Strategic ultrasound-induced stress response of lactic acid bacteria on enhancement of β-glucosidase activity for bioconversion of isoflavones in soymilk.

The purpose of the study is to develop the new methodology of strategic ultrasound treatment on lactic acid bacteria (LAB) to induce stress response for the enhancement of β-glucosidase activity that can be used for the biotransformation of glucosides into aglycones isoflavones in soymilk. Among the five LAB tested, Lactobacillus acidophilus BCRC 10695 showed the best ability to release β-glucosidase for further ultrasonic stimulation to induce proper stress response. With ultrasound (20 kHz, amplitude at 20%) to irradiate on L. acidophilus BCRC 10695 at stationary phase of growth for 2 min and 24 h of re-incubation, the β-glucosidase activity was enhanced to 3.91 U/ml, which was 1.82 times of that without ultrasound treatment. Using the ultrasound-treated L. acidophilus BCRC 10695 to ferment soymilk, the fraction of aglycones in total isoflavones in soymilk was effectively increased from 21.8% initially to 97.9% in 24 h. The strategic ultrasound treatment on L. acidophilus BCRC 10695 demonstrated promotion of β-glucosidase activity, and this methodology had the potential to be applied in the production of functional soymilk by adding probiotics LAB to increase the bioactive isoflavones and nutritional values for human health.