The RSA was generally concomitant with
reducing power. The FRAP activity was defined as the corresponding
FeSO4 concentrations of the UV absorbance at
593 nm in the calibration, namely FRAP value, mmol/L. As
shown in Fig. 5, the FRAP value increased as the heating
time increased from 0 to 10 days, which was in agreement
with the result of the DPPH RSA. The FRAP value of the
fresh garlic samples was 0.08 mmol/L, whereas the reducing
power of the black garlic samples increased gradually to
0.21 mmol/L after 10 days of thermal processing. This result
indicated that the total antioxidant activity of black garlic
extracts was significantly enhanced during the thermal processing,
as the FRAP value of black garlic was approximately
three times higher than that of fresh garlic.
In general, the antioxidant activities of black garlic,
including the DPPH, HO radical scavenging ability and
FRAP activity were increased gradually during the thermal
processing. The representative antioxidant compounds in
black garlic are phenolic acids, flavonoids and sulfur compounds
such as SAC, DAS, DADS and DAT. Specially,
Suleria et al.have reported that the higher amount of
total phenolic content (71.87 ± 1.69%) was obtained in
methanolic garlic extract at 45 min as compared with that
of ethanolic and water extracts, and the antioxidant activity
of methanolic garlic extract had the maximum value of
83.83 ± 0.16% correspondingly, indicating that phenolic
acids are an important group of contributors to the antioxidant
activity of garlic. Furthermore, our previous work suggested
that alliin is converted into SAC when subject to
thermal processing.and other
sulfur compounds have been reported to have antioxidant
effect and are considered to be effectively free-radical scavengers.
In addition, a number of nonenzymatic
browning reactions could lead to the formation of aromatic
compounds such as 5-hydroxymethylfurfural, which possess
strong antioxidant activities.