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Preservation: past, present and futureGrahame W GouldFormerlyUnilever ResearchLaboratory, Colworth House,Bedford, UK

Foods deteriorate in qua lity due to a wide range of reactions including sometha t are physical, some tha t are chemical, some enzymic and somemicrob iological. The various forms o f spoilage and foo d poisoning caused bymicro-organisms are preventable to a large degree by a number of preservationtechniques, most of which act by preventing or slowing microbial gro wth. Theseinclude freezing , chilling , drying, curing, conserving, vacuum packing, mo difiedatmosphere packing, acidifying, ferme nting , and adding preservatives. Incontrast, a smaller number of techniques act by inactivating micro-organisms,predom inantly heating (pasteurization and s terilization). Com plementarytechniques restrict access of micro-organisms to foo d products, e.g. asepticprocessing and packaging. New and 'emerging' preservation techniques includemore that act by inactivation. They include the application of ionizing radia tion,high hydrostatic pressure, high voltage electric discharges, high intensity light,ultrasonication in combination with heat and slightly raised pressure('manotherm osonication'), and the add ition t o foods of bacteriolytic enzymes,bacteriocins, and other naturally-occurring antimicrobials. Major trends, reactingto consumers' needs, are towards the use of procedures that deliver foodproducts that are less 'heavily' preserved, higher quality, more convenient, more'natural', freer from additives, nutritionally healthier, and still with highassurance o f microb iologica l safety.

Correspondence to.Prof. Grahame W Gould

17 DoveRoad BedfordMK41 7AA, UK

With few exceptions, all foods deteriorate in quality following harvest,slaughter or manufacture, in a manner that is dependent on food type andcomposition, formulation (of manufactured foods) and storageconditions. The principal quality deterioration reactions, which are,therefore, the principal targets for preservation, are well known andrelatively few (Table 1). They include some that are essentially micro-biological, others that are chemical, enzymic or physical1. When preser-vation fails, the consequences range from extreme hazard, e.g. if anytoxinogenic m icro-organisms are no t controlled, to relatively trivial loss ofquality such as loss of colour or flavour. The most serious forms of qualitydeteriora tion include those due to m icro-organisms, following the survivaland/or growth of infectious pathogenic bacteria or the growth of toxin-ogenic ones2. The major food poisoning bacteria are listed in Table 2,

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Table 1 Principal quality deteriora tion reactions of foodsMicrob io log ica l Enzymic Chemical PhysicalGrowth or presenceof tox inogenicmicro-organisms

Hydrorytic reactionscatalysed by lipases,proteases, etc

Oxidat ive rancidi ty Mass transfer, movementof low MW compounds

Growth or presenceof infect ive micro-organisms

Rancidity catalysedby l ipoxygenases

Oxidat ive andreduct ivediscolourat ion

Loss of crisp textures

Growth of spoi lagemicro-organisms

Enzymic browning Non-enzymicb row n i ng

Loss of flavours

Destruct ion ofnut r ients

Freeze-mducedstructural damage

Adapted f rom Gould 'along with their abilities to grow at low, chill cabinet/refrigeratortemperatures, and their resistance to heating, e.g. during cooking in thehom e or food service establishment, o r during processing in the factory3.

Table 2 Major food poisoning bacteria and the ir tem perature relationshipsM i n i m um g row t htemperature

Heat resistanceLow* High

Low (0-5 'C or so)Usteria monocytogenes (INF)1

Yersima enterocolitica (INF)Aeromonas hydrophila (INF)

M ed i um (5 -10 'C or so )Salmonella species (INF)Vibrio parahaemolyticus (INF)Eschenchia coli enteropathogenicand verocytotoxigenic strains (INF)Staphylococcus aureus (TOX)

Medium (10-15 C or so)

High (over 3 0 * OCampylobacter jejuni an d coli (INF)

Clostndium botulmum Ean dnon -pro teoly tic B and FfTOX) 1

Bacillus cereus(INF and TOX)Bacillus su btilis (TOX)Bacillus lichenlformis (TOX)

Clostridium botulmumA and proteolyt ic (TOX)Clostridium perfnngens (INF)

