Sterility 63601 Tetra Pak Document

7/28/2019 Sterility 63601 Tetra Pak Document

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An introductionto commercial sterility


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3 Introduction

4 Histor y

4 Single-cellriendsandenemies

5 Thelogarithmicorder

7 Approachingabsolutesterility

8 Aweaklinkbreaksthechain

9 Meetingmarketrequirements

10 Howmanysamples?

11 Increasingsterilizationeciency

12 Amixtureoremedies

13 UHTtreatment

14 Microltration

15 Summar y

Index


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Thepurposeothisbookletistoprovideabasicunder-

standing othe microbiologicalaspects oaseptictechnologywiththeocusontheprocessingpart.We

donotaimtocovereverythingindetail,butinsteadtry

togiveyouagoodoverviewotheimportanceocom-

mercialsterilityandhowtoachieveit.Iyouneedmore

detailedinormation,donothesitatetocontactusat

TetraPak.Wearehappytoassistyouwithourcompe-

tenceandexperience.

Introduction

Allqualityisbasedoncompetence.


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History

The destruction o microorganisms

EvenbeoreLouisPasteurprovedoveracenturyagothatmicroorganismscausedermentationanddis-

ease,NicolasAppert,aParisianconectioner,irst

succeededinpreservingcertainoodsinglassbottles

thathadbeenkeptinboilingwaterorvaryinglengths

otime.Thishappenedintherstdecadeothe1800s

Single-cell riends and enemies

SporesposethemajorchallengetoUHT(ultrahigh

temperature)processinganddeterminethesteriliza-

tionparameters.

Microorganismscarryoutinnumerableunctions

thatareessentialtolieonearth.However,asregards

oodproducts,theyarepublicenemynumberone.

Manybacteria,someowhicharepathogenictoman,

areresponsibleortherapidspoilageoood.Applying

asuitabletemperature(e.g.72Cor15s)degrades

proteins,essentialcomponentsolivingcells,andsoit

isquiteeasytokillvegetativebacteria.

Themajorproblemisbacterialspores.Theseare

diculttokillbecausetheyhavethickprotectivewalls

whichincreasetheirresistancetoheat,drying,reez-

ing,chemicalagentsandradiation.Thisiswhyspores

posethebiggestchallengetoUHTprocessingandde-

terminetheparametersusedorsterilization.

andby1839,tin-coatedsteelcontainerswerewidelyinuse.Thusbeganthebirthohighheattreatmentand

canningasameansopreservingood.

Today,acombinationocontinuousheat-treatment

andasepticpackagingproduceshighqualityliquid

oodinacost-eectiveway.


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The logarithmic order

A never-ending story

Asepticprocessinginvolvesoneormoreseparatester-ilization stepsaimed at killingall microorganisms.

Whenapplyinga sterilizationprocedure,e.g.using

heat,notallbacterialsporesarekilledatthesametime.

Acertainnumberwillbekilledwithinagivenunito

timeasshowninthetablehere.Theprinciplebehind

thekillingprocessisindependentothesterilization

temperature.

Whenthenumberosurvivingsporesisplotted

againsttheirlogarithmicreduction,weobtainastraight

lineasshowninFigure1.Thisstraightlineiscommonly

knownasthelogarithmicorderodeath,andtheslope

othelineindicatestherateodestruction.

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 ef-ciency. The initial microbial load is here assumed to be 100 000 bacterialspores o equal resistance. D is the decimal reduction time.

UNIT SPORES KILLED TOTAL LOGARITHMICOF TIME SURVIVORS PER UNIT TIME ACCUMULATED REDUCTION

0 100000 0 0 01 10000 90000 90000 12 1000 9000 99000 23 100 900 99900 34 10 90 99990 45 1 9 99999 5

6 0.1 0.9 99999.9 6

7 0.01 0.09 99999.99 78 0.001 0.009 99999.999 8

1000000

100000

1000

100

10

1

0.1

1.01

0.001

0.0001

NUMBER OF SPORES

TIME

1 2 3 4 5

D


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Approaching absolute sterility

Asalogarithmicunctionnevercanreachzerobutonly

approachit,absolutesterilitynevercanbereachedorguaranteed.Theeciencyoasterilizationprocessis

expressedbythenumberodecimalreductionsinthe

countoresistantspores.TheD-value(decimalreduc-

tiontime)isnormallyusedtoindicatethekillingrate,

i.e.thetimerequiredatacertaintemperaturetokill

90%othespores,whichisthesameasreducingtheir

numberbyonelogarithmunit.Measuredunderthe

sameconditions,theD-valuewillbeaectedbythe

speciesobacteria.

