In palm oil mills, each minute and bar of steam pressure moves the needle on two make-or-break metrics: free fatty acids (FFA) and DOBI. New evidence shows how to trim cycle time to boost throughput without trashing color and bleachability.
Industry: Palm_Oil | Process: Sterilization
Palm oil sterilization — high‑pressure steam cooking of fresh fruit bunches (FFB) — softens fruit, detaches fruitlets, and deactivates lipase enzymes (naturally occurring enzymes that hydrolyze oil). Done right, it suppresses post‑harvest hydrolysis to lower free fatty acid (FFA) content and preserves the Deterioration of Bleachability Index (DOBI, a proxy for carotene content/oxidation and downstream bleachability).
Standards set the guardrails. Indonesia’s SNI 01‑2901‑2006 caps CPO (crude palm oil) at 5% FFA and 0.25% moisture/impurities (js.bsn.go.id). DOBI isn’t yet in SNI, but refiners often require ≥2–3; proposals in Indonesia argue for ≥2.2 as a minimum, noting average Indonesian CPO DOBI is still low (<2) (js.bsn.go.id). DOBI correlates strongly with carotene (freshness) and inversely — albeit weakly — with FFA (js.bsn.go.id).
High‑performing mills typically target CPO with FFA ≲2–3% and DOBI ≳3. One conventional run — 75 minutes at ~132 °C (≈1.5 bar gauge) — delivered ~2.1% FFA and ~3.4 DOBI (researchgate.net).
Quality specs and industrial benchmarks
Export‑grade targets in practice: keep FFA well under 5% (preferably <3%) and DOBI ≥2–3. A Malaysian quality benchmark is DOBI ≥2.2 (MS 814:2007); in ripe seasons, CPO easily exceeds this if bunches are promptly sterilized (researchgate.net). The SNI cap remains 5% FFA and 0.25% moisture/impurities (js.bsn.go.id), while proposals in Indonesia push for DOBI ≥2.2 to lift average quality above the current <2 national mean (js.bsn.go.id).
Time–temperature dynamics (minutes matter most)
Sterilization time dominates both fruitlet separation and oil quality. At ~1.5 bar steam, extending time from 30→60 minutes steadily improved separation and yield; up to 45 minutes, yield reached ~26.7% of FFB and ~90.4% fruitlet separation with FFA ≤1.25% (akademiabaru.com). But beyond the point where lipases are inactivated, every extra minute erodes DOBI.
One detailed study found DOBI fell dramatically as time increased — from ~4.5 to ~0.5 over the experimental range (researchgate.net). Empirically, 30–45 minutes is the sweet spot for separation in pilot tests; beyond ≈60 minutes, lipases are essentially fully inactivated but DOBI costs mount (researchgate.net).
Alternative heat‑up modes are slow: steam‑bath tests needed >90 minutes at 80–105 °C to push FFA below 1% (researchgate.net). Bottom line for throughput: don’t heat longer than needed — excess time (≳75–90 minutes) yields little further FFA benefit but greatly lowers DOBI (researchgate.net).
Steam pressure, temperature and water use
Pressure sets temperature, which sets killing rates and softening. Industrial practice commonly runs 2.0–3.0 atm (~120–130 °C) for ~60–90 minutes. A standard run at 132 °C (1.5 bar gauge) for 75 minutes gave FFA ≈2.1% and DOBI ≈3.4 (researchgate.net).
Raising pressure can trim time. In an integrated direct‑steam trial, 2.5 bar for 60 minutes achieved “acceptable” FFA/DOBI while slashing water use to ~0.0587 kg water/kg FFB — about 4.3× less than conventional (researchgate.net). In those experiments, time mattered more than pressure for fruit separation, and response‑surface models likewise show sterilization time has a stronger influence on DOBI than temperature (researchgate.net; localcontent.library.uitm.edu.my).
Push too far and heat bites back: very high pressure (>3 bar) or long steaming risks hydrolysis (raising FFA) and oxidation (lowering DOBI). Fits from response‑surface methodology (RSM, a statistical optimization method) found optimum DOBI at relatively low temperatures (100–109 °C) and short times (20–80 minutes, depending on ripeness) (researchgate.net), while optimum low‑FFA for ripe fruit needed 100–120 °C with moderate time (researchgate.net).
The practical compromise many mills use: ~2–2.5 bar steam (≈130–140 °C) for ~60 minutes. Under these conditions, FFA around 1–3% and DOBI ≈3–4 are typical; in the integrated‑steam trials at 2.5 bar/60 minutes, FFA remained <3% and DOBI >3 (researchgate.net).
Ripeness sensitivity and mixed batches
Fruit condition changes the game. RSM optimization showed underripe fruit can hit ~0.62% FFA at 100 °C/20 minutes, while overripe fruit needed 120 °C/20 minutes to reach ~1.75%. Loose fruits (fallen) still yielded ~3.16% at 120 °C/20 minutes, reflecting pre‑existing hydrolysis (researchgate.net). Most mills process mixed batches tilted to ripe FFB, so optimization centers on ripe‑fruit parameters (researchgate.net; researchgate.net).
Throughput–quality optimization guide
Use sufficient pressure/temperature to shorten time. In practice, sterilizer pressures around 2.0–2.5 bar (≈130–140 °C) allow stable steaming; in one direct‑steam mini‑sterilizer, 2.5 bar for 60 minutes gave excellent results with far lower water use (researchgate.net). If capacity is constrained, a small pressure bump (e.g., from ~1.5 to ~2.5 bar) can shave 10–15 minutes off cycle time.
