Fluoride in Tea and Its Risk Assessment

Li NN，Liang YR，Zheng XQ

Zhejiang University Tea Research Institute，Hangzhou 310058，China.

*:Corresponding author:Zheng XQ，e-mail:xqzheng@zju.edu.cn

There was survey showing that F level in black tea was not associated with considerable risks of fluorosis［10］.However，F(xiàn) concentration varied with teas.For adult and children tea drinkers consuming five cups of black tea per day，the F intake from tea consumption will be in the range of 8.0-303%and 12-303%of the SAI(safe and adequate daily intake)，respectively.Excessive F intake with black tea，especially in the regions with its high F level in the drinking water，increases the risk of dental fluorosis in children during the years of tooth development［11］.Regression analysis showed that fluorosis was significantly correlated with the consumption of milk tea made from brick tea solution and the endemic fluorosis occurring in some areas was considered to be related to the heavy consumption of foodstuffs prepared with brick-tea［12］.The F concentration in tea samples collected from UK market ranged from 93 mg/kg to 820 mg/kg in the dry tea and 0.43 mg/L to 8.85 mg/L in the infusions.The UK supermarket economy teas contained elevated F，ranging from 3.60 to 7.96 mg/L in a 2 minute brewing infusion，being comparable to the Chinese brick tea.One liter of tea infusion prepared from the economy tea contained 6.0 mg/L of F，being 75 120%of the DRI(dietary reference intake，4 mg F for an adult)fluoride［13］.The high fluorine content in brick tea might be due to the high content in the raw material［14］.

There was great difference in F concentration of tea leaf between various tea cultivars.Table 2 shows that F concentration in mature shoots with five leaves and a bud of cultivar Jinfeng was more than 3 times of that in cultivar Lvyafoshou，suggesting that selecting proper tea cultivars during establishment of tea gardens is an effective way to control F level in tea products.

Tea plant is considered to be a fluorine(F)accumulator because it takes up a large quantity of F element from acidic soils.F unbalance in the human body can cause diseases of teeth and bones［1］.Increasing attention has been paid to the behavior of F in tea and tea infusions because tea fluoride is a main source of F supplementation［2］.However，fluorosis could be developed for people who consumed a large quantity of brick tea，which was called brick tea-type fluorosis and it was considered to be more severe adverse effects on human health compared with both water-type and coal combustion-type fluorosis that occurred in some areas［3］.There were controversial results about the effects of tea consumption on fluorosis.It was reported that the consumption of Pu'erh brick tea was unlikely to induce fluorosis［4］.Advances in studies on F variations in different kinds of teas and its safety assessment were reviewed in the present paper so as to supply useful information for developing techniques to control the oversupply of F from brick tea.

1.Variations in F level of different kinds of tea

F level in tea depends on the maturity of the tea leaf which is used to prepare the tea.A great variation in F level was found between different kinds of tea prepared using different leaf materials.There was test showing that dark brick tea had the highest F level，which was about 3 times of that in green tea(Table 1)［5］.F levels ranged from 113.18 ～604.20 mg/kg in commodity black teas collected from market［6］.Because the F level showed good correlation with the quality grade of teas，it could be used as a quality indicator for tea quality evaluation［7，8］.

F content in black tea infusions prepared by brewing one tea bag in 100 mL boiling water for 5 min ranged from 0.57 mg/L to 3.72 mg/L，being much higher than those of herbal and fruit infusions(0.02 ～0.04 mg/L)［9］.

Table 1.F concentrations in various kinds of tea［5］.

2.Exposure and associated health risks for tea F

There was survey showing that F level in black tea was not associated with considerable risks of fluorosis［10］.However，F(xiàn) concentration varied with teas.For adult and children tea drinkers consuming five cups of black tea per day，the F intake from tea consumption will be in the range of 8.0-303%and 12-303%of the SAI(safe and adequate daily intake)，respectively.Excessive F intake with black tea，especially in the regions with its high F level in the drinking water，increases the risk of dental fluorosis in children during the years of tooth development［11］.Regression analysis showed that fluorosis was significantly correlated with the consumption of milk tea made from brick tea solution and the endemic fluorosis occurring in some areas was considered to be related to the heavy consumption of foodstuffs prepared with brick-tea［12］.The F concentration in tea samples collected from UK market ranged from 93 mg/kg to 820 mg/kg in the dry tea and 0.43 mg/L to 8.85 mg/L in the infusions.The UK supermarket economy teas contained elevated F，ranging from 3.60 to 7.96 mg/L in a 2 minute brewing infusion，being comparable to the Chinese brick tea.One liter of tea infusion prepared from the economy tea contained 6.0 mg/L of F，being 75 120%of the DRI(dietary reference intake，4 mg F for an adult)fluoride［13］.The high fluorine content in brick tea might be due to the high content in the raw material［14］.

