Effects of Drying Temperature on the Isoflavone Content and Antioxidant Capacity of Fermented Soybean (Thua Nao)

Abstract

Soybeans and soy products are recognized as health food. In the present study, the effect of drying temperature on the content of isoflavones and capacity of antioxidant of Thai fermented soybean (Thua Nao) was studied. The fermented soybean was prepared by inoculating autoclaved soybean with starter powder of the Bacillus subtilis at the ratio 0.1% (w/w), incubated at 42°C for 48 h, and then dried in a hot air oven at 60, 70, and 80°C. The contents of isoflavone -glycosides (daidzin, genistin and glycitin), aglycosides (daidzein, genistein and glycitein) and total phenolic compounds were evaluated. Antioxidant activities in terms of DPPH radical scavenging activity (DPPH) and ferric reducing antioxidant power (FRAP) were also investigated. Drying temperature significantly affected the retention of antioxidants. Although there was no significant difference in total phenolic compounds and isoflavone aglycosides at different drying temperature, Thua Nao dried at 70 and 80°C had higher contents of -glycosides (11 and 11%) and total isoflavones (17 and 20%) than those of dried product at 60°C. In addition, the product dried at 80°C had superior activities of both DPPH and FRAP compared with those at below drying temperature. Genistin was found the most in all dried products, followed by daidzin and daidzein, respectively.