Food Preservation Pulsed Electric Fields

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Food_Preservation_by_Pulsed_Electric_Fields_/1845690583/files/00003___de2918de2602be1feae3eef147bc9ca5.pdfFood preservation bypulsed electric fieldsFrom research to application

Edited byH. L. M. Lelieveld, S. Notermans and

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Food_Preservation_by_Pulsed_Electric_Fields_/1845690583/files/00005___099219de7f4eec1b0153dd6a4721055d.pdfContributor contact details .................................................................. xiiiPreface ................................................................................................... xix1 Preservation of food by pulsed electric fields: An

introduction .................................................................................. 1S. Notermans, Foundation Food Micro and Innovation,The Netherlands1.1 The need to preserve food ................................................ 11.2 Major preservation technologies ....................................... 31.3 Current developments and demands ................................. 51.4 Current needs ..................................................................... 51.5 References .......................................................................... 7

2 History of pulsed electric field treatment ................................ 9S. Toepfl, V. Heinz, Deutsches Institut fr Lebensmitteltechnik(DIL) e.V., Germany, and D. Knorr, Berlin University ofTechnology, Germany2.1 Introduction ........................................................................ 92.2 The evolution of PEF techniques ..................................... 102.3 Research work on PEF applications from 1980s to 2004 192.4 Applications of PEF in food and bio-processing ............. 242.5 Present situation and future industrial exploitation ......... 282.6 Outlook and conclusions ................................................... 302.7 References .......................................................................... 31

Part I Technology

3 Circuitry and pulse shapes in pulsed electric fieldtreatment of food ......................................................................... 43S. W. H. de Haan, Delft University of Technology,The Netherlands3.1 Introduction ........................................................................ 433.2 Requirements ..................................................................... 44

Contents

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Food_Preservation_by_Pulsed_Electric_Fields_/1845690583/files/00006___2f7f0957f254ff4f0a74f32a99c6cd22.pdf3.3 Pulse shapes ....................................................................... 473.4 Circuitry ............................................................................. 483.5 Switches ............................................................................. 573.6 Other components .............................................................. 603.7 Examples of applied systems ............................................ 613.8 Miscellaneous .................................................................... 633.9 Trends in pulsed power technology .................................. 663.10 Sources of further information ......................................... 673.11 References .......................................................................... 67

4 Chamber design and process conditions for pulsed electricfield treatment of food ................................................................ 70H. F. M. van den Bosch (formerly Delft University ofTechnology), The Netherlands4.1 Introduction ........................................................................ 704.2 Chamber geometries .......................................................... 704.3 Electric field calculations .................................................. 754.4 Residence time distribution............................................... 784.5 Wall temperature ............................................................... 794.6 Experimental set-up ........................................................... 844.7 Temperature measurements ............................................... 854.8 Other experimental results ................................................ 894.9 Conclusions and future trends .......................................... 914.10 References .......................................................................... 92

5 Electrochemistry in pulsed electric field treatmentchambers ...................................................................................... 94B. Roodenburg, Delft University of Technology,The Netherlands5.1 Introduction ........................................................................ 945.2 Theory ................................................................................ 955.3 Experiments ....................................................................... 1005.4 Treatment chamber lifetime .............................................. 1055.5 Legislation ......................................................................... 1055.6 Conclusions ........................................................................ 1065.7 References .......................................................................... 106

6 Hygienic design for pulsed electric field installations ........... 108C. Smit and W. de Haan, Stork Food & Dairy Systems,The Netherlands6.1 Introduction ........................................................................ 1086.2 Hygienic demands ............................................................. 1096.3 Construction elements ....................................................... 1096.4 Process aspects .................................................................. 1106.5 Conclusions ........................................................................ 1176.6 References .......................................................................... 117

vi Contents

Food_Preservation_by_Pulsed_Electric_Fields_/1845690583/files/00007___3d643aac445f03ba068361c4de49a848.pdf7 Technical and occupational safety requirements whentreating foods by pulsed electric fields .................................... 118P. H. F. Morshuis, Delft University of Technology,The Netherlands7.1 Introduction ........................................................................ 1187.2 Potential safety hazards ..................................................... 1187.3 Technical safety requirements ........................................... 1207.4 Occupational safety requirements ..................................... 1217.5 Food safety ........................................................................ 1227.6 Conclusions ........................................................................ 1237.7 References .......................................................................... 123

Part II Product safety and quality8 Microbial inactivation kinetics of pulsed electric field

treatment ...................................................................................... 127M. B. Fox, NIZO Food Research, The Netherlands8.1 Introduction ........................................................................ 1278.2 Factors affecting inactivation kinetics .............................. 1278.3 Kinetic models ................................................................... 1308.4 Discussion .......................................................................... 1338.5 Sources of further information ......................................... 1338.6 References .......................................................................... 134

9 Probable mechanisms of microorganism inactivation bypulsed electric fields ................................................................... 138G. Saulis, Vytautas Magnus University, Lithuania, andP. C. Wouters, Unilever Research & DevelopmentVlaardingen, The Netherlands9.1 Introduction ........................................................................ 1389.2 Models used for the description of inactivation

kinetics ............................................................................... 1399.3 Mechanisms of microorganism inactivation by PEF ....... 1409.4 Discussion .......................................................................... 1489.5 Future trends ...................................................................... 1509.6 Sources of further information ......................................... 1509.7 Acknowledgements ........................................................... 1519.8 References .......................................................................... 151

10 Adaptation potential of microorganisms treated by pulsedelectric fields ................................................................................ 156D. Rodrigo, M. Ziga, A. Rivas and A. Martnez, Instituto deAgroqumica y Tecnologa de Alimentos, Spain, andS. Notermans, Foundation Food Micro and Innovation,The Netherlands10.1 Introduction ........................................................................ 15610.2 Pulsed electric field technology........................................ 157

Contents vii

Food_Preservation_by_Pulsed_Electric_Fields_/1845690583/files/00008___de67b0a1b64dd4483257cc901a684bf3.pdf10.3 Sub-lethal damage from PEF ............................................ 15710.4 Possibility of transformation ............................................. 15910.5 Assessment of the risk of transforming Lact. casei by

PEF treatment .................................................................... 16010.6 Results and discussion ...................................................... 16110.7 Conclusions ........................................................................ 16310.8 References .......................................................................... 163

11 Hurdle technology and the preservation of food by pulsedelectric fields ................................................................................ 165I. lvarez, University of Zaragoza, Spain, andV. Heinz, Deutsches Institut fr Lebensmitteltechnik (DIL) e.V.,Germany11.1 Introduction ........................................................................ 16511.2 Combination of PEF and temperature .............................. 16711.3 Combination of PEF and pH ............................................ 16911.4 Combination of PEF and antimicrobials .......................... 17011.5 Combination of PEF and high pressure ........................... 17211.6 Combination of PEF and ultrasound ................................ 17311.7 Future trends ...................................................................... 17411.8 Sources of further information ......................................... 17411.9 References .......................................................................... 175

12 Validating the safety of foods treated by pulsed electricfields ............................................................................................ 178L. Keener, International Product Safety Consultants, Inc, USA12.1 Introduction ........................................................................ 17812.2 Regulatory considerations ................................................. 17912.3 General principles of process validation .......................... 18212.4 Validating PEF-treated products ....................................... 18512.5 Validating the process performance .................................. 19012.6 Future trends ...................................................................... 19512.7 Conclusions ........................................................................ 19612.8 Summary ............................................................................ 19812.9 References .......................................................................... 19812.10 Bibliography ...................................................................... 200

13 Toxicological aspects of preservation of food by pulsedelectric fields ................................................................................ 201A. M. Matser, H. J. Schuten, H. C. Mastwijk, Food TechnologyCentre Wageningen UR, The Netherlands, and A. Lommen,RIKILTInstitute of Food Safety, The Netherlands13.1 Introduction ........................................................................ 20113.2 Sources of possible toxicological hazards ....................... 20213.3 Metal release by electrode degradation ............................ 20313.4 Electrochemistry ................................................................ 20413.5 Possible changes in PEF-treated products: Substantial

equivalence study .............................................................. 205

viii Contents

Food_Preservation_by_Pulsed_Electric_Fields_/1845690583/files/00009___4aea6aaa54b923ebdd483b955f0904db.pdf13.6 Conclusions and future trends .......................................... 20913.7 Acknowledgement ............................................................. 21013.8 References .......................................................................... 210

