*Article* **Ultrasonic Processing Induced Activity and Structural Changes of Polyphenol Oxidase in Orange (***Citrus sinensis* **Osbeck)**

**Lijuan Zhu 1,2, Linhu Zhu 1,2, Ayesha Murtaza 1,2, Yan Liu 1,2, Siyu Liu 3, Junjie Li 1,2, Aamir Iqbal 1,2, Xiaoyun Xu 1,2, Siyi Pan 1,2 and Wanfeng Hu 1,2,\***


Academic Editor: Stefano Serra

Received: 11 April 2019; Accepted: 14 May 2019; Published: 18 May 2019

**Abstract:** Apart from non-enzymatic browning, polyphenol oxidase (PPO) also plays a role in the browning reaction of orange (*Citrus sinensis* Osbeck) juice, and needs to be inactivated during the processing. In this study, the protein with high PPO activity was purified from orange (*Citrus sinensis* Osbeck) and inactivated by ultrasonic processing. Fluorescence spectroscopy, circular dichroism (CD) and Dynamic light scattering (DLS) were used to investigate the ultrasonic effect on PPO activity and structural changes on purified PPO. DLS analysis illustrated that ultrasonic processing leads to initial dissociation and final aggregation of the protein. Fluorescence spectroscopy analysis showed the decrease in fluorescence intensity leading to the exposure of Trp residues to the polar environment, thereby causing the disruption of the tertiary structure after ultrasonic processing. Loss of α-helix conformation leading to the reorganization of secondary structure was triggered after the ultrasonic processing, according to CD analysis. Ultrasonic processing could induce aggregation and modification in the tertiary and secondary structure of a protein containing high PPO activity in orange (*Citrus sinensis* Osbeck), thereby causing inactivation of the enzyme.

**Keywords:** browning reaction; polyphenol oxidase; ultrasonic processing; structural changes; aggregation
