Polyphenol Oxidase (PPO)
Definition
The enzyme in tea leaves responsible for driving oxidation, converting colorless catechins into colored theaflavins and thearubigins. PPO activity is the central chemistry of black, oolong, and pu-erh production.
Why it matters
Polyphenol oxidase (PPO), along with peroxidase (POD), catalyzes the oxidation of catechins when the plant's cell walls are ruptured and oxygen reaches the leaf interior. The enzyme is concentrated in chloroplasts and vacuoles and is widely distributed across the plant kingdom; it also drives the browning of cut apples and bruised bananas. In tea production, processors deliberately manipulate PPO activity. White teas use minimal rupture and slow ambient drying, so PPO is active only briefly. Oolongs use carefully controlled yaoqing bruising to engage PPO at the leaf edges. Black teas use full rolling or CTC to maximize PPO activity. Greens use shaqing (steaming or pan-firing) to denature PPO almost immediately, preserving the green character. The products of PPO-driven oxidation include theaflavins (orange-red, brisk) and thearubigins (red-brown, body-giving), which together account for the color and much of the character of black tea liquor.