Effects of Ultrasound,Ultraviolet,Ultrasound Synchronized Ultraviolet on the Inactivation of Polyphenol Oxidase and Peroxidase of Fresh-cut Lentinus edodes and Its Mechanisms

Enzyme-promoted browning is one of the important factors affecting the quality of fresh-cut vegetables. In the present study,ultrasound treatment,ultraviolet treatment,ultrasound synchronized ultraviolet treatment was applied to inhibit the activity of polyphenol oxidase(PPO)and peroxidase(POD),which were the major enzymes leading to browning of fresh-cut Lentinus edodes. Moreover,the mechanism of inactivation of enzyme was discussed from the aspects of ultraviolet-vis spectra,change in the content of secondary structures and X-ray diffraction spectra. The results showed that ultrasonic treatment or ultrasound synchronized ultraviolet treatment was effective to inhibit the activity of PPO and POD. Having undergone treatment under the parameters of power density of 70 W/L,dual-frequency 20 kHz and 40 kHz at sequential operation for 15 min,the relative activity of PPO and POD were 30.6% and 44.3%,respectively. Ultraviolet treatment at a dosage of 1.8 kJ/m² resulted in reduction of relative activity of PPO and POD,which were 66.5% and 60.8% respectively,and ultrasound synchronized ultraviolet treatment led to a relative activity of PPO and POD with a value of 31.0% and 46.2%,respectively. Ultraviolet-visible spectra showed that hydrophobic groups located at the C end of enzyme molecules were exposed after treatment,irrespective to the kind of treatments. The analysis of secondary structural content and showed that the inhibition of PPO and POD might result from increase in the content of α-helix and random coil,and decrease in the β-sheet content. The results of X-ray diffraction indicated the change in the 3-dimensional conformation as well as intensity of diffraction peaks of PPO and POD might lead to the inactivation of enzymes. In conclusion,the present study can provide theoretical basis for inhibition enzyme in fresh-cut fruits and vegetables.