Fluoride Content of Green Tea
The highest Fluoride concentration reported in any vegetation occurs in tea leaves.
The Fluoride content of Tea leaves can be 1,000 times the soluble Fluoride content of the soil and 2–7 times the total Fluoride content of the soil [Fung 1999]. A measurement of over 21,000 ppm was reported for one sample of black tea leaves [Michalski 2006].
Green Tea is produced from withered leaves that are steamed and rolled before drying to prevent the fermentation of the leaves, which occurs in making Black tea.
Brewed Green Tea has been found to contain very high Fluoride levels, e.g. nearly 7 mg/L (ppm) when made with deionized water [Chan 2010] and over 25 ppm in one Green Tea brew [Lung 2003]. Brick Tea, made from older leaves, has even higher Fluoride content.
Fluoride uptake is increased by the presence of Aluminium, probably due to the uptake of Aluminium–Fluoride complexes. Tea leaves are considered a hyper-accumulator of Aluminium. They can absorb up to 30,000 ppm of neurotoxic Aluminium [Matsumoto 1976, Shu 2003, Wong 2003].
Reports of the Fluoride content of Green Tea varies, with some finding lower Fluoride content than in Black Tea and others higher [Zerabruk 2010, Giljanović 2012, Zhu 2013, Embiale 2014, Atasoy 2016, Das 2017]. Given the Fluoride hazard, it is not surprising that the Tea industry has financially supported some research [Peng 2015].
Various assessments of Tea contribution to total daily intake have been reported [Pang 1992, Heilman 1999, Kao 2000, Malinowska 2008, Chan 2013, Peng 2015, Waugh 2016 ].
In temperate climates such as Britain excessive Tea intake was shown to contribute most to total intake of up to 8.9 mg of Fluoride per day [Walters 1983]. The pH of many brewed and bottled instant teas can be quite low [Simpson 2001, Behrendt 2002, Pehrsson 2011, Lunkes 2014, Reddy 2016] leading to a substantial amount of corrosive and cariogenic Hydrogen Fluoride (HF) in the beverage [Hendricks 2013].
Measurement of Fluoride content by ion selective electrode can be affected by particle size, brewing time, pH and Aluminium [Borjigin 2009] content, so that published values might in some cases represent underestimates of the toxicity hazard. HF is not detected by a Fluoride ion selective electrode.
Tactics used to improve analysis include the use of different ionic strength buffers including chelating agents for Aluminium such as tartaric acid, TRIS (tris(hydroxymethyl) methylamine), or trans-1,2-cyclohexanediamino N,N, N',N'-tetra acetic acid, and the use of the standard addition technique [Yuwono 2005, Reto 2008, Janiszewska 2013].
Ion chromatography using an alkaline eluant has been reported to be convenient for simultaneous measurement of anions present in Green Tea, which include Nitrate, Oxalate, Phosphate and Sulfate [Michalski 2006, Kumar 2008, Mincă 2013, Maleki 2016, Yang 2017].
Chewing gum incorporating Green Tea extract has been reported [Suyama 2011].