Which chemical process turns green tea into black tea?

The chemical process that turns green tea into black tea is oxidation.

All traditional tea types come from leaves of Camellia sinensis, but processing determines whether the finished tea is green, oolong, or black. Green tea leaves are heated soon after picking to deactivate oxidative enzymes, preserving their green color and fresher flavor. For black tea, the leaves are allowed to oxidize extensively instead.

After withering, the leaves are rolled, crushed, or otherwise bruised. This breaks open their cells and lets oxygen interact with enzymes and polyphenols in the leaf. The leaves darken and develop the deeper color, aroma, and brisk flavor associated with black tea. Producers then dry the leaves to halt the process.

Tea production sometimes calls this stage “fermentation,” but that term is chemically imprecise: ordinary black-tea production does not depend on microbial fermentation. The process also distinguishes black tea from oolong, which is only partially oxidized. “Infusion” describes steeping dried tea in water, not manufacturing it, while calcination involves heating minerals or other substances and is unrelated.