Abstract:Xiaoqinggan (XQG), the green small fruit of Citrus reticulata 'Chachi', is the raw material used for processing Citrus Pu’er tea, but many XQG have "white frost" on their surface during storage. Currently, the composition, safety, and formation mechanism of the "white frost" remain unknown. Therefore, the microscopic morphology, surface composition, mycotoxin content, and microbial diversity of normal, white-frosted, and moldy XQG were analyzed. Using scanning electron microscope, the “white frost” on the surface of XQG was found to consist of blocky and needle-like crystals. Through UPLC/Q-TOF analysis, the main components of the surface wash solution from white-frosted XQG were polymethoxyflavones. Through quantitative analysis, the nobiletin content in the surface wash solution of white-frosted XQG was found to be 9.15 times and 14.9 times higher than that of normal and moldy XQG, respectively, while the tangeretin content was 26.9 times and 78.8 times higher, respectively. The diversity analysis of fungi and bacterial communities on the surface of the XQG showed that Ogataea, Penicilliusm, and Alternaria in the moldy XQG increased significantly, while the microbiota levels on the white-frosted XQG surface were found to be similar to those from the normal group. In the mycotoxin analysis, only moldy XQG was found to contain 430 μg/kg of zearalenone. This indicates that the“white frost” on the surface of XQG is formed by the precipitation of polymethoxyflavones, with nobiletin and tangeretin as the main components. Additionally, the microbial profile on the surface of white-frosted XQG is similar to that of normal XQG, with no fungal contamination or no risk of mycotoxin exposure, suggesting that the white-frosted XQG are harmless to humans.