3.Effectors influencing F level in tea

3.1.Tea cultivar

There was great difference in F concentration of tea leaf between various tea cultivars.Table 2 shows that F concentration in mature shoots with five leaves and a bud of cultivar Jinfeng was more than 3 times of that in cultivar Lvyafoshou，suggesting that selecting proper tea cultivars during establishment of tea gardens is an effective way to control F level in tea products.

Table 2.Concentration of F in mature shoots of various tea cultivarsa.

3.2.Maturity of tea shoots

F concentration in tea leaf increases with maturity of the leaf.The F concentration in the first leaf beneath the apex bud on the spring shoot was found to be less than 100 mg/kg and that in the third leaf less than 300 mg/kg.However，the old leaves with age more than 24 months had F level more than 2900 mg/kg(Table 3).The upper limit of F in various brick teas are 300 mg/kg based on Chinese standard GB19965-2005 Fluoride concentration in brick tea.If tender shoots above 3rdleaf were used as brick tea materials，the F concentration can be controlled under the upper limit 300 mg/kg.

Table 3.Effect of leaf maturity on F level(mg/kg).

3.3.Soil conditions

Fluoride was absorbed from soil and accumulated in the leaf by tea plant.Soil F concentration and soil pH affected the F accumulation in tea leaf.There was study showing that foliar F concentration was related to the total F and extractable F concentration in the top soil(0-20 cm).The soil pH and extractable Al level influence the solubility of F in soils，leading to the changes in F level of tea plant tissues［15］.The F－uptake by tea plant roots was an active process which was energy dependent and possibly mediated by a kind of anion transport system，in contradictory to passive or restrict absorption in other plants such as rice，barley，corn and tomato.This explains why tea plants are able to accumulate considerably high F－［16］.The amount of malic acid release in culture medium was linearly correlated with the root malic acid concentration.F treatments decreased the activities of ATPase and acid phosphatase，which resulted in increase in F content in the tea leaf［17］.

3.4.Tea processing

Although the high fluorine content in brick tea was considered to be due to the high content in the raw material and not to be related with the processing procedures［14］，there was study showing that fixation method had great effect on the fluoride contents of brick tea.Fluoride contents in tea leaf by steaming fixation was the highest，that by boiling water fixation the least，with pan-fired fixation in between［18］.If coal is burnt during the drying of tea，F(xiàn) contaimination will occur owing to the high F concentration in the coal.Test showed that F level in the air around the furnace mouth was as high as 0.030-0.184 mg/m3，and the dust on furnace platform had 2510 mg/kg of F［19］.It is recommended to use gas as fuel during tea processing so as to avoid F contaiminant in the tea products.

3.5.Tea infusion preparation

Preparation method affects the fluoride leaching from tea leaf into tea infusion.The higher the ratio of water to tea was during infusion，the more complete the tea fluoride was extracted.The extracted fluoride from tea leaf increased with the extension of extraction time［20］.The cumulative F contents detected in the tea solution prepared by repeated infusion were higher than those prepared by continuous infusion［17］.The leaching process of tea fluoride could be divided into initial fast stage and later slow stage.The duration of the fast stage depended on leaf particle size and temperature.The higher the temperature and the smaller the leaf particle size，the shorter the duration of initial fast stage.A two-step infusion method was developed to control fluorine level in tea liquor，during which the tea was infused in 50℃ water for 20 s，the solution was poured and discarded，and then the tea leaf was re-infused in boiling water for 5 min.The F concentration in the obtained tea solution decreased by 26 percent or more［21］.

Acknowledgements

This work was financially supported by the National Science Foundation of China(Project No.31170643).

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