Submitted to: International Journal of Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 16, 2010
Publication Date: April 30, 2010
Citation: Gurtler, J., Rivera, R.B., Geveke, D.J., Zhang, H.Q. 2010. Selection of surrogate bacteria in place of E. coli O157:H7 and Salmonella Typhimurium for pulsed electric field treatment of orange juice. International Journal of Food Microbiology. 139:1-8. Interpretive Summary: Outbreaks of foodborne illnesses due to the contamination of fruit juices with Salmonella and E. coli has led the U.S. Food and Drug Administration to require that all juice products sold to the public must be pasteurized for the inactivation of the target pathogen of interest. E. coli O157:H7. Traditionally, pasteurization is conducted by the use of heat; however, this process is known to change the aroma and flavor of juices. One alternative to thermal pasteurization that has been studied is a technology known as pulsed electric field (PEF) processing, which generates thousands of microsecond pulses into a liquid food product. PEF technology has been used as an innovative treatment for the reduction of microorganisms in a number of liquid foods and beverages. It is theorized that the electrical impulses generated during PEF processing of liquid foods inactivates bacteria by perforating the membrane of the microorganisms, which induces leakage of intercellular contents. PEF may be used to pasteurize orange juice at lower temperatures than traditional thermal processes, and thus may assist in preserving the sensory attributes. This study examined the ability of PEF to inactivation E. coli O157:H7 and two strains of Salmonella Typhimurium in orange juice. Another goal of the study was to find a non-pathogenic, non-virulent bacterium that would be inactivated in similar fashion to E. coli and Salmonella. This non-pathogenic bacterium is known as a surrogate organism, which it can be used in place of pathogenic bacteria in laboratories where use of the virulent bacteria is not permitted. We found that E. coli O157:H7 was more resistant to PEF treatment in orange juice than Salmonella. We then tested twenty strains of non-pathogenic bacteria by PEF and found one non-pathogenic strain (non-pathogenic E. coli, ATCC #35218) was a suitable surrogate organism since its rate of inactivation was statistically similar to the pathogenic strain of E. coli O157:H7. Results indicated that we eliminated 99.99% of either E. coli strain when orange juice was treated by PEF at 50 deg C with an electrical field strength of 28 kV/cm. Our surrogate bacterium (E. coli ATCC 35218) is now being used in larger PEF studies in a pathogen-free food processing plant. The results of this study will assist in moving the PEF processing technology closer to adoption by the commercial food industry.
Technical Abstract: Pulsed electric field (PEF) technology has been used as an innovative treatment for the reduction of microorganisms in liquid foods and beverages by the electroporation of bacterial membranes. PEF may be used to pasteurize orange juice at lower temperatures than traditional thermal processes, preserving sensory attributes. Enterohemorrhagic E. coli O157:H7 (EHEC) ATCC 43895, two strains of Salmonella Typhimurium, and twenty strains of non-pathogenic bacteria were screened for inactivation in orange juice by pulsed electric field (PEF) at a field strength of 22 kV/cm at 45 deg C (treatment time of 59 us) and 20 kV/cm at 55 deg C (treatment time of 72 us). Higher populations of Salmonella Typhimurium UK-1 were inactivated (2.81 log CFU/ml) at 55 deg C, in comparison with reduction of EHEC (2.22 log). When tested under the same conditions, inactivation of EHEC was slightly higher than that of a non-pathogenic E. coli (NPEC) ATCC 35218 (2.02 log). NPEC was further tested as a surrogate for EHEC by comparison of PEF inactivation kinetics at 45, 50 and 55 deg C and at field strengths of between 7.86 and 32.55 kV/cm (treatment time of 75 us). Regression analysis was performed by fitting inactivation data with a first-order kinetics model. Comparison of slopes by analysis of covariance revealed that EHEC and NPEC were statistically (p > 0.05) indistinguishable (homogeneous inactivation curves) at outlet temperatures of 45 and 50 deg C. Analyzing slopes of inactivation at 55 deg C, however, revealed that EHEC O157:H7 was slightly yet significantly (p < 0.05) more sensitive to PEF than surrogate E. coli strain 35218 at 55 deg C. The slightly higher PEF resistance of non-pathogenic E. coli 35218 at 55 deg C may provide a desirable margin of safety when used in pilot plant challenge studies in place of E. coli O157:H7, as enterohemorrhagic strains of E. coli with greater PEF-resistance may exist.