Combined effects of post-frost harvesting and Aspergillus cristatus fermentation on the flavor and metabolite profiles of mulberry leaf tea

Mulberry leaf tea (MLT) is rich in bioactive compounds but is often associated with bitter, astringent, and green, grassy sensory characteristics. Fermentation with Aspergillus cristatus ( A. cristatus ) can improve the sensory quality of plant teas. However, the sensory and metabolic consequences of post-frost harvesting followed by A. cristatus fermentation in MLT remain insufficiently understood. This study combined sensory evaluation with comprehensive metabolomics profiling using headspace solid-phase microextraction gas chromatography–mass spectrometry (HS-SPME-GC–MS) and ultra-high-performance liquid chromatography-high-resolution mass spectrometry (UHPLC-HRMS). The aim was to identify metabolic changes associated with sensory remodeling and to explore their potential contribution to flavor formation. Compared with pre-frost tea, post-frost harvesting altered the volatile and non-volatile metabolic profiles and reduced grassy aroma intensity, accompanied by changes in flavonoid-, phenolic-, and amino-acid-related metabolites. Fermentation further remodeled this metabolic landscape, decreasing 3-ethyl-2,5-dimethylpyrazine by approximately 87%, rendering several other pyrazines undetectable, and increasing methyl 2-hydroxybenzoate by 7.37-fold relative to post-frost unfermented mulberry leaf tea (PostF-UMLT). These changes were accompanied by increased sweet and fungal-fermented aroma intensity and reduced grassy aroma intensity, while the roasted aroma partially declined after fermentation; post-frost fermented mulberry leaf tea (PostF-FMLT) showed the highest mean overall sensory score. Collectively, post-frost harvesting was associated with a distinct metabolic state that was subsequently further remodeled during A. cristatus fermentation. These findings provide insights into how harvest timing and fungal fermentation reshape the metabolite composition and sensory quality of MLT.