n palm oil mills, the digester — a steam‑heated steel drum with spinning beaters — sets the tone for everything that follows. Keep it near 95–100°C for about 15–20 minutes, and oil flows; miss the window, and losses mount fast.
Industry: Palm_Oil | Process: Digestion_and_Pressing
Deep inside modern mills, oil palm digesters are built like industrial locomotives: large steam‑heated cylindrical vessels, typically 5–10 m long and 2–3 m in diameter in large mills, with a central rotating shaft fitted with beater or plow arms (FAO) (patents.google.com).
Sterile fruitlets (“brondol” — the detached fruits after sterilization) are fed in and continuously agitated — around 25–26 rpm in one reported setup (ejournal.undip.ac.id). In practice, digesters are kept almost full: as digested mash is discharged through perforated holes at the base, fresh fruit enters at the top (patents.google.com).
Throughputs span roughly 3–60 tonnes of fresh fruit bunch (FFB) per hour, with insulated steam jackets and live‑steam injection holding the heat. One modern flow sheet injects about 1.35 t/h of steam at 148 °C, producing digester discharge near 105 °C (mdpi.com).
Temperature control at 90–100 °C
The operating target is high temperature — approximately 90–100 °C (ejournal.undip.ac.id) (patents.google.com). As FAO explains, “pounding…at high temperature helps to reduce the viscosity of the oil, destroys the fruits’ outer covering, and completes the disruption of the oil cells already begun in the sterilization phase” (FAO). In simple terms, the mesocarp (the oil‑rich flesh) softens and the pericarp (outer layers) loosen; at around 100 °C, cell membranes weaken so oil escapes more readily.
Numerical studies indicate fruitlets heat uniformly to roughly 95 °C within a few minutes of steaming — reaching equilibrium in about five minutes (mdpi.com) (FAO). In practice, mills maintain this range with continuous steam injection or jacket heating, and temperature monitoring (e.g., thermocouples — temperature sensors that convert heat to voltage) to stay in the 90–100 °C band (ejournal.undip.ac.id) (mdpi.com).
Residence time and flow regime
Time in the drum matters as much as heat. Continuous mills engineer a residence time — the average hold‑up of material in the vessel — on the order of 15–20 minutes, which tests and patent disclosures describe as needed “for complete digestion” (patents.google.com) (patents.google.com). Level sensors and automatic feed keep the digester full so every fruitlet gets the same heat and mixing exposure.
When flow is erratic or the vessel isn’t kept full, the process can backfire: oil emulsifies in the pulp and is lost with fibre. FAO notes that insufficient loading or interrupted flow makes “emulsified oil loss in the fibre [to be] substantial” (FAO). In well‑managed continuous systems, hot wash water is avoided, and the mash flows steadily to the presses to maximize release before pressing (FAO) (patents.google.com).
Oil release and yield outcomes
Run correctly — about 95–100 °C for roughly 15–20 minutes — digestion alone liberates an intensive release of “virgin crude palm oil” equivalent to about 15–20% of FFB weight, which drains through the perforated base as hot liquor (patents.google.com). Pressing then recovers the balance.
Industrial figures align with the model: one process reports ~20,277 kg/h of digested fruit at 105 °C feeding the screw press, which extracts about 8,582 kg/h of oil‑rich liquor (mdpi.com). Proper digestion reduces mechanical load on presses and kernel breakage; when the pulp is fully disrupted, screw presses need less pressure and yield higher extraction.
Miss the window and losses add up. If the digester runs too cool or too short, oil stays trapped in fibres and cake; FAO reports 2–3% oil may remain in cake (FAO). Cold wash water or early discharge emulsifies oil on fibres, sharply lowering recovery (FAO). In well‑operated mills, overall crude oil extraction from ripe fruit reaches on the order of 22–24% of FFB weight, so each 0.1% change in yield is economically substantial. By contrast, a 1% drop in extraction (e.g., from poor digestion) across a 20 Mt/yr output equals 200,000 t/year lost — worth on the order of hundreds of millions of dollars.
Key operating window, summarized
Efficient digestion in modern mills is achieved at ~95–100 °C with ~15–20 min residence (ejournal.undip.ac.id) (patents.google.com). Under these conditions, digestion releases ~15–20% of FFB weight in oil within the digester (patents.google.com), enabling final oil extraction rates above 22% of FFB and keeping oil loss in fibre/cake to below about 2–3% (targets drawn from FAO reporting) (FAO) (FAO). Maintaining this temperature/time regime — via steam control and level monitoring — underpins yield and throughput.
Sources: FAO processing manuals and industry studies (FAO) (patents.google.com) (patents.google.com) (mdpi.com) (FAO); Indonesian technical regulations and monitoring reports (ejournal.undip.ac.id); greenhouse modeling of digester performance (mdpi.com); and press‑efficiency data (FAO). Inline citations link to precise references.
