10300376_morris

PACKAGING TECHNOLOGY AND SCIENCEPackag. Technol. Sci. 2007; 20: 275–286Published online 9 July 2007 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/pts.789

Non-Thermal Food Processing/PreservationTechnologies: A Review with PackagingImplications#

By Caroline Morris, Aaron L. Brody* and Louise WickerDepartment of Food Science and Technology, University of Georgia, Athens, GA, USA

Non-thermal food processing/preservation methods interest food and foodpackaging scientists, manufacturers and consumers because they exert a minimalimpact on the nutritional and sensory properties of foods, and extend shelf life byinhibiting or killing microorganisms. They are also considered to be more energyefficient and to preserve better quality attributes than conventional thermallybased processes. Non-thermal processes also meet industry needs by offering value-added products, new market opportunities and added safety margins.

This study reviewed non-thermal processing technologies currently available ordevelopmental for the inactivation of microorganisms and thus microbiologicalshelf life in foods, and to identify packaging interactions that might result.Processes include ultra-high pressure, ionizing radiation, pulsed X-ray, ultrasound,pulsed light and pulsed electric fields, high-voltage arc discharge, magnetic fields,dense phase carbon dioxide and hurdle technologies. Copyright © 2007 John Wiley& Sons, Ltd.Received 14 January 2007; Revised 29 May 2007; Accepted 29 May 2007

KEY WORDS: packaging, high pressure processing, non-thermal processes

* Correspondence to: A. L. Brody, Packaging/Brody, Inc., PO Box 956187, Duluth, GA 30095, USA.E-mail: [email protected]# This paper was originally submitted by author Morris as part of her requirements for the degree of Master of Food Technology.

Copyright © 2007 John Wiley & Sons, Ltd.

INTRODUCTION

Thermal inputs for conversion and/or preserva-tion of food dominate the food processing indus-try. Thermal treatments generally, however, causeundesirable changes in food flavour, colour,texture and nutritional attributes such as proteinand vitamin destruction.

On the other hand, non-thermal processingmethods interest food scientists, manufacturersand consumers because they have a minimalimpact on the nutritional and sensory properties offoods, and extend shelf life by inhibiting or killingmicroorganisms. ‘Non-thermal food preservationprocesses are considered to be more energy effi-

cient and to preserve better quality attributes thanconventional processes’.1 Non-thermal processesalso meet industry needs by offering value-addedproducts, new market opportunities and addedsafety margins.

This paper summarizes non-thermal processingtechnologies currently available or developmentalfor the inactivation of microorganisms and thusmicrobiological shelf life in foods, and to identifypackaging interactions that might result. Processesinclude ultra-high pressure, ionizing radiationincluding pulsed X-ray, ultrasound, pulsed lightand pulsed electric fields, high-voltage arc discharge, magnetic fields, dense phase carbondioxide and hurdle technologies.

ULTRA-HIGH PRESSUREPROCESSING (HPP)

HPP, high hydrostatic pressure and ultra-highpressure are all names for the same process. HPPis a cold pasteurization method employed for shelflife extension and pathogen reduction whileretaining the food’s inherent colour, flavour, nutri-ents and texture. HPP can inactivate vegetativemicrobial cells by breaking non-covalent bondsand causing damage to the cell membrane. ‘Highpressure disrupts secondary and tertiary struc-tures of macro-molecules, such as proteins andpolysaccharides, and alters their structural andfunctional integrity in a pressure-dependent way’.2

The process causes non-covalent bonds to break,interrupting cellular function and inactivating thepathogenic bacterial cells. The product can be solidor liquid food, packaged or not, to which 100–1000mega pascals (MPa) of pressure is applied at tem-peratures ranging from 0–110°C, from a milli-second pulse to over 20min, in batch orsemi-continuous systems. The heat generatedduring compression adiabatically raises the tem-perature of a product 3°C for every 100MPa, achange reversed during decompression. Criticalprocess factors for HPP are listed in Table 1.

HPP often yields better results when combinedwith thermal processes. A temperature range of45–50°C is optimal for the inactivation of foodpathogens and spoilage microorganisms. Clostrid-ium botulinum cells, but not their spores, can bedestroyed at 90–110°C and at 500–700MPa. ‘Devel-opments are underway to increase the pressure to800MPa and combine with low or high tempera-ture to possibly achieve ambient temperaturemicrobiological shelf stability’.3

HPP applies very high pressures (up to 6000times atmospheric pressure) to packages or bulkliquid, or to solid foods, in a hydrostatic press.Baskets containing packaged foods are introducedinto steel vessels filled with water, and high pres-sure is applied (through a piston) to the incom-pressible water to be transferred to the food –uniformly from all surfaces.

