Combination effects of ultrasound and citral nanoemulsion against Shigella flexneri and the preservation effect on fresh-cut carrots

This present study evaluated the effect of ultrasound (US), citral nanoemulsion (CLON) and combined treatments on the inactivate effectiveness of Shigella flexneri in phosphate-buffered saline (PBS) and on the surface of fresh-cut carrots. In this study, CLON was produced by ultrasonic nanoemulsification. Results demonstrated that US + CLON treatment significantly (P < 0.05) improved the bactericidal effect against Sh. flexneri, especially 0.15 mg/mL of CLON combined with US (20 kHz, 345 W/cm2) treatment for 9 min decreased the number of Sh. flexneri by 8.55 log CFU/mL. The assay of reactive oxygen species (ROS) and malondialdehyde (MDA) levels indicated that US + CLON aggravated oxidative stress and lipid peroxidation in cell membranes. The leakage of cell constituents (proteins and nucleotide) demonstrated that the permeability and integrity of the cell membrane was significantly (P < 0.05) impaired. Field-emission scanning electron microscopy further showed that US + CLON exacerbates the disruption of the bacteria morphology. Confocal laser scanning microscopic and flow cytometry observed US + CLON treatment enhanced the changes of bacterial cell membrane integrity. Meanwhile, US + CLON was effective in reducing the presence of bacteria on the surface of fresh-cut carrots, and had no negative effects on the quality of fresh-cut carrots in terms of colour, hardness and weight during storage. Overall, this present study revealed ultrasound combined with CLON is a promising and viable cleaning method for improving the microbiological safety of fresh produce.