Submitted to: Food Control
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/26/2011
Publication Date: 3/29/2011
Citation: Gurtler, J., Bailey, R., Geveke, D.J., Zhang, H.Q. 2011. Sodium benzoate, potassium sorbate, and citric acid induce sublethal injury and enhance pulsed electric field inactivation of E. coli O157:H7 and nonpathogenic surrogate E. coli in strawberry juice. Food Control. 22(2011)1689-1694. Interpretive Summary: Pasteurization is traditionally conducted by the use of heat; however, this process is known to change the aroma and flavor of juices. One alternative to thermal pasteurization is pulsed electric field (PEF) processing, which generates thousands of microsecond long electrical pulses into a liquid food product and has been used as an innovative treatment for the reduction of microorganisms in liquid foods and beverages at lower temperatures to preserve flavor and aroma. This study examined the ability of PEF to inactivate E. coli O157:H7 and a non-pathogenic, non-virulent “surrogate” bacterium with or without the common preservatives sodium benzoate, potassium sorbate and citric acid. Non-pathogenic surrogate bacteria can be used in place of harmful pathogenic bacteria in laboratories where use of the virulent bacteria is not permitted. We found that both the pathogenic and surrogate bacteria succumbed to equivalent inactivation when processed by PEF with or without preservatives. The results of this study allows us to use our surrogate bacterium in a larger industrial-scale PEF system in a pathogen-free food processing pilot plant and assists us in moving the PEF processing technology closer to adoption by the commercial food industry.
Technical Abstract: Current FDA regulations require that juice processors effect a 5 log CFU/ml reduction of a target pathogen prior to distributing products. Whereas thermal pasteurization reduces the sensory characteristics of juice by altering flavor components, pulsed electric field (PEF) treatment can be conducted at lower temperatures, and may preserve sensory characteristics. This study was the first known to compare the inactivation of E. coli O157:H7 and a nonpathogenic E. coli in strawberry juice by PEF with or without sodium benzoate, potassium sorbate, and citric acid. E. coli O157:H7 (ATCC 43895) and a nonpathogenic E. coli (ATCC 35218), respectively were inoculated into single strength strawberry juice with or without 750 ppm sodium benzoate, 350 ppm potassium sorbate, and 2.7 citric acid. Juice was treated at outlet temperatures of 45, 50 and 55 deg C at a field strength of 18.6 kV/cm for 150 microseconds with a laboratory-scale PEF unit. Inactivation of surrogate E. coli at 45, 50, and 55 deg C were 2.86, 3.12, and 3.79 log CFU/ml, respectively, in plain juice (pH 3.4), and 2.75, 3.52, and 5.11 with the addition of benzoic and sorbic acids (pH 3.5). Inactivation of E. coli O157:H7 under the same conditions were 3.09, 4.08, and 4.71 log CFU/ml, respectively, and 2.27, 3.29, and 5.40 with antimicrobials. E. coli O157:H7 in juice with antimicrobials and 2.7 percent citric acid (pH 2.7) treated with PEF was reduced by 2.60, 4.32 and 6.95 log CFU/ml at 45, 50 and 55 deg C while the surrogate E. coli decreased by 3.54, 5.69, and 7.13 log under the same conditions. When juice (pH 2.7) was held for 6 h without PEF treatment, higher numbers of E. coli 35218 (7.17 log CFU/ml) were inactivated than of acid-resistant E. coli O157:H7 (3.89 log). Slightly greater PEF inactivation of E. coli O157:H7 than of the surrogate bacterium indicates that E. coli ATCC 35218 provides a margin of safety when used as a surrogate for O157:H7 in plain strawberry juice or in juice + sorbic and benzoic acids at 45 – 50 deg C or with the preservatives and citric acid at 55 deg C. Further studies will be performed on scaled-up PEF equipment in a pilot plant.