The effect of high pressure on dairy proteinsincludes size reduction of micelles, denaturation ofwhey protein, increased solubility of calcium andcolour change. HPP has been found to accelerate

the ripening of some cheeses and eliminate Listeriamonocytogenes in soft cheeses. HPP has been testedwith fruit yogurts to extend shelf life. Severalresearchers have found that moderate pressuretreatment can immediately change the microstruc-ture in fresh curd to that of aged Cheddar and usedfor shreds. Expediting of the cheese-makingprocess by approximately 30 days, eliminatingsteps in the process and decreasing the amount ofrefrigerated storage space are important potentialbenefits.

Other studies on the effects of HPP on liquidwhole eggs and tomato juice thus far have demon-strated 3–5 log reductions in microbiologicalcounts.

The most widely used commercial applicationsfor HPP are in refrigerated guacamole, refrigeratedsalsas, chilled entrees, delicatessen meats andsmoothies, with all but the last in-package pro-cessing. HPP can also be used for non-thermal pro-cessing of avocado halves, applesauce, cured hamand chopped onions. It is employed in the seafoodindustry to pasteurize oysters while maintaining araw designation, as well as to relax the shell foreasier shucking, and to help separate lobster meatfrom its shell. HPP can be used as a post-packag-ing lethality step for the inactivation of L. monocy-togenes on in ready-to-eat meats such as sliced hamand deli meat.

HPP cannot yet successfully deliver low acidfoods that are ambient temperature shelf stable.4

Since a major portion of foods is high acid and socan be stabilized by HPP, and a large fraction is dis-tributed under refrigeration, foods processed by

C. MORRIS, A. L. BRODY AND L. WICKERPackaging Technologyand Science

Copyright © 2007 John Wiley & Sons, Ltd. 276 Packag. Technol. Sci. 2007; 20: 275–286DOI: 10.1002/pts

Table 1. Critical process factors in HPP

• Time to achieve pressure• Time under pressure• Time of decompression• Product temperature• Temperature during process• Vessel temperature• pH• Product composition and water activity• Package integrity• Pre- and post-processing factors

(Anonymous, FDA, Executive, 2000.16)

HPP could become part of the growing proportionof extended shelf life chilled foods.

HPP can cause a shift in the pH of foods andaffects enzymatic reactions by ‘. . . altering thekinetic constants or by producing conformationalchanges in the structure of the enzymes and/orsubstrates’.5

Flexible packages of foods may be subjected tohigh pressure to achieve egg white gelation,increase cheese yield and to extend the shelf livesof milk, juice, jams and jellies.

With HPP, gelatinization of starch occurs even atlow temperatures. The starch maintains its granu-lar character, has limited swelling, with littleamylase release. They also display lower gelstrength and viscosity. These attributes portend apromising future with new and innovative prod-ucts such as flan.

HPP can be used in a continuous process forliquids and can process foods in their packaging.The liquid being processed is pumped through aventure tube to increase the pressure exerted on itand exits to be aseptically packaged. Freezingquality is improved because of smaller ice crystalformation.6

HPP is reported to lead to disruption of flexiblelaminations by virtue of fluctuations of pressuredue to increasing and decreasing during initiationand ending of HPP operations. Blistering, espe-cially of aluminum foil laminations, alters thebarrier properties of such package structures andreduces their effectiveness for protection. Sincemuch HPP is performed with product in sealedpackages, this deficiency will have to be addressedby packaging engineers before universal applica-tion of the process is possible. Already, some lam-inated package structures that do not display thevulnerability to pressure damage have been devel-oped. Coextruded plastics are next on the list forevaluation. Some companies are batch processingthe naked food product in high pressure vesselswith the piston imparting the pressure and subse-quently packaging aseptically, thus avoidingpotential damage attributable to the ultra-highpressure.7

Despite all the drawbacks, and certainly all foodprocessing and packaging encountered challengeswhen developed in laboratories and pilot plants,HPP has demonstrated sufficient benefits ofminimal alteration of the product to lead to a con-

clusion that it will become a more significant foodpreservation force in the future.

IONIZING RADIATION

Ionizing radiation is a non-thermal food pasteur-ization process that reduces or eliminates spoilageand pathogenic microorganisms, such as Salmo-nella, Escherichia coli O157:H7, L. monocytegenes andCampylobacter jejuni, by fragmenting DNA. Irradi-ation processes minimize post-harvest loss,decrease perishability and inhibit sprout formationin products such as potatoes. Post-packagingpotentials for irradiation includes the disinfectionof grains, legumes, spices, fruits, melons, lettuces,vegetables and tubers; colour retention in freshmeats; and microbiological control in eggs, pork,poultry and meat.

Not all foods are suitable for irradiationprocesses. Milk and other protein foods candevelop off-flavour, odour and colour, and somefruits may exhibit softening and discolouration,especially at higher dose levels.