In excess of a 10*-fold inac tivatio n of vege tative micro -organism s by pasteuriz ation , e g at a tem pe ratu re o f abou t 70*C for 2 minb ln excess of a 10*-fold inactivation of spores at temperatures ranging from about 90*C for most heat-sensitrve types to about 120*Cfor 10 mm for the most heat-toler ant types' INF - organisms that may contaminate foods, and may mul t iply in them, and which cause food poisoning by infect ion*TOX - organisms that may contaminate foods and mul t iply in them to form toxins that then cause food poisoning by intoxicat ion.Adapted f rom Russel l and Gould

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Table 3 Changing consumer requirements and foo d industry reactionsTrends in consumer requirementsImproved convenienceHigher qua lity - in prepa ration, storage, shelf-lifeFresher - in flavour, texture , appearance

More natural - wi th fewer additivesNutritionally healthierMinimally packagedSaferFood industryreactionsMilder processing - minimal over-heating- less intensive heating- non-thermal alternatives to heatFewer additives - less 'chem ical' preservatives

Use of 'hu rdle ' technologies or 'combina tion preservation' systemsDevelopment and use of predictive models- grow th models, as a function of pH,awtemperature,preservatives-survival models, as above-thermal death modelsEvaluation of natural antimicrobial systems as food preservativesLess use of salt, saturated fats, sugar; more low calorie foodsReduced,environmentally-friendly packagingElimination of food poisoning micro-organisms

Adapted from Gould*

Changes in the requirements of consumers in recent years haveincluded a desire for foods which are more convenient, higher quality,fresher, more natural and nutritionally healthier than hitherto (Table 3).Food industry reactions to these changes have been to develop lesssevere or 'minimal' preservation and processing technologies (Table 3).However, minimal technologies tend to result in a reduction in theintrinsic preservation of foods, and may, therefore, also lead to a po tentialreduction in their microbiological stability and safety. Thus, an importantchallenge has been to ensure that new and improved technologies retain,or preferably improve on, the effertiveness of preservation and ensuranceof safety that may otherwise be lost.

Major current preservation technologiesThere is a limited range of techniques currently employed to preservefoods. These are commented on below, and listed in Table 4 in such away as to emphasize the fact that most of them act by slowing down, orin some cases by completely inhibiting, microbial growth. Few act by

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Table 4 Major existing technologies for food preservationTechniquesthatsloworpreventthe growth ofmicro organismsReduction in tempe rature - chill storage, frozen storageReduction in water activity - drying, curing with added salt conserving with added sugarReduction in pH - acidification (e g use of acetic, citric acids,etc), fermentationRemoval of oxygen - vacuum or modified atmosphere packagingModified atmosphere packaging - replacement of air w ith COy- O , N2mixturesAddition of preservatives - inorganic (eg sulphite, nitrite)- organic (eg propio nate, sorbate, benzoate, parabens)- bactenocin (e g nisin)- antimycotic (eg natamycin)Control of microstructure - in wa ter-in-oil emulsion foodsTechniquesthatinactivate micro organismsHeating - pasteurization- sterilizationTechniquesthatrestrict accesso fmicro organismstoproducts

PackagingAseptic processingAdapted from Gould1

direct inactivation. A major trend is to apply these techniques in newcombinations, in ways that minimize the extreme use of any one ofthem , and so improve food produ ct quality. This has formed the basis ofthe successful 'hurdle technologies' of Leistner5 that have fostered thedevelopment of new routes to food preservation around the world.While traditional hurdle technologies were developed empirically, newlogical developments are being made supported by the use ofmathematical models6. These are generated using data derived fromlarge multifactorial experiments, and allow confident computer-aidedpredictions to be m ade,e.g.of the effects of parameters such as pH, aw,temperature, preservatives, gas phase, etc. on the growth, survival, andthermal death of specific micro-organisms in foods7.

Low temperatureAs the temperature of a chilled food is reduced, the types of micro-organisms and their rates of growth are reduced also. Two particularlyimportant temperatures are around 12C, which represents the lowerlimit for growth of the strict anaerobes,Clostridiumperfringensand theproteolytic strains ofClostridium botulinunt (types A and some types ofB), and 3C, which is the lower limit for non-proteolytic strains of C.botulinum (types E and some types of B and F). A few years ago, thiswould have been the chill storage temperature below which no foodpoisoning micro-organisms would have been expected to multiply.

