Metals in green coffee beans from major coffee-growing regions of Ethiopia

The metal contents of green coffee beans cultivated in major coffee growing zones of Ethiopia (Wollega, Sidamo, Harar, Bench Maji and Kafa) have been determined in representative samples of the five coffee varieties collected from Coffee Quality Inspection and Liquoring Center located in Addis Ababa, the capital city of Ethiopia. Different sample preparation procedures were tested by varying reagent volumes and types, digestion time, digestion temperature and amount of the sample to decompose the green coffee beans and solubilize the metals. The optimal procedure required 4 h to completely digest 0.5 g of green coffee beans with 5 mL HNO3 (70%) and 1.5 mL HClO4 (70%) at 270 oC. Concentrations of metals (Ca, Cd, Cr, Co, Cu, Fe, K, Mg, Mn, Ni, Pb and Zn) were determined by flame atomic absorption spectrometer employing a four point external calibration curve. The accuracy of the optimized procedure was evaluated by analyzing sample digests spiked with standard solutions. Recoveries of the spiked samples varied from 90 to 110% in green coffee beans. The metals levels observed in green coffee beans are comparable to literature reports. Cd was not detected in any of the five samples while Pb was detected at trace level in only one of the five samples. This indicated that the Ethiopian green coffee beans are free from the toxic metals. The Pearson correlation coefficients indicated strong to medium positive or negative correlation among the metals in the green coffee beans. The analysis of variance results at 95% confidence level suggested that there were significant difference in the concentrations of all the metals except K between the five sampling areas. Thus, this study has revealed variation of metal composition of green coffee beans with the geographical origin of the coffee verities. The variation in composition among coffee sample might be due the differences in mineral contents of the corresponding soils.