All radiation processes must obtain approvalfrom the Food and Drug Administration (FDA)because they are defined as a food additive. Thisruling included package materials, all of which aresubject to the regulation. In 1990, the FDAapproved the use of irradiation of poultry prod-ucts at the level of 1.5–3.0 kilogray (kGy). In 1997,the FDA approved its use for fresh or frozen meatsincluding beef, lamb and pork. The use of irradia-tion, at a dose of 1.5kGy, in conjunction withreduced oxygen packaging and refrigeration, canincrease the shelf life of ground beef to more than15 days, compared to a four-refrigerated-day lifeof non-irradiated. A dose of 1.0kGy, however, isrecommended for ground beef to minimize thedeterioration of sensory qualities.

More than 40 countries have approved irradia-tion in over 100 food items. ‘Of the freshness-enhancing non-thermal technologies, manyconsider irradiation to be the most effectiveapproach to eliminating pathogens and spoilagemicroorganisms from the food supply’.8

Irradiation processes can be gamma fromradioisotopic sources such as Cobalt60 or Cesium137,electrons, X-rays from electron beam accelerators,or ultraviolet (UV) sources.


NON-THERMAL FOOD PROCESSING/PRESERVATION Packaging Technologyand Science

Combinations of modified atmosphere packag-ing (MAP) and irradiation have demonstrated syn-ergistic effects on the shelf life of produce. Instudies cited by Boynton et al.,9 irradiation reducedthe microbiological load in romaine lettuce by 1.5logs CFU/g, and maintained, by the 18th day ofstorage, a 4 log CFU/g difference over that of non-irradiated.

‘However, undesirable sensory quality changessuch as lipid oxidation, off-flavour and pink/redcolour may occur during irradiation that affectproduct quality and acceptability by consumers’.10

Irradiation also causes contraction of meat myofib-rils, yielding a tough texture. Lower-fat foods canbe irradiated at higher doses than foods with ahigher fat content.

In irradiation, foods are exposed to a form ofenergy, which produces free radicals that thenreacts with food biochemicals; alternatively, theradiation directly attacks the cellular nuclei. Formsof ionizing radiation include UV, gamma and betaray or electron beam.3 A high dose (10–74kGy) isrequired for sterilization, which usually damagesthe food, but a lower dose (0.1kGy) may beemployed for pasteurization. ‘Microbial inactiva-tion by all types of ionizing radiation is believedto happen through two main mechanisms: directinteraction of the radiation with cell componentsand indirect action from radiolytic products suchas water radicals H+, OH− and eaq’.11 Ionizing radi-ation’s primary target is the chromosomal DNA,and it exerts a secondary effect on the cytoplasmicmembrane, either of which can cause microbialinhibition or inactivation.

Gamma rays are photons or electromagneticwaves that are emitted from the nucleus of theatom. This energy dislodges electrons from foodmolecules, converting them to electrically chargedparticles or ions. Since gamma rays do not haveenough energy to affect the neutrons in the nuclei,they are incapable of inducing radioactivity.Gamma radiation has high penetrating power.Optimally, irradiation is used in post-packagingwhere pallets of packaged products are conveyedinto a chamber behind a labyrinth. The Co60

gamma ray source is raised from a water pool,allowing the products to absorb gamma radiation.The source then returns to the pool for shielding,and the product exits the chamber.3

The World Health Organization has declaredthat irradiation of any food commodity, up to 10kGy, is not a toxicological hazard. Gamma irradia-tion has been shown to preserve nutritive contentand prolong shelf life by preventing post-harvestinsect and pest infestation of beans and grains. Theadvantage of gamma radiation over chemicals, e.g.fumigation with ethylene oxide, is that the irradi-ation does not leave a chemical residue or induceother adverse effects in the quality of productsbeing treated such as spices.12

Irradiation is widely used for sterilization ofpackage structures for aseptic packaging. Exam-ples include laminated pouches for bag-in-box for food service and industrial containment offluid foods, and thermoformed unit portion-sizeplastic cups for liquid coffee lighteners packagedon aseptic deposit/fill/seal equipment. Ionizingradiation is or has been used commercially in the USA for spice sterilization or reduction of infes-tation, microbial reduction in strawberries andsome other fruits, and pathogen reduction onpoultry and ground beef. Gamma irradiation pro-vides enhanced microbial safety in green onionswith a dose as low as 1kGy, and cilantro with 2kGy, for retention of sensory attributes andincreased shelf life of 14 days, and extends theshelf life of minimally processed gourds by 7days.13

Ground beef products in the form of frozenground beef patties and fresh ground beef chubs,loaves and patties have been irradiated in the USAsince May 2000. Important hurdles to the practicehave been the limited number of package materi-als approved for food irradiation and sensorychanges associated with the irradiation of highoxygen content modified atmosphere packagedcase-ready meats. The high oxygen modifiedatmosphere package has been the preferred case-ready package format. The high oxygen, 70–80%,and carbon dioxide, 20–30%, gas mixture extendsthe shelf life of the product by reducing the rate ofoxymyoglobin oxidation and by inhibiting thegrowth of the spoilage bacteria, respectively. Thepresence of oxygen during the irradiation processresults in the production of ozone, which severelyoxidizes the cherry-red colour (oxymyoglobin) ofground beef to a brown colour (metmyoglobin),which is undesirable to consumers.7