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However, both Listeria monocytogenes and Yerstnia enterocoltttca cangrow at temperatures below 1C, so that indicated shelf-lives and sell-bydates can play an important role in ensuring safety, particularly whentemperature control can not be assured, e.g.in the home8. Many typesof spoilage micro-organisms may continue to grow at sub-zerotemperatures, multiplying slowly at temperatures down to about -7C.Badly stored frozen foods may, therefore, slowly spoil through theactivities of micro-organisms, but not become dangerous if thawing hasnot occurred. At the temperature of properly stored frozen foods,nominally -18C in many countries, microbial growth is completelyprevented, although slow loss of quality may still occur through theactivities of enzymes and through chemical reactions and physicalchanges (see Table 1).

Reduction in water activityWater activity values (aw) are widely used to predict the stability of foodswith respect to the growth of micro-organisms and the chemical,enzymic and physical changes that lead to quality deterioration9. Valuesrange from 1 (pure water) to zero (no water), equivalent to equilibriumrelative hum idities (ERH) on a scale from 10 0 to 0 . The wateractivity of foods is reduced by drying or by adding solutes such as salt,as in cured products, or sugars, as in conserves, or by combinations ofthese treatments. Small reductions, e.g. to about 0.97, are sufficient toprevent the growth of some important spoilage micro-organisms, e.g.Pseudomonasspecies that grow at high aws, and rapidly spoil foods suchas fresh meat stored in air. Cured meats generally have aws sufficientlyreduced to ensure longer Pseudomonas-iree shelf-lives. Slow souring,caused by lactic acid bacteria occurs instead. If the aw is lower still,below ab ou t 0 .95 , as in some salamis and dry-cured meat products, eventhese are inhibited, and slow spoilage by low aw-tolerant micrococcitakes over. These and similar relationships are widely used to explainand predict the storage stability and safety of foods. Of the foodpoisoning micro-organisms,Staphylococcu s aureusis the most toleran t,with a low aw limit for growth of about 0.86 in air, but only 0.91anaerobically, so that it may grow and produce enterotoxin in relativelylow aw foods if other conditions are conducive, e.g. temperature andtime of storage. At awvalues below 0.86, few bacteria, and no bacteriaof public health concern, can grow, and food is spoiled by yeasts ormoulds, some of which can multiply slowly at aws as low as 0.6. Belowthis aw, no micro-organisms are able to grow. Shelf-stable dried foodsare generally formulated around aw0.3, where lipid oxidation and otherchemical changes are minimal.

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An interesting extrapolation of aw-control of microbial growth into theclinical area was made by Herszage and his colleagues in Buenos Aires10.He built on the ancient uses of honey and other highly soluble solutes bypromoting the treatment of infected wounds with cane sugar. The sucrosewas not highly absorbed into underlying tissues, but served to reduce theawwithin a wound, and apparently without interfering with macrophageactivity, sufficiently to prevent the growth of pathogens, including Staph.aureus.Efficacy was dem onstrated in a number of clinical studies11, andthe procedure was said to have potential value, e.g. where particularlyantibiotic-resistant micro-organisms were involved, or in third worldcountries where sugar is much cheaper than antibiotics.

Vacuum and modified atmosphere packaging MAP )The effectiveness of vacuum and MA P derive firstly from the removal ofoxygen, with the consequent inhibition of strictly oxidative micro-organisms. Fermentative organisms continue to multiply but they do somore slowly and, for some types of foods, they have less unpleasantconsequences for food quality. Special attention is always given to thepossibility of encouraging the growth of strictly anaerobic foodpoisoning m icro-organisms, such as C.botultnum,so that for foods suchas 'sous vide' products, which are vacuum packed and pasteurizedrather than sterilized, minimal heat treatments and tight temperaturecontrol in distribution are recommended12 . Carbon dioxide is widelyused in MAP foods because it has a specific antim icrobial activity, actingas a preservative that uniquely dissipates when the food pack isopened13 . For example, much supermarket meat is packed in gasmixtures containing about 70 O 2and 30 C OrThe O2maintains themeat in the b right red oxym yoglobin colour th at consum ers prefer, w hilethe CO 2 slows down the growth of Gram-negative spoilage bacteria soas to about double the useful shelf-life.

