As a distiller, understanding the nuances of rum fermentation is not just about mixing molasses and adding yeast; it’s about building the foundations for the flavours ahead.
This article delves into the heart of molasses fermentation, guiding new distilleries and managers through the essentials of process, equipment, and a few tips for what to focus on.
Whether you’re aiming for a light, subtle rum or a rich, heavy variant, conquering fermentation is key to creating a product that will resonate with drinkers.
Understanding molasses in rum fermentation
Molasses, the thick, dark syrup derived from sugarcane, is the backbone of rum production. It contains sugars, primarily sucrose, glucose and fructose, along with essential minerals and other trace elements.
The sugar content and the presence of these non-sugar components significantly influence the fermentation process and the flavour profile of the resulting rum.
The type of molasses used can greatly impact the taste and quality of the rum. Blackstrap molasses, with its robust flavour and higher mineral content, is a popular choice.
Understanding the sugar composition and impurity levels of different molasses types helps in predicting fermentation behaviour and flavour outcomes.
You can find more about molasses on our article Mastering Molasses.
Choosing the right yeast & the right type of fermentation for you
Once you have your base ingredient – molasses – the first part of a successful ferment is choosing the right yeast.
You need both alcohol and flavour to be created at this stage. For light rums, strains of Saccharomyces cerevisiae are preferred for their ability to produce subtle flavours and high alcohol yields. For heavy rums, yeasts like Schizosaccharomyces pombe can be used to achieve a more complex flavour profile.
We’ll just repeat it again as it’s a really big factor in the end profile of a rum. The yeast not only converts sugars into alcohol but it also contributes hugely to the rum’s aroma and taste.
Once your strain has been selected, the type of fermentation needs to be considered. Here are two main types of fermentation processes used in rum production.
Batch fermentation
Batch fermentation is the traditional and straightforward approach to rum production.
In this method, all the ingredients – molasses (once clarified / optimised), water, and yeast – are mixed together in a fermenter at the start of the process. The yeast then begins to convert the sugars in the molasses into alcohol and other by-products.
Temperature and pH are controlled throughout the fermentation process. This ensures that the yeast operates efficiently and helps prevent the growth of unwanted bacteria. Batch fermentation typically lasts between 24 to 72 hours, depending on the yeast strain and desired rum profile.
This method is known for its simplicity and ease of control, making it a popular choice for smaller distilleries.
Incremental feeding fermentation
Incremental feeding fermentation offers a more complex approach. Unlike batch fermentation, incremental feeding involves adding more molasses and nutrients in stages rather than all at once and can be used in a continuous process if desired.
Through gradual addition and by controlling the introduction of sugars and nutrients, this method helps manage osmotic stress on the yeast. This can lead to a more consistent fermentation process and can potentially increase the final alcohol yield.
In some situations, it is also possible for incremental feeding to allow for the development of more complex flavour profiles.
This method allows for more flexibility in adjusting the process mid-fermentation, which can be handy for those working with wild yeast, dunder or muck. However, mostly, the advantage of this policy lies in its ability to propagate yeast at scale. The goal is to increase the yeast population at first so that when a greater volume of juice, containing more sugars, is introduced, there are enough yeasts available to effectively process these sugars. This approach avoids overwhelming the initial yeast population with excessive food at once.
Incremental feeding fermentation requires more careful monitoring and control than batch fermentation. It also demands a deeper understanding of yeast metabolism and fermentation kinetics, making it more suitable for experienced operators.
Both batch and incremental feeding fermentation have their unique advantages and challenges. The choice between them depends on the distiller’s objectives, the desired rum profile, and the level of control they wish to exert over the fermentation process.
For those wanting to know more about the role of Dunder, Esters and a different angle on the Intricacies of rum fermentation, click through on each for more articles.
What to focus on as a new distiller when starting rum fermentation – monitoring and control
Establish a yeast propagation system
Developing an in-house yeast propagation system allows for greater control over yeast quality and consistency. This can lead to more predictable fermentation outcomes and helps maintain the unique character of the rum.
A system doesn’t have to be convoluted or automated – it just needs to be replicable. How are you going to store yeast? How are you going to do condition checks? Where, how are you going to weigh it, and add it to the ferment identically each time? What equipment will you use and how will you keep it free of contamination?
Set yourself up for success by having protocols in place and a system anyone can follow.