Three operations in the USA employed radiationpasteurization for the extended chilled shelf life of ground beef, but all have been closed. Onereopened on a limited basis in 2006. Other facili-ties reduce the pathogenic microbial load andretard oxidation of ground beef by packaging it inbarrier expanded polystyrene trays, conveying itthrough a tunnel where it receives a flush of nitro-gen and X-ray irradiation, and finally being sealedin a modified atmospheric barrier package. Thissystem minimizes the potential adverse effects ofionizing radiation on the package structures.3

It has been found that at low doses, irradiationhas little effect on nutritional and organolepticfood qualities, but can minimally affect some vita-mins such as thiamin. At higher doses, ionizingradiation forms compounds in meats and poultry,known as 2-alkylcyclobut-anones, which havebeen claimed to be carcinogenic.

ELECTRON BEAM (EB)

EB radiation can be applied to fresh fruits in orderto reduce infestation, lower respiration rate andextend shelf life. It provides high throughput, andis reportedly efficient. The cost of EB radiation islower than gamma or X-ray. Electron beams havelower penetration. ‘Penetration of the acceleratedelectron beam is improved by passing the beamthrough a metal film . . . producing X-rays, whichhave penetrating power similar to gamma rays’.14

EB irradiation has been studied and performedcommercially on hamburger patties, strawberries,mangoes, blueberries, cantaloupes, romainelettuce hearts and sweet cherries.15 Two entry EBradiation was employed in the short-lived groundbeef irradiation programme during the early2000s: EB was applied top and bottom to achievethe penetration that otherwise could not haveoccurred with EB. This double entry enhanced thedamage to package materials, but was evidentlyaccepted by regulatory authorities.3

EB sterilization of plastic bottle interiors foraseptic packaging [in place of chemical steriliza-tion or treatment for extended shelf life (ESL)] wasdeveloped on a pilot basis, but this interestingprocess was not seen on the commercial scene.3

UV RADIATION

In UV light processing, radiation is obtained froman electromagnetic spectrum’s UV region. Expo-sure of 400 joules per square metre (J/m2) in allparts of the product must be obtained to achievemicrobiological inactivation. ‘Critical factorsinclude the transmissivity of the product, the geo-metric configuration of the reactor, the power,wavelength and physical arrangement of the UVsource(s), the product flow profile and the radia-tion path length’.16 UV also reacts synergisticallywith oxidizing agents, such as ozone, and so canbe used to treat fruit juice and cider microbiologi-cally, although, of course, ozone can oxidize desir-able moieties. UV radiation suffers from aninability to penetrate food – all action is on thesurface. UV is not infrequently employed to irra-diate the surfaces of food package materials osten-sibly to sterilize or at least sanitize them prior toaseptic or ESL packaging. Some of the microbici-dal effects are attributable to the generation ofozone, a powerful oxidizing agent. In some ESLoperations for dairy products, UV is applied insynergy with chemical sterilants on package mate-rial surfaces to increase the microbicidal effect andto reduce the residual chemicals.3

PULSED X-RAY

Pulsed X-ray involves a high voltage, which ispulsed through foods at ambient or refrigeratedtemperatures. In pulsed X-ray, the source of radia-tion is electrically driven, making it easier to incor-porate into an existing operation. It does notrequire permanent massive shielding, as doradionuclide sources. Instead, a solid state-opening switch is used to generate pulses of EB X-rays with high intensity. ‘The practical applicationof food irradiation in conjunction with existingfood processing equipment is further facilitated by(a) the possibility of controlling the direction of theelectrically produced radiation; (b) the possibilityof shaping the geometry of the radiation field toaccommodate different package sizes; and (c) itshigh reproducibility and versatility’.11 We can onlypostulate that pulsed high-intensity electromag-



netic energy could result in chemical and physicalalterations to package polymer structures beyondthose of steady low-dose gamma rays that couldadversely alter the functionalities and alsoproduce possibly undesirable components thatwould fall under regulatory scrutiny.

With all the promise and study over the past 60years, and the repeated calls for ionizing radiationto solve problems of microbiological pathogens infoods, the challenges of the past continue to hauntthe technology: consumer concern (despite docu-mented reports that consumers are willing to buyirradiated foods), labelling, potential sensorychanges, a paucity of acceptable package materi-als, special structures and economics. Since no full-scale food irradiation facility has ever operated,time costs remain a mystery. Irradiation has beencharacterized by starts and stops, each boosted byan external driver, e.g. military field rations, spacefood, pathogen reduction in poultry and groundbeef, banning of chemical pesticides, etc. Neverhas irradiation been fully tested, and so thepromise continues, as scientists and technologistscontinue to study the means to overcome the objec-tions. Whether the reduction in pathogenic micro-biological growth on meats or fresh produce,which undergo no other overt processing, can bejustified by the costs is a question that must beaddressed.