AcidificationMany yeasts and moulds are able to multiply at very low pH values, i.e.well below pH 2, so that they predominate in the flora of spoilingacidified foods. Few bacteria grow below about pH 3.5 or so. Thosethat do are adapted to acid environments, e.g. the lactic acid bacteria,and indeed are employed in numerous acid-generating food ferment-ations such as those for yoghurts, cheeses and salamis. A particularlyimp ortant pH for food safety is pH 4.5 , because it is the pH below whichC.botulinumis unable to multiply. Consequently, in thermal processing,

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Preservatives

Table5 Most-used fo od preservat ivesPreservatives Examplesoffoodsinwhich theyareusedWeak hpophilic organic acids andesters

SorbateBenzoateBenzoate esters e g methyl , propyl)Propionate

Organic acid acidulantsAcetic, lactic, citric, malic,etc

Mineral acid acidulantsPhosphoric hydrochloric

Inorganic anionsSulphite (SOj, metabisulphrte)Ni t r i te

AntibioticsNisinNatamycin (pimancin)

Smoke

Cheeses, syrups, cakes, dressingsPickles, soft drinks, dressingsMarinaded f ish productsBread, cakes, cheese, grain

Acidulantsfor lowpH sauces, mayonnaises, dressings,salads, drinks, fruit juices and concentratesAcidulants,asabove

Fruit pieces, dried fruits, wine, meat (Brit ish freshsausages)Cured meats

Cheese, canned foodsSoft f rui t dry-cured meatsMeats and f ish

Adapted from Russelland Gould15

it isnotnecessarytoheat foods that are more acid than thistothe sameextent ashigher pH low acid' foods. Below about pH 4.2, other foodpoisoning and spoilage bacteria are mostly controlled. However, recentlythe spore-forming bacterium licyclobacillus acidoterrestris, capableofgrowthatpH values as low as 2, has caused spoilage problems ('disinfect-ant taints')insome low pH foods.Survivalofmicro-organismsatlow pH maybeimportant, eveniftheyare unabletomultiply. For example,Eschenchia coh01 57 hasanacidtolerance that may have contributed to some food poisoning outbreaksin which thevehicle wasalowpHfood, e.g. American (non-alcoholic)apple cider. Fu rthermore, acid tolerance may aid passageofsuch o rgan-isms through the stomach. Food processors are aware that acid toler-ance maybeincreased by prior exposure to mild acidification, orevenby seemingly unrelated stresses, suchasmild heating14 .

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the inorganic ones (sulphite, nitrite) . All are more effectiveatlow ratherthanathigh pH15 . Indeed, with the possible exceptions of the alkyl estersof p-hydroxybenzoate ( 'parabens'), there are no wide-spectrumantimicrobial food preservatives that areeffective at near-neutralpH.There is a well-established rationale for theeffectiveness of theweakacidsand for their synergy with hydrogen ions, i.e.with low pH. Thisderives from the fact thatin their unionized forms, which are favouredat low pH, theyareableto readily equilibrate across the microbial cellmembrane andaccess thecytoplasm of thecell.The pKvalue of thecommon weak acid preservatives range from 4.2 (benzoic) to 4.87(propionic), sothatat pH values much above these activity isgreatlyreduced. At the pH ofmost foods, micro-organisms maintain an internalpH higher than thatoftheir surroun dings. Consequently, on entering thecytoplasm, the undissociated acids tend to dissociate, deliveringhydrogen ions along with the particular anion. The additional hydrogenions may be exported by the micro-organisms, but this is energy-demanding, so cell growth is restricted. Ifthe energy supply is overcome,thenthe pH ofthe cytoplasm eventually falls to alevel thatis too lowfor growth tocontinue.Inaddition, theaccumulated anion may havespecific antimicrobial effects16.From the pointofviewofpractical food preservation,itis, therefore,sensible to include a weak organic acid whenever possible, then toacidify the food product asmuch as is organoleptically acceptabletocapitalizeon theweak acid-lowpHsynergy, thentovacuum pack it ifpossible because this will restrict the amountofenergy that is availablefor theextrusion of hydrogen ions, then toreduce the awasmuchaspossible, because this will place additional energy requirements on thecell,and soon.Inthis way, many empirical preservation 'combinationtechnologies' can be rationalized, and new, logically-based ones sought.