14 Impact of pulsed electric fields on food enzymes and shelf-life ................................................................................................. 212P. Elez-Martnez, O. Martn-Belloso, Universitat de Lleida,Spain and D. Rodrigo, F. Sampedro, Instituto de Agroqumica yTecnologa de Alimentos, Spain14.1 Introduction ........................................................................ 21214.2 Enzyme inactivation by PEF ............................................ 21214.3 Shelf-life of food processed by PEF ................................ 22714.4 Future trends ...................................................................... 24114.5 Nomenclature ..................................................................... 24214.6 References .......................................................................... 242

Part III Applications15 Public acceptance of pulsed electric field processing ............ 249

L. Frewer and A. Fischer, Social Sciences Group WageningenUR, The Netherlands15.1 Introduction ........................................................................ 24915.2 A historical perspective on risk communication .............. 25015.3 The psychology of risk ..................................................... 25215.4 The introduction of GM foods in Europe ........................ 25315.5 Consumer perceptions of risk management ..................... 25315.6 Other issues of relevance to the introduction of PEF

technology .......................................................................... 25415.7 References .......................................................................... 254

16 Economic aspects of pulsed electric field treatment of food 257H. Hoogland, Unilever Research & Development Vlaardingen,The Netherlands, and W. de Haan, Stork Food & DairySystems, The Netherlands16.1 Introduction ........................................................................ 25716.2 PEF for pasteurisation ....................................................... 25816.3 PEF as a processing aid .................................................... 26216.4 Quality ................................................................................ 26316.5 Conclusions ........................................................................ 26416.6 Acknowledgements ........................................................... 26416.7 References .......................................................................... 265

17 Applications of pulsed electric fields for food preservation 266B. Altunakar, S. R. Gurram and G. V. Barbosa-Cnovas,Washington State University, USA17.1 Introduction ........................................................................ 26617.2 Historical overview ........................................................... 267

Contents ix

Food_Preservation_by_Pulsed_Electric_Fields_/1845690583/files/00010___69532b652de113e7a62e40ada1b46151.pdf17.3 Preservation of foods with pulsed electric fields ............. 27017.4 The hurdle concept and combination studies ................... 28617.5 Conclusions ........................................................................ 28717.6 References .......................................................................... 288

18 Pitfalls of pulsed electric field processing ............................... 294H. L. M. Lelieveld (formerly Unilever Research & DevelopmentVlaardingen), The Netherlands, H. C. Mastwijk, FoodTechnology Centre Wageningen UR, The Netherlands, andH. F. M. van den Bosch (formerly Delft University ofTechnology), The Netherlands18.1 Introduction ........................................................................ 29418.2 Control and hygiene .......................................................... 29418.3 Position of treatment chamber .......................................... 29518.4 Particulate foods and emulsions ....................................... 29818.5 Gas bubbles ........................................................................ 29918.6 Measuring field strength and pulse shape ........................ 29918.7 Conclusions ........................................................................ 299

19 Technologies related to pulsed electric field processing andtheir potential .............................................................................. 300H. L. M. Lelieveld (formerly Unilever Research &Development Vlaardingen), The Netherlands, and S. W. H.de Haan, Delft University of Technology, The Netherlands19.1 Introduction ........................................................................ 30019.2 Pulsed magnetic field treatment ....................................... 30019.3 High-voltage arc discharges .............................................. 30119.4 Submerged streamers ........................................................ 30219.5 Ohmic heating ................................................................... 30319.6 Microwave and radio frequency heating .......................... 30319.7 Electron beam treatment ................................................... 30419.8 Pulsed intense light ........................................................... 30419.9 Cold gaseous plasma treatment ........................................ 30519.10 PEF in-pack ....................................................................... 30519.11 References .......................................................................... 306

20 Future potential of pulsed electric field treatment:Treatment of bacterial spores, emulsions and packedproducts ........................................................................................ 307H. C. Mastwijk and P. V. Bartels, Food Technology CentreWageningen UR, The Netherlands20.1 Introduction ........................................................................ 30720.2 Current PEF technology .................................................... 30720.3 Opportunities ..................................................................... 31620.4 Conclusion ......................................................................... 31820.5 Symbols ............................................................................. 31820.6 References .......................................................................... 319

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Food_Preservation_by_Pulsed_Electric_Fields_/1845690583/files/00011___52833ddd68aa374b4de241daeec0182a.pdf21 Definitions and guidelines for reporting on pulsed electricfield experiments ......................................................................... 320H. C. Mastwijk, K. Gulfo-van Beusekom, I. E. Pol-Hofstad,H. Schuten, M. Boonman and P. V. Bartels, Food TechnologyCentre Wageningen UR, The Netherlands21.1 Introduction ........................................................................ 32021.2 Parameters, units and dimensions ..................................... 32121.3 Electric conductivity and resistance ................................. 32321.4 Electric field strength, temperature and treatment time .. 32621.5 Energy balance .................................................................. 33521.6 Processing, pre-sterilisation, start-up ............................... 34021.7 Microbiological inactivation, ring-test assessment .......... 34221.8 Conclusions ........................................................................ 34321.9 References .......................................................................... 345

22 Scaling-up of equipment for pulsed electric field treatmentof foods ......................................................................................... 346H. F. M. van den Bosch (formerly Delft University ofTechnology), The Netherlands22.1 Introduction ........................................................................ 34622.2 Continuous versus batch treatment ................................... 34622.3 Parallel versus normal field chamber ............................... 34722.4 Flow cross-section of parallel and normal field chamber 34922.5 Plateplate chamber versus co-axial chamber ................. 35022.6 Effect of scale on wall temperature .................................. 35022.7 Conclusions ........................................................................ 35122.8 Acknowledgement ............................................................. 351

23 Regulatory acceptance ................................................................ 352M. Smith (formerly Unilever Research & DevelopmentVlaardingen, The Netherlands), Philip Morris InternationalResearch & Development, Neuchtel, Switzerland23.1 Introduction ........................................................................ 35223.2 Current European legislation ............................................ 35323.3 US regulatory position ...................................................... 35423.4 Conclusions ........................................................................ 35623.5 References .......................................................................... 356Index ............................................................................................ 358

Contents xi

Food_Preservation_by_Pulsed_Electric_Fields_/1845690583/files/00012___806bb195ba4650514df300e946bfadb5.pdfxii

Food_Preservation_by_Pulsed_Electric_Fields_/1845690583/files/00013___9a9e0d9a1660186ac50ed0b77617d2c5.pdfEditorsH. L. M. LelieveldEnsahlaan 113723 HT BilthovenThe Netherlands

email: [email protected]

S. NotermansFoundation Food Micro and

InnovationObrechtlaan 173723 KA BilthovenThe Netherlands


S. W. H. de HaanDelft University of TechnologyElectrical Power Processing Unit

(EPP)Mekelweg 42628 CD DelftThe Netherlands


Contributor contact details

Chapter 1S. NotermansFoundation Food Micro and



Chapter 2S. Toepfl* and V. HeinzDeutsches Institut fr

Lebensmitteltechnik (DIL) e.V.Professor-von-Klitzing-Str. 749610 QuakenbrueckGermany


D. KnorrBerlin University of TechnologyDepartment of Food Biotechnology

and Food Process EngineeringKoenigin-Luise-Str. 2214195 BerlinGermany

(* = main contact)

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Food_Preservation_by_Pulsed_Electric_Fields_/1845690583/files/00014___465ffc800f98f682005ed46679060e7d.pdfChapter 3S. W. H. de HaanDelft University of TechnologyFaculty EEMCS / EPPMekelweg 42628 CD DelftThe Netherlands


Chapter 4H. F. M. van den BoschSchoolstraat 175438 AC GasselThe Netherlands


Chapter 5B. RoodenburgDelft University of TechnologyFaculty: EWIBart RoodenburgRoom LB03-620Postbus 50312600 GA Delft


Chapter 6C. Smit* and W. de HaanStork Food & Dairy SystemsKetelstraat 21021 JX AmsterdamThe Netherlands


Chapter 7P. H. F MorshuisDelft University of TechnologyFaculty of Electrical Engineering,

Mathematics and ComputerScience

Mekelweg 42628 CD DelftThe Netherlands


Chapter 8M. B. FoxNIZO Food ResearchKernhemseweg 26718 ZB EdeThe Netherlands


Chapter 9P. C. WoutersUnilever R&D VlaardingenPO Box 1143130 AC VlaardingenThe Netherlands


G. SaulisVytautas Magnus UniversityDepartment of Biology58 K. Donelaicio str.LT-44248 KaunasLithuania


xiv Contributor contact details

Food_Preservation_by_Pulsed_Electric_Fields_/1845690583/files/00015___f4eb918f10920aaa90da6b4cb9a5bb1b.pdfChapter 10D. Rodrigo, M. Ziga, A. Rivas,A. MartnezInstituto de Agroqumica y

Tecnologa de AlimentosApartado de Correos 7346100 BurjassotValenciaSpain

S. Notermans*Foundation Food Micro and



Chapter 11I. lvarez*University of ZaragozaC/Miguel Servet, 17750013, ZaragozaSpain


V. HeinzDeutsches Institut fr

Lebensmitteltechnik(DIL) e.V.