ULTRASONICS

Ultrasound utilizes the energy produced fromsound waves, with at least 20000 vibrations persecond, to achieve a bactericidal effect in microor-ganisms and cause enzyme inactivation by celllysis. Ultrasound is one of the simplest and mostversatile methods for cellular disruption and forfood extract production. Critical processing factorsfor ultrasound include the wave amplitude; type,exposure and contact time of microorganisms; andthe composition and volume of food to beprocessed. Ultrasound affects cell membrane per-meability in fruits, like plums, grapes and mango,and can improve the dispersion stability in juices,and thus reduce settling. Ultrasonic extraction dis-rupts plant tissue phenolic compounds from their

vacuolar structures, can be utilized to extract pro-teins, lipids, and oils from beans and seeds such assoybeans, and can also be used to help extract redand yellow pigments from foods such as beets.Ultrasound can also be used in emulsification, dispersing, homogenizing and crystallizationprocesses.

This technology works best when used in con-junction with heat and pressure, but it can be usedalone for fruit juices, sauces, purees and dairyproducts. Ultrasound treatment has been found tobe more effective when combined with otherprocesses such as mano- and thermo-sonication,pressure and/or heat. ‘Ultrasonication uses high-frequency (>16kHz) sound waves to lyse bacteriathrough cavitation [and] . . . could be utilized inthe food industries because it can radiate throughlarge volumes of liquids in the 20- to 500-kHz frequency range’.17 Foods with particulates andother interfering substances do not react well toultrasound.

Ultrasonics has not yet been demonstrated toachieve major beneficial effects that warrantserious consideration for processing or packaging.Perhaps in the future, some food or food packag-ing technology will be developed for this intrigu-ing source of non-thermal energy.

PULSED VISIBLE LIGHT

Pulsed light (PL) involves fleeting but intense,pulses of broad spectrum light that can inactivatemany microorganisms in just a few flashes, andwithin a fraction of a second. This process is uti-lized for the microbiological inactivation on foodpackage material surfaces and in package naturalsterilization. PL can reduce the need for preserva-tives and chemical package sterilants, for producequality and extension of shelf life. In packagematerial surface sterilization, PL is superior tochemical sterilization, e.g. hydrogen peroxide orperacetic acid, because it does not leave an unde-sirable chemical residue. The disadvantage of PLis that it can only be used on product surfaces.Foods in which PL may be employed includebaked goods for mould inactivation, and shrimpand fish for chilled shelf life extension. Other prod-



ucts that may benefit from PL include chickenwings, hot dogs, eggs and cottage cheese.

PL operates by the multiplication of power, fromthe electrical energy accumulated in a storagecapacitor, releasing the storage energy in a veryshort time (millionths or thousandths of a second),thus magnifying the power that is applied, whichexpends only moderate power consumption.Intense and short duration, pulsed, broad-spec-trum ‘white light’ pulses have a power density of0.01–2500J/m2, at a frequency of 170–2600nm,with a pulse duration of 1ms–0.3millisecond,numbering 1–20 pulses/s. Pulsed light wave-lengths range from UV to visible, to the nearlyinfrared spectrum. Shorter UV wavelengths of200–320nm have higher energy levels and yield ahigher rate of microbiological inactivation than dolonger wavelengths. The lethality of the lightpulses varies at different wavelengths. Wave-lengths that are known to produce undesirableattributes are eliminated by filtering, and so aselected wavelength or a full spectrum may beemployed to treat various foods.

Critical process factors in pulsed light includethe light characteristics, e.g. intensity, duration,wavelength and number of pulses, as well as theattributes of the packaging and/or food, e.g. type,colour and transparency. ‘Due to failure of light topenetrate opaque and irregular surfaces, there isgenerally less microbial inactivation with pulsedlight, compared to other technologies’.18 Irre-versible microbial inactivation occurs from chemi-cal modifications and DNA cleavage affectingenzymes, proteins, membranes and the nucleicacid. ‘The antimicrobial affects of these wave-lengths are primarily mediated through absorp-tion by highly conjugated carbon-to-carbon doublebond systems in proteins and nucleic acids’.18 Formost applications, a few pulsed light flasheswithin a fraction of a second will yield high levelsof microbial inactivation.

