Metabolomics analysis, combined with enzyme and in-vitro simulation system analysis, provide new insights into the browning resistance mechanism of goji berries（Lycium barbarum. L)

Enzymatic browning is one of important factor causing quality deterioration of Goji berries (GB). To understand the enzymatic browning process of GB and its mechanism, changes of L*, a*, b* values, and the total color difference and whiteness index during the air contact storage were evaluated. Then changes of moisture content, total phenolics and the activities of the key browning enzymes, peroxidase (POD), Polyphenol oxidase (PPO), phenylalanine ammonia-lyase (PAL) and Superoxide dismutase (SOD), were determined during storage. Finally, the chemical constituents of GB were determined by targeted metabolomics and HPLC, and the possibility that these constituents were endogenous substrates of key enzymes for browning were evaluated. The results show that brightness and reddish-yellowness diminished are the general characteristics of GB discoloration. The activity of browning related enzymes (POD, PPO, PAL) was consistent with that of phenolics and flavonoids, which increased first and then decreased during storage. The content of chlorogenic acid decreased most rapidly under the action of the enzyme, which degraded 75% within 5min, followed by ferulic acid and rutin. Therefore, it is suggested that chlorogenic acid is the main endogenous substrate of GB enzymatic browning. In conclusion, these studies provide experimental basis for the browning mechanism of GB.