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ARS Home » Northeast Area » Wyndmoor, Pennsylvania » Eastern Regional Research Center » Food Safety and Intervention Technologies Research » Research » Publications at this Location » Publication #215357
Title: Membrane Damage and Viability Loss of Escherichia coli K-12 in Apple Juice Treated with Radio Frequency Electric Fields

Author
item Ukuku, Dike
item Geveke, David
item Cooke, Peter
item Zhang, Howard

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/14/2007
Publication Date: 4/1/2008
Citation: Ukuku, D.O., Geveke, D.J., Cooke, P.H., Zhang, H.Q. 2008. Membrane Damage and Viability Loss of Escherichia coli K-12 in Apple Juice Treated with Radio Frequency Electric Fields. Journal of Food Protection. 71(4):684-690.

Interpretive Summary: The need for a nonthermal intervention technology that can achieve a microbial safety without altering nutritional quality of liquid foods led to the development of radio frequency electric fields (RFEF). In order to understand the mechanism of inactivation of bacteria by RFEF, apple juice purchased from a wholesale distributor was inoculated with high level of Escherichia coli K-12 and then treated with RFEF. The RFEF treatment chamber was operated at 20 kHz, 30 kV/cm for 170 µs and, a flow rate of 540 ml/min. Treatment condition was periodically adjusted to achieve an optimal outlet temperature. At 55C, most of the bacteria were killed by the treatment. Bacterial population reduction at lower RFEF strength and the same temperature was much lower suggesting that the reduction observed in samples treated with higher RFEF field strength was entirely due to nonthermal effect. The RFEF treatment resulted in membrane damage of the bacterial population leading to the leakage of intracellular materials. The results of this study suggest that the mechanism of bacterial inactivation by RFEF is due to membrane damage and leakage of intracellular materials.

Technical 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 radio frequency electric fields (RFEF) process. In order to understand the mechanism of inactivation of bacteria by RFEF, apple juice purchased from a wholesale distributor was inoculated with Escherichia coli K-12 at 8.0 log CFU/ml and then treated with RFEF. The inoculated apple juice was passed through an RFEF chamber operated at 20 kHz, 30 kV/cm for 170 µs and, a flow rate of 540 ml/min. Treatment condition was periodically adjusted to achieve optimal outlet temperatures of 40, 45, 50 and 55C. Bacterial inactivation and viability loss occurred at all temperatures tested with 55C treatment leading to 4 log reduction. No significant effect was observed on bacterial population in control samples treated at 55C with a low RFEF (0.3 kV/cm) field strength. These observations suggest that the 4 log reduction in samples treated at 30 kV/cm was entirely due to nonthermal effect. RFEF treatment resulted in membrane damage of the bacteria leading to the efflux of intracellular ATP and UV-absorbing materials. Populations of injured bacteria recovered immediately (< 30 min) from the treated apple juice averaged 0.43 log and were below detection after 1 h of RFEF treatment and determination using selective TSA5 plates. The results of this study suggest that mechanism of inactivation of RFEF is by disruption of the bacterial surface structure leading to the damage and leakage of intracellular materials.