Journal of Microbial & Biochemical Technology

Abstract

Effect of Thermal and Radio Frequency Electric Fields Treatments on Escherichia coli bacteria in Apple Juice

Dike O. Ukuku, David J. Geveke and Peter H. Cooke

The need for a nonthermal intervention technology that can achieve microbial safety without altering nutritional quality of liquid foods led to the development of the radio frequency electric fields (RFEF) process. However, insight into the mechanism of bacterial inactivation by this technology is limited. In this study, we investigated membrane damage of Escherichia coli bacterial (7.8 log CFU/ml) and leakage of intracellular membrane materials in RFEF treated apple juice at 25 kV/cm and operated at 25°C, 55°C and 75°C for 3.4 milliseconds at a flow rate of 540 ml/min. Damage to cell membrane was detected with Transmission Electron Microscopy (TEM) and leakage of cellular materials was determined with ATP luminometer (20 D) and electrostatic and hydrophobic interaction chromatography used to characterize changes in bacterial cell surfaces. RFEF treatment caused a significant decrease in bacterial cell surface hydrophobicity and loss of relative negative ions compared to heat treatment alone at 55°C and 75°C. Leakage of cellular materials into the media indicated cell damage and TEM observation showed altered intracellular membrane structure in RFEF treated E. coli cells. The results of this study suggest that the mechanism of inactivation of RFEF is by disruption of the bacterial cell surface hydrophobicity and loss of relative negative ions which led to injury and leakage of cellular materials and death.