Sterility 63601 Tetra Pak Document

 

 An introduction

 An introduction

to commercial sterility

to commercial sterility

 3 Introduction 4 Histor y 4 Single-cellriendsandenemies 5 Thelogarithmicorder 7 Approachingabsolutesterility 8 Aweaklinkbreaksthechain 9 Meetingmarketrequirements 10 Howmanysamples? 11 Increasingsterilizationeciency 12 Amixtureoremedies 13 UHTtreatment 14 Microltration 15 Summar y

Index 



Thepurposeothisbookletistoprovideabasicunder-standing othe microbiologicalaspects oaseptic technologywiththeocusontheprocessingpart.We donotaimtocovereverythingindetail,butinsteadtry togiveyouagoodoverviewotheimportanceocom-mercialsterilityandhowtoachieveit.Iyouneedmore detailedinormation,donothesitatetocontactusat TetraPak.Wearehappytoassistyouwithourcompe-tenceandexperience.

Introduction

Allqualityisbasedoncompetence.



History

The destruction o microorganisms

EvenbeoreLouisPasteurprovedoveracenturyago thatmicroorganismscausedermentationanddis-ease,NicolasAppert,aParisianconectioner,irst succeededinpreservingcertainoodsinglassbottles thathadbeenkeptinboilingwaterorvaryinglengths otime.Thishappenedintherstdecadeothe1800’s

Single-cell riends and enemies

SporesposethemajorchallengetoUHT(ultrahigh temperature)processinganddeterminethesteriliza-tionparameters. Microorganismscarryoutinnumerableunctions thatareessentialtolieonearth.However,asregards oodproducts,theyarepublicenemynumberone. Manybacteria,someowhicharepathogenictoman, areresponsibleortherapidspoilageoood.Applying asuitabletemperature(e.g.72°Cor15s)degrades proteins,essentialcomponentsolivingcells,andsoit isquiteeasytokillvegetativebacteria. Themajorproblemisbacterialspores.Theseare diculttokillbecausetheyhavethickprotectivewalls whichincreasetheirresistancetoheat,drying,reez-ing,chemicalagentsandradiation.Thisiswhyspores posethebiggestchallengetoUHTprocessingandde-terminetheparametersusedorsterilization. andby1839,tin-coatedsteelcontainerswerewidelyin use.Thusbeganthebirthohighheattreatmentand canningasameansopreservingood. Today,acombinationocontinuousheat-treatment andasepticpackagingproduceshighqualityliquid oodinacost-eectiveway.



The logarithmic order

 A never-endi ng story

Asepticprocessinginvolvesoneormoreseparatester-ilization stepsaimed at killingall microorganisms. Whenapplyinga sterilizationprocedure,e.g.using heat,notallbacterialsporesarekilledatthesametime. Acertainnumberwillbekilledwithinagivenunito timeasshowninthetablehere.Theprinciplebehind thekillingprocessisindependentothesterilization temperature. Whenthenumberosurvivingsporesisplotted againsttheirlogarithmicreduction,weobtainastraight lineasshowninFigure1.Thisstraightlineiscommonly knownasthelogarithmicorderodeath,andtheslope othelineindicatestherateodestruction.

Fig. 1. A typical graph o the logarithmic order o death or bacterial spores during sterilization. The steeper the slope, the higher the rate o killing ef-ciency. The initial microbial load is here assumed to be 100 000 bacterial  spores o equal resistance. D is the decimal reduction time.

UNIT SPORES KILLED TOTAL LOGARITHMIC

OF TIME SURVIVORS PER UNIT TIME ACCUMULATED REDUCTION

0 100 000 0 0 0 1 10 000 90 000 90 000 1 2 1 000 9 000 99 000 2 3 100 900 99 900 3 4 10 90 99 990 4 5 1 9 99 999 5 6 0.1 0.9 99 999.9 6 7 0.01 0.09 99 999.99 7 8 0.001 0.009 99 999.999 8 1000000 100000 1000 100 10 1 0.1 1.01 0.001 0.0001 NUMBER OF SPORES TIME 1 2 3 4 5 D



 Approaching absolute sterility

Asalogarithmicunctionnevercanreachzerobutonly approachit,absolutesterilitynevercanbereachedor guaranteed.Theeciencyoasterilizationprocessis expressedbythenumberodecimalreductionsinthe countoresistantspores.TheD-value(decimalreduc-tiontime)isnormallyusedtoindicatethekillingrate, i.e.thetimerequiredatacertaintemperaturetokill 90%othespores,whichisthesameasreducingtheir numberbyonelogarithmunit.Measuredunderthe sameconditions,theD-valuewillbeaectedbythe speciesobacteria.

