The microbial Succession: An Ecological Timeline
A fermentation pile isn't random chaos. It follows predictable ecological succession—like how bare rock becomes lichen, then moss, then plants over decades. In tea, this happens over 45 days.
This succession is driven by:
- Oxygen availability: External (aerobic top), internal (anaerobic bottom)
- pH shifts: As microbes metabolize, they produce acids that change community composition
- Temperature: As heat rises, heat-loving (thermophilic) organisms replace mesophiles
- Nutrient depletion: Early colonizers consume easily-available sugars; later colonizers break down complex compounds
Phase 1: Inoculation (Days 1-5) - The Environmental Stage
Temperature: 20-25°C | pH: 5.5-6.0 | Dominant Organisms: Environmental microbes
What's Happening
Freshly processed green tea leaves are moisture-adjusted (water added to ~70% humidity) and piled in a fermentation chamber. The pile is inoculated naturally—using microbes from the air, the processing equipment, previous fermentations, or added culture.
These first organisms are primarily:
- Non-sporulating bacteria: From the environment, leaf surface, equipment
- Environmental yeasts: Saccharomyces and wild Hanseniaspora species
- Aspergillus spores: From air and previous fermentations (dormant, not yet active)
Microbial Activity Level: Low
Little fermentation is occurring. Microbes are simply establishing on the substrate. Temperature remains ambient because microbial respiration hasn't yet accelerated. The pile smells fresh, like moist green tea.
Phase 2: Enzyme Release & Fermentation (Days 5-15) - The Yeast-Bacteria Stage
Temperature: 25-40°C (rising) | pH: 4.8-5.5 (declining) | Dominant Organisms: Yeasts and lactic acid bacteria
What's Happening
Environmental microbes begin exponential growth. Aspergillus niger starts enzyme production, degrading cell walls. Yeasts ferment simple sugars anaerobically, producing ethanol and CO₂. Lactic acid bacteria (LAB) produce organic acids, lowering pH.
Temperature Rising
Microbial metabolism generates heat. If environmental controls are good (humidity ~70%, pile turning), temperature rises steadily. By Day 10-12, the pile's interior reaches 30-35°C. This heat acceleration is critical—it selects for thermophilic organisms and accelerates chemical reactions.
Aroma Change
Days 5-8: Slightly fruity, yeasty aroma emerges (ethanol production). Days 10-15: Increasingly "fermented" smell—sour, slightly vinegary notes as LAB organic acids accumulate.
Microbial Activity Level: Rising
This is an active, rapid growth phase. Doubling times for bacteria are 20-30 minutes. The community is still diverse, but yeasts and LAB are dominant.
Phase 3: Thermophilic Peak (Days 15-30) - The mold-Dominated Heat Stage
Temperature: 45-60°C (peak, Days 18-25) | pH: 3.5-4.5 (acidic) | Dominant Organisms: Aspergillus niger, Blastobotrys, thermophilic bacteria
What's Happening
This is the critical transformation phase. Aspergillus niger and Blastobotrys adeninivorans reach peak activity. Temperature rises to 50-60°C (often with hotspots exceeding this).
The Heat Generation
Blastobotrys adeninivorans performs aerobic respiration at maximum rate. Each cell uses glucose to generate ATP, releasing massive amounts of heat (exothermic fermentation). This is why experienced fermenters must turn the pile regularly—to prevent thermal runaway exceeding 65°C (which kills desired organisms and creates off-flavors).
Chemical Transformation Accelerates
At these elevated temperatures, catechins oxidize to theaflavins and theabrownins at 10-100x normal rates. Maillard reactions occur (browning), creating dark color and savory compounds. Volatile aromatics condense or evaporate—determining final tea aroma profile.
Color Change: Green to Brown
By Day 20-22, visual inspection shows dramatic darkening. Fresh green leaves become brown. By Day 25-28, they're dark brown to nearly black. This is the visible marker of successful fermentation.
Aroma Peak
Days 15-20: Rich, earthy, mushroom-like aroma. Days 20-30: Increasingly mellow, less sharp. The peak fermentation smell is quite strong—some describe it as "petrichor" (earthy after rain) or "forest floor."
Microbial Activity Level: Peak
Maximum metabolic activity. Enzyme production is highest. If turning is poor or environmental control fails, temperature can exceed safe limits, and pathogenic bacteria may colonize.
