Why Fermentations Get Stuck: Lessons from a Rhum Distillery
- Jul 5
- 10 min read
Fermentation has a habit of making confident distillers humble.
Two tanks can be filled with sugarcane juice, pitched with the same yeast, fermented in identical vessels, and started only hours apart. One will finish cleanly in just over a week. The other will slow unexpectedly, stop short of completion, and leave a surprising amount of sugar behind.
The instinctive response is often to blame the yeast. In our experience, that is usually the wrong place to start.
Over the past few sugarcane harvests at Choeng Doi Distillery, we've watched dozens of fermentations develop under different conditions. Some have finished in seven days. Others have continued for more than a month. We've seen commercial yeast stop fermenting while wild microorganisms continued working below pH 3.0. We've also learned that by the time a fermentation appears to be stuck, the real cause often occurred days earlier.
A stuck fermentation is rarely a single event. It is usually the result of a chain of small changes that began long before the airlock stopped bubbling.
What Is a Stuck Fermentation?
A stuck fermentation occurs when yeast stops converting sugar into alcohol before fermentation is complete. The obvious consequence is lower alcohol production, but that is only part of the story.
Residual sugar means reduced spirit yield, longer tank occupancy during harvest, and less efficient use of raw materials. During a busy sugarcane season, every extra day a fermentation occupies a tank reduces the number of batches that can be processed before the harvest ends. For craft distillers producing limited-run sugarcane spirits, this efficiency loss directly affects both output and the quality of each vintage.
Not every slow fermentation is stuck, however. Healthy fermentations naturally become quieter as sugar is consumed. Carbon dioxide production slows, bubbling through the airlock becomes less frequent, and visible activity decreases. None of these changes necessarily indicate a problem.
The more useful question is whether the fermentation is still making meaningful progress.
We Originally Thought the Problem Was the Yeast
Like many new distilleries, our early record keeping focused on the obvious production numbers.
We logged starting gravity, finishing gravity, fermentation dates, batch volume, and monitored daily gravity and temperature. Today, gravity and temperature are automatically recorded through our RAPT Pill system, allowing us to focus on the measurements that require manual observation.
When our first commercial sugarcane fermentations began stalling, our assumption was simple. The yeast had stopped working.
That seemed logical. After all, yeast performs fermentation. But after several batches, it became increasingly clear that replacing or adding more yeast was not solving the problem. The yeast wasn't necessarily failing. The environment was.
This realisation forced us to abandon the most common troubleshooting approach taught to new distillers: when fermentation slows, add more yeast. That advice works when yeast populations are truly depleted. It fails entirely when the issue is environmental stress.
The First Fermentation That Changed Our Thinking
One batch in particular forced us to rethink what was happening.
During the 2025 to 2026 harvest, Batch 3.10 appeared to be fermenting normally before slowing dramatically. It eventually finished after 24 days with a final specific gravity of 1.034, substantially higher than our healthier fermentations, which typically finished between 1.012 and 1.018.
The fermentation wasn't obviously spoiled. It didn't produce unpleasant aromas. It simply stopped converting sugar efficiently. That single batch prompted a different question. Instead of asking why the yeast had stopped, we began asking what had changed inside the fermentation before it stalled.
That shift eventually led us to focus much more closely on pH, a variable we had previously treated as secondary information rather than a critical production parameter.
How pH Controls Fermentation Efficiency
Our previous article explored the role of pH in sugarcane fermentation in detail. What surprised us most was not that pH influences fermentation, but how quickly it changes.
In our sugarcane fermentations, pH commonly falls by around one full unit during the first 48 hours.
That rapid acidification is one of the defining characteristics of sugarcane juice. The raw material has minimal buffering capacity, meaning organic acids accumulate quickly and microbial populations become established far more rapidly than in many other fermentation systems. The environment changes far faster than distillers working with other substrates might expect. The biggest lesson we have learned is that most fermentation problems begin during this early period.
By the time a fermentation appears stuck several days later, the conditions that caused it were often established within the first 18 to 24 hours. This timing is critical for any distillery working with sugarcane spirits. The window for intervention is much narrower than conventional fermentation troubleshooting suggests.
Commercial Yeast Has Practical Limits
Every commercial yeast strain operates within a practical range.
For our Blanc, we use Lallemand RM, a strain developed specifically for sugarcane juice fermentation with a published operating range of approximately pH 3.3 to 5.3. Our own production records align remarkably closely with those figures.
