Several food microorganisms such as Bacillus subtilis, Lactobacillus
brevis, and Aspergillus kawachii were tried to elevate the antioxidative
status of yellow A. cepa (data not shown). Significant
changes in anti-oxidative activity were observed only after fermentation
with A. kawachii.
The antioxidative effect was evaluated three ways. The first
one was with the most widely used DPPH assay. DPPH is aradical-generating substance widely used to detect the anti-oxidative
effect of various antioxidants. DPPH, a radical form, has a characteristic
absorption at 517 nm in ethanol, which disappears with
the acceptance of an electron from antioxidant samples (CevallosCasals
and Cisneros-Zevallos, 2003). The DPPH radical scavenging
effect of EtOAc soluble fraction from fermented onion (ESFF-O)
was 79.06 ± 1.48% at 100 lg/mL, whereas the control (inactivated
enzyme-treated onion) showed only negligible activity at the same
concentration (18.24 ± 2.10%: Fig. 1A), indicating that DPPH radical
scavenging activity was increased about 4.3 times by fermentation.
The second evaluation method was TEAC assay. The TEAC value
is based on the ability of the antioxidant to scavenge the radical
cation ABTS using spectrophotometric analysis. The activity of
the tested samples was expressed as Trolox equivalent antioxidant
capacity (TEAC) values; the TEAC value is defined as the concentration
of standard Trolox with the same antioxidant capacity as a
1 mM concentration of the antioxidant sample. ESFF-O showed
higher ABTS radical scavenging effect than the inactivated crude
enzyme-treated control. In particular, at 50 lg/mL, the ABTS radical
scavenging effect of ESFF-O was 0.77 mM Trolox equivalent
(TE), while that of a control was 0.46 mM TE (Fig. 1B). This result
clearly suggested that fermentation enhanced the ABTS radical
scavenging effect of onion