3.3. Antioxidant activity
The antioxidant activity of the tea infusions was assessed using
two independent assays, ABTS and inhibition of LDL oxidation
(Fig. 4). The results obtained from the ABTS assay reported in
Fig. 4A show that all cold teas have significantly higher antioxidant
activity (17–30 mmol/L TE) compared to their respective hot
infusions (5–16 mmol/L TE). The highest activities were observed
for cold Bai Mu Dan (29.8 1.5 mmol/L TE), Xue Ya
(29.2 2.2 mmol/L TE) and White Salima Peony (28.1 0.6 mmol/
L TE), while the antioxidant activity of the Bvumbwe Bsp tea is
significantly lower than all the others (cold: 5.3 0.1 and hot:
17.1 1.7 mmol/L TE). Furthermore, a good correlation was found
between the results obtained with the ABTS assay and TPC and TFC
reported in Fig. 1A and B (r2 = 0.8518, p = 0.000 and r2 = 0.8789,
p = 0.000, respectively).
The lag times reported in Fig. 4B concerning the ability of the
teas to suppress copper-induced LDL oxidation, are in accordance
with the trend of antioxidant activity evaluated using the ABTS
assay reported in Fig. 4A (r2 = 0.8026, p = 0.000). The results also
show that cold tea infusions have a greater suppressive effect
against LDL oxidation compared to hot tea infusions which did not
significantly prolong the lag time (except for Bai Mu Dan and White
Lung Ching) compared to the control without tea (70 3 min). The
lowest activities were observed for Yhin Zhen Bai Hao (cold: 172 30
and hot: 88 17 min) and Bvumbwe Bsp teas (cold: 174 24 and
hot: 96 22 min).
3.4. Metal chelating activity
The chelating activity of the tea infusions measures how
effective the compounds in them can compete with ferrozine for
ferrous ions. Fig. 5 shows that the hot teas have the lowest
chelating activity (0.29–0.55 mmol/L EE) with no differences
among each other, except for Yhin Zhen Bai Hao
(0.29 0.06 mmol/L EE) that is significantly lower than White Lung
Ching, White Salima Peony and Bvumbwe Bsp (0.47–0.56 mmol/L EE).
Amongst the cold teas, Bvumbwe Bsp shows a significantly higher
chelating power (1.08 0.15 mmol/L EE) compared to all the others.