Microbes in the Pile: Aspergillus niger, Yeasts, and Bacillus
Ripe pu'er is often mistaken for "tea that's been left to go moldy." In fact, wodui is a microbial fermentation with a division of labor and a rotation of roles, much like brewing soy sauce or making miso: at controlled temperature and humidity, specific microbes transform the raw material into the flavors you want.
Which Microbes Are in the Pile?
Over the past decade or so, researchers have analyzed the microbes in wodui using culturing and gene sequencing. Common members include:
| Group | Common members | General role |
|---|---|---|
| Aspergillus fungi | Aspergillus niger and related species | Dominant early on, secreting a range of enzymes |
| Yeasts | Blastobotrys adeninivorans and others | Increase in the middle and later stages, involved in transforming constituents |
| Heat-tolerant fungi | Thermomyces and others | Appear when the pile is hottest |
| Bacteria | Bacillus, Pantoea, and others | Pantoea is abundant early, Bacillus takes over later |
Some studies that re-identified the strains concluded that part of what was once identified as Aspergillus niger is actually related species in the same genus, such as Aspergillus luchuensis. Results differ from study to study, because factory, material, season, and pile size all affect the makeup of the microbial community.
Dominant Microbes at Each Stage
Researchers who followed the full wodui process saw a very clear "change of shift":
- Early stage: after watering, the pile is moist and warming, and Aspergillus fungi multiply rapidly. In the 2025 study by Hu et al., Aspergillus niger dominated the early stage. Among bacteria, a 2026 study comparing four fermentation methods found Pantoea abundant early on.
- Middle stage: as the pile rises above 50–60°C, the proportion of heat-tolerant species increases.
- Late stage: Bacillus, heat-tolerant fungi (such as Thermomyces), and Blastobotrys yeasts become dominant.
Turning changes the pile's temperature and oxygen, which in turn shifts who is active when. So the master's timing of each turning is, in a sense, conducting this change of shift. For the details of turning, see Watering and Turning.
What the Microbes Do
Microbes secrete various enzymes that act on the leaf's constituents:
- Tea polyphenols transform: in one measured 30-day wodui, tea polyphenols went from about 107 mg per gram to about 47 mg, a drop of more than half.
- Pigments form: theabrownin rose from about 18 mg per gram to about 61 mg, and the liquor turned from yellow-green to deep red.
- Aroma shifts: fresh floral and grassy notes decrease, while woody and aged aroma compounds increase.
- Mouthfeel shifts: bitterness and astringency soften, and the liquor becomes mellow and smooth.
For the chemistry of the pigments, see Theaflavins, Thearubigins, and Theabrownin.
Are the Microbes Added?
Traditional wodui doesn't add any starter. The microbes come from the material itself, the air, the water, the floor and walls of the fermentation room, and the tools. A fermentation room used for decades at an established factory has long since developed a stable microbial community, which is one reason each factory's ripe tea has its own "factory taste."
In recent years, researchers and some factories have tried "inoculated fermentation": adding selected strains to the pile to make fermentation more controllable and faster. This is still a new technique, and most ripe tea on the market is still naturally fermented. See Modern Fermentation.
How Fermentation Differs from Mold
| Wodui fermentation | Mold | |
|---|---|---|
| Conditions | Deliberately controlled temperature, humidity, and turning | Accidental damp, uncontrolled |
| Microbes | A set rotation of dominant microbes | Molds from the environment growing at random |
| Process | A start and an end, halted by drying | Keeps growing as long as it's damp |
| Result | Deep red liquor, ripe sweet aroma | Pungent moldy smell, mold spots on the surface |
Finished ripe tea is dried to below 13–14% moisture, and the microbes are no longer very active. In storage, keep it dry, ventilated, and free of odors, and protect it from damp. If ripe tea shows mold spots on its surface or smells pungent, something has gone wrong in storage, and you shouldn't drink it.
FAQ
Which microbes are in the pile? Commonly Aspergillus fungi such as Aspergillus niger, yeasts such as Blastobotrys, heat-tolerant fungi, and bacteria such as Bacillus.
Are wodui microbes added? Traditionally no. They come from the material, the air, the water, and the fermentation room. Inoculated fermentation is a recent development.
Are the dominant microbes the same early and late? No. Aspergillus dominates early on, and Bacillus, heat-tolerant fungi, and yeasts take over later.
Is wodui the same as mold? No. Wodui is a fermentation with controlled temperature and humidity and a set rotation of microbes; mold is random growth after accidental damp.