Abstract

Aglycone isoflavones are more potent in their biological activities than their corresponding glycosides. The objective of this research was to explore the suitable temperature and time to obtain optimum conversion of daidzein and genistein using far infrared irradiation (FIR) as a thermal source on black soybean. The changes were monitored and quantified by high performance liquid chromatography (HPLC). The alteration in total phenolics (TP) and antioxidant activity due to FIR in different temperature and time was also evaluated. The increase in temperature with increasing exposure time in FIR caused gradual increase in diadzein and genistein production. The average maximum increase in aglycone isoflavone (sum of diadzein and genistein) content was achieved in 30 min of exposure with 341 mg/100 g dw at 196±6°C which were about 9.89 and 5.08 fold higher than those of the control for genistein and diadzein, respectively. The TP contents also altered due to the increase in temperature and exposure time and reached a plateau (1385.31 mgTAE/100 g dw) after about 30 min of irradiation at 196°C which was 2.87 fold higher than the control. In antioxidant assay, the 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical inhibition was increased with temperature and time. However, the metal chelating property decreased with increasing temperature. Overall, the FIR can be used as a convenient tool for chemical conversion of isoflavone glycosides in soybean.

Key words: Antioxidant, conversion, isoflavone, far infrared irradiation, high performance liquid chromatography, soybean.