Oxidation is the most fundamental process in tea making. When the cell walls of a fresh leaf are broken — by rolling, bruising, or crushing — the enzyme polyphenol oxidase contacts catechins and oxygen, triggering a chain of chemical reactions that darken the leaf and create the flavour compounds characteristic of oolong and black tea. Green tea avoids this by heating ('kill green') to deactivate the enzyme. White tea oxidises minimally due to gentle handling. Pu-erh undergoes microbial fermentation, a different but related process.
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