Change of polyphenol oxidase activity, color, and browning degree during storage of cloudy apple juice treated by supercritical carbon dioxide

The effects of supercritical carbon dioxide (SCCO2) treatment of 8, 15, 22, and 30 MPa for 60 min at 55 degrees C on polyphenol oxidase (PPO) activity, color, and browning degree in cloudy apple juice during storage at 4 degrees C for 4-weeks were investigated. The SCCO2 treatment had significant effects on inactivation of PPO and the least residual activity of PPO was 38.50% at 30 MPa. The restoration of PPO residual activity after SCCO2 treatment was also observed, which was dependent on the pressure level. A greater reduction of lightness L and a minor increase of redness a of cloudy apple juices after SCCO2 treatment occurred. Moreover, the total color difference (Delta E), which was significantly less than that of untreated sample, was decreased by enhancing the pressure level. The changes of lightness L and browning degree A during storage were well fitted to a first-order kinetic model. The rate constants of k(L) and k(A) of samples subjected to SCCO2 treatment reduced from 4.75x10(-2) to 0.42x10(-2) and 37.19x10(-2) to 8.02x10(-2), respectively, when pressure increased from 0 MPa (untreated sample) to 30 MPa.