Low frequency ultrasound enhanced the antioxidant activity and isoflavones accumulation of soybean sprouts by inducing oxidant stress

Soybean sprouts as one of year-round vegetable are widely consumed due to its high nutrition values and short growth period. In this study, soybean seeds were treated by the ultrasound during soaking periods at different processing modes. After germination, the morphological changes, antioxidant capacity, isoflavones and key genes of phenylpropanoid metabolic pathway in soybean sprouts were quantified. Results showed that ultrasonic treatment significantly increased germination rate, sprout length sprouts, protein content, as well as antioxidant activities of soybean sprouts. Moreover, a significantly increased in isoflavones content was observed, especially for the daidzin, glycitin, glycitein and genistein which was increased by 30.48%, 20.28%, 84.58%, 52.53%, respectively after ultrasound treatment. This was consistent with the up-regulation of isoflavones synthesis-related genes (PAL, C4H, 4CL). Additionally, the SOD (superoxide dismutase), POD (peroxidase) and CAT (catalase) activities in soybean sprouts were also markedly increased. The significant increase of MDA (malondialdehyde) content in soybean seeds after ultrasound treatment indicated that an oxidative stress damage was occurred, which consequently enhancing the generation of isoflavones by regulating the genes in phenylpropyl metabolic pathway and improving antioxidant activities of soybean sprouts. Consequently, ultrasound treatment could be considered as an effective way to improve the nutritional value of soybean sprouts.