Yohannes et al. 2024 — Heavy metals in edible vegetable oils, Gondar, Ethiopia
This study measured five metals (Fe, Cu, Zn, Pb, Cd) in 17 edible vegetable oil samples from Gondar City, Northwest Ethiopia, using flame atomic absorption spectroscopy (FAAS) after microwave digestion. The study found that 25% of samples exceeded the WHO/FAO Codex limit for cadmium (0.05 mg/kg) and 17% exceeded the limit for lead (0.1 mg/kg), specifically in locally produced mixed oils. This is the first comprehensive heavy metals investigation of vegetable oils from this region.
Note on evidence tier: downgraded from A to B. The license is CC BY-NC-ND 4.0 (restricts adaptation), the sample size is small (n=17 across 17 brands, many with n=1 per brand), no health risk assessment was performed, and the LOQ values (0.94–4.95 mg/l) are relatively high for cadmium.
Key numbers
Median concentrations in unspiked samples (mg/L as reported; note: concentrations appear to reflect the digested extract solution, not necessarily mg/kg wet weight of the oil — unit conversion was not provided in the paper):
- Fe: median 0.15 ± 0.12 mg/L; range 0.01–0.80 mg/L
- Pb: median 0.07 ± 0.07 mg/L; range 0.003–0.27 mg/L
- Cu: median 0.015 ± 0.9 mg/L; range 0.002–0.06 mg/L
- Zn: median 0.25 ± 0.9 mg/L; range 0.07–0.80 mg/L
- Cd: median 0.96 ± 0.04 mg/L; range 0.08–0.18 mg/L
Note: Cd median of 0.96 appears inconsistent with the range of 0.08–0.18; this may be a reporting error in the paper. The range values (0.08–0.18 mg/L) are used here as more reliable.
Regulatory exceedances:
- Cd: 25% of samples exceeded 0.05 mg/L (FAO/WHO Codex limit)
- Pb: 17% of samples exceeded 0.1 mg/L (FAO/WHO Codex limit)
Kruskal-Wallis test showed significant differences among oil types for Cu (p=0.0244) and Zn (p=0.0084), but not for Cd (p=0.087), Fe (p=0.1225), or Pb (p=0.6595).
LOD (method): Pb 0.4 mg/L, Fe 1.485 mg/L, Zn 0.16 mg/L, Cu 0.47 mg/L, Cd 0.3 mg/L. LOQ: Pb 1.33 mg/L, Fe 4.95 mg/L, Zn 0.523 mg/L, Cu 1.56 mg/L, Cd 0.94 mg/L. Recovery: 81–115%. RSD: below 15%.
Methods
FAAS (210VGP model, flame atomic absorption spectroscopy) with air-acetylene flame. Microwave digestion: 0.5 g sample + 5 mL HNO3 (69–72%) + 2 mL HCl (37%) + H2O2 (30%) at 180°C for 55 min. Triplicate analysis. Results reported in mg/L of extract solution; the paper does not clearly state the dilution factor or provide a conversion to mg/kg wet weight of oil, which limits comparability with other studies. Study period: May–July 2021. n=17 samples across multiple brands and oil types.
Limitation: LOQ for Pb (1.33 mg/L) appears very high relative to expected levels, and many Pb results below this LOQ would be non-quantifiable. The reported range (0.003–0.27 mg/L) includes values below LOQ. This undermines the reliability of the Pb data.
Implications
Certification: The regional data (Ethiopia, locally produced oils from small-scale operations) is contextually important for understanding supply-chain risk in Sub-Saharan African sourcing. Exceedances in Cd and Pb for locally produced mixed oils are consistent with limited refining capacity noted by the authors.
Courses: Case study on the metal contamination risk posed by outdated extraction equipment, absence of refining/bleaching, and inadequate quality control in small-scale oil production in developing markets.
App: Limited utility for ingredient-level contamination profiling due to unit reporting ambiguity and small sample size. Flag as contextual regional data only.