Professor-von-Klitzing-Str. 749610 QuakenbrueckGermany

Chapter 12L. KeenerInternational Product Safety

Consultants, Inc.4021 W. Bertona StreetSeattle 98199-1934WashingtonUSAemail: [email protected]

[email protected]

Chapter 13A. M. Matser*, H. J. Schuten,

H. C. MastwijkFood Technology CentreWageningen URPO Box 176700 AA WageningenThe Netherlands


A. LommenRIKILTInstitute of Food SafetyBornsesteeg 456700 AE WageningenThe Netherlands

Chapter 14P. Elez-Martnez and O. Martn-

Belloso*Department of Food TechnologyUniversitat de LleidaAv. Alcalde, Rovira Roure, 19125198 LleidaSpain


D. Rodrigo and F. SampedroInstituto de Agroqumica y

Tecnologa de AlimentosApartado de Correos 7346100 BurjassotValenciaSpain

Chapter 15L. Frewer* and A. FischerSocial Sciences GroupWageningen URBode 87Postbus 81306700 EW WageningenThe Netherlands


Contributor contact details xv

Food_Preservation_by_Pulsed_Electric_Fields_/1845690583/files/00016___8c118a9f565b508c162f5ad53ba346ff.pdfChapter 16H. Hoogland*Unilever R&D VlaardingenPO Box 1143130 AC VlaardingenThe Netherlands


W. de HaanStork Food & Dairy SystemsKetelstraat 21021 JX AmsterdamThe Netherlands

Chapter 17B. Altunakar, S. R. Gurram and

G. V. Barbosa-Cnovas*Center for Nonthermal Processing

of FoodPullmanWA 99164-6120USA


Chapter 18H. L. M. Lelieveld*Ensahlaan 113723 HT BilthovenThe Netherlands


H. F. M. van den BoschSchoolstraat 175438 AC GasselThe Netherlands


xvi Contributor contact details

H. C. MastwijkFood Technology CentreWageningen URPostbus 91016700 HB WageningenThe Netherlands


Chapter 19S. W. H. de Haan*Delft University of TechnologyElectrical Power Processing Unit

(EPP)Mekelweg 42628 CD DelftThe Netherlands

email: S.W.H.deHaan@ tudelft.nl

H. L. M. LelieveldEnsahlaan 113723 HT BilthovenThe Netherlands


Chapter 20H. C. Mastwijk* and P. V. BartelsFood Technology CentreWageningen URPostbus 91016700 HB WageningenThe Netherlands


Food_Preservation_by_Pulsed_Electric_Fields_/1845690583/files/00017___4c3ed7ba7b582129233bdf0e4c0f4ea5.pdfChapter 21H. C. Mastwijk,* K. Gulfo-van

Beusekom, I. E. Pol-Hofstad,H. Schuten, M. Boonman andP. V. Bartels

Food Technology CentreWageningen URPostbus 91016700 HB WageningenThe Netherlands


Chapter 22H. F. M. van den BoschSchoolstraat 175438 AC GasselThe Netherlands


Chapter 23M. SmithDirector Toxicological Risk

Assessment and CommunicationPMI R&DQuai Jeanrenaud 562000 NeuchtelSwitzerland


Contributor contact details xvii

Food_Preservation_by_Pulsed_Electric_Fields_/1845690583/files/00018___8ab0966bfa5a3c0cfc7233cf24d9b711.pdfxviii

Food_Preservation_by_Pulsed_Electric_Fields_/1845690583/files/00019___2c6ced711747413e300759cc07115d1c.pdfPreface

Pulsed electric field (PEF) processing seems to be an ideal and relativelysimple solution to the problem of producing shelf stable food products thatretain the characteristics of fresh food. Preservation by pulsed electric fieldsdestroys the microorganisms in the food, but colour, flavour and levels ofvitamins and antioxidants are unaffected. The technology was conceivedalmost 100 years ago,1 but was not pursued seriously until the 1960s. At thattime, commercial application was far off and even scaling up of the technologymust have seemed close to impossible. In the 1980s, however, when consumersstarted to question the quality of canned and other foods preserved by thermalmethods, novel preservation technologies gradually received more attentionfrom several research groups. Early results tempted Maxwell Technologiesin the USA (through a subsidiary named PurePulse Technologies) to marketPEF equipment for the preservation of food. It transpired, though, that thismove was premature. The inactivation of microorganisms by PEF was morecomplex than envisaged and results of pilot plant studies were disappointing.Maxwell closed down PurePulse Technologies in 2002. In particular, theinfluence of the equipment on the microbiological results had been greatlyunderestimated. Attempting to meet consumer demands in Europe and militaryrequirements in the USA, governments supported further research incollaboration with the food industry. In the Netherlands, a large research anddevelopment consortium was established, consisting of several researchinstitutes and R&D departments of multinational food companies. They workedin co-operation with engineers from the High-Voltage Laboratory of theTechnological University of Delft, who were to deal with the electrotechnicalchallenges involved in PEF preservation of food. The project resulted in thedevelopment and testing of a fairly large scale pilot plant and the establishmentof rules for scale-up to production size.

Despite the significant progress made this decade, which is due to the co-

1 A.K. Anderson and R. Finkelstein, Electro-Pure process of treating milk, Journal ofDairy Science 2 (1919), 374407.

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Food_Preservation_by_Pulsed_Electric_Fields_/1845690583/files/00020___68d66917e29a51d6ad11e3329ca4f368.pdfoperation of many scientists, engineers and technologists, in particular inEurope and the USA, application is still very limited. It is hoped that thisbook will show that the technology is maturing rapidly and that it will helpindustry to overcome its hesitations regarding preservation of food by pulsedelectric fields.

For a new food preservation technology to be introduced, insight is notonly required into the effects of the technology on the inactivation ofmicroorganisms and enzymes and on product nutritional and organolepticcharacteristics; this book therefore also covers occupational safety,toxicological aspects, consumer acceptance, regulatory requirements and thepromising economic aspects of PEF technology. Combining the technologywith other preservation methods has not been forgotten and last, but notleast, we have included a chapter discussing the potential future developmentsof PEF technology, which may include the preservation of food by PEF afterpackaging.

Huub Lelieveld, Serv Notermans and Sjoerd de Haan

xx Preface

Food_Preservation_by_Pulsed_Electric_Fields_/1845690583/files/00021___49c1c0b76e57fdc3ad0feed4c4f2b7b1.pdf1.1 The need to preserve food1.1.1 Historical aspectsThe need to preserve food has a long history. Problems with deteriorationmust have been a continuous preoccupation of early humans, once theybegan their hunting and food-gathering activities, and domestic productionof food animals and crops. Although the exact timing is uncertain, organisedfood production probably started between 18 300 and 17000 years ago, whenbarley production is said to have flourished in the Egyptian Nile Valley(Wendorf et al., 1979). During that time, there was a need to preserve thegrain and keeping it in a dry condition was an obvious precaution. Attemptsto preserve other foods were based mainly on experience gained in linkingthe spoilage of the food with the manner in which it had been prepared andstored. Increasingly, it became clear that food could only be maintained in anacceptable condition if the product was kept dry and away from contact withair. Some foods were treated with honey and later with olive oil (Toussaint-Samat, 1992). This led to the development of additional preservative measures,such as heating and salting. Once the preservative capability of salt wasdiscovered, the value of this substance increased, since it was not availablein sufficient quantity to meet the demand. According to Toussaint-Samat(1992), the large amount of salt in the Dead Sea was one of the reasons forthe interest of the Romans in Palestine.