As fermentation approaches pH 3.3, activity begins slowing significantly. Once it reaches that point, the transition can be surprisingly rapid. Fermentation may appear healthy one day, then show very little further progress the next.
Importantly, this is not always obvious from smell alone. Our fermenters remain sealed under airlock throughout fermentation, so we avoid opening them unnecessarily. When samples are taken, stalled fermentations often smell surprisingly clean. They have begun developing the natural acidic aroma expected near the end of fermentation, yet they retain a noticeably syrupy texture and residual sweetness.
The sugar simply hasn't been converted. The yeast has not failed. The conditions have moved beyond its practical working range.
Four and a Half Percent Doesn't Sound Like Much
When a fermentation stops prematurely, the impact is measured in yield.
Across our own batches, a stalled commercial fermentation typically leaves around 4.5 percent ABV worth of potential alcohol unfermented. In a single batch, that may not appear catastrophic. Across an entire harvest, however, those losses accumulate quickly.
Every litre of alcohol left in the fermenter represents sugar that was grown, harvested, transported, milled, and fermented without ever becoming spirit. For a craft distillery focused on terroir and raw material quality, that wasted potential is particularly frustrating.
At Choeng Doi, where our sugarcane comes from specific estates in Northern Thailand, each batch represents a year of agricultural work by our farming partners. Recovering that yield became one of our biggest priorities, not merely for efficiency but as a matter of respect for the raw materials themselves.
Wild Fermentation Plays by Different Rules
One of the most interesting observations from our sugarcane programme has been the behaviour of wild fermentations. As of the 2025 - 2026 season, all of our single estates are fermented with wild yeast isolated from the different farms.
Commercial yeast generally begins struggling around pH 3.3. Wild fermentations often do not. We've recorded mixed and wild fermentations continuing below pH 3.0, with some reaching approximately pH 2.9 before finally finishing. That doesn't necessarily make wild fermentation better or worse.
In fact, these fermentations often become extremely slow. While a healthy commercial fermentation usually finishes within seven to ten days, some of our wild batches have continued for weeks.
Our longest wild fermentation remained active for 38 days. The longest commercial fermentation we allowed to continue lasted 37 days. Those experiences taught us another valuable lesson. Survival and efficiency are not the same thing.
Wild microorganisms may tolerate much harsher environments than commercial yeast, but they generally do so at the expense of speed and predictability. For a contemporary sugarcane spirit producer focused on consistency and quality control, this trade-off rarely makes sense. However, understanding how wild fermentation behaves differently provides insight into the mechanisms that limit commercial yeast, regardless of what production approach a distillery chooses.
The Biggest Misconception About Stuck Fermentations
When people encounter a stuck fermentation, the first suggestion is often simple.
"Add more yeast."
In our experience, that rarely addresses the real problem. If the environment has already become too acidic, introducing fresh yeast simply places new cells into the same hostile conditions that stopped the original population. The question should never be whether there is enough yeast. The question should be whether the yeast can still function.
This distinction has profound implications for how distilleries approach fermentation troubleshooting. Many producers spend months and money trying increasingly exotic yeasts or bacterial supplements when the actual solution lies in controlling the environment during the first 24 to 48 hours of fermentation. Once the pH has dropped beyond a strain's practical range, no amount of additional inoculant will rescue the batch.
Learning When to Intervene
One of the biggest changes we've made over the years has been deciding when intervention should happen.
Originally, we waited. If fermentation appeared stuck for four or five days, we would begin trying to understand why.
Today, we approach the problem completely differently. We monitor pH closely during the early stages of fermentation, often taking readings every few hours. If pH begins falling too rapidly, intervention happens during the first 18 to 24 hours, before commercial yeast reaches its practical limit.
The objective is no longer rescuing a stalled fermentation. It is preventing it from becoming stalled in the first place.
This shift from reactive troubleshooting to proactive environmental management represents the most significant operational change we've made since establishing our sugarcane spirit programme. It requires discipline, consistent monitoring, and a willingness to intervene early rather than waiting for obvious symptoms.
Buffering Improves Yield, Not Character
One concern many distillers have is whether buffering pH changes the flavour of the final spirit.
Our experience suggests that its primary effect is on efficiency rather than aroma. Buffered fermentations are far more likely to ferment dry, recovering alcohol that would otherwise remain trapped as residual sugar.
