Study on the white frost formation mechanism during storage of Phyllanthus emblica Linn. fruit based on component analysis and spatial metabolomics

The Phyllanthus emblica Linn. fruit (PEF) is a well-known medicinal and food homologous item in tropical Southeast Asian. During the drying and storing processes, some PEF will grow white frost on its surface, which is typically taken as a sign of greater quality. However, the material basis and formation mechanism of white frost on PEF surfaces are currently unclear, and there is no sufficient evidence to support the correlation between white frost on PEF surfaces and their quality. In this paper, high-performance liquid chromatography (HPLC) was used to study the differences in active ingredient content of PEF medicinal materials with and without frost. The microstructure and elemental composition of white frost were studied using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The Fourier transform infrared spectroscopy (FT-IR) was used to analysis the main functional groups in white frost. The ultra-performance liquid chromatography quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF-MS/MS) combined with UNIFI database, EDS and FT-IR results, and reference materials were used to identify the chemical composition of white frost. The exocarp of PEF before and after drying and storage was analyzed by spatial metabolomics using desorption electrospray ionization (DESI) mass spectrometry imaging system to reveal the formation mechanism of white frost on the surface of PEF. The results found that the PEF with frost have higher levels of active ingredients than those without frost. EDS and FT-IR results show that white frost is mainly composed of C, O, K elements, and contains a large number of phenolic hydroxyl, carboxyl etc. UPLC-Q-TOF-MS/MS results found that the main components of white frost were organic acids, fatty acids, and tannins, including quality markers such as gallic acid and ellagic acid etc. Spatial metabolomics research found that the white frost formation mechanism mainly involved in the ascorbate and aldarate metabolism, cutin, suberin and wax biosynthesis, citrate cycle (TCA cycle) and biosynthesis of unsaturated fatty acid. This study reveals the material basis, formation mechanism, and relationship between the surface white frost of PEF and the quality of medicinal materials, providing valuable information for the quality evaluation of PEF.