Set the minimum effective time. Around ~130 °C/2.0 bar, ~60 minutes typically yields ~2–3% FFA and DOBI ~3 (researchgate.net; researchgate.net). If quality is met, avoid running longer than 60 minutes. Trial 50–55 minutes at full pressure and check FFA/DOBI; times below 45 minutes are rarely used commercially because separation/yield drops and FFA may rise.
Monitor real‑time quality metrics. Sample CPO each shift; verify FFA and DOBI, and keep records to correlate conditions with quality. If FFA creeps above spec or DOBI falls below ~2.2, raise time or pressure slightly; if comfortably within limits, shorten the cycle. For laboratory workflows that require precise titrants or reagents, facilities often specify accurate chemical dosing.
Consider integrated or continuous sterilizers. Designs that combine boiler and sterilizer or use direct steam injection reduce heat losses and water usage (researchgate.net). Upgrades that rationalize steam and water service typically involve supporting equipment for water treatment.
Maintain post‑sterilization handling. Even after sterilization, delays before pressing allow FFA to rise; move fruit into digesters/presses immediately and maintain temperature.
Adhere to standards. Indonesian SNI permits FFA up to 5% (js.bsn.go.id), but markets often require <3%. Target DOBI ≥2.2 (proposed in Indonesia, js.bsn.go.id) or ≥3 for premium quality.
Example guidelines: operating at ~2–2.5 bar, many mills use ~60–75 minutes. Designers can target ~60 minutes at ~2.0 bar as a starting point; ripe FFB often yields ≈2% FFA and ≈3–3.5 DOBI (researchgate.net). If throughput demands rise, test 50 minutes at 2.5 bar and verify FFA/DOBI. Conversely, with underripe or fatty fruit, up to 80 minutes may be needed to control FFA. In short, adjust time to the minimum that still meets FFA and DOBI targets.
Sterilization and oil quality trade‑offs
Free Fatty Acids (FFA). Steaming inactivates lipase, so hotter/longer runs reduce post‑sterile FFA. Examples from RSM: underripe fruit ~0.62% FFA at 100 °C/20 minutes; overripe fruit ~1.75% at 120 °C/20 minutes; loose fruits ~3.16% even at 120 °C/20 minutes (researchgate.net). Generally, ripe bunches achieve ~1–2% FFA under ~60–75 minutes. Exceeding ~80 minutes brings diminishing benefits and can even slightly increase FFA by thermal pathways; aim for ~2–3% rather than chasing <1%.
DOBI. Longer and hotter steaming lowers DOBI. Time increases caused DOBI to crash (e.g., ~4.5→~0.5) in one study (researchgate.net), while models predict near‑maximum DOBI (~5) if both time and temperature are kept minimal (researchgate.net). In mills, trimming steam time 10–20% typically bumps DOBI while modestly raising FFA; if FFA rises from ~2% to ~2.5% but DOBI stays >3, the trade‑off often remains in spec.
Mixed outcomes by ripeness. Under‑ripe and over‑ripe fruit respond differently to heat; over‑ripe fruit may require higher temperature for enzyme kill, and under‑ripe fruit already has lower FFA but also lower carotene (researchgate.net; researchgate.net).
Industrial benchmarks. Export‑quality CPO commonly accepts FFA up to ~3% and DOBI ≳3; a Malaysian benchmark is DOBI ≥2.2 (MS 814:2007) (researchgate.net). Indonesian mills should ensure DOBI >2.2 and FFA <5% (SNI), with a preferred FFA <3% (js.bsn.go.id).
Key takeaways for mill settings
“Kevin relationships.” Sterilization time is the main driver of fruit separation and oil quality. Up to ~45 minutes, longer steaming boosts yield (akademiabaru.com); beyond that, it erodes DOBI (researchgate.net). Steam pressure speeds sterilization but has subtler effects.
Throughput vs quality. Higher pressure/temperature can reduce time but risks overheating that degrades DOBI and can raise FFA. Balanced conditions (≈2 bar, 60–75 minutes) typically maximize yield while keeping FFA <3% and DOBI >3.
Data‑driven optimization. Use published RSM as a guide; test any new setting on yield and oil quality. Incremental moves (±5–10 minutes or ±0.5 bar) allow fine‑tuning (localcontent.library.uitm.edu.my).
Practical example. One mill started at 132 °C (1.5 bar) × 75 minutes, yielding ~2.1% FFA and ~3.4 DOBI; it improved to 2.5 bar × 60 minutes with no quality loss and much lower water use (researchgate.net; researchgate.net). Mills can calibrate similarly: raise pressure or shorten time until quality metrics approach limits, then hold.
Sources
Empirical studies and reviews on palm oil milling, including Hanifarianty et al. (2020) (akademiabaru.com), Wae‑Hayee et al. (2021) (researchgate.net), Mat Jusoh et al. (2015; DOBI vs time/temperature) (researchgate.net), Mat Jusoh et al. (2013; optimization by ripeness) (researchgate.net), Hasibuan (2014; SNI, DOBI proposals) (js.bsn.go.id), Sarah et al. (2014; enzymatic inactivation via steam bath) (researchgate.net), and response‑surface summaries (localcontent.library.uitm.edu.my; researchgate.net; researchgate.net).
