Potentiometric flow-injection estimation of tea fermentation degree

A potentiometric flow-injection (FI) system was developed for estimating tea fermentation degrees; the method is based on the redox potentials of tea infusions. The infusions were extracted with phosphate buffer (10 mM, pH7.0), and injected (200 μL) into the FI system (1 mL min−1). The redox potentials (vs. Ag/AgCl) of carrier stream were monitored with a home-made electrochemical flow cell equipped with a platinum indicating electrode. Flow-injection strategy successfully decreased the problem of electrode fouling with increased reproducibility (S.D. < 1.5 mV) and elevated sample throughput (>20 determination h−1). During indoor withering process (0–24 h) for making pouching tea, the redox potential of tea infusions rose (110–130 mV) along with the depletion of tea catechins (100–75 mg g−1 dry tea powder, by HPLC). The system can also be used to estimate the fermentation degrees of commercial tea samples. For commercialization, the platinum electrode can be replaced with a screen-printed carbon-paste disposable electrode with similar performance.