Over many millennia, mankind has also learned to select edible plant andanimal species, and to produce, harvest and prepare them in a safe mannerfor food purposes. This was done mostly on the basis of trial and error fromlong experience. Many of the lessons learned, especially those relating to

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Preservation of food by pulsed electricfields: An introductionS. Notermans, Foundation Food Micro and Innovation,The Netherlands

1

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adverse effects on human health, are reflected in various religious taboos,which include a ban on eating specific items, such as pork, in the Jewish andMuslim religions (Tannahill, 1973). Other taboos showed a more generalappreciation of food hygiene. In India, for example, religious laws prohibitedthe consumption of certain unclean foods, such as meat cut with a sword,or sniffed by a dog or cat, and meat obtained from carnivorous animals(Tannahill, 1973). Most of these food safety requirements were establishedthousands of years ago when religious laws were likely to have been the onlyones in existence. The introduction of control measures in civil law was ofa much later date. A more recent example of discovering that preservationwas an effective tool in preventing disease was in the time of the cholera andplague epidemics. It was noted that beer-drinkers did not fall ill and thebrewing and drinking of beer became very popular. The main reason for thiswas that beer, due to the production process used, is a well-preserved productand does not contain any pathogenic organisms.

Because the underlying causes of food spoilage and food-borne illnesswere unknown, spoilage and poisoning were recurrent problems. However,the situation changed after 1795, when the French government, driven bywar, offered a substantial reward for anyone developing a new method ofpreserving food. It was Nicolas Appert, a Parisian confectioner, who acceptedthe challenge and developed a wide-mouth glass bottle that was filled withfood, before being corked and heated in boiling water for about six hours. In1810, Durand in England patented the use of tin cans for thermal processingof foods, but neither Appert nor Durand understood why thermally processedfoods did not spoil (Hartman, 1997), despite the fact that in 1677 vanLeeuwenhoek had discovered his little heat-sensitive animalcules (Dobell,1960).

It was Louis Pasteur who provided the scientific basis for heat preservationin the period 18541864. During that time, he showed that certain bacteriawere either associated with food spoilage or caused specific diseases. Basedon Pasteurs findings, commercial heat treatment of wine was first introducedin 1867, to destroy any undesirable micro-organisms, and the process wasdescribed as pasteurisation. Another important development occurred inGermany, when Robert Koch introduced a method of growing micro-organismsin pure culture and, with colleagues, first isolated the cholera vibrio in 1884,during a worldwide pandemic (Chung et al., 1995). Over the next 100 yearsor more, laboratory isolation and study of pure cultures of microbes remainedamong the predominant activities of food microbiologists (Hartman, 1997).

Following the discovery of micro-organisms and recognition of theirpathogenic potential towards the end of the 19th century, food preservationdeveloped on more scientific lines. Preventing spoilage and keeping foodsafe were no longer the only reasons for preservation. Increasingly, preservationhas become important in maintaining product quality and especially foodflavour. Clear examples are the production of sauerkraut, kimchi, severaltypes of ham and sausage and many dairy products. Nevertheless, preventing

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spoilage and ensuring product safety are still the main objectives of foodpreservation. Despite this, EU Council Directive No. 95/2 EC describespreservatives as substances which prolong the shelf-life of foodstuffs byprotecting them against deterioration caused by micro-organisms.

1.1.2 Quality loss and food safetyLoss of quality in foods is caused by a wide range of reactions. Theseinclude processes that are predominantly physiological (respiration of fruitand vegetables), physical (changes in texture following freezing), chemical(oxidation of fatty acids, loss of colour), enzymatic (enzymatic browning),biological (damage by insects) or microbiological (food spoilage). Changesin organoleptic characteristics, freshness and suitability for human consumptionmust also be taken into account. Of all the adverse reactions in foods thataffect product quality, the microbiological ones are the most severe. It hasbeen estimated that, currently, about 25% of the worlds food supply is lostfrom microbial spoilage (Huis int Veld, 1996). The available means ofcombating the deleterious effects of micro-organisms are to preventcontamination, kill the organisms in situ or slow their growth. Therefore,preservation processes are increasingly being integrated into the food-production chain. Such an approach allows the use of mild preservationtreatments, thus avoiding unnecessary quality loss caused by the treatment itself.

Whilst most preservation techniques aim to control all forms of qualityloss that can occur, the overriding priority is always to minimise the occurrenceand growth of micro-organisms, particularly those that can cause food poisoningor food-borne infections (Russell and Gould, 2003). The main reason is thehigh levels of reported food-borne illness (see, for example, Mead et al.,1999), which, incidentally, has stimulated an interest in the use of preservativetreatments (Rombouts et al., 2003). Preservatives may contribute to thecontrol of food-borne illness, but there are other important factors that alsoinfluence the growth and survival of micro-organisms in foods (Jejuneja andSofos, 2001).

1.2 Major preservation technologiesOver the years, many food preservation technologies have been developedand are described in detail by Russell and Gould (2003). Here, they will besummarised only briefly:

Chilling. Primarily, the effect of low temperature is to reduce the growthrate of micro-organisms and this, in turn, delays spoilage and growth ofany pathogens. Deep chilling to temperatures around 0 C allows relativelylong storage times for many food products, including meat, fruit andvegetable products.

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Freezing. Freezing is an effective form of food preservation because itprevents the growth of both spoilage and pathogenic organisms, whilemany of the latter decrease in numbers. Several methods of freezing havebeen developed, including immersion in very cold liquids, such as liquidnitrogen.Reduction in water activity. This approach is based on reducing theavailability of water, which is essential for microbial growth. It may beachieved by drying or adding salt or sugar to the food. A desirable side-effect of using salt or sugar as a preservative is the pleasant flavour eachcompound confers on the final product. Drying is a natural means ofpreventing spoilage. Today, a variety of dehydrating techniques are used,including vacuum-drying, spray-drying and freeze-drying.Acidification. Between pH 4.5 and 4.2 almost all pathogenic micro-organisms stop growing. With the exception of some acid-tolerant bacteria,yeasts and moulds, the growth of spoilage organisms is also stronglyinhibited.Fermentation. Fermentation is a process in which microbial growth occursand a characteristic flavour may develop in the food. The product alsotends to show good keeping quality. In many products, it is the lactic acidproduced that causes the preservative effect. Fermentation is the key toproducing new products such as different kinds of cheese.Chemical preservatives. Most of the effective and widely-used preservativesare acids, for example, the weak lipophilic organic acids, such as sorbic,benzoic and propionic acids and their sodium salts. Inorganic preservativesinclude sulphite and nitrite. All are most effective at pH values < 5.5.Smoking is a variant of chemical preservation. Today, the smoking processhas become a sophisticated method of food preservation, with both hotand cold systems being used.Compartmentalisation. This technique is important in preserving water-in-oil emulsion products, such as margarines. When emulsions are preparedcarefully, the microbes that are present are confined to a few water dropletsthat are too small and usually contain insufficient nutrients to allowsignificant multiplication.Heating. Heating of food is an effective means of preservation, because itkills the great majority of harmful organisms. Both pasteurisation (killingvegetative cells) and sterilisation (killing both vegetative cells and spores)are applications of this principle.Vacuum and modified-atmosphere packaging. Vacuum packaging is basedon removal of oxygen, so microbes that are oxygen-dependent will notgrow. Removing oxygen from the pack also prevents oxidation of fattyacids and other oxidative reactions in the product. The introduction of agas with an inhibitory effect on bacteria, such as CO2, is an example ofmodified-atmosphere packaging.Physical technologies. These can be grouped in two main categories:thermal and non-thermal processes. Examples of thermal processes are

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microwave and ohmic heating. However, a major advantage of non-thermaltechnologies is that they cause less loss of flavour, nutrients and vitaminsduring processing (Barbosa-Cnovas and Gould, 2000). Available techniquesinclude the application of irradiation (both and UV) and hydrostaticpressure. Among other techniques being developed are the use of ultra-sound, cold plasma technology and pulsed electric fields. The last mentionedis now ready for a break-through in commercial application and is thesubject of this book.

1.3 Current developments and demandsIn recent years, the most obvious trend has been the ever-increasing demandfor high-quality foods with a marked degree of user convenience. Naturalfreshness and flavour are highly valued, particularly in foods that are ready-to-eat. It is obvious that the food manufacturer will try to meet consumerpreferences. These have helped to determine the need for more hygienicmeans of producing primary products and improved hygiene during processing.Foods should also be healthy from the nutritional viewpoint, which is ofteninterpreted as a need for lower levels of additives, such as salt and otherpreservatives (Gould, 1992). Such requirements lead to milder processingand preservation treatments, but food safety and shelf-life cannot becompromised. It is in this situation that new preservation strategies are urgentlyneeded (Rombouts et al., 2003).