HeatPasteurization at times and temperatures sufficient to inactivatevegetative micro-organisms,andsterilizationattimes and temperaturessufficient to inactivate bacterial spores, remain the bases of largeindustries around theworld17. With theslow acceptanceofirradiationfor food preservation in most countries, heat remains the onlysubstantial meansfor inactivating micro-organisms in foods. However,mostofthe new and 'emerging' technologies that have been investigatedand promoted inrecent yearsact byinactivation, but without the needfor substantial heating.

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Newandemerging food preservation technologiesNatural additives

A few natural additivesare widely used (Table 6)18-19.Forinstance,eggwhite lysozymeisemployedatlevelsinexcessof 100tonnesperannumto prevent 'blowing', by lysing vegetative cells of Clostridiumtyrobutyricum outgrowing from spores in some cheeses. Activationofthe lactoperoxidase system has been shown to beuseful to extend theshelf-life of bulk milk inthose countries in which pasteurization soonafter milking is not possible and refrigerated transport systems arepoorly developed. The small post-transcriptionally modified peptidebacteriocin, nisin, is increasingly used to prevent spoilage of somecheeses and to prevent spoilageof some canned foods by thermophilicspore-forming bacteria such as Bacillus stearothermophtlus andClostridium thermosaccharolyticum. More than 40 other bacteriocinshave been discovered and some are being evaluated for food use.Hundreds of herb, spiceand other plant-derived compounds have beendescribed and shown to have antimicrobial properties in laboratorystudies20 . While some of them are effective in foods, their efficacy isoften reduced because of bindingof thecompounds to food proteins,partition into fats,etc.

New physical proceduresItislikely thatnew physical procedures will providethe most effectivealternatives toheat. Someofthemarealreadyincomm ercialuse,whileother are attracting substantial research and development support(Table6)4.High hydrostatic pressureThe applicationof high hydrostatic pressureis nowwell-establishedforthe non-thermal inactivation ofvegetative bac teria, yeastsandmouldsinfoods, by 'pressure pasteurization'21 . Vegetative forms of micro-organismsaregenerally sensitivetopressuresin theregionof400-600MPa (Megapascals) or so (equivalent to 4000-6000 atmospheres),though with large differences in thesensitivitiesof different speciesandsometimes large strain-to-strain variations too.Foodssotreated includejams, fruit juices, dressings, and avocado dip (guacamole). Theadvantageof thetreatments isthat, whereas pressuremaygreatly alterthe state of macromolecules in foods, such as proteins and poly-sacchandes, it haslittle effect on small molecules,so that flavoursandodours remain relatively unaltered and 'fresh-like'.

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Table 6 New and emerging technologies for foo d preservationNatural addKivasAnimal-derived antimicrobials - lysozyme- lactoperoxidase system

- lactofernn, lactoferricinPlant-denved antimicrobials - herb and spice extractsMicrobial products - nisin- pediocin- other bactenocins and culture products

Physical process**Gamma and electron beam irradiationHigh voltage electric gradient pulses ('electroporation')High hydrostatic pressureCombined ultrasonics, heat and pressure ('manothermos onication')Laser and non-coherent light pulsesHigh magnetic field pulses

High p ressure has so far been exploited m ainly for the preservation offoods in which spores are not a problem, e.g.foods in which the pH istoo low for spores to outgrow, or which are stored for limited tunes atchill temperatures. These limitations result from the fact that bacterialspores are far more tolerant to pressure than are vegetative cells.However, it has been found that pressure can be highly synergistic withmild heating for the inactivation of spores. This seems to occur becausepressure, in some as yet unknown manner, actually triggers spores togerminate. Having germinated, they lose their resistance to pressure, andto heat, so that the two physical processes applied together inactivatemany more spores than either alone. Further development along theselines, and the possibility of other synergies e.g. pressure has been shownto be synergistic with nisin) may eventually allow it to be used as an alter-native to heat-sterilization of foods, and possibly of some pharmaceuticalstoo.Pressure was first evaluated for vaccine production.UltrasonicationUltrasonication at high enough intensities has long been known toinactivate vegetative bacteria and to reduce the heat resistance of spores;the effect is amplified by increasing the temperature. However, as thetemperature is increased, the relative magnitude of the amplificationbecomes reduced. It is thought that this occurs because, as the vapourpressure rises, it has the effect of reducing the effectjveness of cavitation(the rapid formation and collapse of tiny bubbles), /which is the mainvehicle of killing. However, application of a slight overpressure i.e.a fewatmospheres) has been reported to overcome this fall in effectiveness, sothat the amplification is maintained at higher temperatures. The

