How cocoa beans are fermented, and why six days matters

There is a persistent idea that cocoa flavour comes from the tree. Genetics and terroir set the ceiling, certainly. But the difference between a bean that tastes of red fruit and caramel and one that tastes of flat, bitter cardboard is almost never the tree. It is what happened in the six days after the pod was opened.

The bean arrives with no chocolate flavour at all

Cut open a fresh cacao pod and taste a bean. It is sweet, mucilaginous, faintly floral — and it tastes nothing like chocolate. The compounds a chocolate maker eventually roasts are not present. They exist only as precursors: proteins, peptides and sugars locked inside the cotyledon. Fermentation is the process that kills the seed and unlocks them.

What actually happens in the box

Wet beans, still coated in sugary pulp, go into wooden boxes and are covered. Three overlapping microbial phases follow.

Days 1–2, yeasts. Anaerobic conditions inside the mass favour yeasts, which convert pulp sugars into ethanol. The pulp begins to break down and drain away as sweatings. Temperature starts to climb.

Days 2–4, lactic and acetic acid bacteria. As pulp drains, air enters the mass. Acetic acid bacteria oxidise the ethanol into acetic acid, and that reaction is strongly exothermic. The mass climbs to 45–50 °C. Hot acetic acid penetrates the seed coat and kills the embryo — the single most important event in the whole process. Cell walls inside the cotyledon break down, enzymes and substrates that were physically separated come into contact, and the precursor reactions begin.

Days 4–6, enzymatic development. With the seed dead and the internal structure broken open, proteases cut storage proteins into the peptides and amino acids that will become chocolate flavour under roasting. Polyphenols oxidise, which is what turns the bean from deep purple to brown and drops astringency to a tolerable level.

Stop at day three and you get a purple, astringent bean with a dead-flat roast profile. Run to day nine and you get over-fermentation: ammonia, hammy off-notes, mould risk. Six days, with turning, is where our beans land.

Why the turning interval is not a detail

A fermentation mass is not uniform. The centre runs hot and anaerobic; the outer layer is cooler and better aerated. Left alone, you finish with a lot that is simultaneously over-fermented in the middle and under-fermented at the edges — and the cut test will show both defects at once.

Turning solves it. We turn every 48 hours, moving the outside of the mass to the centre and the centre outward. It also introduces the oxygen the acetic phase needs. Turn too often and you lose the heat that drives the whole reaction; turn too rarely and the lot stratifies. Every 48 hours across a six-day ferment gives two turns, which for our box size and climate is what produces the most even result.

Wooden boxes matter too. Wood insulates — it holds the heat the acetic phase generates, where plastic or metal would sink it. Boxes also carry a resident microflora from previous ferments, which is one reason a given station tends to produce a consistent house character over time.

Then drying, which can undo all of it

Fermented beans leave the box near 55% moisture and must reach roughly 7% to be stable. Do it too fast and acetic acid gets trapped inside the bean instead of volatilising off, leaving a sour lot. Too slow, and mould takes hold — the defect with the largest sensory penalty and a genuine food-safety concern.

We dry on raised racks under shelter for 11 to 14 days, spread thin and turned by hand. The shelter is the part worth underlining. In a region with unpredictable rain, an open-yard lot caught by an afternoon downpour at day four can be effectively lost. Raised racks also give airflow underneath, which ground-drying on tarpaulin does not.

Reading the result

All of this resolves into one number set: the cut test. Three hundred beans per lot, cut lengthwise, scored. Well-fermented beans are brown and fully fissured. Purple beans are under-fermented and astringent. Slaty beans are grey, compact and never fermented at all — bitterness with no cocoa character. Mouldy and insect-damaged beans are counted separately.

Industry standards allow up to 3% each for slaty, mouldy and insect-damaged. Our lots typically run 88 to 92% well fermented, with slaty and mouldy at 1 to 3%. Those figures travel on the lot sheet with every shipment, because they are the numbers that tell a chocolate maker what they are actually buying — long before anyone tastes anything.