Newer techniques of processing and storage that are already in use includechilling at very low temperature (02 C), modified-atmosphere and vacuumpackaging, use of edible films and coatings, active packaging, high-temperature/short-time treatment combined with aseptic packaging, ohmicheating, microwave heating, high hydrostatic-pressure pasteurisation andthe use of bacteriocins and antimicrobial enzymes (Gould, 1996). Many ofthese are aimed at specific products, while others can be used to preserve abroad range of items. Other techniques that are currently being exploredinclude use of high-intensity pulsed electric fields, intense light pulses andultrasonics (Hoover, 1997). More recently, cold plasma technology hasbeen considered for surface decontamination. Some of the above may findapplication eventually, most likely in combination with other techniques. Ina number of cases, not all the micro-organisms present are killed; some aremerely inhibited.

1.4 Current needsAs mentioned previously, the ever-increasing demand for high-qualityconvenience foods with both natural freshness and flavour is an important

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factor in the development of mild preservation techniques. Fruit, fruit juicesand vegetables are examples of the many different foods in this category.

Table 1.1 gives an overview of the pathogenic micro-organisms that arepotentially present on fruit and vegetables. Many pathogens, including bacteria,parasites and viruses, are involved in outbreaks of food-borne illness that areattributed to fresh fruit and vegetables, and an increasing number of outbreaksassociated with fresh produce is being reported (Sivapalasingam et al., 2004).

Following a report from the European Food Safety Authority (EFSA) onzoonoses and zoonotic agents in the European Union in 2004 (EFSA, 2006),it is evident that more people were involved in food-borne disease outbreaksfrom fruit and vegetables than any other food commodity (see Table 1.2).

It is clear that the newer consumer demands require different preservationstrategies to ensure that the foods in question remain safe and have an

Table 1.1 A general overview of the microbial hazards associated with fruits andvegetables (Geldreich and Bordner, 1971; Nichols et al., 1971; Nguyen-The andCarlin, 1994; Beuchat, 1995; Beuchat, 1998; Francis et al., 1999; Johannessen et al.,2002).

Bacteria SalmonellaShigellaEscherichia coli (pathogenic)CampylobacterListeria monocytogenesVibrio spp.

Parasites CryptosporidiumCyclosporaGiardiaEntamoebaToxoplasmaNematodesPlathhelminthes

Viruses Hepatitis ANorovirusRotavirusEnterovirus

Table 1.2 The contribution of different commodities in reported outbreaks of food-borne disease in the EU during 2004 (source: EFSA, 2006).

Commodity Proportion of affected individuals (%)Fruit and vegetables 25Egg and egg products 23Water 21Bakery products 11Poultry meat 10Pork 4

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acceptable shelf-life. The application of pulsed electric fields is one of thetechnologies that would be suitable, at least for some of the consumer productsconsidered here.

1.5 ReferencesBarbosa-Cnovas G V and Gould G W, (2000) Innovations in Food Processing, Technomic

Publishing Co., Lancaster, PA, USA.Beuchat C R, (1995) Pathogenic microorganisms associated with fresh produce. J.

Food Protection, 59, 204216.Beuchat L R, (1998) Surface Decontamination of Fruits and Vegetables Eaten Raw: A

review. Food Safety Unit, World Health Organisation, WHO/FSF/FOS/98.2.Chung K-T, Stevens S F and Feris D H, (1995), A chronology of events and pioneers of

microbiology SIM News, 1995, 45, 313.Dobell C (1960), Antony van Leeuwenhoek and his Little Animals, Dover Publications,

New York, USA.EFSA (2006), EFSAs First Community Summary Report on Trends and Sources of

Zoonoses, Zoonotic Agents and Antimicrobial resistance in the European Union in2004. http://www.efsa.eu.int/science/monitoring_zoonoses/reports/1277_en.html

Francis G A, Thomad C and OBeirne T, (1999) The microbiological safety of minimallyprocessed vegetables Int. J. Food Sci. Technol., 34, 122.

Geldreich E E and Bordner R H, (1971) Faecal contamination of fruits and vegetablesduring cultivation and processing for market. A review. J. Milk Food Technol., 34,184195.

Gould G W, (1996), Methods for preservation, Int. J. Food Microbiol., 33, 5164.Gould G W, (1992), Ecosystem approaches to food preservation, J. Applied Microbiol.,

Symposium Supplement, 73, 58S68S.Hartman P A, (1997), The evolution of food microbiology, in Doyle M P, Beuchat L R

and Montville T J, (eds.), Food Microbiology: Fundamentals and Frontiers, ASMPress, Washington, 313.

Hoover D G, (1997), Minimally processed fruits and vegetables: reducing microbialload by nonthermal physical treatment, Food Technology, 51, 6671.

Huis in t Veld J H J, (1996), Microbial and biochemical spoilage of foods: An overview,Int. J. Food Microbiol., 33, 118.

Jejuneja V K and Sofos J N, (2001), Control of Foodborne Microorganisms, MarcelDekker, New York, USA.

Johannessen G S, Loncaevic S and Kruse H, (2002), Bacteriological analysis of freshproduce in Norway, Int. J Food Microbiol., 77, 199204.

Mead P S, Slutsker L, Dietz V, McCaigh L F, Bresee J S, Shapiro C, Griffin P M andTauxe R V, (1999), Food-related Illness and Death in the United States, EmergingInfectious Diseases. Volume 5, No. 5. http://www.cdc.gov/ncidod/eid/vol5no5/mead.htm

Nguyen-The C and Carlin F, (1994), The microbiology of minimally processed freshfruits and vegetables, Crit. Rev. Food Sci. Nutr. 34, 371401.

Nichols A A, Davies P A and King K P, (1971), Contamination of lettuce irrigated withsewage effluent, J. Hort. Sci., 46, 425433.

Rombouts F M, Notermans S H W and Abee T, (2003), Food Preservation FutureProspects, in Russell N J and Gould G W (eds), Food Preservatives, 2nd ed., KluwerAcademic, New York, 348370.

Russell N J and Gould G W (2003), Food Preservatives, 2nd ed., Kluwer Academic, NewYork, 348370.

Sivapalasingam S, Friedman C R, Cohen L and Tauxe R V, (2004), Fresh produce: A

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growing cause of outbreaks of foodborne illness in the United States, 1973 through1997, J. Food Prot., 67, 23422353.

Tannahill R (1973), Food in History, Stein and Day, New York, USA.Toussaint-Samat M (1992), History of Food, Blackwell, Cambridge, UK.Wendorf F R, Schild R, Hadidi N, Close A E, Kobusiewics H, Wieckowska H, Issawi B

and Haas H, (1979) Use of barley in the Egyptian late paleolithic, Science, 205,13411348.

Food_Preservation_by_Pulsed_Electric_Fields_/1845690583/files/00029___968334c6e53c35423dc29ee33da7aa66.pdf2.1 IntroductionThough often regarded as a novel technique, the concept of applying electricalcurrent for food treatment dates back to the end of the 19th century. Since the1960s, applications of pulsed electric fields (PEFs) have been reported.Pioneering work was carried out by the German engineer Heinz Doevenspeck,and this was followed by the research work of Sale and Hamilton in theUnited Kingdom. In the 1980s, Krupp Maschinentechnik, Germany, developedtwo processes, ELCRACK and ELSTERIL, based on Doevenspecks work.Technical scale prototypes as well as four industrial ELCRACK plantshave been realised for the treatment of fish slurry. These prototype projects(which implemented not only PEFs, but also other ambitious processingtechniques such as ultrafiltration) failed and the equipment was dismantled.Subsequently the technique found its way back to research labs and universities,where the mechanisms of the action of PEFs and the key requirements forPEF processes have been investigated, mainly in batch and lab-scale continuousequipment. Promising applications have been identified, such as the pre-treatment of fruit and vegetable tissue prior to extraction, drying or juicewinning, enhancement of mass transport rates in fish and meat tissue, microbialdecontamination of liquid foods, and treatment of waste and processingwater. Since 1995 several attempts have been made to industrialize a PEFapplication once again, and commercialisation of a PEF pasteurization treatmentwas achieved in 2005. At present, techniques to employ PEF as a pre-treatmentin fruit and vegetable processing and meat processing are in the process ofbeing commercialised. In the following sections, the evolution of the techniquewill be presented, along with an overview of possible applications and adiscussion of costs of operation and technical feasibility.

