What Is Cacao Fermentation? The Five Days On The Farm
Cacao fermentation is a microbial process that happens on the farm, within hours of the pod being opened, before any roasting or grinding takes place. Beans and their sticky white pulp are heaped up or boxed, and a sequence of yeasts, lactic acid bacteria and acetic acid bacteria works through the sugars in the pulp over roughly five days.
It is the step the rest of the industry cannot undo. A roaster can adjust a profile, a grinder can change particle size, a maker can retemper a batch. Nobody downstream can re-ferment a badly fermented bean, which is why a farm's fermentation practice sets the ceiling for everything that happens afterwards.
What is cacao fermentation doing?
Three populations arrive in order, each feeding on what the last one produced. Ordoñez-Araque and colleagues reviewed the microbiology for Foods and Raw Materials in 2020, screening 350 articles and including 50 that met their criteria. Their timings, drawn from that literature, look like this.
| Stage | When | Peak population |
|---|---|---|
| Yeasts | Active about 48 hours, peaking at 24 hours | 108 CFU per gram of pulp |
| Lactic acid bacteria | Appear between 16 and 48 hours, peaking at 36 hours | 6.4 × 107 CFU per gram of pulp |
| Acetic acid bacteria | Appear around 24 hours, peak at 88 hours, undetectable after 120 hours | 1.2 × 107 CFU per gram of pulp |
Yeasts go first, in a pulp with almost no oxygen in it, converting the sugars to ethanol and carbon dioxide. As the pulp breaks down and drains, air gets in. Acetic acid bacteria then oxidise that ethanol, by way of acetaldehyde, into acetic acid.
The heap warms as this happens. The review records the temperature rising above 37 °C once oxygen enters, and the mass reaching as much as 50 °C once the alcohol and lactic acid have turned to acetic acid. It gives 28-30 °C as the optimum range for the acetic stage itself, which is a reminder that the heap is routinely running hotter than ideal.
The moment the bean dies
Everything above is happening outside the bean. The event that matters is what the acetic acid and the heat do together: according to the review, they kill the bean's germ, which stops its metabolism.
That is the actual purpose of the exercise. A living cacao seed is a seed. Once it can no longer germinate, its internal chemistry is open to the reactions that a roaster will later turn into chocolate flavour. Fermentation is not seasoning the bean. It is ending the bean's life in a controlled way.
The acidity shifts as this goes on. The review puts the starting pulp pH at 3.6, with the value moving into a 3.5 to 5.0 band as oxygen increases and an optimum of 4.5 for acetic fermentation.
Where it goes wrong
Two failures sit on either side of a narrow window. Leave the mass too long and the Ordoñez-Araque review notes that at 45-50 °C, Bacillus species may appear and can produce off-flavours. Quintana-Fuentes and colleagues, reviewing the evidence on spontaneous fermentation for Fermentation in 2025, likewise report that fermentation continued beyond about six days can tip into over-fermentation.
Cut it short and the opposite happens, and the window is genotype-dependent. That 2025 review gives roughly four days for Criollo, about five for Forastero and anywhere from 1.5 to 10 days for Trinitario, with a Mexican study it cites finding Criollo optimal at three days at pH 3.7 against Forastero and Trinitario at five days and pH 3. One schedule does not fit one farm's planting, let alone a region's.
Between those two is a judgement call made by a person with a thermometer and a shovel, turning a heap at intervals, in a shed, in the humidity of a producing region. There is no instrument that reads "done". It is why experienced fermenters are worth paying for and why origin is not only about genetics or soil. Our comparison of how Ecuadorian and Peruvian cacao differ and the background in Peru's criollo growing tradition both sit on top of this step.
Heaps, boxes and the five-day clock
The timings in the table imply a working window. Acetic acid bacteria are undetectable after 120 hours, so by day five the microbial work is essentially finished whatever the farm does next. Drying follows, as a separate stage: the 2025 Fermentation review cites work on a Hybrid C 70 clone in which eight days of fermentation plus six days of drying brought moisture below 7.5%.
