Abstract: This paper aimed to explore the mechanism by which high pressure processing (HPP) accelerates browning of pear juice. The effect of HPP, applied at 500 MPa treatment for 5 min on the color of fresh Cuiguan pear juice during its shelf life was investigated. Untreated samples were used as control. Dissolved oxygen concentration and total phenol concentration in the pear juice were measured before and after HPP treatment, together with enzymatic characteristics of polyphenol oxidase (PPO) such?as?optimal pH, optimal temperature, thermal stability, and Michaelis constant (Km). The results showed that obvious browning in the HPP-treated juice occurred at 6 h (ΔE = 1.61), while no browning was observed in the untreated juice at 24 h (ΔE = 0.56). In the fresh juice there were two forms of PPO: PPO1 (pH 5.0) and PPO2 (pH 7.0), and the accelerated browning of HPP-treated juice was associated with the activation of the two PPOs; the activity of PPO1 was increased from 6.5 to 14.1 U/mL, by 2.17 folds, and the activity of PPO2 from 2.6 to 25.5 U/mL, by 9.8 folds. The potential PPO activities themselves were very weak or even nearly zero, which could be activated by external factors such as HPP, sodium dodecyl sulfate (SDS) and heat treatment. The Km was decreased, and the substrate affinity of PPO was increased by HPP, thereby enhancing the rate of enzymatic reaction. The substrate affinity (1/Km) of PPO1 was increased from 2.353 to 46.909 L/mol, by 19.9 folds, and the substrate affinity of PPO2 from 2.852 to 62.857 L/mol, by 22.0 folds. The maximum reaction rate Vmax was decreased after HPP treatment. Inactivation will occur if the Vmax has a greater effect on the rate of enzymatic reaction than does the Km. To sum up, HPP treatment accelerates browning of fresh pear juice by activating potential PPO, and increasing the substrate affinity of PPO and consequently the enzymatic reaction rate.

Key words: fresh pear juice, high pressure processing, browning, polyphenol oxidase, potential enzyme activity, activating effect