The result o a heat sterilization process depends on: •theinitialnumberomicroorganisms; •thedecimalreductiontime(heat-resistance) omicroorganisms; •thetimeandtemperatureoheatexposure. AnotherimportantparameteristheZ-value, whichisthechangeintemperatureneeded toaltertheD-valuebyaactoroone logarithmunit.

Absolutesterility

canneverbereached

orguaranteed.

Fig. 2. The Z-value is the increase in temperature needed to achieve  the same lethal action or the same eect in one-tenth o the time.

1000 100 10 1 0.1 D-VALUE TEMP °C 120 130 140 150 160 Z 



 A weak link breaks the chain

Asepticprocessingandpackagingisrathercomplex becausea minimum othree dierentsterilization stepsare required toachieveaseptic llingo the liquidoodproduct.Sterilizationiscarriedout: • ontheequipmentpriortoproduction; • ontheliquidoodproduct; • onthepackagingmaterialsurace. Theperormanceleveloanasepticplantmustbere-gardedastheperormancelevelotheentireprocess ratherthanthatoasinglecomponentotheproduc-tionline.Eachlinkintheprocessingandpackaging chainmustbeequallystrong.

Eachlinkintheprocessingandpackaging

chainmustbeequallystrong.

Raw  material Pre-processing UHT treatment  Aseptic packaging Sterilization Packaging material



Meeting market requirements

Theprocessingresultisnotdeterminedbytheprocess- ingactorsalonebutalsobythequalityorawmate-rial,pre-processingprocedures,environmentaland operativeparametersaswellasmaintenance.Absolute sterility–denedasthetotalabsenceomicroorgan-isms–isnotpossibleorthereasonmentionedearlier andthuscannotbeguaranteed. Commercial sterility AccordingtotheWHO/FAO,commercialsterility olow-acidoodisdenedasollows:

“Commercial sterility means the absence o microorganisms capable o growing in the ood at normal non-rerigerated conditions at which the ood is likely to be held during manuacture, distribution and storage.” (Re: Codex Alimentarius Commission (WHO/ FAO) CAC/RCP 0-)

TheEUdenestheheattreatmentnecessaryto achievingcommercialsterility.

“UHT treatment is achieved by a treatment: (i) involving a continuous ow o heat at a high temperature or a short time (not less than °C in combination with a suitable holding time) such that there are no viable microorganisms or spores capable o grow-ing in the treated product when kept in an aseptic container at ambient temperature, and (ii) sufcient to ensure that the products remain microbiologically stable ater in-cubating or  days at 0°C in closed con-tainers or or seven days at °C in closed containers or ater any method demonstrat-ing that the appropriate heat treatment has been applied.” (Re: Commission Regulation (EC) No /00 (amending Regulation (EC) No /00)).

Speciying quality levels

Thepracticalconsequencesoabsolutesterilitybeing unobtainable means establishing microbiological specicationsortheend-product.Thisisnecessaryin ordertodevelopaviablequalitycontrolprogramme. Suchspecicationsoracceptancequalitylevels(AQL) ormanintegralpartoacompany’squalitypolicy.

Amicrobiological AQLor long-lieproducts shouldbeexpressedasamaximalacceptabledeect rate.Acleardistinctionhastobemadebetweenthe maximalacceptabledeectiveratesandthegoalo production.Regardlessoestablishedmaximalaccept-abledeectrates,thegoaloproductionmustalways beperection. Sampling procedure Amaximalacceptabledeectrateoaround1per1000 to10000samplesisgenerallyapproved.Inorderto detectsuchdeectrates,samplingstatisticsmustbe introduced.Thisincludesdeningtheleveloproba-bility(condencelevel)tobeusedwhencheckingor deectrates.Usually,a90–95%condencelevelo achievingacorrectresultisused.

0

How many samples?

More samples will improve the detection level Todeterminehowmanysamplesweneedtotaketo detectadeectiveproduct,let’slookatatypicalex-ample.Howmanysamplesarerequiredtodetect adeectrateo1per1000with90%probability? Thisproblemissolvedwiththehelpotherandom samplingcurveor90%probabilityshowninFigure3. Iweollowthe1%gureuptowherethecurveinter-sectsthey-axiswegettheresult240.For0.1%(1per 1000)theanswertothequestionis2400.Thestatistical resultsareaectedonlybythesamplingsizeandnot bythetotalnumberounitsproduced. Let’stakeanotherexampleusingthesame90% probabilitycurve.Iwetake50samplesromabatch andnoneothemaredeective,whatisthedeect rate? Inthiscasewetakeahorizontallinerom50sam-plesonthey-axis,andwhereitintersectsthe90% curveisthedeectrateatthisleveloprobability:less than5%,orabout5samplesper100units.Moresam-pleswillimprovethedetectionlevel. Todetectadeectrateo1per1000with95% probability,wewouldneedtotake3000samples.

