Exploring the mysterious effect of piling fermentation on Pu-erh tea quality formation: Microbial action and moist-heat action

Moist-heat action and microbial action are essential to the formation of Pu-erh tea's quality. To explore the impact of piling fermentation (PF) environment on Pu-erh tea quality, samples subjected to spontaneous PF (SPPF) and sterile PF (STPF) from the same batch of raw material were processed, and their sensory quality and metabolic profile dynamics were determined. Sensory evaluation indicated that infusions of the SPPF samples were mainly brownish-red, stale-mellow taste, and stale aroma, whereas infusions of the STPF samples were mainly orange-red, sweet-mellow taste, and woody aroma. Metabolomics analysis identified 465 key metabolites, including 33 microbial markers and 39 moist-heat activity markers. Enzyme and pathway analyses revealed that the production of microbial enzymes accelerates the degradation of flavonoids, flavonols, and lipids and also indicated an exclusive influence of microorganisms on the lipid metabolite pathway. This study elucidated the mechanism underlying the formation of Pu-erh tea's stale flavour.