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combination procedure ('manothermosonication'), therefore, has beenclaimed to have potential for reducing pasteurization and sterilizationtemperatures for pumpable liquid and semisolid foods22.High voltage electric dischargesHigh voltage electric discharges ('electroporation') are most effective forthe inactivation of vegetative bacteria, yeasts, and moulds, while sporesare much m ore toleran t. The cell mem brane is one of the most im portantstructures controlling many of the vegetative cell's homeostaticmechanisms. It is not surprising, therefore, that electroporation, whichbreaches this structure, has such a lethal, and essentially non-thermal,effect on vegetativecells.Voltage gradients in the region of 2 0- 60 kV/cmare used, delivered in a series of microsecond pulses, at pulse repetitionrates sufficiently low to avoid too much hea ting. Foods such as milk andfruit juices can be pasteurized using this technique in flow-throughcontinuous treatment cells23. The reason for the resistance of spores isnot know n for certain, but probably results from the fact that the centralcytoplasm of spores is thought to be relatively dehydrated. This wouldreduce its conductivity, and make difficult the development of asufficiently high voltage gradient to breach the surrounding membrane.High intensity lightHigh intensity laser and non-coherent light pulse generators have beendeveloped for the decontamination of surfaces of foods and packagingmaterials, and possibly transparen t foods also

24, as well as in dentistry

25.The killing effect results partially from the UV content for someapplications and partially from intense but local heating for others.Additional non-UV and non-thermal effects have been claimed by someresearchers.

High intensity magnetic field pulsesExposure to high intensity oscillating magnetic fields has been reportedto have a variety of effects on biological systems ranging from selectiveinactivation of malignant cells26 to the inactivation of bacteria onpackaging materials and in foods27. Treatment times are very short,typically from 25 ms to a few milliseconds, and field strengths are veryhigh, typically from 2 Tessla to about 100 Tessla at frequencies betweenabout 5-500 kHz. Efficacies of treatments did not exceed about 100-fold reductions in numbers of vegetative micro-organisms inoculatedinto milk Streptococcus thermophilus), orange juice {Saccharomycesspp.), bread rolls (mould spores) and no inactivation of bacterial sporeshas been reported27 , so the practical potential for the technique, as it hasbeen developed so far, appears to be limited28.

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IrradiationThe use of ionizing radiation, including gamma radiation from isotopessuch as ^Co, and electrons and X-rays from machine sources, is legal fordismfestation, to prevent sproutmg of bulbs and tubers, and forantimicrobial pasteurization of foods in nearly 40 countries. Dosesallowed have generally been up to 10 kGy (kilogray). Recently, the WorldHealth Organization recommended that there are no toxicological orother hazards associated with higher doses, so that there should be noupper dose limit imposed for the irradiation of foods29 . The technology isrelatively simple to apply, with straightforward inactivation kinetics andgeometry that makes dose control and processing requirements mucheasier than for m any he at processes. Th e potential value to consum ers, inthe area of prevention of food poisoning through the elimination ofpathogens such as Salmonella and Campylobacter from som e foods ofanimal origin and some sea foods, is substantial. However, this is notwidely recognized by consumers, so that slow acceptance by the publiccontinues to restrict its introduction in most parts of the world.

ConclusionsWhile the most-employed preservation technologies have a long history ofuse, there is currently a real need for improved techniques, to meet thedeveloping needs of consumers. Some improvements are being derivedfrom the use of established techniques in new combinations or underimproved control, and other improvements are being derived essentiallyfrom the development of new techniques. These are finding, at first, newand attractive, but niche, markets. It is expected that these will expand asexperience in the new techniques is gained. If the resistance of bacterialspores to some of the new techniques could be overcome, and in a ma nne rthat was widely proven and accepted to be safe, then the potential marketscould be immeasurably larger. A particular attraction of the newertechniques is that they act by inactivation rather than by inhibition. Withregard to reducing the incidence of food poisoning disease, theintroduction of effective inactivation techniques that lead to theelimination of the pathogens must be the ultimate target of primary foodproducers, processors, distributors, and retailers. Occasional lapses ofhygiene will continue to occur in the food service establishme nt an d in th ehome, but would be of no public health consequence if the organisms ofconcern did not enter these premises in the first place.

