Synergistic efficacy of pulsed magnetic fields and Litseacubeba essential oil treatment against Escherichia coli O157:H7 in vegetable juices

Pulsed magnetic fields (PMF) as a non-thermal sterilization has broad application prospects, but at this stage its bactericidal effect is not satisfactory. The essential oils have been proved with good antimicrobial effects. However, they will cover the original flavor of the food products at high dosages. In order to overcome the defects of PMF and essential oils, the bactericidal effects of pulsed magnetic fields in combination wdrith Litseacubeba essential oil (LC-EO) treatments were assessed in cucumber, carrot, spinach and bitter gourd juices. The optimal synergistic effects were found using PMF (3 times under 8T, 60 pulses) treatments combined with 1.5 mg/mL of LC-EO. Transmission electron microscopy (TEM) was employed to determine morphological changes and the laser scanning confocal microscopy (LSCM) deducted the intracellular nucleic acid changes. The obtained results from TEM and LSCM indicated that the synergistic treatment can completely destroy E. coli O157:H7 cells via breaking the cell membrane. The detection of some intracellular substances, intracellular enzymes and metabolic capacity of bacteria also showed that synergistic treatment inhibits the activity of various intracellular enzymes of bacteria. The synergistic treatment of PMF and LC-EO maintained the number of bacteria close to 1 Log CFU/mL when the concentration of essential oil reduced from 3 mg/mL to 1.5 mg/mL and the PMF was 3 times under 8T, 60 pulses. Similarly, the combined treatment could also maintain the number of bacteria at a very low level in vegetable juice. Besides, the sensory evaluation exhibited that the combined treatment had no impact on the quality of vegetable juices. This work highlighted the huge prospect of collaborative processing.