugar cane juice destined for Agricole Rum, or Espadin based aguamiel on its way to becoming Mezcal…
Wild yeast is neither negative or positive by default. It’s a factor to contend with and by being conscious of it and the decisions they make, distillers find the optimum balance for their process.
Tips for beginners starting to work with fermentation
Track everything! Set parameters for what is acceptable and replicate processes exactly wherever possible. So many poor batches could be prevented by distillers prioritising good quality raw materials and following well-defined procedures.
Avoid the main sources of microbial contamination such as leaving the unfermented liquor too long. Ensure you have the correct yeast supply and add the right quantity at the right time. The same is true once complete – distil it promptly!
Pay attention to temperature controls, not exceeding 35°C and having the right volume of fluid, correct level of fermenter filling and suitable headspace.
Clean! Maintain a clean plant, pipework, and valve systems.
An entire world of intricacy within a small part of the spirits production process
Mastering fermentation is essential for any aspiring distiller.
The journey from sugar to spirit is not just about yeast converting sugars into ethanol; it’s an intricate partnership involving various sugar sources, a plethora of yeast strains, and a delicate balance of temperature and timing.
Getting it right involves understanding the different stages of fermentation and the role of both beneficial and harmful micro-organisms. Meanwhile, opting for wild yeasts and their potential unique flair or reliable cultivated strains for consistency are choices that will impact the final flavour profile.
Crafting high-quality spirits with distinct and desirable characteristics starts at fermentation stage. No wonder then, that it’s a difficult process to master and a challenging set of variables to wrangle to your will.
