Effect of ultrasound combined with malic acid on the activity and conformation of mushroom (Agaricus bisporus) polyphenoloxidase

• Combination of ultrasound and malic acid effectively inactivated PPO.
• The inactivation kinetics of combination treated PPO followed a first-order kinetics.
• Conformation of PPO was unfolded with change of secondary and tertiary structure.
• Combined treatment disrupted structure of PPO and molecules were connected together.

Polyphenoloxidase (PPO) plays an important role in the browning of vegetables, fruits and edible fungi. The effects of ultrasound, malic acid, and their combination on the activity and conformation of mushroom (Agaricus bisporus) PPO were studied. The activity of PPO decreased gradually with the increasing of malic acid concentrations (5–60 mM). Neither medium concentrations (10, 20, 30 mM) malic acid nor individual ultrasound (25 kHz, 55.48 W/cm2) treatment could remarkably inactivate PPO. However, the inactivation during their combination was more significant than the sum of ultrasound inactivation and malic acid inactivation. The inactivation kinetics of PPO followed a first-order kinetics under the combination of ultrasound and malic acid. The conformation of combination treated PPO was changed, which was reflected in the decrease of α-helix, increase of β-sheet contents and disruption of the tertiary structure. Results of molecular microstructure showed that ultrasound broke large molecular groups of PPO into small ones. Moreover, combined treatment disrupted the microstructure of PPO and molecules were connected together.