Pulsed high-intensity light has shown somepromise in package material and product microbi-ological destruction. Transparent products, such aswater and ophthalmic solutions, have been treatedwith pulsed high-intensity light with good results.The concept of sterilizing flexible package materi-als in aseptic packaging was proposed and testedwith good technical results, but little if any com-mercial application.3

PULSED ELECTRIC FIELD(PEF)

During PEF processing, energy is stored in a capac-itor, retrieved from a high-voltage power supply,and is discharged through foods that are eitherstatic or are flowing through a treatment chamber.PEF uses short bursts of electricity (sub-microsec-onds to milliseconds), yielding few to no detri-mental affects on quality attributes in pumpablefoods. This process pulses high voltage (10–80kV/cm) into foods placed between two electrodes,for less than one second, near ambient tempera-ture, then packaged aseptically and distributedrefrigerated. This process attains a 5 log reductionon most pathogenic bacteria by rupturing the cellmembranes in liquid media. It causes onlyminimal detrimental changes to the physical andsensory properties in foods, helps retain ‘fresh’quality and assists in nutrient retention.

PEF can be applied to the pasteurization ofliquid products, in continuous systems, such asmilk, yogurt, juices, liquid eggs, soups, brines andother products that can withstand high electricfields. High electric field pulses can be employedto aid in the extraction of polysaccharides and pep-tides. PEF has limited effects on microbial spores,cannot be used on products that contain or couldform air bubbles, and cannot be used on foods thathave higher or variable electrical conductivity.‘Pressure is applied to inhibit the formation of airbubbles in which electrical arcing could occur withfields above 20000V/cm’.4 Since PEF kills cells andimpairs water retention, it can aid in filtrationmethods and can also be used for the extraction ofsugars and starches from root vegetables. PEF onlyaffects a few enzymes, a concern in the juice indus-try. Enzymes negatively affect juice processing by reducing pectin, which aids in fruit particle suspension, and may cause sedimentation, dis-colouration and flavour degradation.

Critical factors that can affect the inactivation ofmicroorganisms using PEF include process vari-ables, media and microbial factors, which are listedin Table 2. PEF processing variables include pulsewave and width, electric field intensity, tempera-ture and time of exposure. Electric fields are pro-duced on equipment that can be compared to thatof radar. ‘The most typical equipment generates a



short square wave and reverses polarity, in part to avoid erosion of electrodes’.4 Two other waveforms can be produced, which include sinusoidaland exponential decay. In reverse polarity, bipolargenerators are twice the cost of non-polar units,making this an expensive process at this time. Oneoperation has received FDA approval and is beingused by Genesis Juice Corp. for fresh juice pro-cessing. The sinusoidal wave form uses equipmentcomparable to that of a radio and is less difficult togenerate. A square wave can deliver more energyper cycle because the sinusoidal only reachesmaximum power for a split second.

Although PEF is a non-thermal process, anincrease in temperature occurs in the processingchamber. ‘A typical temperature change is about30°C for orange juice and less for apple juice . . .[and] processes typically operate at 35–50°C’.4

Time of exposure depends on several factors, firstof which is the chamber design of which there aretwo categories: flowing and non-flowing. Flowingprocesses include co-axial, parallel plate or co-field. In the co-field method, the electric field iscycled 1000 times a second through various treat-ment chambers, separated by ceramic or polymerinsulators, while receiving multiple pulses. In non-flowing, the process is static and can be applied tosolids.

PEF imposes a strong electric field on pumpablefoods for a very short time to kill vegetative cells.Critical field strengths of about 15000V/cm areused on foods, whereas at 35000V/cm, PEF isused as a disinfectant. Under PEF, cell membranepores develop or enlarge, and can be reversible orirreversible. Pores affect membrane permeabilityby allowing external matter to enter, causing a lossof cellular content, thereby killing the cell. Perfo-

ration of cell membranes caused by PEF in fruitand vegetable cell walls can yield improved extrac-tion of juice from cells.

Disadvantages that must be overcome in orderto commercialize PEF ‘. . . are (a) scale up of thesystem . . . in such a way that profitable produc-tion is possible, (b) the presence of bubbles, whichmay lead to non-uniform treatment as well asoperational and safety issues, (c) treatment of sus-pensions with solid particles, with a minimum riskof breakdown and (d) availability of commercialunits’.19

If bubbles are present in the PEF treatmentchamber, dielectric breakdown will occur. Thishappens because the spherical gas bubbles elon-gate, causing the ends to have up to a five timesmore intense electric field. The bubbles grow largeras the electric field overcomes the dielectricstrength of the bubbles, causing partial discharge,and eventually connecting the two electrodes,causing a spark. Vacuum de-gassing and pressur-ized treatment during processing can minimize the presence of gas bubbles. Concerns must beaddressed when considering PEF for the treatmentof suspensions with particulates: include thepotential for dielectric breakdown on the surfaceof particulates; uniform treatment distribution ofthe applied electric field; the manufacturing of atreatment chamber and feed pump systemdesigned for particulates; control of heat inducedby the process; and the particle size must besmaller than the gap in the treatment region toensure proper processing.19

