. The residual activities of both enzymes decreased significantly with treatment time and pressure during the first 5 min (Pb0.05). For PPO residual activity, it decreased fast to slowly with treatment time. This might be due to two isoenzymes existing in PPO of watermelon juice. One isoenzyme was sensitive to pressure and the other was stable. When pressure was applied, the liable one could be inactivated easily but the stable one could not.