Food chemistry

• Freezing-Factors weakened the effect of Pressure-Factors on PPO denaturation. • Freezing declined the activation degree of PPO treated via high pressure. • PPO with the same activity might have different molecular morphology. The mechanism of enzyme protein denaturation induced by high pressure freezing is complicated and unclear as this process involves Pressure-Factors (pressure and time) and Freezing-Factors (temperature, phase transition, recrystallization, and ice crystal types). In this study, the thermodynamics and conformation changes of mushroom polyphenol oxidase (PPO) under high pressure freezing treatments (HPF, 100,150,200,300,400,500MPa P -20°C/30min ) and high pressure processes (HPP) followed with normal pressure immersion freezing (HPP-IF, 100-500MPa P 25°C/30min – 0.1MPa P -20°C/30min ) are investigated as compared with that processed under high pressure processes (HPP, 100-500MPa P 25°C/30min ) and normal pressure immersion freezing process (IF, 0.1MPa P -20°C/30min ). The results suggested that the treated PPO with the same enzyme activity may have various thermodynamic characteristics and conformations; Pressure-Factors play the main roles in the denaturation of the PPO during the HPF treatment, and Freezing-Factors can weak the effect of Pressure-Factors on PPO denaturation; The treated PPO may be transferred into a partially fold intermediate state.

Impacts of high pressure assisted freezing on the denaturation of polyphenol oxidase