RESEACRH ON MICROWAVE TREATMENT FOR FIXATION OF POLYPHENOL OXIDASE IN PROCESSING OOLONG TEA IN VIETNAM
This study was aimed to optimize a microwave treatment to fix enzymes after Oolong tea fermentation based on concentrations of tannin, total polyphenols and the remaining activity of PPO (polyphenol oxidase) enzyme. Three factors including power levels (540 - 720W), radiation time (160-195 sec) and sample loading density (0.026 - 0.078 g.cm-2) were considered for the optimization. Results showed that the optimal conditions were at power of 630W, radiation duration of 190 s and the sample loading density of 0.06 g.cm-2. At these conditions, dry matter of treated tea had 23.14% tannin, and 24.10% total polyphenols, and the remaining activity of PPO was at 11.19%. The concentrations of tannin of the samples treated by microwave were higher than those of the conventionally roasted sample (21.10%). Remained total polyphenols of the former were also higher and the remaining PPO activity was lower. The color of the microwave-treated sample was better whereas the flavor and taste were less preferred as compared to the conventionaltreated tea. The results showed that microwave had high potential to inactivate enzymes in fermented tea. However, flavor and taste of the microwave-treated tea needs to be improved.