Multi-scale insights: Impact of drying on water status, antioxidant mechanisms, and phenolic metabolomics in Osmanthus fragrans var. thunbergii

This study systematically evaluated the effects of ultrasound-assisted vacuum freeze-drying (Ul-FD), variable-temperature and variable-speed hot air drying (VT/VS-HD), and modulated microwave drying (Mo-MD) on the quality of Osmanthus fragrans var. thunbergii ( O. fragrans ). The results indicated that Ul-FD best preserved chromatic attributes and cellular structure, retaining the highest levels of total phenolics (1.08 μg GAE/mg DW), total flavonoids (0.80 μg RE/mg DW), and antioxidant capacity (DPPH scavenging activity of 94.57%), significantly outperforming the other treatments. Mo-MD significantly reduced the drying time to 20 min and exhibited the fastest dynamic process of moisture migration and bound water transformation. HRLC-MS identified 82 phenolic compounds, with Ul-FD best preserving polyphenol diversity, Mo-MD enriching heat-resistant aglycones, and VT/VS-HD promoting the formation of small-molecular acids. PCA and PLS-DA identified 17 differential metabolites, demonstrating that drying methods significantly reshape the polyphenome. KEGG enrichment analysis indicated that the phenylpropanoid and flavonoid biosynthesis pathways were the main responsive routes. This study links the drying process to phenolic metabolism and bioactive quality. It identifies Ul-FD for high-value, Mo-MD for industrial, and VT/VS-HD for general applications, providing a basis for optimizing O. fragrans drying and product development.