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

Dike O Ukuku,David J Geveke

doi:10.4172/1948-5948.1000074

Abstract

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.

Highlights

Physical and chemical treatments have been used in food processing to eliminate or reduce the presence of pathogenic and spoilage microorganisms in foods [1,2,3,4,5]
To investigate the effect of heat treatment alone, the radio frequency electric fields (RFEF) treatment chamber was changed from 0.2 cm gap (25 kV/cm) to 20 cm gap (0.25 kV/cm) and the chamber temperature set at 25°C, 55°C and 75°C
We investigated the effect of minimal heat treatment alone by allowing inoculated apple juice samples to pass through the 20 cm RFEF chamber

Summary

Introduction

Physical and chemical treatments have been used in food processing to eliminate or reduce the presence of pathogenic and spoilage microorganisms in foods [1,2,3,4,5]. The authors reported that pulsed electric fields, a similar non-thermal processing treatment to RFEF led to changes in E. coli cell surface charge. We used hydrophobic interaction chromatography (HIC) and electrostatic interaction chromatography (ESIC) to estimate changes on the net negative charge and surface hydrophobicity of E. coli cells treated with RFEF with or without heat.

Results

Conclusion