Potentiometric Determination of Fluoride Release from Three Types of Tea Leaves

The purpose of this study is to investigate the fluoride release properties from three types of tea leaves which are commonly consumed in Hong Kong and Hangzhou. Totally 72 drinking water samples of 50 ml each were collected from Hong Kong Island and Hangzhou. The drinking water collected and deionized (D.I.) water were used to brew puerh tea leaves, black tea bags, and green tea leaves by two infusion methods (repeated and continuous). Fluoride concentration of drinking and D.I. water and tea infusion samples taken at specific time intervals was measured by fluoride-ion selective electrode. In tea infusion samples, pH value was recorded as well. Fluoride concentration (ppm) of drinking water samples collected from Hong Kong (95% CI = 0.48-0.54) was significantly higher than Hangzhou (95% CI= 0.19-0.20). Black tea bags (about 10 ppm) released significantly higher fluoride content than puerh tea leaves (about 5 ppm), which was significantly higher than green tea leaves (about 2 ppm) (p<0.05). Prepared by repeated infusion method, original fluoride level in brewing water (D. I. water< drinking water) and round of infusion (1(st)<2(nd)<3(rd)) also statistically related to fluoride content in tea infusions. The pH values of all tea infusion samples were below 7.0 and had no significant relation with fluoride release. Fluoride concentration in tea infusion could saturate within several minutes after brewing. It seems the dynamics of fluoride release from tea leaves may be affected by original ionic level of brewing water, brewing time, and round of infusion.