Development of fermented surimi products using simulated tofuyo processing: associated changes in chemical composition, antioxidant activities and angiotensin-converting enzyme inhibition

Traditional Japanese koji-fermented tofu (tofuyo) processing was simulated to make fermented seafood. The koji fermentation process at room temperature was applied to steamed surimi and kamaboko to develop new types of food, extend their shelf life and improve their health-promoting effects. Extractive nitrogen and sugar contents, 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging, copper-reducing activities, and angiotensin-converting enzyme (ACE) inhibitory activity significantly increased after fermentation. These increases were the highest in kamaboko. Kamaboko fermented for 6 months had the highest DPPH radical scavenging, copper-reducing activities, and ACE inhibition. Kamaboko fermented for 6 months showed statistically the same copper-reducing power and ACE inhibition to tofuyo, but significantly higher DPPH radical scavenging activity. Size-exclusion chromatography showed the same peak(s) analyzed as Lowry-reactive substances, ninhydrin-reactive substances, sugar, and DPPH radical scavenging activity. The most antioxidative fraction had a molecular weight of 900–1300 and a sugar and amino acid structure(s). There was no specific ACE inhibitory fraction(s). The koji fermentation process at room temperature did not need additional energy, led to different foods with a sweet and a umami-rich taste and a soft texture. Additionally, koji fermented steamed surimi and kamaboko effectively prolonged best-before periods of up to 6 months.