3.2. Evaluation of total antioxidant capacity of tea extracts by off-line
DPPH assay
DPPH radical scavenging capacities of tea samples were shown in
Table 1. The antioxidant activities of different tea samples were
compared with that of Trolox, and were expressed as mM of Trolox
equivalent per g of dry weight. The Trolox equivalents among the
different tea samples ranged from 1321 mM Trolox for Milanxiang
(S11) to 2550 mM Trolox for Xinyangmaojian (S6), showing a 1.9
fold difference in antioxidant activity. Green teas possessed significantly
(p b 0.05) higher mean Trolox equivalents (2143 mM Trolox)
than white teas (1410 mM Trolox) and oolong teas (1372 mM
Trolox). Meanwhile, IC50 values of different tea samples were also determined.
The lower IC50 value implies the higher antioxidant activity.
Green teas (IC50 values in the range of 24.58–32.84 μg/mL) also
exhibited significantly (p b 0.05) higher antioxidant activity than
white teas (IC50 values in the range of 32.79–33.08 μg/mL) and
oolong teas (IC50 values in the range of 33.95–35.80 μg/mL) (Table 1).
Comparable DPPH results from investigations by Unachukwu et al.
(2010) on green teas havemean DPPH IC50 value of 23.26 μg/mL, slightly
lower than that obtained in our investigation; however, the preparation
methods of tea sample solutions slightly differed. Manian et al.
(2008) observed mean DPPH IC50 value for green tea extracts of
19.50 μg/mL with a different extraction method and solvent. These results
indicated that all the tea samples possessed a good free radical
scavenging capacity, and the antioxidant capacities of the three types
of tea mentioned abovewere significantly different (p b 0.05). Generally,
the processing methods for production of the three types of teawere
different. Green tea is an unfermented tea (degree of fermentation is 0),
which is particularly rich in polyphenol compounds. Nevertheless,
white tea and oolong tea are partially fermented teas, their degrees of
fermentation are about 20–30% and 30–60% respectively; the polyphenolic
compounds contained in them were partially oxidized during fermentation
(Harbowy & Balentine, 1997; Hsiao, Chen, & Cheng, 2010).
Kim et al. (2011) and Unachukwu et al. (2010) reported that antioxidant
activity of tea samples was positively correlated with the amount
of phenolic compounds. Therefore, green tea revealed higher antioxidant
activity than white tea and oolong tea did.