New insights into optimal storage conditions for raw Pu-erh tea: modulating post-fermentation flavor via microbial metabolism under controlled water activity

Introduction 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 influenced   by water activity (A w ). However, its specific effects on microbial community dynamics and associated metabolic contributions during RPT storage remain unclear. Objectives This study aims to elucidate the mechanisms by which different A w levels shape microbial communities and drive flavor compound formation during RPT storage. Methods RPT samples were stored under defined A w 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 A w conditions were conducted to investigate the relationship between microbial dynamics and flavor evolution. Results Controlled A w conditions significantly influenced the microbial composition and flavor chemistry of RPT. A w 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 A w  > 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, A w levels exceeding 0.70 promoted the formation of undesirable volatiles like N , N -dimethyl-propanamide, toluene, and isophorone, indicating potential quality deterioration. Conclusion A w 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 A w regulation as a practical and novel strategy for improving aging quality.