A comprehensive study on the inactivation of Escherichia coli under nonthermal technologies: High hydrostatic pressure, pulsed electric fields and ultrasound

The inactivation of Escherichia coli under selected treatments using high hydrostatic pressure (HHP), pulsed electric fields (PEF), and ultrasound (US) was studied in apple juice. HHP was carried out from 300 to 600 MPa (0.5-7 min, 21 °C), PEF was applied from 23.07 to 30.76 kV/cm (20-40 °C) using up to 21 pulses, and US was performed in continuous and pulsed mode at 120 μm and temperature from 50 to 60 °C (0-30 min). The best treatment was HHP that inactivated 7 log of bacterial cells in less than 1 min. US also showed positive results (>6 log reduction) after 5 min when temperature was kept at 60 °C regardless of the operation mode. Inactivation with PEF showed about 5 log reduction using the highest electric field strength. The color of the juice showed some changes after processing; the L* value parameter was significantly different (p < 0.05) in almost all the samples treated with the three technologies. Weibull and Fermi's equations were used to describe the inactivation curves from high hydrostatic pressure, pulsed electric fields and ultrasound, showing that first equation described better the PEF survivor's curve (R ≥ 0.9898) and the second one fitted better the HHP and US data (R ≥ 0.9969). Finally, electron microscopy was able to show the lethal damage on the cells after processing with nonthermal technologies. Using SEM injures in cell surface such as broken membranes, perforations, and cell debris were observed. Meanwhile, TEM depicted some internal damage in the cells, such as the lack and agglomeration of cytoplasm content and the change in cellular shape. Indeed these nonthermal technologies represent viable options for apple juice pasteurization achieving the 5-log reduction FDA-required standard for alternative pasteurization methods.