Effects of a mixed koji culture of Aspergillus oryzae HG-26 and Aspergillus niger HG-35 on the levels of enzymes, antioxidants and phenolic compounds in soy sauce during the fermentation process

Soy sauce contains a number of bioactive components, which have been shown to possess strong antioxidative properties. The objective of this study was to compare the enzyme activities, antioxidant activity and phenolic compounds content of soy sauces prepared from a mixed koji (SSAON, inoculated with Aspergillus oryzae HG-26 and Aspergillus niger HG-35), with those of soy sauces made from a koji culture containing only Aspergillus oryzae HG-26 koji (SSAO). The total phenolic content (TPC), total flavonoid content (TFC), content of three isoflavone aglycones (daizein, glycitein and genistein) and the antioxidant activity of the SSAON were found to be higher than those of the SSAO during moromi fermentation. In addition, they showed strong positive correlations with the antioxidant potential of the soy sauce. This study has demonstrated the potential of a mixed-culture koji, based upon A.orzyae and A.niger, for the production of soy sauce with an increased level of bioactive components.