ABSTRACT
Approximately 78% of the 2.5 million metric tons of tea leaves produced worldwide was used for the preparation of black tea, which is mainly consumed in Western nations, and about 20% was used for the preparation of green tea, which is mainly consumed in Asian countries and in some parts of North Africa. Green tea mainly contains polyphenols such as
flavanols
(
catechins
),
flavonols
,
flavandiols
, and
phenolic acids
. These polyphenols account for about 25 to 30% of the solids in water extracts of green tea leaves. The major green tea catechins are
(–)- epigallocatechin-3-gallate
(EGCG),
(–)-epigallocatechin
(EGC),
(–)-epicatechin-3-gallate
(ECG),
(–)-epicatechin
(EC), and
(+)-catechin
. In the manufacture of black tea, the polyphenols in tea leaves undergo polyphenol oxidasecatalyzed oxidative polymerization, which leads to the formation of
bisflavanols
,
theaflavins
,
thearubigins
, and other oligomers in a process commonly known as “tea fermentation.” Theaflavins (about 1 to 2% of the solids in water extracts of black tea leaves) include
theaflavin
,
theaflavin-3-gallate
,
theaflavin-3
′-gallate
, and
theaflavin-3
,
′-digallate
, and these substances contribute to the characteristic color and taste of black tea. A substantial portion of the solids in water extracts of black tea leaves represents
thearubigens
, which are even more extensively oxidized and polymerized than the aflavins. The thearubigens have a wide range of molecular weights and are poorly characterized. Thus, it has been assumed that there are significant differences in antioxidant properties between green and black teas.
