Comparison Between Emerging and Conventional Methods for Edible Oils Bleaching

Overview

Abedi and colleagues review industrial oil bleaching (IB) and four emerging alternatives — ultrasound-assisted bleaching (UAB), high-voltage electric field-assisted bleaching (HVAB), microwave-assisted bleaching (MWAB), and membrane-assisted bleaching (MAB) — across pigment removal, sterol and tocopherol retention, oxidative indices, heavy metals removal, clay consumption, and adsorption kinetics and thermodynamics. Heavy-metal-relevant content is confined to four short subsections (one per bleaching technology) plus scattered table entries. The authors summarise that bleaching clays remove trace metals (notably Fe, Cu, and a broader set including lead (Pb), cadmium (Cd), nickel (Ni), chromium (Cr), Co, Al) from crude vegetable oils by adsorption, that higher temperatures and ultrasonic or electric-field assistance enhance the metal-removal efficiency relative to conventional bleaching, and that incomplete neutralisation upstream can leave residual Ca, Mg, Pb, and P in the bleached oil. The review reports no original measurements and presents no occurrence distributions for heavy metals in finished retail edible oils.

Key numbers

All values below are reported by the review as drawn from cited primary studies. The review does not contribute new measurements.

Section 2.4 — Industrial bleaching effect on heavy metals (p. 8, citing Silva et al. 2014 and Abedi et al. 2021)

Oil	Treatment	Element	Reported reduction or value	Cited primary source
Palm oil	3% acid-activated bleaching clay	Phosphorus	Reduced to 2.7 ± 0.1 mg/kg (post-bleaching final value; pre-bleaching value not quoted in this review)	Silva et al. 2014
Palm oil	3% acid-activated bleaching clay	Iron (Fe)	Reduced to 0.2 ± 0.1 mg/kg (post-bleaching final value; pre-bleaching value not quoted in this review)	Silva et al. 2014
Soybean oil	Industrial bleaching, temperature 35 °C → 65 °C	Iron (Fe)	Decrease of 3% to 24% (depending on operating conditions)	Abedi et al. 2021
Soybean oil	Industrial bleaching, temperature 35 °C → 65 °C	Copper (Cu)	Decrease of 12.5% to 24%	Abedi et al. 2021

The review further states that increasing bleaching temperature by about 10 °C can roughly double the reaction rate and enhance trace-metal adsorption onto acid-activated bleaching clay (Abedi et al. 2021).

Section 3.1.5 — Ultrasound-assisted bleaching (UAB) effect on heavy metals (p. 15, citing Abedi et al. 2015, 2021)

The review states that applying ultrasound during soybean oil bleaching produced a “notable decrease in the levels of Fe and Cu compared to the IB method” via cavitation-induced abrasion of the clay surface, exposing fresh adsorption sites. No numeric Fe or Cu reduction value is supplied for the UAB-vs-IB comparison in this subsection beyond the IB values above; the review notes a comparable mechanism for phosphorus and soap removal (Abedi et al. 2015, 2021).

Section 3.2.7 — HVEF-assisted bleaching effect on heavy metals (p. 19, citing Abedi et al. 2016, 2020 and Mousavifard et al. 2024)

The review states that the mechanism for metal removal under HVAB relates to the medium’s ionic strength in the presence of Fe²⁺ and Cu²⁺ and that convection in the electric field enhances cation transport to the electrode surfaces. No quantitative reduction percentages or before/after values are reported in this subsection.

Section 3.3.4 — Microwave-assisted bleaching (MWAB) effect on heavy metals (p. 20, citing Sarafraz Ardakani et al. 2023)

The review reports that microwave treatment at 600 W of sesame oil during refining was associated with removal of metal cations (Al, K, Fe, Pb, Cd, Ni, Cu, Cr, Co) via interaction with bleaching clays. Post-bleaching analysis in the cited primary study revealed elevated concentrations of Mg, Ca, Pb, and P ions, which the review attributes to potential contributions from wash water and adsorbents and to incomplete removal of these elements during the neutralisation stage. No concentration values, sample sizes, or analytical limits of detection are quoted in the review for the Sarafraz Ardakani et al. 2023 measurements; the review’s heavy-metal-specific content for MWAB consists of a single paragraph and a qualitative mechanism statement.

Tabulated entries with explicit heavy-metal numbers in the review’s data tables

Table 2 (USAB conditions and outcomes, pp. 9–13) lists three rows that include explicit metal-reduction percentages copied from the underlying primary studies:

Oil	USAB conditions	Reported metal effect	Cited source (per Table 2)
Olive oil	20 kHz, 750 W, 1% Tunisian acid-activated clay, 60–80 °C, 13–45 min	”Reduction in phosphor by 300%; Reduction in iron by 54.5%; Reduction of calcium by 64.2%; Increase in magnesium by 150%“	De Jesús-Hernández et al. 2023
Olive oil	20 kHz, 750 W, 1% bleaching earth bentonite, 28% amplitude, 30 °C, 13–45 min	”Reduction in phosphors by 93%; Reduction in iron by 41.12%; Reduction in calcium by 48.91%; Increase in magnesium by 43.66%“	Gharsalli et al. 2025
Olive oil	20 kHz, 750 W, 1% bleaching earth bentonite, 28% amplitude, 30 °C, 30–60 min	”Reduction in phosphors by 93%; Reduction in iron by 41.12%; Reduction in calcium by 68.54%; Increase in magnesium by 43.66%“	Essid et al. 2016

(The “300%” phosphorus reduction in the first row reproduces what the review prints; a reduction greater than 100% is not possible in the literal sense and is presumably a typesetting carry-over from the review’s table — see Verification notes.)