The result o a heat sterilization process

depends on:theinitialnumberomicroorganisms;

thedecimalreductiontime(heat-resistance)

omicroorganisms;

thetimeandtemperatureoheatexposure.

AnotherimportantparameteristheZ-value,

whichisthechangeintemperatureneeded

toaltertheD-valuebyaactoroone

logarithmunit.

Absolutesterilitycanneverbereachedorguaranteed.

Fig. 2. The Z-value is the increase in temperature needed to achievethe same lethal action or the same eect in one-tenth o the time.

1000

100

10

1

0.1

D-VALUE

TEMP C

120 130 140 150 160

Z


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A weak link breaks the chain

Asepticprocessingandpackagingisrathercomplex

becausea minimum othree dierentsterilizationstepsare required toachieveaseptic llingo the

liquidoodproduct.Sterilizationiscarriedout:

ontheequipmentpriortoproduction;

ontheliquidoodproduct;

onthepackagingmaterialsurace.

Theperormanceleveloanasepticplantmustbere-

gardedastheperormancelevelotheentireprocess

ratherthanthatoasinglecomponentotheproduc-

tionline.Eachlinkintheprocessingandpackaging

chainmustbeequallystrong.

Eachlinkintheprocessingandpackaging

chainmustbeequallystrong.

Rawmaterial

Pre-processing

UHTtreatment

Asepticpackaging

Sterilization

Packagingmaterial


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Meeting market requirements

Theprocessingresultisnotdeterminedbytheprocess-

ingactorsalonebutalsobythequalityorawmate-rial,pre-processingprocedures,environmentaland

operativeparametersaswellasmaintenance.Absolute

sterilitydenedasthetotalabsenceomicroorgan-

ismsisnotpossibleorthereasonmentionedearlier

andthuscannotbeguaranteed.

Commercial sterility

AccordingtotheWHO/FAO,commercialsterility

olow-acidoodisdenedasollows:

Commercial sterility means the absence o

microorganisms capable o growing in the

ood at normal non-rerigerated conditions

at which the ood is likely to be held during

manuacture, distribution and storage.

(Re: Codex Alimentarius Commission (WHO/

FAO) CAC/RCP 0-)

TheEUdenestheheattreatmentnecessaryto

achievingcommercialsterility.

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) sufcient to ensure that the products

remain microbiologically stable ater in-

cubating or days at 0C in closed con-

tainers or or seven days at C in closed

containers or ater any method demonstrat-

ing that the appropriate heat treatment has

been applied. (Re: Commission Regulation

(EC) No /00 (amending Regulation

(EC) No /00)).

Speciying quality levels

Thepracticalconsequencesoabsolutesterilitybeingunobtainable means establishing microbiological

specicationsortheend-product.Thisisnecessaryin

ordertodevelopaviablequalitycontrolprogramme.

Suchspecicationsoracceptancequalitylevels(AQL)

ormanintegralpartoacompanysqualitypolicy.

A microbiolog ical A QLor long- li ep r oducts

shouldbeexpressedasamaximalacceptabledeect

rate.Acleardistinctionhastobemadebetweenthe

maximalacceptabledeectiveratesandthegoalo

production.Regardlessoestablishedmaximalaccept-

abledeectrates,thegoaloproductionmustalways

beperection.

Sampling procedure

Amaximalacceptabledeectrateoaround1per1000

to10000samplesisgenerallyapproved.Inorderto

detectsuchdeectrates,samplingstatisticsmustbe

introduced.Thisincludesdeningtheleveloproba-

bility(condencelevel)tobeusedwhencheckingor

deectrates.Usually,a9095%condencelevelo

achievingacorrectresultisused.


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0

How many samples?

More samples will improve the detection level

Todeterminehowmanysamplesweneedtotaketodetectadeectiveproduct,letslookatatypicalex-

ample.Howmanysamplesarerequiredtodetect

adeectrateo1per1000with90%probability?

Thisproblemissolvedwiththehelpotherandom

samplingcurveor90%probabilityshowninFigure3.

Iweollowthe1%gureuptowherethecurveinter-

sectsthey-axiswegettheresult240.For0.1%(1per

1000)theanswertothequestionis2400.Thestatistical

resultsareaectedonlybythesamplingsizeandnot

bythetotalnumberounitsproduced.

