Effect of different fermentation conditions on antioxidant capacity and isoflavones content of soy tempeh

Tempeh is a traditional fermented soybean product widely consumed as part of the staple Indonesian diet. Besides its role as a protein source, the consumption of tempeh has been associated with health benefits, such as those from antioxidants. The fermentation of soybean by tempeh starter (Rhizopus oligosporus) determines the antioxidant capacities of tempeh. Updated studies reporting the fermentation conditions effect on tempeh are limited. Therefore, this research aimed to evaluate various fermentation conditions (lactic acid addition, fermentation time, fermentation temperature) on antioxidant capacities and physical characteristics of tempeh. In this study, soybean was soaked for 4 h, dehulled and boiled in water containing lactic acid with different concentrations (0.8%, 0.9% and 1.0%) for 30 mins. After cooling, the tempeh starter (106 CFU/g of Rhizopus oligosporus) was inoculated into the soybean. Afterward, the soybean was incubated at three different temperatures (25, 30 and 35 ℃) for 2, 3 and 4 days. For each fermentation condition, the physical characteristic (L*a*b* color value), antioxidant capacities and isoflavones contents of the tempeh powder were observed. The results showed that the tempeh fermented for more than 2 days was overripe, and higher incubation temperature could intensify the speed of fermentation, resulting in darker tempeh. A significant decrease in the L* value and increases in the a* and b* values were observed on tempeh stored at longer incubation time at various storage temperatures and various lactic acid concentrations (p < 0.05). Tempeh incubated at a higher temperature and longer incubation time exhibited lower EC­ 50 values of 2, 2'-Azino-Bis-3-Ethylbenzothiazoline-6-Sulfonic Acid (ABTS) scavenging and ferrous chelating ability, increase total phenolic contents and significantly increase aglycone isoflavones (p < 0.05). Therefore, the highest antioxidant capacity, the highest total phenolic content and the highest aglycone isoflavones were found in the tempeh incubated for 4 days at 35 ℃. The 1% lactic acid-tempeh had the highest antioxidant of chelating ability, highest total phenolic content and highest aglycone isoflavone increase.