Changes in isoflavone profile, antioxidant activity, and phenolic contents in Taiwanese and Canadian soybeans during tempeh processing

Soybean seeds of eight different varieties were processed to tempeh. Factors affecting each step of tempeh making include isoflavone profile, FRAP (ferric ion reducing antioxidant power) and DPPH (1,1-diphenyl-2-picrylhydrazyl radical scavenging ability) assayed antioxidant activity, and TPC (total phenolic content) in the prolonged fermentation were investigated respectively. The total isoflavone contents in the dry seeds of the tested varieties ranged from 893.3 to 1386.0 μmol/100 g, and more than 99% was in the form of isoflavone glucosides. Tempeh fermentation increased the isoflavone aglycone contents by more than 20 times. Principle component analysis provided evidence of the significant correlation (p < 0.05) between isoflavone aglycone content and FRAP and DPPH assayed antioxidant activities. Molar concentration (μmol/100 g) was the preferred unit to precisely present the conversion from isoflavone glucosides to aglycones, and the total isoflavone content (μmol/100 g) was not decreased after 10 d of fermentation in most varieties. In the prolonged fermentation, daidzein kept increasing throughout but genistein decreased after day 2. For extracting a higher amount of daidzein, longer fermentation could be considered. Our results provide information of preferable fermentation periods and soybean varieties and show that tempeh may be a functional food with readily bioavailable form of isoflavones.