2

History of pulsed electric field treatmentS. Toepfl, V. Heinz, Deutsches Institut fr Lebensmitteltechnik(DIL) e.V., Germany, and D. Knorr, Berlin University ofTechnology, Germany

9

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2.2 The evolution of PEF techniques2.2.1 First applications of electrical current for food treatmentInvestigations into the effects of electric current on biological cells beganshortly after electricity became commercially available and have been carriedout ever since. At the end of the 19th century, the bactericidal effects ofdirect and alternating electrical current were investigated for the first time byProchownick and Spaeth (1890). A direct current of 300 mA was found notto inactivate Staphylococcus aureus in suspension (Prochownick and Spaeth,1890), but it was noticed that there were variations in acidity in the treatedmedia at different points in the treatment chamber. When microbes wereattached to agar gel to investigate the impact of an electrically generateddrop in pH on their viability, it was found that samples taken from the anodewere sterile, in contrast to samples taken from the cathode. In the 1920s, aprocess called Electropure, one of the first attempts to use electricity formilk pasteurization, was introduced in Europe and the USA (Beattie andLewis, 1925; Fetterman, 1928; Moses, 1938). The process involved theapplication of a 2204200 V alternating current within a carbon electrodetreatment chamber. Electropure was fundamentally a thermal method, usingelectric energy to directly heat milk. The electrical chamber consisted of arectangular tube and opposing carbon electrodes. The milk was preheated to52 C and subsequently electrically heated to 71 C and held at this temperaturefor 15 s. About 50 plants were in operation until the 1950s, serving about50 000 consumers. At this time, there were only a few studies reporting onmicrobial inactivation below thermal death points during this process (Beattieand Lewis 1925). The technique was accepted as a safe pasteurization stepin six states in the US. The units were mainly provided by Trumbell ElectricManufacturing Co (Getchell, 1935; Edebo and Selin, 1968). Due to risingenergy costs and competition with mild, novel thermal preservationtechnologies such as UHT, these plants were replaced (Reitler, 1990). It wasnot until the 1980s, when interest in ohmic heating was revived, that somefurther industrial applications of this technology were achieved, includingpasteurization of liquid eggs and processing of fruit products. Recently,ohmic heating (also termed moderate electric field treatment) has also receivedattention as a method of pre-treatment prior to drying, extraction and expressionor reduction in water use during blanching (Reznick, 1996; Cousin, 2003;Sensoy and Sastry, 2004; Lebovka et al., 2005; Praporscic et al., 2006a). Inaddition to thermal effects, based on the mechanism of ohmic (joule) heating,occasional lethal effects when subjecting food to low voltage alternatingcurrents, such as the hydrolysis of chlorine, were reported by Pareilleux andSicard (1970). Tracy (1932) reported the deadly effect of low voltage alternatingcurrent on yeast cells, at a minimum lethal temperature of 46 C. The formationof free chlorine or other toxic substances was responsible for the deadlyeffect. Inhibition of cell division of Escherichia coli was first described in1965 by Rosenberg et al. (1965). Further information on the impact of electricity

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on cells and the possibilities of cell electromanipulation can be found inPalaniappan et al. (1990), Chang et al. (1992), and Zimmermann and Neil(1996). In 1949, Flaumenbaum reported the application of direct and alternatingcurrent for electroplasmolysis of fruit and vegetable tissue (Flaumenbaum,1949). An increase in juice yield of up to 10 % was found.

2.2.2 Electrohydraulic treatmentPulsed discharge application of high voltage electricity across two electrodesfor microbial inactivation has been under investigation since the 1950s(Fruengel, 1960; Allen and Soike, 1966; Edebo and Selin, 1968), resulting ina process called electrohydraulic treatment. During this process, the electrodesare submerged in a liquid medium within a pressure vessel and electric arcsare generated by high voltage pulses forming transient pressure shock wavesup to 250 MPa and ultraviolet light pulses. The method is capable of up to95% inactivation of E. coli, Streptococcus faecalis, Bacillus subtilis,Streptococcus cremoris and Micrococcus radiodurans suspendend in steriledistilled water (Gilliland and Speck, 1967a). In this study, the electrode gapwas between 1.6 to 6.4 mm and the peak voltage was 15 kV. Allen and Soikereported that electrohydraulic treatment was most effective using a capacitanceof 6 F and a voltage of 5 kV (Allen and Soike, 1967). It was concluded thatelectrohydraulic treatment is a quick, effective and inexpensive non-thermalmethod for sterilization of water and sewage. With electrochemical reactions,shock waves and ultraviolet light forming freely, it was claimed that highlyreactive radicals were responsible for the bactericidal effect. Operating withcopper core electrodes resulted in a certain amount of residual toxicity in thetreatment media, but this effect was not found when iron or aluminiumelectrodes were used. When a double chamber system, separated by adiaphragm, was employed, it was revealed that mechanical action alone wasnot responsible for microbial inactivation (Gilliland and Speck, 1967b). Edeboand Selin (1968) investigated the impact of plasma photon emission, towhich they attributed microbial inactivation. Varying the electrode material,a higher efficiency was reported for copper than for iron, steel or aluminiumelectrodes. Though these early studies gave promising results, the technologywas never developed to a point where an application in food technology wasachieved. Disintegration of food particles and electrodes, causing foodcontamination, appear to have prohibited industrial applications of this processother than in wastewater treatment (Jeyamkondan et al., 1999).

2.2.3 First application of Pulsed Electric Fields the pioneeringwork of Heinz DoevenspeckThe secondary effects of electrochemical reactions and hydraulic pressureare less relevant when short, homogeneous pulses without arcing are applied.The first application of pulsed currents of high voltage were reported by

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Gossling (1960), with the goal of inducing artificial mutation. He reported apartial microbial kill, dependent on treatment intensity, for Streptococcuslactis, and recultivated the survivors to find mutations. He suggested a batchas well as a continuous treatment chamber in small scale. The German engineerHeinz Doevenspeck reported pioneering experiments into the application ofpulsed electric fields in food processing, resulting in a patent in 1960(Doevenspeck, 1960) describing the application of pulsed electric fields fordisruption of cells in food material to improve phase separation (Doevenspeck,1961). From 1961 to 1971 (Doevenspeck, 1975) he investigated the changeof pH in a solution subjected to pulsed electric fields, reporting a colourchange of neutral to red at the electrode surfaces. The pH at the anode wasmeasured as 6.8, whereas at the cathode it was increased to a value of 8.After mixing, this change was shown to be reversible and the initial pH of7.2 was restored. When Lactobacillus delbrckii in beer stained with methyleneblue was treated, colour was taken up, indicating cell permeabilization. Growthof microbes and spoilage of beer samples was prevented after treatment withpulses of 6 kV using a 2.5 F capacitor. Subjecting cells of E. coli to pulsedelectric fields, it was found that the application of electric fields with lowfield strength (soft pulses, below 200 V/mm) led to enhanced growth, whereasincreasing electric field strength (hard pulses) resulted in cell death. Treatmentof fish tissue revealed an improvement in separation of the solid and liquidphases. PEF-treated fish slurry was found to be 100 % digestible, comparedto a value of 97 % for conventionally available fish meal. No detrimentaleffect was found when treating concentrates of vitamin A, B1, 2, 6, 12 andfolic acid. The potential to enhance the production of biogas was investigatedat the waste water treatment plant in Nienburg and a 20 % increase wasreported (Doevenspeck, 1963). A picture of Doevenspeck and his pulsegenerator at the facilities of Krupp in the 1980s is shown in Fig. 2.1.

A typical unit for PEF treatment of food consists of a high voltage pulsegenerator and a treatment chamber in which the media are exposed to theelectrical field. In Doevenspecks patent of 1960 (Doevenspeck, 1960), thesetup of a pulse modulator as well as a continuously operated treatmentchamber was described. To summarise, the pulsed power is generated byrepetitive discharge of energy stored in a capacitor bank across a high voltageswitch, and mercury switch tubes were suggested. As shown in Fig. 2.2,Doevenspeck proposed different treatment chamber geometries. A centrifugecoated with carbon, containing a carbon coated sieve as well as a mixingtank with a carbon coated agitator were outlined for batch treatment. Forcontinuous treatment, it was suggested that the product could be conveyedby a screw press through cylindrical electrodes in a coaxial setup. Applicationexamples presented in the patent range from waste and tap water treatmentto the cleaning of gases, as well as extraction from animal tissue. As alreadydescribed, another effect reported was the inactivation of pathogenicmicroorganisms. A 96 % inactivation of microbes suspended in marinationbrine, as well as inactivation of Salmonella in egg powder suspensions, was

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Fig. 2.1 Heinz Doevenspeck and his pulsed power generator at the facilities ofKrupp Maschinentechnik in the 1980s (Sitzmann, 2006).