References1 Gould GW. (Ed) Mechanisms of Action of Food Preservation Procedures. Barking: Elsevier, 19892 Lund BM, Baird-Parker AC, Gould G W (Eds) The M icrobiological Safety and Quality of Foods.Gaithersburg, MD: Aspen, 2000

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3 Gould GW, Abee T, Granum PE, Jones MV. Physiology of food poisoning microorganisms andthe major problems in food poisoning control. Int J Food Microbiol 1995; 28 121-28

4 Gould GW. (Ed) New Methods of Food Preservation. Glasgow: Blackie, 19955 Leistner L, Gorris LGM. Food preservation by hurdle technology. Trends Food Sci Technol

1995; 6: 41-66 Baranyi J, Roberts TA. Mathematics of predictive microbiology. Int J Food Microbiol 1995, 26:199-2187 Baranyi J, Roberts TA. Principles and application of predictive modeling of the effects of

preservative factors of microorganisms. In: Lund BM, Baird-Parker AC, Gould GW (Eds) TheMicrobiological Safety and Quality of Foods. Gaithersburg, MD: Aspen, 2000; 342-58

8 Herbert RA, Sutherland JP. Chill storage. In: Lund BM, Baird-Parker AC, Gould GW. (Eds) TheMicrobiological Safety and Quality of Foods. Gaithersburg, MD: Aspen, 2000; 101-21

9 Christian JHB. Drying and reduction in water activity. In: Lund BM, Baird-Parker AC, Gould GW.(Eds) The Microbiological Safety and Quality ofFoods.Gaithersburg, MD: Aspen, 2000; 146-74

10 Chirife J, Scarmato G, Herszage L. Scientific basis for use of granulated sugar in treatment ofinfected wounds Lancet 1982, I: 560-1

11 Sehvyn S, Durodie J. The antimicrobial activity of sugar against pathogens of wounds and otherinfections of man. In. Simatos D, Multon JL. (Eds) Properties of Water in Foods. Dordrecht:MartinusNijhoff, 1985; 293-308

12 Notermans S, Dufrenne J, Lund BM. Botulism risk of refrigerated processed foods of extendeddurability. J Food Protect 1990; 53: 1020-24

13 Mohn G. Modified atmospheres In: Lund BM, Baird-Parker AC, Gould GW. (Eds) TheMicrobiological Safety and Quality of Foods. Gaithersburg, MD: Aspen, 2000; 214-34

14 Wang G, Doyle MP. Heat shock response enhances acid tolerance of Eschenchta coh O157-H7.Lett Appl Microbiol 1998; 26: 3 1 ^

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levels. J Appl Bactenol 1983; 54: 383-917 Pflug IJ, Gould GW. Heat treatment In. Lund BM, Baird-Parker AC, Gould GW. (Eds) The

Microbiological Safety and Quality of Foods. Gaithersburg, MD: Aspen, 2000; 366418 Davidson PM, Brannen AL. (Eds) Antimicrobials in Foods New York Marcel Dekker, 199319 Dillon VM, Board RG (Eds) Natural Antimicrobial Systems and Food Preservation.

WalLngford, Oxon: CAB International, 199420 Hoover DG. Microorganisms and their products in the preservation of foods. In: Lund BM,

Baird-Parker AC, Gould GW. (Eds) The Microbiological Safety and Quality of Foods.Gaithersburg, MD: Aspen, 2000; 251-76

21 Ledward DA, Johnston DE, Earnshaw RG Hasting APM. (Eds) High Pressure Processing ofFoods. Nottingham: Nottingham University Press, 1995

22 Sala FJ, Burgos J, Condon S, Lopez P, Raso J. Effect of heat and ultrasound on microorganismsand enzymes. In: Gould GW. (Ed) New Methods of Food Preservation. Glasgow: Blackie, 1995:176-204

23 Zhang Q, Qin BL, Barbosa-Canovas GV, Swanson BG. Inactivation of E. coh for foodpasteurization by high strength pulsed electric fields. J Food Proc Pres 1995; 19 103-18

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