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
(–)-epigallocatechin3-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 oxidase-catalyzed 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-3,3
′
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 (see Hogg and Chakravarthy, 2004).