(d)

(c)

(b)(a)

Fig. 2.2 Treatment chamber geometries suggested by Doevenspeck in 1960:(a) Rotating carbon-coated sieve electrode; (b) carbon-coated mixing electrode;(c) and (d) screw press with co-axial treatment chamber of carbon electrodes

(Doevenspeck, 1960).

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described (Doevenspeck, 1961). An industrial scale plant with a capacity ofup to 2500 kg/h was erected for processing of beef and pork material as wellas fish waste material as early as 1961 in a fat smeltery in Germany. On hisquest for possible applications of the technique, Doevenspeck, active as aconsulting engineer, came into contact with Mnch, Technical Director ofanimal material processing at Krupp Maschinentechnik in 1985, whorecognized the techniques potential. Following restrictions in the use ofperchloroethylene for fat extraction, Krupp Maschinentechnik was seekingalternative processing techniques to induce cell disintegration and to improvephase separation of fish slurry in a screw press (Sitzmann, 2006). Guided byDoevenspeck, a work group consisting of Mnch, Sitzmann and othercoworkers developed the processes ELCRACK and ELSTERIL (Sitzmannand Mnch, 1988b, 1989).

2.2.4 Early PEF applications in the UK and UkraineFollowing the application of direct and alternating current for electroplasmolysisof apple mash in the 1940s (Flaumenbaum, 1949), in 1965 an industrialprototype of 6 to 15 t/h capacity was erected in a canning factory in theSoviet Republic of Moldawia (Flaumenbaum, 1968). An increase in juiceyield of 1012% was found and the products were described to be lighter incolour and less oxidized than after a heat or enzymatic pre-treatment (McLellanet al., 1991). At this time, the application of pulsed direct current was alsoreported, but no further details about pulse parameters or results regardingapplications were mentioned. The first systematic studies in the UK toinvestigate the non-thermal lethal effect of homogeneous pulsed electricfields on microbes were conducted at Unilever Research, Colworth House,Bedford by Sale and Hamilton (1967). To investigate the effect of pulsedelectric fields, a pulse generator connected to a batch treatment chamber wasdeveloped. The carbon electrodes were separated by a polythene spacer, anda U-shaped sample container was obtained. The pulse voltage was adjustableup to 10 kV with a pulse length of 2 to 20 s. The maximum electric fieldstrength was limited to 3 kV/mm by the electrical breakdown of air abovethe sample (Sale and Hamilton, 1967). They demonstrated that electric fieldstrength, total treatment time, and the product of pulse width and numberwere the most important factors affecting microbial inactivation. By treatingmicroorganisms in a gel impermeable for electrolytic products, they showedthe insignificance of electrolysis on the lethal effect of direct current (DC)pulses. Damage to the cell membrane, causing an irreversible loss of itsfunction as a semipermeable barrier between the cell and its environment,was proposed as the cause of cell death. After treatment, leakage of ions, lossof cytoplasmic content as well as changes in membrane morphology and celllysis were reported (Sale and Hamilton, 1968). The deadly effect of PEF wasstated to be independent of current density; thus it was concluded thatinactivation was due to non-thermal effects. Electric field intensity was

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identified as one of the most important factors, with a critical field strengthof 1.0 to 1.5 kV/mm for most microorganisms.

2.2.5 Fundamental effects and mechanisms ofelectropermeabilizationFollowing the first empirical descriptions of Gossling (1960) and Doevenspeck(1960; 1961) in the 1980s interest in the use of electroporation in medicalscience and genetic engineering greatly increased. Whereas studies of thedisruptive effect of electric fields on biological cells were previously mainlyempirical, the fundamental effects and mechanisms of electropermeabilizationwere becoming the focus. The dielectric rupture theory was introduced inparallel by Zimmermann et al. (1974) and Neumann and Rosenheck (1972).Two effects thought to be triggered by the electric field had been described;the ionic punch-through effect (Coster, 1965) and the dielectrical breakdownof the membrane (Zimmermann, et al., 1973). Cells were subjected to electricfield pulses of s duration and an increase in membrane conductivity wasobserved when a transmembrane potential of approximately 1 V was induced.The phenomenon was termed dielectric breakdown, borrowing the expressionfrom solid state physics (Zimmermann et al., 1976). An electro-mechanicalmodel was developed, implying that the presence of an electrical field acrossthe membrane resulted in a mechanical compression. The cell membranewas considered to act as a capacitor containing a perfectly elastic dielectric.Electric compressive forces were described as being balanced by the restoringmechanical force. If the compression was increased by increasing thetransmembrane potential, mechanical instability could occur. Crowley (1973)reported a good agreement between predicted breakdown volume and assumedelastic parameters for a model system of phosphatidylcholine bimolecularlipid layers. Electrical breakdown has been shown for algal cells as well asfor bacteria and human red blood cells, measuring size distribution with aCoulter counter. The conclusion was that the most important applicationwould probably be to load cells with substances for which the cell membraneis normally impermeable (Zimmermann et al., 1976). To the present day,electro-mechanical instability is still used to explain the effect of externalelectrical fields on biological cells and this is one of the most acceptedtheories.

Other theories to explain the increase in permeability of biological cellscaused by external electrical fields include the occurrence of membranedeterioration and reorientations in the lipid bilayer and the protein channels.Dipolar reorientation of phospholipids and transition from hydrophobic tohydrophilic pores has been described by Tsong (1991), assuming a change inmembrane structure by Joule heating within a conductive pore. In addition,the presence of protein channels, pores and pumps has been considered, inparticular as their functionality is influenced by the transmembrane potential.The gating potential for protein channels is in the 50 mV range, considerably

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smaller than the dielectric strength of a phospholipid bilayer. However, thoughopening of protein channels is induced, this may not be sufficient to preventthe development of a transmembrane potential above the breakdown potentialof the lipid bilayer. Based on experiments with model systems such as liposomesor protoplasts, large eukaryotic cells and microbes, several theories havebeen developed or proposed to explain the underlying mechanism of poreformation and resealing (Neumann and Rosenheck, 1972; Zimmermannet al., 1974; Sugar and Neumann, 1984; Weaver and Powell, 1989; Changet al., 1992; Ho and Mittal, 1996; Kinosita and Tsong, 1997; Neumann et al.,1998; Weaver, 2000). Pore formation might also occur as a consequence ofstructure defects within the cell membrane, expanding spontaneously formedpores in the presence of an electric field (Tsong, 1991).

An unprecedented amount of research work has been published in thefield of genetical and bioengineering (Prasanna and Panda, 1997; Pavlinet al., 2002; Valic et al., 2003; Puc et al., 2004) and a comprehensive reviewof knowledge (or the lack of it) regarding cell permeabilization mechanismshas been published by Teissie et al. (2005). Pulse generators as well as batchtreatment cuvettes in micro-litre scale are commercially available fromEppendorf, Bio-Rad, BTX and Genetronics (Puc et al., 2004). Recently, areview has been published comparing the microfluidic devices available forelectroporation (Fox et al., 2006). Even if underlying mechanisms of actionare the same and microfluidic units are very helpful for mechanism elucidation,in contrast to food application, treatment intensity is much lower and in mostcases a very small volume is treated. A microfluidic cell for pasteurizationuse has been presented by Fox et al. (2005). This has a 50 m wide channelwith a 10 m deep, 30 m long constriction for electric field focussing.

