Heavy metal analysis, nutritional values and polyphenol levels of indigenous onion cultivars from Ethiopia: variability implication for its health concerns

Onions are rich in essential minerals and polyphenols supporting health and immunity, yet data on the nutritional and metal contents of onion cultivars remains limited. This study measured Zn, Cu, Fe, Mn, Ni, Pb, Cd, and total polyphenols in onion bulb. Samples were digested in HNO3:HClO4 (3:2) and Inductively coupled Plasma optical emission spectrometry (ICP-OES) technique, with polyphenols determined via Folin-Ciocalteu reagent. The method was validated with LOD and LOQs. Recoveries ranged from 87.77% to 95.00% also ensured validation of the employed method. Concentrations (mg/kg) of Zn, Cu, Fe, Mn, and Cd ranged from 0.45–31.03, 12.42–28.57, 132.01–334.50, 4.59–19.90, and 0.11–0.36, respectively; Ni and Pb were not detected. However, Cd concentrations exceeded the permissible limits set by the WHO/FAO and EFSA across onion samples. Attention should be paid to regions with higher concentrations of Cd metal. Polyphenol content (10.14–26.15 mg GAE/g) indicated its antioxidant potential in the onion which could be ascribed for the bioactivities of phenolic acids, flavanols, and flavonols in the onion extracts enhancing healthy conditions. Chemometric analysis identified moderate positive correlations (p < 0.05) between Mn-Zn and Zn-Fe, suggesting shared metal sources. PCA revealed three components explaining 73.78% of the variance.