Levels of selected heavy metals and fluoride in tea (camellia sinensis) grown, processed and marketed in Kenya

Abstract

Tea-drinking is a habit that has over time spread globally. The chemical composition of tea is very complex and is currently a subject of broad medical and toxicological scientific studies. Thus, the accurate quantification of the levels of both essential and non-essential elements in tea is very important in assessing it’s standard and quality as they are directly related to health and disease. This study examined the levels of Iron (Fe), Zinc (Zn), Copper (Cu), Lead (Pb), Cadmium (Cd) and fluoride in tea from various regions in Kenya as well as other tea producing countries in East Africa (Rwanda, Uganda and Tanzania). The levels of these heavy metals were quantified using Flame Atomic Absorption Spectroscopy (FAAS) whereas the fluoride levels were determined potentiometrically using a Fluoride Ion Selective Electrode (FISE) method. The levels of heavy metals in unprocessed tea were found to be in the range 54.6 - 123.3µg/g for Fe, 15.4 - 37.5µg/g for Zn, 10.3 - 14.8µg/g for Cu, 0.12 - 0.28µg/g for Pb and 10.0 - 27.1μg/kg for Cd. For black tea, the levels were in the range 81 - 369µg/g for Fe, 17.1 - 44.9µg/g for Zn, 9.0 - 17.8µg/g for Cu, 0.12 - 0.41µg/g for Pb and 9.1 - 40.0µg/kg for Cd whereas the concentrations were in the range 2.2 - 12.5µg/ml for Fe, 1.5 - 5.9µg/ml for Zn, 0.7 - 3.0µg/ml for Cu, 0.02 - 0.08µg/ml for Pb and below detectable limit (BDL) - 7.0µg/L for Cd in black tea liquors. The general accumulation pattern and extractability of the elements in the unprocessed, black tea and tea liquors was in the order Fe > Zn > Cu > Pb > Cd and the levels of these metals in the tea liquors were proportional to the respective total levels in unprocessed and black tea. The fluoride levels in tea liquors were found to range from 0.11 to 1.35µg/ml. These results confirm that tea consumption is indeed an important dietary source of Fe, Zn, Cu and fluoride. Based on their heavy metal and fluoride contents, Kenyan teas were comparable with those from Uganda, Rwanda and Tanzania, and all the samples analysed conformed to international standards for tea. However, regional variations in heavy metal and fluoride contents were evident.