Levels of major, minor and toxic metals in tubers and flour of Dioscorea abyssinica grown in Ethiopia

Abstract

Dioscorea abyssinica, commonly known as yam, is an indigenous plant in Ethiopia. Its root tuber is used as staple and co-staple food in South Nation Nationality People and Regional State of Ethiopia. This study was carried out to analyze the selected mineral nutrients in the tuber and flour of D. abyssinica grown in different parts of the country for human consumption. Samples collected from five different areas were analyzed for eleven metals (Ca, Mg, Fe, Zn, Cu, Mn, Co, Cr Ni, Cd and Pb) by flame atomic absorption spectrometry and two metals (K and Na) by flame emission spectrometry. Known weight of oven-dried sample was wet-digested using 3 mL of (69–72%) HNO3 and 1 mL of (70%) HClO4, for 2 h at variable temperature (120–270 oC). The validity of the optimized procedure was evaluated by the analysis of spiked samples whose recovery was in the range of 92–105%. The mean concentration range (in μg/g) of each metal in D. abyssinica samples were K (8,469–13,914), Na (133–405), Ca (172–448), Mg (180–354), Fe (28.3–144.5), Mn (12.0–14.5), Zn (12.3–44.5), Cu (7.26–17.6), Co (1.91–8.68), Cr (0.86–3.41) and Ni (2.43–5.31). D.abyssinica could be good sources of essential trace metals to the individuals, more particularly Fe and Zn were higher than the entire trace metals in the samples investigated while Cd and Pb were in minor quantities and hence the plant is free from toxic heavy metals. Consuming 100 g of this tuber per day contribute the following concentration range of Fe, Zn, Cu, Mn, Co, Ni and Cr in mg/100 g per day:2.8–14.4, 1.2–4.5, 0.73–1.8, ND–1.5, ND–0.87, 0.24–0.53 and 0.09–0.34 mg/100 g, respectively. Statistical analysis revealed that significant variations in metal concentrations among the sampling sites were observed for some of the metals when pair wise test was made while insignificant variation were seen for the remaining metals. D. abyssinica could be a better source of K, Ca, Fe and Zn to human compared to the common cereal flours (barley, wheat and red teff flour) as well as tuber food products like potato, cassava, yam, and enset (Ensete ventricosum), while it could be an alternative source of the other essential mineral nutrients to the individual daily mineral intake.