Temperature and pH control
The fermentation environment’s temperature and pH significantly affect the yeast’s performance and the quality of the rum. Maintaining optimal conditions ensures efficient fermentation, prevents the growth of unwanted bacteria, and controls the formation of congeners (flavour compounds).
Set parameters for min / max temperatures. Consistent temperature control is crucial for preventing unwanted bacterial growth and ensuring a steady fermentation rate.
Set parameters for min / max pH over time. Maintaining a pH between 4.5-5.5 is vital for optimal yeast activity.
Set a timeline of when a batch will be checked and what will be recorded and monitored (alcohol, brix, gravity etc.). You should have detailed logs of each batch, including ingredient specifics, fermentation conditions, and any anomalies.
This not only helps in maintaining the desired fermentation conditions but also in detecting and addressing any issues promptly. With a broader lens and when looking at the process more holistically, you’ll need repetitive data to aid in quality control when you do reviews.
Dealing with contaminants
Contamination can lead to off-flavours and spoilage. Good sanitation practices are crucial to prevent contamination and spoilage. Regular cleaning and sterilisation of equipment, as well as maintaining a clean fermentation environment, are essential.
Techniques for maintaining consistency and quality
Regular testing of samples for sugar content, acidity, and alcohol levels helps in maintaining product consistency. Either set yourself up with the relevant lab equipment you need to do it in house, or work with a supplier who can assist as and when needed.
Sensory evaluation should be a big part of the fermentation process. Regular tasting and aroma checks by experienced team members can detect any deviations in flavour profile. It’s very easy to look at monitoring fermentation as a numbers game as there is so much potential data to gather. But the best place to start is with your nose and senses!
See Distillery Hygiene, Process Control, Sensory Assessment & Quality Control articles for more on how to do this well.
Once you’ve gone through your process and made your first batch, don’t be afraid to try different fermentation methods, like varying the duration, adjusting the temperature, or experimenting with different yeast strains. It’s worth experimenting the first few times before settling into consistency as it can lead to the development of unique flavour profiles that set your rum apart.
What to focus on when getting up and running – equipment considerations
Choosing the right fermenters – conical fermenters are your friend!
The type and size of fermenters used in rum production significantly impact the fermentation process. Stainless steel fermenters are popular due to their durability and ease of cleaning, which helps prevent contamination. The size should match the scale of production – work backwards from the size of your still and optimise your process to account for the timeline you want to work to.
There are several reasons why conical fermenters are often considered better than flat-based ones for molasses fermentation.
Yeast collection and reuse
Conical fermenters allow the yeast to settle at the bottom into the cone after the fermentation process is complete.
This design makes it easier to collect and potentially reuse the yeast, which can be a significant advantage for cost-efficiency. For those not reusing, the conical bottom allows for more efficient removal of sediment and dead yeast cells (lees).
Pressure control
Most conical fermenters are designed to handle pressure, allowing for secondary fermentation or carbonation within the same vessel. While this is more relevant for beer production, the knowing they are designed to control far greater pressure than what will be needed here can also be a bonus safety feature.
Bolt-ons for temperature and analytics
It may be worth adding in features that can aid with temperature control. After-all, consistent temperature control is crucial for optimal yeast activity and flavour development.
Automated cooling and heating systems can help maintain the desired temperature range throughout fermentation. Thermal jackets and closed loop circuits are not that much more expensive, and for those in warmer climates, can be save significant sums and wastage over their lifespan.
Meanwhile, tools for measuring alcohol content, sugar concentration, pH, and temperature can be built in. Not only does this mean the data is being recorded automatically, avoiding human error – consistently accurate measurements allow for timely adjustments and ensure the fermentation is proceeding as desired. Many measurement tools can be set against alarms and prompts when they detect an input that falls outside of the desired margin.
The craft of rum fermentation
The mastery of molasses fermentation is both an art and a science. Each step, from selecting the right molasses to the careful monitoring of fermentation, contributes significantly to the character and quality of the rum.
The choices made in yeast selection, fermentation process, and equipment use are not just technical decisions. They are expressions of the a distillery’s vision and skill.
As we have explored, understanding and controlling these elements is key to producing rum that is not only enjoyable to drink but also a reflection of the distiller’s dedication to their craft. You can’t distil quality from a poor-quality wash.
Whether you are a new distillery starting out or an experienced distillery manager looking to start making a new spirit, the pursuit of excellence in fermentation is a rewarding and defining aspect of crafting truly memorable rum. In many ways – fermentation is where rum is truly made!
