Antioxidant activities of soybean as affected by Bacillus-fermentation to kinema

The antioxidant activities of methanolic extract of kinema, fermented using Bacillus subtilis, and cooked non-fermented (CNF) soybean were evaluated by four in vitro methods, namely stable DPPH^{{radical dot}}-scavenging activity, Fe³⁺-reducing power, Fe²⁺-chelating activity, and activity in linoleic acid emulsion system. The total phenol content of kinema was 144% higher than that of CNF soybean (3.3 mg g^-1 dry weight). At all the tested concentrations (10-50 mg ml^-1), kinema extract was found to be a better free radical scavenger, which increased in a time and dose-dependent manner, than the CNF soybean extract. The reducing power of kinema was 147% higher than that of CNF soybean (3.8 mg ascorbic acid equivalents g^-1 dry weight). At 10 mg ml^-1, the kinema extract exhibited 64% metal-chelating effect which was much higher than the effect shown by CNF soybean (22%). At 50 mg ml^-1, after 24 h, while kinema exhibited antioxidant activity with 44% inhibition of linoleic acid peroxidation, CNF soybean exhibited 36% inhibition. The inhibition of both of them was found to decline with time and reached 26% after 72 h. The total phenol contents of kinema and CNF soybean were positively correlated (P < 0.01) with the respective values of free radical-scavenging activity (r = 0.96 and 0.82), reducing power (r = 0.94 and 0.82), metal-chelating activity (r = 0.95 and 0.76), and lipid peroxidation inhibitory activity (r = 0.91 and 0.82). Whereas the total phenol content in CNF soybean accounted for 68% of each of free radical-scavenging activity and reducing power, 58% of Fe²⁺-chelating activity and 66% of lipid peroxidation inhibitory activity, that in kinema could reflect 83-92% of all the antioxidant parameters studied. The data also revealed a significant correlation (P < 0.01) between any two of these five parameters, indicating while total phenol content as the independent variable, all other parameters as the dependent variables. Thus, kinema may be exploited as a functional food to alleviate oxidative stress.