The last method used was determination of ROS levels in cells. To assess the inhibitory effects of onion extracts against glutamateinduced oxidative stress on HT22 cells, CM-H2DCFDA was used as a ROS detection reagent. CM-H2DCFDA is a cell-permeable oxidative stress indicator that is nonfluorescent until removal of the acetate groups by intracellular ROS (Fekete et al., 2008). ESFF-O effectively reduced glutamate-mediated intracellular ROS generation. When HT22 cells were exposed to 10 mM glutamate for 12 h, intracellular ROS level was increased up to 17.46-fold over the control. This level was reduced to 1.24-fold after treatment with 50 lg/mL of ESFF-O. On the contrary, the ROS levels were not changed by inactivated crude enzyme-treated extract. Trolox (50 lM), a well known antioxidant, also effectively decreased the ROS levels by 2.19-fold (Fig. 1C). As a consequence, fermentation effectively increased the antioxidant effect of onion in three assay models.