The Chemistry of Oxidation
Enzymatic oxidation in tea is a two-stage process. In stage one, polyphenol oxidase (PPO) catalyses the addition of oxygen atoms to catechin hydroxyl groups, producing o-quinones. In stage two, these quinones spontaneously couple — with each other and with other catechins — forming theaflavins (two catechin units) and ultimately thearubigins (large polymers). This process requires intact, active PPO and atmospheric oxygen, but no biological agents.
The process is thermally controlled: heating to 70°C inactivates PPO and stops oxidation. Cooling increases its rate. Atmospheric oxygen must be available — rolling in anaerobic conditions slows oxidation significantly. The entire chemistry is understood at the molecular level and can be monitored by measuring the disappearance of EGCG and the appearance of theaflavins over time.
🧠 Expert Tip: Correct Terminology
When describing oolong and black tea production, always use "oxidation" or "enzymatic oxidation." Reserve "fermentation" exclusively for pu-erh, kombucha, and similar teas where microbial activity is the primary process. This is not pedantry — the distinction explains fundamental differences in the compounds produced and the resulting tea character.
The Chemistry of Fermentation in Pu-erh
True tea fermentation — as in pu-erh production — involves a complex microbial consortium. In "wo dui" (pile fermentation) for shou pu-erh, wet maocha (compressed raw tea) is piled to 50–70cm depth and periodically turned for 40–60 days. The moist, warm, partially anaerobic interior of the pile hosts sequential microbial succession: Aspergillus niger first acidifies the pile and produces cellulolytic enzymes; then various bacteria (Lactobacillus, Bacillus) take hold; finally yeasts contribute. Each microbial wave produces different chemical transformations.
| Process | Driver | Oxygen? | Products | Resulting Taste/Colour |
|---|---|---|---|---|
| Oxidation (black tea) | Polyphenol oxidase (enzyme) | Required | Theaflavins, thearubigins | Orange-red amber, bright, astringent |
| Oxidation (oolong) | PPO (controlled) | Required | Partial theaflavin formation | Gold to orange, complex |
| Microbial fermentation (pu-erh) | Aspergillus, bacteria, yeasts | Low (micro-aerobic) | Theabrownins, organic acids, microbial metabolites | Dark brown-black, earthy, complex |
| Anaerobic (GABA tea) | Plant enzyme (GAD) | None (N₂ atmosphere) | GABA from glutamic acid | Slightly altered, umami-forward |
Why the Confusion Has Real Consequences
Using "fermented" to describe black tea leads to confusion in several practically important contexts: (1) Dietary restrictions — some religious traditions restrict fermented foods but not oxidised ones; (2) Probiotic marketing — consumers may incorrectly believe black tea contains live cultures; (3) Health research — study participants who drink "fermented tea" may be consuming either pu-erh or black tea, producing irreconcilable results.
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