Letstakeanotherexampleusingthesame90%

probabilitycurve.Iwetake50samplesromabatch

andnoneothemaredeective,whatisthedeect

rate?

Inthiscasewetakeahorizontallinerom50sam-

plesonthey-axis,andwhereitintersectsthe90%

curveisthedeectrateatthisleveloprobability:less

than5%,orabout5samplesper100units.Moresam-

pleswillimprovethedetectionlevel.

Todetectadeectrateo1per1000with95%

probability,wewouldneedtotake3000samples.

Analytical methods

Therearemanymethodsavailableordetectingbac-

teriaandestimatingtheirnumber.Themostcommon

onesaregiveninthetablebelow.

Analytical method Accuracy Cost

Bacteriologicalplating BacterialATP

Sensory

pH

Fig. 3. The relation between deective product samples

in percent and the total number o samples to be analysedor bacteria needed to fnd one or more deects in samplesat 90 and 95% levels o probability.

Highlysensitivemethodsdetectthemostbacteriabut

arealsothemostexpensive.Inbacteriologicalplating,asampleoliquidoodisspreadoutonnutrientagar

platesandanycoloniesthatariseromindividualbac-

teriaaterincubationarecountedtoestimatethecon-

centrationobacteriaintheproduct.Severalmethods

areavailablethatdisruptthebacterialwallsothata

compoundcalledadenosinetriphosphate(ATP)present

invegetativecellscanleakoutandbemeasuredto

estimatethenumberoviablebacteria.SensoryandpH

methodsarethecheapestbuthavepooraccuracy.

Whichmethodischosendependsonabalance

betweenthemaximalacceptabledeectrate,analytical

costandthecompetenceocompanypersonnel.

AccordingtotheEuropeanCouncilDirective,random

samplesoUHTmilkmustbecheckedusingthebacte-

riologicalplatecounttechnique,althoughthenumber

osamplesisnotstated.

Thestatisticalresultsareaectedonlybythesamplingsizeandnotby

thetotalnumberounitsproduced.

300

200

100

0

NUMBER OF SAMPLES

DEFECT RATE %

1 1.5 2 3 4 5 6 7 8 910


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Increasing sterilization efciency

Theeciencyoasterilizationprocesscanbeexpressed

asthenumberodecimalreductionsinmicrobialpopulationachievedbytheprocess.

This is mainly determined by two actors:

1. Thetemperatureandthelengthotime

i ti sa ppl ie d.

2. Theheat-resistanceobacterialspores

p resent ,D- value.

Someotheractors,e.g.thecomposition,viscosity,uni-

ormityandpHotheliquidoodproduct,willalso

aectthesterilizationeciency.FortypicalUHTpro-

cessingequipmentitcanbeassumedthatanaverage

o910decimalreductionscanbeachievedwithbac-

terialsporesgrowingatambienttemperatures.

Initial spore Spore load/l UHT decimal Spores/l Defect

load /ml reduction in product rate10000=10 4 107 9D 0.01 1:100

100=102 105 9D 0.0001 1:10000

More spores, more problems

Inadditiontotheactorsmentionedabove,thee-

ciencyotheUHTprocessisinfuencedbythespore

contentotheliquidood,i.e.thetotalsporeload,ed

intothenalheat-treatmentsectionotheUHTplant.

Whatistheeectodierentinitialsporeloadson

end-productquality,assumingidenticalprocessing

conditions?Itheproductislledinto1-litrepackages,

thedeectratecanbedeterminedromtheinorma-

tiongiveninthetable.

Figure4illustratestheeectoincreasedsterilization

temperatureoninitialsporeload,andtheeectodi-

erentsporeloadsonthesametemperature.

Fig. 4. Increasing the temperature or a particular initial

spore load achieves a quicker killing rate. Increasing theinitial 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

Sterilizationeciencycanbeexpressedasthenumberodecimalreductionsin

microbialpopulation.


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A mixture o remedies

Theimportanceoinitialsporeloadonsterilization

eciency,andhenceend-productquality,isobvious.Butwhatcanbedonetoimprovethesituation?The

mostimportantmethodsoachievingthisaregiven

belowandsummarizedinthetable.

Improved hygiene

Ahighconcentrationosporesmayalreadybepresent

inmilkatthearm,orbeintroducedintorawmaterials

asaresultopoorpreprocessingroutines.Theanswer

hereistoimprovethelevelohygienebothatthearm

andtheprocessingplant.However,itisdiculttocon-

trolwhathappensonaarmanditisnotalwayspos-

sibletochangemilksuppliers.

