Processing procedures of brick tea and their influence on fluorine content.
China is the only country that produces brick tea, and more than 90% of the brick tea is consumed in the western minority nationality regions of China. The high fluorine content of brick tea is possibly associated with the special processing procedures, but no investigation has been conducted in this field. To explore the characteristic features of brick tea manufacturing and the alterations in fluorine content during the processing procedures, we performed a field survey involving two brick tea factories and the nearby tea plantations. For the fluorine contents of the initial, intermediate and final processing products, altogether eight types of specimens were collected and determined by using the ion-selective electrode standard curve technique. It was found that the raw material tea leaf for brick tea processing was old, coarse and not the tender delicate tea leaf used for ordinary green or black tea processing. For the fluorine content of the raw material tea leaf, the intermediate and the final products showed that the fresh raw leaf contained a fluorine content as high as 489.31-512.68 mg/kg. During one fermentation-like processing procedure, the fluorine content rose by 4.67% and 1.88% in the specimens from the two factories, respectively, which revealed no statistical significance (P>0.5). These results suggest that the high fluorine content in brick tea might be due to the high content in the raw material and not related with the processing procedures.