The vessel matters too. That review reports wooden boxes as the most widely used method worldwide, with piles, trays, baskets and platforms also in use, and 24 or 48 hours as the commonest turning frequency. In one cited study, bitterness and astringency fell by 20-30% across fermentation, with the highest flavour intensity after five days in a 20 kg box.
Pods picked on a Tuesday and opened on a Thursday have already begun to differ from pods opened the same afternoon. Lot consistency is a logistics problem dressed as a flavour problem, which is where single-origin flavour description starts.
What's actually in a Chaski tube
Chaski is a 600 g tube of dark chocolate buttons at 77% cacao, made from single-origin Peruvian and Ecuadorian cacao with coconut sugar, Lion's Mane as a fruiting body extract and Cordyceps sinensis mycelium. The cacao is low-temperature ground, with no emulsifiers, no seed oils, no palm oil and no added cane sugar.
The Peruvian farm is one our founder has visited, and he has met the farmers working it. Fermentation happens three thousand miles from a UK production line, under someone else's hands, and the only way to know anything about it is to go. Every batch is tested for lead, cadmium and pesticides, and the daily dose on the pack is four buttons. The full declaration is on the Chaski Cacao product page; the rest of our sourcing and process notes are in the Chaski guides library.
Frequently Asked Questions
How long does cacao fermentation take?
Roughly five days for the microbial part. Ordoñez-Araque and colleagues, reviewing the literature for Foods and Raw Materials in 2020, report that acetic acid bacteria appear at about 24 hours, peak at 88 hours and are no longer detectable after 120 hours. Yeasts are active for about 48 hours and peak at 24, with lactic acid bacteria appearing between 16 and 48 hours. Drying follows fermentation and adds further days, but that is a separate stage.
Why does fermented cacao get hot?
The microbial activity itself generates heat in the heap. The 2020 Foods and Raw Materials review records the temperature rising above 37 °C once oxygen enters the mass, and reaching as much as 50 °C after the alcohol and lactic acid have been converted into acetic acid. The review also gives 28-30 °C as the optimum range for the acetic stage, so the heap commonly runs hotter than the ideal. Turning the heap is one way farms manage that.
What happens to the bean during fermentation?
The bean is killed. The 2020 review describes acetic acid and heat together killing the germ of the bean, which stops its metabolism. Until that point the seed is alive and capable of germinating. The pulp around it is where the microbes work: the review gives an initial pulp pH of 3.6, moving into a 3.5 to 5.0 range as oxygen increases, with 4.5 given as the optimum for acetic fermentation. The bean's own interior changes follow from that.
Can cacao be over-fermented?
Yes, and it shows up as off-flavours rather than as spoilage you can see. The 2020 Foods and Raw Materials review notes that at 45-50 °C, Bacillus species may appear in the mass and can produce off-flavours. Under-fermentation is the opposite failure, where the process is cut short. Neither can be corrected later: roasting, grinding and conching all work with whatever the farm handed over. Fermentation is judged by people, not instruments, which is why practice varies.
Is unfermented cacao sold as a product?
Some cacao is sold with very short or minimal fermentation, often under "raw" descriptions. Fermentation length is one of the variables the 2025 Fermentation review treats as decisive, and it cites work in which bitterness and astringency fell by 20-30% over the course of fermentation. Whether a given product was fully fermented is not usually stated on a label, and no reserved description covers it, so it is a question to put to the seller directly rather than something you can read off the pack.
Sources
- Ordoñez-Araque R H, Landines-Vera E F, Urresto-Villegas J C and Caicedo-Jaramillo C F, Foods and Raw Materials, 2020, 8(1), 155-162
- Quintana-Fuentes L F, García-Jerez A, Rodríguez-Negrette A C, Hoyos-Merlano N T and Alvis-Bermúdez A, Fermentation, 2025, 11(7), 377
See where the beans came from
Chaski Cacao — 600 g tube, 77% dark, single-origin Peruvian and Ecuadorian cacao