 Analy tical me thods

Therearemanymethodsavailableordetectingbac-teriaandestimatingtheirnumber.Themostcommon onesaregiveninthetablebelow.

Analytical method Accuracy Cost Bacteriologicalplating ••• •• BacterialATP ••• •••

Sensory •• •

pH • •

Fig. 3. The relation between deective product samples in percent and the total number o sampl es to be analysed  or bacteria needed to fnd one or more deects i n samples at 90 and 95% levels o probability.

Highlysensitivemethodsdetectthemostbacteriabut arealsothemostexpensive.Inbacteriologicalplating, asampleoliquidoodisspreadoutonnutrientagar platesandanycoloniesthatariseromindividualbac- teriaaterincubationarecountedtoestimatethecon-centrationobacteriaintheproduct.Severalmethods areavailablethatdisruptthebacterialwallsothata compoundcalledadenosinetriphosphate(ATP)present invegetativecellscanleakoutandbemeasuredto estimatethenumberoviablebacteria.SensoryandpH methodsarethecheapestbuthavepooraccuracy. Whichmethodischosendependsonabalance betweenthemaximalacceptabledeectrate,analytical costandthecompetenceocompanypersonnel. AccordingtotheEuropeanCouncilDirective,random samplesoUHTmilkmustbecheckedusingthebacte-riologicalplatecounttechnique,althoughthenumber osamplesisnotstated.

Thestatisticalresultsareaected

onlybythesamplingsizeandnotby

thetotalnumberounitsproduced.

300 200 100 0 NUMBER OF SAMPLES DEFECT RATE % 1 1.5 2 3 4 5 6 7 8 910



Increasing sterilization efciency

Theeciencyoasterilizationprocesscanbeexpressed asthenumberodecimalreductionsinmicrobial populationachievedbytheprocess.

This is mainly determined by two actors: 1. Thetemperatureandthelengthotime i t i s a ppl ie d. 2. Theheat-resistanceobacterialspores present,D-value. Someotheractors,e.g.thecomposition,viscosity,uni-ormityandpHotheliquidoodproduct,willalso aectthesterilizationeciency.FortypicalUHTpro-cessingequipmentitcanbeassumedthatanaverage o9–10decimalreductionscanbeachievedwithbac-terialsporesgrowingatambienttemperatures.

Initial spore Spore load/l UHT decimal Spores/l Defect load /ml reduction in product rate 10000=104 ₁₀7 _{9D 0.01 1:100}

100=102 ₁₀5 _{9D 0.0001 1:10 000}

More spores, more problems

Inadditiontotheactorsmentionedabove,thee-ciencyotheUHTprocessisinfuencedbythespore contentotheliquidood,i.e.thetotalsporeload,ed intothenalheat-treatmentsectionotheUHTplant. Whatistheeectodierentinitialsporeloadson end-productquality,assumingidenticalprocessing conditions?Itheproductislledinto1-litrepackages, thedeectratecanbedeterminedromtheinorma-tiongiveninthetable. Figure4illustratestheeectoincreasedsterilization temperatureoninitialsporeload,andtheeectodi-erentsporeloadsonthesametemperature.

Fig. 4. Increasing the temperature or a particular initial  spore load achieves a quicker killing rate. Increasing the  initial spore load or a particular temperature leads to a longer time required to achieve the same spore reduc-tion as or a lower spore load.

105 104 103 102 101 100 10-1 10-2 10-3 10-4 10-5 NUMBER OF SPORES TIME High temperature Low temperature

Sterilizationeciencycanbeexpressed

asthenumberodecimalreductionsin

microbialpopulation.



 A mixture o remedies

Theimportanceoinitialsporeloadonsterilization eciency,andhenceend-productquality,isobvious. Butwhatcanbedonetoimprovethesituation?The mostimportantmethodsoachievingthisaregiven belowandsummarizedinthetable. Improved hygiene Ahighconcentrationosporesmayalreadybepresent inmilkatthearm,orbeintroducedintorawmaterials asaresultopoorpreprocessingroutines.Theanswer hereistoimprovethelevelohygienebothatthearm andtheprocessingplant.However,itisdiculttocon- trolwhathappensonaarmanditisnotalwayspos-sibletochangemilksuppliers. Spore reduction Agoodwayoimprovingtheend-productistoreduce thesporecontentorawmaterialsbeoretheliquid oodisheat-treated.Processessuchasbactougation andmicroltrationarespeciallydesignedtogreatly reducethesporecontentomilk.

