لخّصلي

Tea (Camellia sinensis) is native to the southern regions
of China and parts of India, Laos, Thailand, Vietnam,
and Myanmar [1]. Tea is said to have first been discovered
as a drink and medicine in China around 2737 BC. It
was then introduced to Japan during the early 13th century
and to Europe in the 16th century, then to America, Africa
and other regions of the world [1–3]. Tea is presently
cultivated in over 30 countries around the world and the tea
beverage is second only to water in terms of worldwide
consumption [4].
Based on the oxidation of the polyphenols in the tea
leaves during the fermentation process, tea has been classified into three types: green tea, black tea, and oolong tea
[4]. Green tea refers to non-fermented tea, in which the
oxidation of the tea polyphenols, called catechins, is
prevented by quickly heating the leaves after harvest to
inactivate the main oxidising enzyme, polyphenol oxidase
(PPO), and thus, most of the catechins are preserved during
the processing. Black tea refers to fully fermented tea and
oolong tea is semi-fermented tea. In these teas, aerobic
oxidation of the tea leaf polyphenolics is allowed to occur
and the catechins are enzymatically catalysed to form theaflavins and thearubigins. For black tea, the fermentation
reaction is promoted to maximize the oxidation of the
catechins but for oolong tea, it is usually stopped half-way
before completion [1, 4].
Green tea is a rich source of catechins, which account
for up to 30% of the leaf dry weight [4]. A typical composition of catechins (polyphenolics) and other components in
the green tea is shown in Table 1. As catechins can donate
hydrogens from the hydroxyl groups in their structure, they
have been found to have excellent antioxidant activities,
expressed through their free radical scavenging ability being
more powerful than vitamin C, vitamin E, or b-carotene
[5–7]. They have also been shown to chelate transition metal
ions, modulate oxidant and antioxidant enzymes, and affect
gene expression [5].
The catechins are receiving considerable interest for
their potential benefits on human health. The recent in vivo
and epidemiology studies have linked the green tea catechins with the prevention of some skin and liver cancers
[8–11]. Other studies have linked the catechins with a
reduced development of lung, gastric, and breast cancers
[12–14]. In addition, green tea and its catechins have been
linked with reductions in cardiovascular disease, dental
decay, obesity, diabetes, and an improvement in the
immune system