Effect of pH on Pressure and Thermal Inactivation of Avocado Polyphenol Oxidase: A Kinetic Study

Abstract

High-pressure and thermal inactivations of avocado polyphenol oxidase (PPO) were studied for enzyme systems with pH 4−8. Both pressure and temperature inactivation of the enzyme followed first-order kinetics in the pH range 5−8 but showed deviation from simple first-order kinetics at pH 4. Threshold pressure and temperature for PPO inactivation, as well as pressure and temperature dependence of the inactivation rate constants, were influenced by pH. The minimum inactivation pressure increased from ∼450 MPa at pH 4 to ∼850 MPa at pH 8. The threshold temperature increased from ∼40 at pH 4 to ∼65 °C at pH 6 and was approximately constant at higher pH values. The zp value and the absolute value of the activation volume, both reflecting the pressure dependency of the inactivation rate constant, respectively increased and decreased with increasing pH, in the pH range 5−8. The zt value and activation energy, both reflecting the temperature dependency of the inactivation rate constant, respectively decreased and increased with increasing pH in the pH range 5−7. At pH 8, the activation energy was slightly lowered. Keywords: Polyphenol oxidase; thermal stability; pressure stability; pH; avocado (Persea americana Mill.)