Chemistry Reference
In-Depth Information
HO
OH
O
O
CH
3
O
O
CH
3
Keto Form
HO
OH
O
O
CH
3
O
OH
CH
3
Enol Form
FIGURE 13.7
Structure of curcumin representing keto-enol tautomerism.
13.9
TEA
13.9.1 t
he
G
rowinG
and
p
roCeSSinG
of
t
ea
Green, black, and white teas are all derived from
Camellia sinensis
, an evergreen
shrub of the Theaceae family. Unlike black tea, which is formed by oxidation of
green tea after picking, green tea is harvested and usually supplied in its natural
state with no further processing. Green tea may be consumed in the form of a brewed
beverage. Successful tea cultivation requires moist humid climates typically found
on the slopes of Northern India, Sri Lanka, Tibet, and Southern China. Green tea is
consumed predominantly in China, Japan, India, and a number of countries in North
Africa and the Middle East, whereas black tea is consumed predominantly in
Western and some Asian countries.
After the leaves are picked, they undergo one or more of the following processes,
which determine whether the final tea product will remain green or white, or trans-
form to oolong, or black teas.
Figure 13.8
describes the processing of tea leading to
determining the type of tea and the characteristic flavonoid content.
Withering
—Fresh, green leaves and buds are softened by withering. The
leaves are allowed to air dry in the sun or are placed on racks in a large,
heated room. During this stage, the starch in the leaves begins to convert to
sugar. For white tea, leaves are withered for 4 to 5 hours; for green, oolong,
and black tea, almost twice as long.
Rolling—
After withering, leaves are rolled, a process originally performed by
hand but now handled by machine, that twists and crushes the leaves, releas-
ing the sap and exposing it to oxygen, which stimulates further oxidation.
Oxidation—
For white and green tea, this step is skipped. All black teas are
fully oxidized. Oolong tea, however, is only partially oxidized. Rolled
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