Table 1 (IB conditions and outcomes, pp. 5–6) lists two rows that include metal-reduction figures:

Oil	IB condition	Reported metal effect	Cited source (per Table 1)
Palm oil	Acid-activated bleaching earth, 105 °C, 30 min, 50 mmHg pressure, 0.5%–3.0% w/w	”Decrease in carotenes from 258 mg/kg to 10 mg/kg by increasing acid-activated earth from 0.5% to 3%; Decrease in phosphorus (<3 mg/kg) and iron (<0.3 mg/kg) contents using 3% acid-activated bleaching earth”	Silva et al. 2014
Soybean oil	0.5%–2% w/v activated bentonite clay, 35–65 °C, 0–30 min	”Decrease in iron and copper levels about 3%–24% and 12.5%–24%, respectively by increasing temperature from 35 °C to 65 °C”	Abedi et al. 2021

The review’s body text in Section 2.4 paraphrases these two table entries.

Other elements named in the review

The text and tables also mention Mn (magnesium increase patterns in UAB), Ca, Mg, K, and P. Of these, only K is a candidate HMI analyte category and only when speciated as a context element; Ca, Mg, and P are nutritional minerals outside the HMI corpus and are tracked here only because they co-occur with the Fe, Cu, Pb, Cd, Ni, Cr, Co, aluminum (Al) cations in the cited primary work. The review does not measure or quantify them.

Evidence Fitness

This is a narrative review of edible oil bleaching technologies. It contributes no primary heavy-metal measurements and no occurrence distributions. The quantitative heavy-metal figures it presents are second-hand summaries of the underlying primary studies (Silva et al. 2014, Abedi et al. 2021, Sarafraz Ardakani et al. 2023, De Jesús-Hernández et al. 2023, Gharsalli et al. 2025, Essid et al. 2016, and others). Public evidence label “Context only” is appropriate.

The source supports process-context claims about how bleaching unit operations in edible-oil refining adsorb trace metals from crude oils into bleaching clay residues, and points to specific primary studies where the underlying observations live. It does not support occurrence claims for Pb, Cd, Ni, Cr, Co, Al, Cu, or Fe in finished retail edible oils and should not be used as the basis for HMTc threshold work or for ingredient contamination_profile values on any vegetable-oil ingredient page. Threshold work on edible oils should rely on the primary occurrence studies referenced here.

Methods (brief)

Narrative literature review without an explicit search-and-selection protocol or PRISMA-style inclusion criteria. The review compiles published characterisation and process-outcome data on industrial and emerging bleaching technologies from cited primary studies; analytical methods for the underlying primary measurements are not re-described in detail in the review’s tables and would need to be retrieved from each cited study (e.g., Silva et al. 2014 for palm oil P/Fe figures, Abedi et al. 2021 for soybean oil Fe/Cu figures, Sarafraz Ardakani et al. 2023 for sesame oil Pb/Cd/Ni/Cr/Co/Al figures). The review separately describes the adsorption isotherm, kinetics, and thermodynamic frameworks applied across the cited bleaching studies (Langmuir, Freundlich, Temkin, Hill-de Boer, Toth, BET isotherms; pseudo-first-order, pseudo-second-order, Elovich, Weber–Morris intraparticle diffusion kinetics; Van’t Hoff thermodynamic parameters), but again does not contribute new measurements.

The review’s data tables present results in mixed bases (mg/kg, ppm, percentage reductions, and percentage increases for some mineral elements). No single analyte distribution is reported on a consistent unit basis suitable for cross-study pooling within the review itself.

Implications

Certification: Process context only. The review supports the general claim that the bleaching step of edible-oil refining adsorbs trace Fe and Cu (and, where neutralisation is upstream and complete, Pb, Cd, Ni, Cr, Co, and Al) from crude oil into the bleaching clay, and that ultrasound and electric-field assistance can enhance this adsorption relative to conventional bleaching. The review provides no occurrence values for finished retail edible oils that should inform HMTc thresholds; primary occurrence studies of bottled or bulk retail oils remain the appropriate basis.

Courses: Useful as background reading for explaining the edible-oil refining sequence (degumming, neutralisation, bleaching, deodorisation) and the role of bleaching clay as a trace-metal sink. The Sarafraz Ardakani et al. 2023 reference is the only Pb/Cd/Ni/Cr/Co-quantifying study cited in the review and is worth pursuing as primary evidence for edible-oil heavy-metal occurrence in sesame oil refining.

App: No ingredient contamination_profile impact. The review contributes no concentration values usable for the consumer-app pipeline.

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