Polyphenol Oxidase: The Master of Oxidation
Polyphenol oxidase (PPO, also called catechol oxidase or phenolase) is a copper-containing metalloprotein found in the chloroplasts and cytoplasm of tea leaf cells. Its biological role in the plant is wound response: when a leaf is damaged by insects or disease, PPO rapidly oxidises phenolic compounds into quinones, which polymerise into dark, antimicrobial compounds that seal the wound site.
In tea processing, this wound-response machinery is deliberately triggered at scale. Rolling, bruising, and crushing break cell membranes, allowing PPO to contact its primary substrates — the catechins (EGCG, EGC, ECG, EC). The enzyme catalyses the addition of oxygen atoms to create o-quinones, which spontaneously condense into dimers and larger polymers: theaflavins (two catechin units) and thearubigins (large, structurally complex polymers). This is the chemical basis of tea oxidation.
🧠 Expert Tip: Processing Insight
The degree of cell rupture during rolling determines the oxidation rate. A light roll for oolong breaks relatively few cells — producing 15–40% oxidation. A heavy CTC machine ruptures virtually all cells, driving near-complete oxidation rapidly. The skill of orthodox rolling is in controlling this rupture progression to create the desired compound profile.
Peroxidase: The Secondary Oxidation Engine
Tea peroxidase complements PPO but uses a different mechanism. Rather than using atmospheric oxygen directly, peroxidase uses hydrogen peroxide (H₂O₂) — itself a product of cellular stress — as its oxidant. Peroxidase can oxidise catechins, polyphenols, and other substrates, contributing phenolic oxidation products that differ subtly from pure PPO products.
Critically, peroxidase is more thermostable than PPO. While PPO is largely inactivated at 65–70°C, peroxidase persists to 80°C or higher. This means that even after kill-green steaming is applied, some residual peroxidase activity continues in the leaf, potentially contributing to colour and flavour development during drying. In some green teas — particularly those with a slight oxidative character like aged white teas — peroxidase accounts for the subtle development that occurs during long low-temperature storage.
| Enzyme | Substrate | Products | Optimal Temp | Inactivation Temp | Key Role |
|---|---|---|---|---|---|
| Polyphenol oxidase | Catechins (EGCG, EGC, ECG, EC) | Theaflavins, thearubigins (brown/red) | 25–35°C | 65–70°C | Colour formation, oxidation |
| Peroxidase | Polyphenols, lipids | Oxidised polyphenol derivatives | 30–40°C | 80–85°C | Secondary oxidation |
| Beta-glucosidase | Glycoside-bound terpenes | Free volatile terpenes | 35–45°C | 70–75°C | Aroma liberation |
| Lipoxygenase | Unsaturated fatty acids | Aldehydes (hexenal, hexenol) | 25–35°C | 65–70°C | Green/grassy aromas |
| Protease | Leaf proteins | Free amino acids | 40–50°C | 70°C | Amino acid release during withering |
Beta-Glucosidase: The Aroma Liberator
Beta-glucosidase is arguably the most important enzyme for tea aroma quality. Many of tea's key volatiles — terpene alcohols including linalool, geraniol, and nerolidol — are stored in the fresh leaf as chemically stable glycoside conjugates. In this bound form, they contribute nothing to aroma. Beta-glucosidase cleaves the glycosidic bond between the terpene alcohol and its sugar carrier (usually glucose), releasing the free volatile which then evaporates and contributes to the characteristic floral and fruity aroma of quality teas.
This enzymatic liberation occurs predominantly during withering and rolling, when cell disruption brings beta-glucosidase into contact with the glycoside substrates. The size and timing of this terpene release contributes significantly to why slow traditional withering — allowing enzyme activity to proceed steadily over 12–18 hours — produces more aromatically complex tea than rapid withering. The enzyme needs time.
🧠 Expert Tip: Withering Wisdom
Traditional tea masters assess withering quality not just by moisture loss (the standard 20–30% weight reduction) but by the fragrance rising from the withering beds. When a distinct floral note becomes perceptible above the fresh green smell, it signals that beta-glucosidase activity has reached its productive phase and rolling can begin.
Lipoxygenase: The Grass and Green Note Generator
Lipoxygenase (LOX) acts on unsaturated fatty acids in the leaf membrane lipids, particularly linoleic and linolenic acids. The resulting hydroperoxides decompose to produce six-carbon aldehydes — (Z)-3-hexenal, (E)-2-hexenal, and hexanol — that account for the characteristic fresh green, grassy, cut-grass notes in tea. These compounds are present in highest concentrations immediately after leaf disruption, declining as they are further metabolised.
In Japanese green tea production, the rapid steaming that kills polyphenol oxidase also preserves more of these LOX products — which is why sencha has a pronounced seaweed and green note that pan-fired Chinese greens lack. The LOX-derived aldehydes are more abundant because the heat came quickly, preserving the fresh-wound chemistry.
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