The post-fermentation storage of raw Pu-erh tea (RPT) is a key process that contributes to the development of its characteristic aged flavor. Microbial activity plays a vital role in this transformation which strongly influencedby water activity (Aw). However, its specific effects on microbial community dynamics and associated metabolic contributions during RPT storage remain unclear. This study aims to elucidate the mechanisms by which different Aw levels shape microbial communities and drive flavor compound formation during RPT storage. RPT samples were stored under defined Aw conditions, during which microbial communities were characterized using Internal Transcribed Spacer (ITS) sequencing, and flavor metabolites were analyzed through gas chromatography-mass spectrometry (GC-MS), liquid chromatography-mass spectrometry (LC-MS). Correlation analyses and core strain-inoculated fermentations under simulated and realistic Aw conditions were conducted to investigate the relationship between microbial dynamics and flavor evolution. Controlled Aw conditions significantly influenced the microbial composition and flavor chemistry of RPT. Aw above 0.60 enriched fungal genera such as Papiliotrema, Hannaella, Toxicocladosporium, and Pestalotiopsis, which were strongly correlated with the accumulation of aging-associated terpenoids (e.g., α-terpineol, terpinen-4-ol, cedrol, linalool) and kokumi-active compounds including flavoalkaloids and amino acid derivatives. A core yeast isolate, Papiliotrema flavescens, exhibited metabolic activity at Aw > 0.60 under both simulated and realistic storage conditions. It produced β-glucosidase, which promoted the hydrolysis of glycosidic precursors and the accumulation of linalool and methyl salicylate, confirming its role in microbial-driven flavor development. However, Aw levels exceeding 0.70 promoted the formation of undesirable volatiles like N,N-dimethyl-propanamide, toluene, and isophorone, indicating potential quality deterioration. Aw is a pivotal parameter that connects storage safety with microbial-driven flavor development in RPT. An optimal range of 0.60-0.70 promotes beneficial transformations while minimizing off-odor risks. These findings highlight Aw regulation as a practical and novel strategy for improving aging quality.

The thick and mellow flavor of green tea serves as a core sensory indicator for assessing its quality grade and market value. This characteristic fundamentally represents the dynamic equilibrium of multiple flavor-contributing substances within the tea leaves, coupled with the synergistic effects of multi-sensory perception. Currently, the characteristics, chemical composition, and interactions underlying the thick and mellow flavor of green tea are the subject of intense research, yet no consensus has been reached within the academic community. This review systematically details the research progress on the chemical and quality characteristics and interactive effects of green tea’s thick flavor. First, the physiological basis for perceiving this thick and mellow flavor—the multisensory integration mechanism involving taste, smell, and oral tactile sensations—is explored. Second, evaluation methods for the thick and mellow flavor of green tea, including sensory analysis, instrumental analysis, and the integrated application of artificial intelligence technologies, are outlined. Third, the chemical basis of the thick and mellow flavor of green tea and the mechanisms of its interactions are systematically summarized. This review aims to systematically analyze the formation mechanisms and regulatory characteristics of the thick and mellow flavor of green tea, providing valuable reference for standardizing green tea flavor quality control.

Storage post-fermentation contributes to tea flavor development, but the functions of individual microorganisms that remain active under moisture-limited storage are still poorly understood. Papiliotrema flavescens RPT (Pf-RPT), a tea-derived osmotolerant yeast previously shown to remain metabolically active under storage-relevant low-water-activity conditions, was inoculated into crude Pu-erh tea (PT) and Yunnan white tea (WT) produced from the same fresh leaves through contrasting primary processing routes. Under controlled incubation fermentation, Pf-RPT enhanced kokumi at several stages and showed a tendency to restrain sourness accumulation, while the global metabolome remained primarily determined by processing derived substrate background. Time series metabolomics identified 45 and 50 Pf-RPT responsive non-volatile metabolites (NVMs) in PT and WT, respectively, involving flavonol glycosides, methylated catechins, flavoalkaloids, amino acid derivatives, and lipids. Correlation analysis and CaSR docking prioritized four kokumi associated candidates that accumulated in response to Pf-RPT, including 8-C-R-ECG-cThea, 8-C-R-EC-cThea, GCG-3″-Me, and EGCG-3″-Me. In an application trial, Pf-RPT formulated as active dry yeast (ADY) enhanced volatile metabolites (VMs) accumulation during tea cake storage. These findings support Pf-RPT as a promising microbial tool for improving storage post-fermentation quality.

Storage is regarded as a continuation of raw Pu-erh tea (RPT) processing, improving sensory quality and market value. However, studies based on naturally aged teas from different batches confound storage effects with variation in raw materials, processing, and storage history. Here, the same RPT batch was stored for three years at controlled temperature and water activity (Aw) levels of 0.53, 0.61, and 0.71. Storage time defined the aging trajectory of 122 annotated non-volatile metabolites (NVMs), characterized by puerin, amino acid derivative, and lipid accumulation, together with catechin and catechin oligomer depletion. Higher Aw accelerated the aging trajectories of 73 NVMs. Fungal communities became increasingly dominated by Cladosporium, Setophoma, and Papiliotrema with storage time and increasing Aw. Papiliotrema flavescens RPT (PF-RPT) inoculation under natural storage conditions accelerated L-theanine depletion and puerin accumulation, supporting a partial microbial basis for Aw-accelerated RPT aging and its potential for improving aging efficiency and process controllability.