Fermentation as a chemical process
When freshly harvested cacao beans are piled under banana leaves or in wooden boxes, one of the most complex biochemical processes in the food industry begins. Cacao fermentation is a microbial ecological succession where different communities dominate at different phases.
Phase 1 (0–24 h): yeasts and ethanol
Fresh cacao pulp contains 10–15% sugars and has an initial pH of 3.5–4.0. Yeasts (Hanseniaspora, Saccharomyces cerevisiae) perform alcoholic fermentation: C₆H₁₂O₆ → 2 CH₃CH₂OH + 2 CO₂. The ethanol produced (up to 5–7% v/v) is the primary substrate for acetic acid bacteria in the next phase, and its diffusion into the cotyledons contributes to cell death in the bean.
Phase 2 (24–72 h): lactic acid bacteria
As ethanol rises and simple sugars are depleted, lactic acid bacteria (LAB) dominate: Lactobacillus fermentum, L. plantarum, Fructobacillus pseudoficulneus. They ferment residual sugars via homofermentative and heterofermentative routes, accumulating lactic acid (pKa 3.86) at concentrations of 200–400 mmol/kg. Lactic acid's main role is lowering pulp pH to 3.2–3.5, activating the endogenous proteases that generate flavor precursors.
Phase 3 (48–120 h): acetic acid bacteria and the decisive acid
With oxygen entering through bean mass turnovers, acetic acid bacteria (Acetobacter pasteurianus) oxidize ethanol to acetic acid in two steps: ethanol → acetaldehyde → acetic acid. Acetic acid (pKa 4.76) reaches 400–800 mmol/kg and is the most important acid for two reasons: it penetrates the cotyledon (higher lipophilicity allows membrane diffusion), and its intracellular acidification triggers the enzymatic cascade producing chocolate flavor precursors.
The forgotten acid: succinic
Succinic acid (butanedioic acid, pKa₁ 4.21) is produced at 50–150 mmol/kg. Its dicarboxylic nature gives it significant buffering capacity in the pH 4–6 range. Sensorially, it contributes an acid-umami-slightly salty character — one of the compounds responsible for the "clean acidity" of premium Colombian single-origin cacaos.
pH as a quality lever
| Final bean pH | Implication | |---|---| | < 4.5 | Excess acetic acid — vinegar notes | | 4.5 – 5.0 | Optimal — complete precursor development | | 5.0 – 5.5 | Short fermentation — more polyphenols preserved | | > 5.5 | Insufficient fermentation — raw purple flavor |
The next article enters the most spectacular transformation in cacao processing: the Maillard reaction during roasting — how the amino acids and reducing sugars produced in fermentation become the hundreds of molecules that make up chocolate aroma.