Phase 4: Cooling & Stabilization (Days 30-45) - The Bacterial Transition
Temperature: 35-45°C (declining) | pH: 3.5-4.2 (stabilized) | Dominant Organisms: Thermophilic bacteria, residual molds
What's Happening
Temperature naturally declines as the pile's nutrient-rich substrate is progressively depleted. Yeasts and Blastobotrys slow their activity. Bacillus and thermophilic actinomycetes (heat-loving bacteria) become dominant. These organisms consolidate the chemical transformations and stabilize flavor compounds.
Final Aroma Development
Days 30-40: The sharp fermentation aroma mellows. Pleasant fruity, woody notes emerge. Days 40-45: The tea smells "aged"—earthy, slightly sweet, mellow.
Flavor Stabilization
This phase locks in the flavor profile. Late-stage microbial enzymes create the final compound cocktail. Lovastatin production peaks during this phase. Remaining polyphenols stabilize in their oxidized forms.
Microbial Activity Level: Declining
Growth slows, then plateaus. The community stabilizes into a mature state. By Day 40, significant activity has ceased. The pile is considered "fermented" by professional standards at Day 35-40, though some makers extend to Day 45 for additional aging.
Detailed Fermentation Timeline Table
| Days | Temp (°C) | Phase | Dominant Microbes | Key Changes |
|---|---|---|---|---|
| 1-5 | 20-25 | Inoculation | Environmental microbes, spores | Colonization. Mild moisture absorption. No color change. |
| 5-10 | 25-32 | Yeast Bloom | Yeasts (S. cerevisiae, others), early Aspergillus | Temperature rises. Slight fruity aroma. Early color darkening. |
| 10-15 | 32-42 | LAB Dominance | Lactic acid bacteria, Aspergillus niger | Rapid enzyme production. Sour aroma. Continued darkening. pH drops. |
| 15-20 | 42-55 | Heat Peak Begins | Blastobotrys adeninivorans, A. niger (peak) | Rapid temperature rise. Earthy aroma. Color turns to dark brown. Major polyphenol oxidation. |
| 20-25 | 50-60 | Heat Peak | Blastobotrys (peak), thermophilic bacteria | Maximum temperature. Maillard reactions. Deep brown to black color. Rich aroma. |
| 25-30 | 45-55 | Heat Decline Begins | Blastobotrys (declining), Bacillus (rising) | Temperature moderates. Flavor compounds stabilize. Aroma mellows. Lovastatin production peak. |
| 30-35 | 40-50 | Bacterial Phase | Bacillus, thermophilic actinomycetes | Continued cooling. Aroma becomes aged and mellow. Chemical reactions slow. Fermentation largely complete. |
| 35-45 | 35-42 | Stabilization | Thermophilic bacteria (Bacillus, others) | Final consolidation. Flavor stabilizes. Aroma fully developed. Microbial activity plateaus. Ready for rest and packaging. |
The Critical Window: Days 20-25
This is where fermentation success or failure is determined. If temperature reaches 65°C+, unwanted microbes colonize and the tea develops harsh, off-flavors. If temperature stays below 45°C, fermentation stalls and the tea remains bitter. Professional fermenters monitor this window obsessively.
Environmental Factors That Alter This Timeline
Humidity
Too dry (<60% RH): Fermentation slows. Microbes struggle to metabolize. Timeline extends to 60+ days.
Optimal (65-75% RH): Microbial growth is optimal. Timeline follows schedule.
Too wet (>80% RH): Pathogenic bacteria (Bacillus, Clostridium) may colonize. Off-flavors develop.
Temperature Control
Cool environment (15°C): Timeline extends to 60+ days. Slow fermentation.
Warm environment (25°C): Fermentation accelerates. Timeline may compress to 35-40 days.
Pile Turning
Regular turning (every 3-5 days) aerates the pile, prevents hotspots, and ensures even fermentation. Lack of turning creates anoxic zones and uneven microbial colonization.
How to Identify Fermentation Progress (Practical Signs)
Days 5-10: Slight fruity smell. Leaves beginning to brown. Temperature rising to 28-32°C.
Days 10-15: Sour, fermented aroma. Leaves noticeably darker. Temperature 35-40°C.
Days 15-20: Rich earthy smell. Leaves dark brown. Temperature 45-55°C.
Days 20-25: Peak aroma. Leaves nearly black. Temperature 50-60°C. This is the critical decision point.
Days 25-35: Aroma mellowing. Leaves fully dark. Temperature declining to 40-45°C.
Days 35-45: Aged, mellow aroma. Leaves black. Temperature 35-40°C. Fermentation visually complete.
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