Eective heat treatment

Anincrease intemperatureand/orprolongingthe productholdingtimeduringheattreatmentwillresult inamoreeectivesterilizationprocess.Asaconse-quence,thenumberodecimalreductionswillincrease andthedeectlevelwilldecrease.

Spore reduction remedies

Process Remedial action to reduce spore count Ef fectiveness

Milking Improve arm hygiene Helpul but hard to control Change suppliers

Preprocessing Improve plant hygiene Helpul Sporereduction Bactougation,microltration

or other measure Eective UHTtreatment Increasetemperature

Prolong holding time Very eective

SporereductionandimprovedUHT

treatmentarethemosteectivemeans

oincreasingsterilizingeciency.



UHT treatment

Two principles

TherearetwodierentUHTmethodsusedinthe liquidoodindustry– directand indirect heating. Whichonetousedependsonanumberoactor ssuch asdesiredendproductqualitylevelandproduction economy. Direct heating Whenusingdirectheating,theproductisbroughtinto directcontactwithhotsteamunderstrictlycontrolled conditions.Thesterilizationtemperature(140°Cor4 sec)israpidlyreachedandaterholding,thetempera-tureisloweredbyfashcoolinginavacuumvessel.The rapidheatingandcoolinggivesminimalheat-loadon theproductwithahigh-qualityproductastheresult. Indirect heating Inthismethodapartitionisplacedbetweentheprod-uctandtheheatingorcoolingmedium.Theprinciple isthatahotmediumisfowingononesideothepar-titionandtheproductontheother.Thepartitionis heatedupandtranserstheheatovertotheproduct lowwithoutanydirectcontactbetweenthehot mediumandtheproduct.Thesameprincipleapplies tocooling. Thisprocedureisdoneinwhatisreerredtoasaheat exchanger.Whichonetochoosedependsonseveral actors,e.g.productfowrate,physicalpropertieso theliquid,requiredrunningtime,cleaningrequire-mentsetc. Theollowingthreetypesoheatexchangersarethe mostwidelyused: • Plateheatexchangers(PHE)aremainlyused orsmoothlowviscousliquidproducts. • Tubularheatexchangers(THE)canalsohandle productswithparticlesuptoacertainsizeand oerlongerrunningtimes.Themaximumsize oparticlesdependsonthediameterothetube. • Scraped-suraceheatexchangers(SSHE)are speciallydesignedorheatingandcoolingo viscous,stickyandlumpyproducts,withor withoutparticles. TheeciencyandfexibilityoUHTprocessingarecon-tinuouslybeingimproved.OnetypeoUHTmodule incorporatesboth directand indirect heating with tubularheatexchangerstoprovidedierentcombina-tionsoheattreatmentinonesystem.



Microfltration

Microltrationminimizesthebacterialandspore

contentomilkinacost-eectiveway.

AcombinationomembraneltrationandUHTtreat-mentisproventobethemosteectivemethodo increasingsterilizingeciency. Byusingamicroltrationmembrane,99.5-99.95% obacteriaandsporesareeectivelyremovedinacost- ecientway.Thisisachievedbylettingtheliquidsub-stancepassoveramembraneathighspeedunder pressureinaclosedsystem. Themembranehasveryspecicpropertiesthat onlyallowthebacteriaandsporesoa“maximumsize” topassthroughthemembrane.Sincethesizeand weightotheunwantedbacteriaandsporesareknown, thelterusedisconstructedwithappropriatepore sizestoremovethesepartsromtheliquidandletthe smallerprotein,lactose,mineralandwatermolecules passontonalUHTtreatment. Themainbenetsothisprocedurearethatthe productwillbemorepuriedbeoreUHTtreatment andthenalproductwillnotcontainanydeadspores. Thisgivesanend-productwithhigherquality,better tasteandlongershel-lie.



Summary

• SporesposethebiggestchallengetoUHTprocessing anddeterminethesterilizationparameters. • Absolutesterilitycanneverbereachedorguaranteed. • Eachlinkintheprocessingandpackagingchainmust beequallystrong. • Acceptancequalitylevels(AQL)ormanintegralpart oacompany’squalitypolicy. • Statisticalresultsareaectedonlybythesamplingsize andnotbythetotalnumberounitsproduced. • Moresampleswillimprovethedetectionlevel. • Sterilizationeciencycanbeexpressedasthenumber odecimalreductionsinmicrobialpopulation. • SporereductionandimprovedUHTtreatmentarethe mosteectivemeansoincreasingsterilizingeciency.



Tt Pk, n ProTecTs whaT’s good

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