2.2.6 Development of technical and industrial scale equipment atKrupp MaschinentechnikBased on Doevenspecks work at Krupp Maschinentechnik, a technical scaleunit with a capacity of up to 200 kg/h for treatment of meat or fish slurry,sugar beet, palm fruit, oil seeds and fruit mashes was developed in the1980s. This is shown in Fig. 2.3, along with the treatment chamber and apicture of Werner Sitzmann. In cooperation with the University of AppliedSciences, Hamburg, several diploma theses were finished, investigating thedisintegration of oil seeds, the degree of cell permeabilization, and the potentialto optimize treatment chamber geometry and process control. After performanceof very promising technical scale tests, further industrial equipment wasproduced by Krupp to be installed in a fish factory in Norway. The processconsisted of: the ELCRACK system, subsequent separation of free liquid,a screw press for further separation, a decanter centrifuge and separators toseparate the fluid into water and oil phases and finally protein removal fromthe water by ultrafiltration. In 1988 a brochure was released by Krupp (1988)to describe the technology and the application of ELCRACK in fish

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processing. Pictures of the equipment taken from this brochure can be foundin Fig. 2.4. After installation of the first equipment, many problems aroseconcerning the electrode stability, subsequent liquidsolid separation, andprotein recovery after the treatment. The installation was dismantled after afew months of operation. From todays point of view, it can be seen that thefailure of this first installation was not solely related to the ELCRACKtechnique, which was only a small part of the total installation. Because toosophisticated a separation technology had been designed and implementedand the whole fish processing unit realised without prior experience, theequipment had to be taken back by Krupp (Sitzmann, 2006). Two further setsof industrial scale equipment were designed, but, after the experience obtainedwith the unit installed in Norway, they were never installed at their destination.After 1986, an ELSTERIL pilot plant was developed, consisting of a high-

(a)

(b) (c)

Fig. 2.3 (a) Pilot-scale PEF system at Krupp Maschinentechnik; (b) Werner Sitzmann(standing), diploma student Volker Stemper (front); (c) ELCRACK treatment

chamber.

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voltage pulse generator with a peak voltage of 15 kV and a repetition rate of22 Hz. The storage capacity varied between 0.5 and 5 F and an ignitron wasused to discharge the electrical energy stored (Grahl, 1994). Five differentbatch- as well as continuous-treatment chambers were developed, equippedwith two parallel plate carbon electrodes. The electrode gap was 5 or 12 mmand a flow rate of 165 L/h was used (Grahl, 1994). In cooperation withFMC Europe in 1990, orange juice was processed in this pilot plant and nodetrimental effects on juice quality were found. A picture of the generator, abatch, and a continuous treatment chamber can be found in Fig. 2.5. Afterthe failure with the first unit, financial support by Krupp was reducedsubstantially and the work group was going to be closed when Krupp andHoesch merged. Through their efforts to publish and to commercialize thetechnology during and after their activities at Krupp Maschinentechnik,Sitzmann and Mnch were able to interest international research groups inthis technique (Sitzmann, 1987; Sitzmann and Mnch, 1987, 1988a, 1989).Sitzmann continued activity in the field of PEF applications subsequentlyrunning his own businesses, DWS and Nafutec GmbH, (Anonymous, 1995;Sitzmann, 1995).

In 1993, the development of a novel electroshock technique in the UnitedStates was reported (Nldechem, 1993), utilizing prior developments at Krupp.The ELSTERIL unit on which it was obviously based was placed at BerlinUniversity of Technology, used for experiments concerning the extractability

a

b

dc

e f g h

Fig. 2.4 Fish processing by ELCRACK pictures of industrial equipmentinstalled in Norway from a Krupp Maschinentechnik brochure (Krupp, 1988):

(a) switch boxes; (b) control unit; (c) press outlet; (d) screw presses; (e) HV-switch;(f) capacitor bank; (g); screw press, dismantled; (h) ultra filtration unit.

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of carrot tissue (Geulen et al., 1992) and the enhancement of potato drying(Angersbach and Knorr, 1997).

Mertens and Knorr (1992), as well as Knorr et al. (1994), reviewed potentialapplications of PEF for food processing. During this time, interest in PEFapplications increased at a research level, and numerous work groups inuniversities and commercial activities were the result.

2.3 Research work on PEF applications from 1980s to 2004The basic mechanisms and effects of pulsed electric fields on biologicalcells were explored empirically in the 1960s, but it was only later that thetechnique received attention at a university level. Research diverged intotwo areas: the reversible and the irreversible effects of electroporation. Neumannand Rosenheck, as well as Zimmermann, investigated the potential to achievereversible permeabilization on a cellular level for applications in bioengineering.Other groups focused on irreversible, lethal effects for preservation purposes.In the 1980s the research group led by Hlsheger developed mathematicalmodels to describe microbial inactivation by PEF, including electric fieldstrength and treatment time (Hlsheger and Niemann, 1980; Hlshegeret al., 1983). A 10 kV pulse generator was developed, discharging a 1 Fcapacity across a spark gap with a pulse repetition rate of 1 pulse in 5 s. Thetreatment chamber consisted of a cylindrical glass tube closed by two roundbrass electrodes with a gold coating. Maximum field was limited to 2 kV/mm,

Fig. 2.5 ELSTERIL prototype equipment (Grahl, 1994). The equipment consists ofa power supply (back right), a capacitor box (back left) including an ignitron and a

treatment chamber.

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as an electrode gap of 5 mm and an electrode area of 800 mm2 was used. Inthe former German Democratic Republic, Jacob et al. investigated the microbialimplications of electric field effects (Jacob et al., 1981) at a maximum fieldof 3.5 kV/mm. In the United States, Dunn, Hofmann and Bushnell had beeninvestigating PEF applications since 1982. In 1987, Dunn and Pearlmanfiled a patent assigned to Maxwell Laboratories, describing an apparatus forextending the shelf-life of fluid food products. Batch as well as continuoustreatment chambers were proposed, followed later by a patent (Bushnell etal., 1996). Since 1988, the ELSTERIL process has been researched at theTechnical University Hamburg-Harburg (Grahl, 1994), investigating the impactof treatment intensities on a variety of microbes and comparing the effects ofbatchwise and continuous operation.

In Japan, in 1980, Sakarauchi and Kondo reported lethal effects of highelectric fields on microorganisms, using a disk-shape parallel plate treatmentchamber with platinum electrodes. A 2 kV pulse generator was used. In 1988,Mizuno and Hori (1988) reported the destruction of living cells by highvoltage, using parallel plate as well as needle plate, wire-cylinder and rodrod shaped electrodes. A rotary spark gap system was used for pulse generation,operating at 20 kV peak voltage and a repetition rate of 25 Hz. Investigatingthe efficiency of different treatment chamber geometries, the maximuminactivation was found when using rodrod shaped electrodes and producingan arc discharge. In 1996, the Japanese Research and Development Associationfor Application of Electronic Technology in the Food Industry was founded,reporting activities in the field of PEF food preservation as well as meatprocessing (Anonymous, 1998). The work was conducted by Mitsubishi andthe Toyohashi University of Technology. Since 1992, Berlin University ofTechnology (Knorr) and Washington State University (Barbosa-Cnovas andSwanson) performed research work in this field, followed by Ohio StateUniversity (Zhang) in 1994. First reviews of the technology were publishedin the 1990s (Palaniappan et al., 1990; Tsong, 1990; Mertens and Knorr,1992; Knorr et al., 1994; Ho and Mittal, 1996; Jeyamkondan et al., 1999) andthe first PEF book was published in 1999 (Barbosa-Cnovas et al., 1999).

In 1995, Pure Pulse Technologies, a subsidiary of Maxwell Laboratories,developed a continuous processing system called CoolPure for treatment ofup to 2000 L/hr. For research use, a pilot system called CoolPure Jr. wasavailable, to be operated at a flow rate of 6 to 10 L/h and at a maximum fieldstrength of 5 kV/mm. A Pure Pulse brochure described the two non-thermaltechnologies PureBright (Pulsed Light) and CoolPure (PEF); a picture ofthe CP2 unit is shown in Fig. 2.6. In the same year, a letter of no objectionwas released by the FDA for the use of PEF technology for food preservation,and in 1996 the treatment of liquid egg was approved, though with certainconditions. Schoenbach et al. investigated the impact of PEF on aquaticnuisance species, such as zebra mussels, hydrozoans or barnacles (Schoenbachet al., 1996). After treatment of tidal water at a field strength of 1.2 kV/mm,biofouling was prevented, indicating that a PEF treatment can be utilized to

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protect lake or river water operated cooling systems from clogging due tobiofouling. Abou-Ghazala and Schoenbach (2000) showed that even attreatment intensities as low as 100 V/mm and energy inputs in the range of16 kJ/kg, 90% of barnacles could be inactivated. A 100% protection againstfouling was obtained after a treatment at 600 V/mm and an energy input of560 kJ/kg. Energy efficiency was found to be increased when reducing thepulse width of the rectangular pulses from 10 to 0.5 s.

In the United States in 1997, a collaboration between OSU, WSU, EPRIand ARMY was initiated (Mermelstein, 1998), and a series of PEF workshopswas held from 1997 to 1998. Protocols were developed for microbial challengetests and a laboratory PEF system designed. Subsequently, in a

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