Definition:
Wo Dui (渥堆, “wet piling”) is the controlled microbial fermentation technique first developed in 1973–1974 that defines shou puerh production — in which moistened maocha is stacked in large indoor heaps (1–2m high), covered with cloth to retain heat and moisture, and left to undergo intense fungal, bacterial, and yeast-driven fermentation at internal pile temperatures of 55–65°C for 45–60 days — converting astringent polyphenols into theabrownins, smoothing the character, darkening the leaf to near-black, and developing the earthy complexity characteristic of shou puerh. It is one of the most technically demanding and industrially significant steps in Chinese tea manufacturing.
In-Depth Explanation
Microbial ecology of the wo dui pile: Research since the 2000s has characterized the complex microbial succession that drives wo dui:
- Fungi: Aspergillus niger is consistently identified as the dominant and most important species. It secretes cellulases, amylases, and polyphenol oxidases that break down the leaf polymer structure and convert catechins into theabrownins. Other Aspergillus species (A. fumigatus, A. glaucus) appear at various pile stages.
- Bacteria: Various Bacillus species contribute proteolytic and saccharolytic activity; Lactobacillus species from later stages contribute to flavour smoothing.
- Yeasts: Saccharomyces and related species produce ethanol and CO2, modifying the pile environment and contributing ester-type aroma compounds.
Pile management: Skilled wo dui requires:
- Correct initial moisture (30–35% post-wetting)
- Appropriate pile height (too high = overheating and burning; too low = insufficient microbial activity)
- Turning the pile every 5–7 days to ensure even temperature and prevent anaerobic zones that cause off-flavours
- Monitoring internal temperature and adjusting turning schedule accordingly
- Knowing when to stop — over-fermented puerh develops severe off-notes that cannot be corrected
Factory hygiene: The specific microbial communities in different factories are influenced by the history and environmental biology of the fermentation space. Factories with decades of wo dui history have established beneficial microbial communities in their walls and floors. This is one reason veteran factories produce more consistent shou than newer producers.
Post-wo dui: After 45–60 days, the tea is spread out to dry and air. The “new shou” smell — a sometimes unpleasant earthy-musty note — dissipates over 6–12 months of resting. Many collectors prefer shou puerh that has been rested 1–3 years before drinking for this reason.
See Also
Related Terms
Research
- Zhao, M., et al. (2011). Characterization of the microbial community and biochemical changes during the wo dui (wet-piling) process of pu-erh tea. Journal of Agricultural and Food Chemistry, 59(14), 7840–7847.
[Provided the most comprehensive mapping to date of the microbial succession and metabolic contributions during the full 60-day wo dui process.]
- Liu, Z., et al. (2012). Analysis of the chemical changes in catechins, theaflavins, and theabrownins during pu-erh tea fermentation using HPLC. Food Chemistry, 131(4), 1337–1342.
[Tracked theabrownin accumulation rate and catechin depletion through successive wo dui stages; confirmed theabrownins as the primary pigment and flavour compound generated by the process.]