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Bull Vet Inst Pulawy 48, 371-373, 2004

EFFECT OF PULSED ELECTRIC FIELD (PEF)

ON ESCHERICHIA COLIWITHIN THE LIQUID WHOLE EGGADAM MALICKI, MACIEJ OZIEMBOWSKI

1, JERZY MOLENDA,

TADEUSZ TRZISZKA1ANDSZYMON BRUEWICZ

2

Department of Food Hygiene and Consumer Health, Veterinary Medicine Faculty,1Department of Animal Products Technology, Faculty of Food Science,

Wrocaw Agricultural University, 50-375 Wrocaw, Poland2Department of Hygiene, Wrocaw Medical University, 50-345 Wrocaw, Poland

e-mail: malicki@ozi.ar.wroc.pl

Received for publication May 12, 2004.

Abstract

The objective of the present study was to evaluate

the efficiency of pulsed electric field (PEF) againstEscherichia coli contaminating the liquid whole egg (LWE).The samples of LWE were inoculated with the test bacteria

and subsequently treated for 30 s by the different number ofpulses (20-180) of PEF (32.89 kV x cm-1). Application of PEF

resulted in statistically significant reduction of the testmicroorganisms, proportional to the number of pulses used.Depending on the studied strain, treatment with 150-160 PEF

pulses was required to obtain the reduction of initial bacterial

level by 4 log units. Considering the obtained results, PEFseems to be an effective technique, improving themicrobiological status of LWE, and consequently its industrial

application is highly advisable.

Key words:pulsed electric field, liquid wholeegg,Escherichia coli.

Use of the liquid whole egg (LWE) as asubstitute for shell eggs requires procedures preventing

its spoilage and growth of pathogenic bacteria within theproduct. Contamination with Enterobacteriaceae and

particularly with Salmonella sp. and Escherichia coliconstitutes the main microbiological problem related toLWE (8). Consequently, pasteurisation of the product isnecessary to obtain the microbiological state required by

Polish legislation (11). The thermal treatment, however,significantly decreases the nutritional value, mainly due

to protein coagulation (4).Application of pulsed electric field (PEF) seems

to be the profitable alternative for thermal treatment offood products. Pulsed electric fields were successfullyapplied for preservation of numerous liquid products,and the resulting bacterial reduction was comparable

with that obtained by means of thermal treatment (12).

The effect of PEF is related to the applicationof high voltage for very short periods of time (in therange of nano- or microseconds). Exposure of bacterialcells to the field changes of the sufficient amplitude

affects the electrical properties of the cell membrane,

reflecting in the decrease in its resistance and theincrease in conductance. Consequently, permeability ofthe membrane is altered, which is known aselectroporation (6, 7).

The objective of the present study was toevaluate PEF efficiency against the test strains of E.coli

contaminating LWE.

Material i Methods

The experiment was performed on the liquidwhole egg (LWE) obtained from the consumer eggs.

The material was inoculated with two strains ofE. coli,PCM 2057 and PCM 224, kindly provided by the

Institute of Immunology and Experimental Therapy,Polish Academy of Sciences, Wrocaw.

The inoculum was prepared from the 18-hculture in TSB (Oxoid), incubated at 37C. The LWE

samples were homogenised by 150 rotations per min for4 s, inoculated with 7 x 107 CFU x ml-1 of the test

bacteria and again homogenised for 4 s.Subsequently, the samples were placed in the

impulse sterilizer and exposed to different number (20,60, 100, 140 or 180) of PEF pulses (working voltage 25

kV, current 32.89 kV x cm-1

). The process, lasting 30 s,was performed at ambient temperature (about 20C).

Microbiological examination of the entire material wascarried out after the treatment.E.colicells were restoredon chromogenic solid medium (Chromocult ColiformAgar, Merck). For additional identification, all thecolonies stained from dark blue to violet were treatedwith Kovacs reagent for indol. The change of coloration

to cherry-red, appearing in a few seconds (positive resultof indol test), confirmed the isolation ofE.coli.

Logarithmic transformation of the bacterial

counts and their statistical analysis were done with theaid of MicrosoftExcel 2000 and Statistica 5, Version

97 software. The D, 4D and 6D values, i.e. the numberof field pulses required for reduction of the initial

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bacterial level by 1, 4 and 6 log units, respectively, werecalculated from the regression analysis. The importanceof the mean value differences was established with theaid of Students test (P

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coli, treated by PEF in the model systems or in the otherfood products (1, 5, 9, 10, 12, 13).

Analysis of two different strains of E. colirevealed some differences in their susceptibility to PEF,evident particularly by lower number of field pulses.

Some strain or species properties, like shape and size of

the bacterial cell and its growth stage are known toinfluence the microbial resistance to PEF treatment (1,5-7, 13).

The efficiency of PEF significantly depends onphysico-chemical properties of the treated food

products. Recent experiments failed to confirm theprotective effect of proteins and lipids on E. coli

exposed to PEF (9). The effectiveness of the PEFtreatment was also proved in the present study on highprotein environment of LWE.

Low conductivity of the suspending medium is

crucial for the efficiency of PEF treatment (3, 13). Sincethe conductivity of LWE is relatively high (6),

sufficiently elevated parameters of PEF are required toobtain the expected level of bacterial reduction. In thepresent study the objective was achieved by increasingthe number of PEF pulses, by the constant, relatively

low electric current. Such approach is quite reasonable,since the excessive current of PEF results in the

undesirable heating of the treated product, andconsequently the process does not fit the concept of non-thermal technique (1). On the other hand, thetemperature of the product exposed to PEF should not be

too low. Phospholipids are less ordered and the cellularmembrane has a liquid-crystalline structure at the

temperatures close to 30C. Accordingly, the bacterial

membranes are more susceptible to electroporation andthe antimicrobial efficiency of PEF treatment is maximal(1).

Synergistic action of PEF and low pH of thesuspending medium against bacteria was proved in

several studies (1, 13). The efficiency of PEF against E.coli in the model system was the highest at pH valuesclose to 4.0 (1). Consequently, LWE as a slightlyalkaline product requires elevated parameters of thetreatment, which was proved in the present study.

Concluding, application of PEF in case of the

LWE contaminated with E. coli, seems to be theeffective technique for microbiological status

improvement, providing that the specific features of theproduct are considered during the process parameteradjustment.

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