Technical issues that must be addressed in orderto fully commercialize PEF as a food processingmethod include consistent generation of high-strength PEF; reliable data acquisition systems and



Table 2. Factors in PEF for microbial inactivation

Process Media Microorganism

Pulse wave and width pH TypeElectric field intensity Antimicrobials ConcentrationTemperature Ionic compounds Growth stageTime Medium ionic strength

Electrical conductivity

(FDA, Exec, 2006.)

measuring devices; identification of the critical,maximum and optimum field strengths for micro-bial destruction; flow rate and dosages; tempera-ture control and minimization of heat productionduring processing; the potential for gas bubblesand interference from suspended particles; thedesign of a full-scale treatment chamber; andaseptic packaging systems that are compatiblewith the process. ‘To date, PEF has been appliedmainly to improve the quality of foods . . . Appli-cation of PEF is restricted to food products that canwithstand high electric fields’.20 PEF may be clas-sified as an interesting laboratory research process.

HIGH VOLTAGE ARCDISCHARGE (HVAD)

HVAD ‘ . . . promotes the formation of an arc in themedia (liquid food) while the pulse is applied’.21

HVAD is the application of electricity to pasteur-ize fluids by rapidly discharging through an electrode gap, generating intense waves and electrolysis, thereby inactivating the microorgan-isms. This ‘ . . . chemical action is a complex effectand depends not only on the voltage applied, butalso on the type of microorganism, initial concen-tration of cells, volume of the media used, dis-tribution of chemical radicals and electrodematerial’.22 The use of arc discharge on liquid foodsmay be deterred because of electrolysis and theformation of highly reactive chemicals, whichsometimes occurs during the discharge.

Fresh-squeezed grapefruit juice processed withHVAD was demonstrated to have a fresh flavourand a shelf life for more than 100 days. Research isalso being conducted in the use of pulsed high-voltage arc discharge for surface contamination infood and beverages because it has been shown tobe a highly efficient and effective method formicrobial destruction. With indirect arc discharge,energy from the electric field can be converted toplasma, then to shock waves, generating free rad-icals and oxidizing agents within the product. Theplasma generation is a non-thermal process,thereby retaining the nutritional and organolepticproperties in foods, especially liquid products.

High-voltage electrical pulses can be used as ameans of non-thermal pasteurization and steril-

ization because it demonstrates no thermal effects,and because 90% of microorganisms are destroyedwithin 10 discharges. Charge-reversed electricalpulses are applied to food that is between two elec-trodes within a treatment chamber. Each electricalpulse has a pulse width from 1 to 5s, increasing thevoltage to a peak. This is followed by a decrease involtage, and continues until the voltage peaks atthe opposite polarity. The vertical pulses are 0.1–25J/pulse with field strengths of 15–120kV/cm. Inac-tivation of enzymes occurs with HVAD due to freeradicals and oxidation reactions. ‘The major draw-backs of this electrical method, however, are cont-amination of the treated food by chemical productsof electrolysis and disintegration of food particlesby shock waves’.22 HVAD appears to be a relativeof PEF requiring marriage to aseptic or ESL pack-aging technologies, and still a research curiosity.

MAGNETIC FIELDS

Magnetic field technologies include static mag-netic fields (SMF) where magnetic wave intensityis applied at a constant strength over time, andoscillating magnetic fields (OMF) where magneticwaves alternate amplitudes. With OMF, food issealed in a package and receives 1–100 pulses at afrequency of 5–500kHz, within 25–100ms, andbetween 0–50°C. The intensity of each pulsedecreases 10% with each sequential pulse. OMFhas been shown to inactivate microorganisms inbread roll dough, juice, milk and yogurt. However,factors involved in magnetic fields are not yet fullyunderstood.23

DENSE PHASE CARBONDIOXIDE (DPCD)

DPCD or pressurized carbon dioxide gas is amethod of cold, i.e. below ambient temperature,pasteurization. DPCD affects enzymes andmicroorganisms by subjecting foods to pressurizedCO2, below 50MPa, and thus does not exposefoods to the adverse effects of heat. Critical pro-cessing factors are pressure and temperature.Treatments include batch, continuous and semi-continuous systems. It is believed that DPCD inac-



tivates microorganisms by several mechanismssuch as oxygen elimination, lowering of pH, inhi-bition of certain enzyme systems, rapid physicaldisruption of cells, modification and penetration ofcell membranes, cytoplasmic acidification inhibit-ing metabolic activity, and the extraction of intra-cellular substances such as phospholipids. DPCDhas been shown to eliminate vegetative forms ofspoilage and pathogenic bacteria, moulds, yeasts,and can inactivate some enzymes at temperaturesthat are ineffective in thermal processing. Theseenzymes include polyphenol oxidase, whichcauses browning of fruits, juices, seafood and veg-etables; pectinesterase in fruit juice, which causescloud loss; peroxidase, which causes food dis-colouration; and the enzyme lypoxygenase, whichdestroys chlorophyll and contributes to the devel-opment of off-flavours in frozen vegetables.24

