Vella et al. 2024 — Heavy metals in Maltese olive cultivars and mill waste via FAAS
This study measured iron, copper, cobalt, nickel, zinc, cadmium, and strontium in four olive cultivars (Carolea, Cipressina, Leccino, Bidni) grown in Malta, using Flame Atomic Absorption Spectrometry (FAAS) with the method of standard additions. Analytes were measured across four fractions — skin, pit, solid waste, and liquid waste — to characterise distribution within the olive and its processing byproducts. The key finding on cadmium is methodologically compromised: FAAS was insufficiently sensitive at the low concentrations regulated for table olives (EU limit: 0.02 mg/kg; Codex limit for vegetables: 0.2 mg/kg), producing standard errors equal to or larger than the measured values, so no defensible conclusion on regulatory compliance could be drawn for cadmium. For all other metals, results were deemed valid, with no significant difference in concentration between cultivars but a significant difference between fractions for several metals, particularly between the liquid waste fraction and the solid fractions.
Key numbers
All concentrations reported as mean ± SE in ppm (mg/kg), representing averages across all four cultivars and media (Table 4 of the paper):
| Metal | Average across all cultivars and media (ppm) |
|---|---|
| Fe | 6.3 |
| Cu | 12.6 |
| Co | 4.7 |
| Ni | 5.3 |
| Zn | 23.0 |
| Cd | 1.3 (unreliable — SE ≥ concentration) |
| Sr | 2.2 |
Selected skin (edible fraction) results by cultivar:
- Carolea skin: Fe 30.0 ± 3.3, Cu 14.9 ± 4.1, Co 6.9 ± 2.7, Ni 5.7 ± 3.0, Zn 22.9 ± 4.7, Cd 1.2 ± 0.4, Sr 6.5 ± 2.7 ppm
- Cipressina skin: Fe 3.7 ± 2.6, Cu 15.0 ± 2.0, Co 5.5 ± 2.8, Ni 5.2 ± 2.8, Zn 25.2 ± 4.8, Cd 1.7 ± 1.0, Sr 0.8 ± 0.7 ppm
- Leccino skin: Fe 5.2 ± 2.8, Cu 51.0 ± 1.5, Co 5.7 ± 2.8, Ni 12.8 ± 3.7, Zn 21.2 ± 4.2, Cd 1.8 ± 1.0, Sr 1.9 ± 0.7 ppm
- Bidni skin: Fe 1.3 ± 0.6, Cu 12.7 ± 1.3, Co 4.8 ± 2.6, Ni 5.3 ± 3.0, Zn 23.3 ± 5.0, Cd 1.6 ± 0.9, Sr 1.6 ± 1.4 ppm
Note: cadmium in Bidni pit had SE larger than the measured value and was flagged as invalid. All cadmium results carry SE of 0.2–1.1 mg/kg, which equals or exceeds both the EU (0.02 mg/kg) and Codex (0.2 mg/kg) limits, making regulatory comparisons impossible with this method.
Liquid waste consistently showed the lowest metal concentrations across all cultivars. No significant difference in metal concentration between cultivars was found (all H0 accepted at α = 0.05 for cultivar comparisons). Cadmium showed significant differences between most media pairs, unlike the other metals.
Comparison with prior literature (Table 4): concentrations obtained by FAAS in this study are generally higher than results reported using ICP-MS or ICP-AES in other cultivars, which the authors attribute to different cultivar and cultivator practices rather than instrument bias per se, since FAAS with standard additions should not systematically overestimate.
Methods
Sampling: olives and mill waste from a single Maltese cultivator, harvested October (ripe stage). Four cultivar types; two-phase continuous centrifugation mill used. FAAS (flame AAS) with method of standard additions to minimise matrix effects. Aqua regia hot-plate digestion (37% HCl + 69.5% HNO3, 3:1 v/v). Freeze-drying of solid samples before pulverisation. Seven-point calibration curves per element per medium per cultivar (112 total calibration curves). Elements: Fe (248.3 nm), Cu (324.8 nm), Co (240.6 nm), Ni (231.9 nm), Zn (213.8 nm), Cd (228.8 nm), Sr (460.6 nm). Air-acetylene flame for all elements. r² threshold for calibration curve acceptance: ≥0.990.
Key limitation: FAAS is designed for ppm-range detection. Cadmium in table olives is regulated at 0.02–0.2 mg/kg (20–200 ppb), well below FAAS sensitivity. Authors recommend GFAAS, ICP-MS, or ICP-OES with microwave aqua regia + HF digestion for cadmium at these concentrations. The study’s non-cadmium results (Fe, Cu, Co, Ni, Zn, Sr) are in the FAAS-appropriate range and are treated as valid.
Evidence tier assigned as B due to: single-cultivator sampling (no geographic diversity), small number of distinct samples, journal is a practitioner-oriented applied research publication rather than a primary peer-reviewed food science journal, and the cadmium data — the most policy-relevant analyte for table olives — is methodologically unusable for regulatory comparison.
Implications
Certification: This source does not provide usable cadmium values for table olives. The skin fraction Ni values (5–13 ppm dry weight) are informative as context. The study underlines the method-selection problem: FAAS is unsuitable for regulatory cadmium monitoring in olives; ICP-MS or GFAAS is required.
Courses: Useful illustration of method-selection consequences — a study can produce valid calibration curves and still fail to answer the regulatory question because the analytical technique’s LOQ is above the regulatory limit of interest. Case study material for testing-method selection in the certification context.
App: Not suitable as a primary data source for contamination_profile values due to B-tier evidence and compromised cadmium data.