On Food and Cooking

presses the leaves to break open their cells,
and then allows the leaves to sit for some time
while the enzymes do their work.
There are two general kinds of enzymatic
transformation in making tea. One is the
liberation of a large range of aroma
compounds, which in the intact leaf are bound
up with sugars and so can’t escape into the air.
When the cells are crushed, enzymes break
the aroma-sugar complex apart. This
liberation makes the aroma of oolong and
black teas fuller and richer than the aroma of
green teas.
The second transformation builds large
molecules from small ones, and thereby
modifies flavor, color, and body. The small
molecules are the tea leaf’s abundant supply
of three-ring phenolic compounds, which are
astringent, bitter, and colorless. The leaf’s
browning enzyme, polyphenoloxidase, uses
oxygen from the air to join the small phenolic
molecules together into larger complexes (p.