Current applications of DPCD for the inhibitionof mould growth on products include whole hon-eydew melon, cucumbers and strawberries.However, it has been known to cause tissuedamage at low pressure in several fruits. Instudies, DPCD has been applied to fruit juices toimprove colour, detention of ascorbic acid, andcloud formation and stability. DPCD has beenadded to cottage cheese, ice cream, yogurt andricotta cheese because CO2 is highly soluble inlipids and aqueous solutions. When it dissolves inwater, it forms carbonic acid, causing a reductionin pH, thereby increasing cell permeability thatinterferes with cytoplasmic enzymes and influ-ences metabolism. The gas decreases the growthrate of microorganisms, displaces oxygen therebyminimizing rancidity, and can be combined withbarrier packaging to extend shelf life, in some casesby triple. ‘To date, there is no commercial foodproduct processed by DPCD’.24 Other non-thermalprocesses include compressed carbon dioxide(cCO2) and supercritical carbon dioxide (ScCO2).

HURDLE TECHNOLOGIES

Hurdle technologies employ several methods,some of which may include mild heat in synergyto preserve foods. Hurdle technologies include theuse of MAP, active packaging, cryogenic cooling,antioxidants, ozonation and enzymes in conjunc-tion with the aforementioned and other technolo-

gies. In MAP, the CO2 level is increased within thepackage, providing a shelf life markedly greaterthan that of traditional packaging. O2 is oftenreduced and other gases may also be added. TheCO2 exhibits a microbistatic effect. When employ-ing MAP, packaged food products should bestored at temperatures under 5°C.

Active packaging is the addition of absorbing oremitting agents that limit product degradation ormicrobial growth by controlling oxygen, moisture,carbon dioxide and odours.3 Cryogenic coolingand freezing may be employed to rapidly chill aproduct, thus extending shelf life. Hurdle tech-nologies may also employ combinations such asantimicrobials, moderately high temperatures(<55°C) and PEF to provide a synergistic effect,and are being studied to eliminate microorganismsin such products as apple cider, grape juice, mangojuice and tomato juice. In Gauri Mittal’s research,a hurdle approach was employed using a temper-ature of 44°C, acidity at pH 3.5, PEF of 80kV/cmand 100U nisin/ml. They achieved a 6 log reduc-tion in orange juice that had a shelf life of 28 dayswithout the use of aseptic packaging, and no sig-nificant differences were found in aromatic com-pounds analyzed by gas chromatography.25

Antioxidants as hurdles have demonstratedeffectiveness in the minimization and retardationof lipid oxidation. Combinations of antioxidantsfrom plant extracts and irradiation have beenshown to reduce oxidation in chicken, anddecrease warmed-over flavour in ground beef.Other forms of non-thermal food preservationmethods include the use of enzymes to inactivateor inhibit the functions of other enzymes, due totheir antimicrobial and antioxidant affect.26 Pack-aging also plays a major role as a non-thermalpreservation process in that it can extend shelf lifeand help preserve freshness in concert with othernon-thermal processes. Thus, hurdle technologiesappear to the best method to achieve results thatthe non-thermal technologies individually havenot been able to accomplish.

CONCLUSION

Non-thermal technologies are being investigateddue to consumer demand for food products that



are minimally processed, of high quality, and areconvenient and safe. Non-thermal processes offershelf life extension without the use of preserva-tives or additives, while still retaining colour,flavour, texture, nutritive and functional qualities.‘To expand the use of non-thermal processes in thefood industry, combinations of these technologieswith traditional or emerging food preservationtechniques are being studied’.27

In order to file for a new or novel manufactur-ing process with the FDA, the following require-ments must be met: first, communicate with theFDA during every step of the process design;second, have the FDA conduct a site visit at thepilot and production facilities; third, draft the pro-posed filing and submit a copy to the FDA; fourth,identify the resistant organisms that are of mostconcern for public health and commercial viability;and last, identify the least lethal treatment zonewithin the system.

The problems associated with non-thermalmethods include spore injury instead of death, andthe rise in product temperature associated with theprocessing method. In HPP, spore injury can occurunder decompression, thus skewing quality assur-ance results, and other issues may arise using PEF,HVAD, PL and OMF.

Currently, non-thermal technologies can beemployed for acidic foods, e.g. fruit juice, but moreresearch is needed for the processing and packag-ing of shelf-stable low acid foods. High pressureprocessing is commercially used for entrees, gua-camole, salsa and fruit juices, but this process willincrease greatly in the future. Little food is irradi-ated in the USA. The other non-thermal processesdiscussed are still in development stages with con-siderable potential.

REFERENCES

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