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 mu mol/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 (mu mol/100 g) was the preferred unit to precisely present the conversion from isoflavone glucosides to aglycones, and the total isoflavone content (mu mol/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.

Automated summary

This study investigated the factors affecting the process of making tempeh from soybeans, including isoflavone content, antioxidant activity, and phenolic content. The results showed that tempeh fermentation increased the non-glycoside form of isoflavones by over 20 times. There was a significant correlation between the non-glycoside form of isoflavones and antioxidant activity. Molar concentration was the preferred unit for presenting the conversion from glycoside to non-glycoside isoflavones. The study provides important information on the preferred fermentation periods and soybean varieties for making tempeh, and suggests that tempeh may be a functional food with readily bioavailable form of isoflavones.