The resulting spirit retains the same broad aromatic profile, but the fermentation completes more effectively. For our Single Estate Sugarcane Spirits, maintaining the terroir character of the base juice while optimising fermentation completion has proved essential. Buffering accomplishes this balance by preserving fermentation conditions without masking the raw material character.
For us, buffering is therefore a production decision rather than a flavour decision.
Record Keeping Changed Everything
Looking back, the most valuable improvement we made was not changing yeast or altering fermentation temperatures.
It was improving our records. In our early seasons, we tracked the basic production numbers needed to operate the distillery. Today, pH has become one of the most closely monitored variables throughout fermentation. If we could go back and change one thing, it would be introducing continuous pH tracking from the very beginning. No single graph has taught us more.
Looking across multiple harvests, the pattern is remarkably consistent. Fermentations that experience rapid early pH decline are far more likely to struggle later, regardless of whether the symptoms appear several days afterwards.
That insight fundamentally changed how we think about fermentation. For any distillery working with sugarcane, the pH curve during the first 48 hours functions as an early warning system. Deviations from expected patterns signal problems long before fermentation visibly stalls.
Why This Matters for Thai Craft Spirits
Stuck fermentation is not unique to our operation or to sugarcane fermentation generally. Distillers working with Thai rice spirit, Lao Khao, and other traditional Thai spirits encounter similar challenges when scaling production or modernising fermentation methods.
What distinguishes contemporary Thai craft spirits production is the deliberate approach to raw materials and fermentation control. Rather than accepting fermentation as an unpredictable natural process, modern distilleries throughout Thailand have begun treating fermentation as a managed system where understanding the environment precedes troubleshooting.
This shift represents a maturation of the Thai spirits category itself. As Thai craft spirits move beyond small-scale household production toward professional distillery operations, documentation and process control become non-negotiable. Understanding why fermentations stick, and preventing stalls before they occur, separates craft producers who can scale predictably from those who remain dependent on inconsistent variables.
Key Takeaways For Distillers
The lessons from stuck fermentation extend beyond simply preventing yield loss.
First, trust your records more than your intuition. A stalled fermentation may smell normal and may even produce spirit that tastes acceptable. Visual and olfactory assessment alone cannot tell you whether fermentation has truly completed. Gravity and pH measurements provide objective evidence.
Second, intervene early or not at all. The most common mistake is waiting too long before acting. If fermentation is showing signs of stress within the first 24 to 48 hours, that is the moment to make changes. Waiting until day four or five means waiting until the damage is already done.
Third, understand your yeast's limits before you need to. Every commercial yeast strain has a practical operating range. Know those limits for the strains you use. Fermentation slowing at pH 3.3 is not a sign that your fermentation is broken. It is a sign that your yeast is operating at its boundary.
Finally, view stuck fermentations as information, not failures. Some of our most valuable production improvements have come from batches that misbehaved. A stalled fermentation tells you something important about your raw materials, your fermentation environment, or your approach to inoculation. Learning to interpret that information is more valuable than simply preventing the problem from happening again.
Every Stuck Fermentation Tells a Story
It is tempting to think of a stuck fermentation as a failed batch.
We no longer see it that way.
Some of our most valuable production improvements have come from fermentations that didn't behave as expected.
The first stalled batch led us to question our assumptions.
The first season of detailed pH tracking showed us that intervention needed to happen far earlier than we had imagined.
Allowing several fermentations to continue naturally for more than a month taught us how commercial yeast and wild microorganisms respond differently to increasingly acidic conditions.
None of those lessons would have come from perfect fermentations.
Conclusion: Understanding the System
The biggest misconception about stuck fermentation is that it begins when the bubbling stops.
Our experience suggests the opposite. Most stuck fermentations are already becoming stuck long before any visible symptoms appear. The first day often determines how the following week will unfold.
For us, understanding stuck fermentation has been less about finding a cure and more about learning to recognise the earliest signs that the system is moving in the wrong direction. That understanding has improved yield, reduced unnecessary intervention, and fundamentally changed how we approach sugarcane spirit production at Choeng Doi.
It has also reinforced a broader principle that shapes our approach to distillation: fermentation is not simply about converting sugar into alcohol. It is about understanding the environment in which yeast is asked to work. Raw materials matter. Fermentation conditions matter. Control and documentation matter.
In the emerging Thai craft spirits category, distilleries that develop this depth of understanding will be those that can scale production while maintaining consistency. Fermentation is where that consistency is built.
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