Post-fermentation refers to the fermentation of tea leaves under the action of damp heat after drying, deactivating enzymes and drying. For example, the important technology of black tea is heap fermentation, that is, tea leaves are piled up under hot and humid conditions, which promotes the physical and chemical changes of tea leaves and forms the quality characteristics of black tea. The natural aging process of Pu 'er raw tea is also a slow post-fermentation process.