Effect of ultrasound-assisted freeze-dried on microstructure, bioactive substances, and antioxidant activity of Flos Sophorae Immaturus

Abstract

Flos Sophorae Immaturus (FSI) extract and its main active secondary metabolites have health benefits including antioxidant activity, but the drying process may negatively affect their quality and quantity. To investigate the effect of ultrasound-assisted freeze-drying (UAFD) on FSI, the drying rate, colour, and microstructure samples were analysed. Furthermore, the flavonoids and antioxidant activity of dried products were evaluated. The results showed that the drying rates were significantly enhanced with UAFD. At 600 W and 15 min, the drying time of UAFD was 40% shorter than that of freeze-drying (FD) alone. The micrographs showed that USAD created more porous structures that allowed water to migrate outward from inside the FSI tissue. Processing conditions of 600 W ultrasonic power and 5 min ultrasonication time produced better antioxidant activity in dried products than in other conditions. The highest characteristic properties were DPPH (75.51%), ABTS (95.14%), FRAP (83.05%), acceptable drying time (3 h), and low colour difference (1.09). Under these conditions, the contents of total flavonoids (298.34 mg/g), rutin (256.66 mg/g), quercetin (20.13 mg/g), kaempferol (19.64 mg/g), isorhamnetin (17.77 mg/g), and genistein (13.97 mg/g) were determined to be as indicated. The results confirmed the obvious benefits of incorporating the ultrasonic effect into the FD method.