An optimized smartphone-based colorimetric detection method for evaluating the fluoride released in tea extracts from different brewing conditions

A smartphone-based colorimetric detection method was optimized for measuring the fluoride content in tea extracts using zirconium-xylenol orange reagent. The images of the formed color complex in the presence of fluoride were captured and analyzed their color components through the RGB (Red, Green, Blue) vector values. To maintain the capturing conditions, a colorimetric box (150 × 70 × 90 mm) was self-designed by optimizing the smartphone camera-sample holder/cuvette distance and inside brightness. The calibration curve was constructed from 0.5 to 25 mg/L with the goodness of linearity (R² > 0.995). Favorable repeatability and reproducibility were obtained (RSD_r = 5.01% and RSD_R = 5.02%), according to the Appendix F of AOAC (2016). UV–Vis and F ion-selective electrode were simultaneously conducted as reference methods for the developed smartphone-based method, demonstrating comparable fluoride content results in tea extracts obtained from these methods. Using 2.5 g tea brewed in 100 mL of deionized water at 90 °C for 60 min, a descending fluoride content order was achieved: black tea (3.31–3.85 mg/L) > oolong tea (2.38–2.65 mg/L) > green tea (2.25–2.97 mg/L) > white tea (2.28–2.31 mg/L). The smartphone-based method was also applied to evaluate the effects of different brewing conditions on the fluoride content in tea extracts, indicating the higher brewing temperature and prolonged time could accelerate the release of fluoride in tea extracts. This study provides a quality control tool for tea consumers and manufacturers as well as contributes to enrich the applications of smartphone-based methods in the food sections, especially for Vietnam with the diversity of agricultural products.