The fresh tea leaf has a bitter and astringent taste due to the abundant phenolic substances present. Aromatic molecules are locked up in nonvolatile compounds with sugar. The key to making tea is encouraging the leaf's own enzymes to transform these austeric molecules into pleasant ones.
The best tea is made from a plant's young shoots and unopened leaf buds as they contain the highest concentration of phenolic compounds and related enzymes. These young leaves are harvested and allowed to wither. Withering causes a shift in their metabolism that accounts for a change in flavor and physical fragility. The fragile leaves are rolled or pressed to break down tissue structures that contain cell fluids. Enzymes spill, react with oxygen, and break the aroma-sugar complexes apart. Simple phenolic compounds such as catechin react into larger compounds. The browning enzyme, polyphenoloxidase, uses oxygen to join small phenolic molecules into large complexes that are brown and not astringent at all. The deep, complex of tea is developed by this enzymatic transformation, often referred to as "fermentation," even though no significant microbial activity is involved.
Once the desired flavor is developed, the leaves are heated to inactive their enzymes. Furthermore, dry heat is used to develop different depths of flavor and to preserve leaves for long keeping. The dry leaves are sieved and graded, and prepared for packaging and consumption.
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