Abstract:Xiaoqinggan (XQG), the immature fruit of Citrus reticulata ‘Chachi,’ is a raw material used for preparing citrus Pu’er tea. During storage, a substance known as “white frost” often forms on the surface. However, its composition, safety, and formation mechanism remain unknown. Therefore, the microscopic morphology, surface composition, mycotoxin content, and microbial diversity of normal, white-frosted, and moldy XQG were analyzed. Scanning electron microscopy indicated that the “white frost” on the surface of XQG comprised blocky and needle-like crystals. UPLC/Q-TOF analysis revealed that the main components of the surface wash solution from white-frosted XQG were polymethoxyflavones. According to quantitative analysis, the nobiletin content in the surface wash solution of white-frosted XQG was 9.15 times and 14.94 times higher than that of normal and moldy XQG, respectively, whereas the tangeretin content was 26.87 times and 78.65 times higher, respectively. The diversity analysis of fungi and bacterial communities on the surface of the XQG showed that Ogataea, Penicillium, and Alternaria increased significantly in the moldy XQG. In contrast, the microbiota levels on the white-frosted XQG surface were similar to those in the normal group. Mycotoxin analysis showed that only moldy XQG contained 430 μg/kg of zearalenone. This indicated that “white frost” on the surface of XQG is formed by the precipitation of polymethoxyflavones, with nobiletin and tangeretin being the main components. Additionally, white-frosted XQG are harmless to humans. The results provide a theoretical basis for establishing product standards and product quality control in XQG.