Spore reduction

Agoodwayoimprovingtheend-productistoreduce

thesporecontentorawmaterialsbeoretheliquid

oodisheat-treated.Processessuchasbactougation

andmicroltrationarespeciallydesignedtogreatly

reducethesporecontentomilk.

Eective heat treatmentAnincrease intemperatureand/orprolongingthe

productholdingtimeduringheattreatmentwillresult

inamoreeectivesterilizationprocess.Asaconse-

quence,thenumberodecimalreductionswillincrease

andthedeectlevelwilldecrease.

Spore reduction remedies

Process Remedial action to reduce spore count Ef fectiveness

Milking Improvearmhygiene Helpulbuthardtocontrol

Changesuppliers

Preprocessing Improveplanthygiene Helpul

Sporereduction Bactougation,microltration

orothermeasure Eective

UHTtreatment Increasetemperature

Prolongholdingtime Veryeective

SporereductionandimprovedUHTtreatmentarethemosteectivemeans

oincreasingsterilizingeciency.


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UHT treatment

Two principles

TherearetwodierentUHTmethodsusedintheliquidoodindustry directand indirect heating.

Whichonetousedependsonanumberoactor ssuch

asdesiredendproductqualitylevelandproduction

economy.

Direct heating

Whenusingdirectheating,theproductisbroughtinto

directcontactwithhotsteamunderstrictlycontrolled

conditions.Thesterilizationtemperature(140Cor4

sec)israpidlyreachedandaterholding,thetempera-

tureisloweredbyfashcoolinginavacuumvessel.The

rapidheatingandcoolinggivesminimalheat-loadon

theproductwithahigh-qualityproductastheresult.

Indirect heating

Inthismethodapartitionisplacedbetweentheprod-

uctandtheheatingorcoolingmedium.Theprinciple

isthatahotmediumisfowingononesideothepar-

titionandtheproductontheother.Thepartitionis

heatedupandtranserstheheatovertotheproduct

lowwithoutanydirectcontactbetweenthehotmediumandtheproduct.Thesameprincipleapplies

tocooling.

Thisprocedureisdoneinwhatisreerredtoasaheat

exchanger.Whichonetochoosedependsonseveralactors,e.g.productfowrate,physicalpropertieso

theliquid,requiredrunningtime,cleaningrequire-

mentsetc.

Theollowingthreetypesoheatexchangersarethe

mostwidelyused:

Plateheatexchangers(PHE)aremainlyused

orsmoothlowviscousliquidproducts.

Tubularheatexchangers(THE)canalsohandle

productswithparticlesuptoacertainsizeand

oerlongerrunningtimes.Themaximumsize

oparticlesdependsonthediameterothetube.

Scraped-suraceheatexchangers(SSHE)are

speciallydesignedorheatingandcoolingo

viscous,stickyandlumpyproducts,withor

w it houtpart icles.

TheeciencyandfexibilityoUHTprocessingarecon-

tinuouslybeingimproved.OnetypeoUHTmodule

incorporatesboth directand indirect heating with

tubularheatexchangerstoprovidedierentcombina-tionsoheattreatmentinonesystem.


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Microfltration

Microltrationminimizesthebacterialandsporecontentomilkinacost-eectiveway.

AcombinationomembraneltrationandUHTtreat-

mentisproventobethemosteectivemethodoincreasingsterilizingeciency.

Byusingamicroltrationmembrane,99.5-99.95%

obacteriaandsporesareeectivelyremovedinacost-

ecientway.Thisisachievedbylettingtheliquidsub-

stancepassoveramembraneathighspeedunder

pressureinaclosedsystem.

Themembranehasveryspecicpropertiesthat

onlyallowthebacteriaandsporesoamaximumsize

topassthroughthemembrane.Sincethesizeand

weightotheunwantedbacteriaandsporesareknown,

thelterusedisconstructedwithappropriatepore

sizestoremovethesepartsromtheliquidandletthe

smallerprotein,lactose,mineralandwatermoleculespassontonalUHTtreatment.

Themainbenetsothisprocedurearethatthe

productwillbemorepuriedbeoreUHTtreatment

andthenalproductwillnotcontainanydeadspores.

Thisgivesanend-productwithhigherquality,better

tasteandlongershel-lie.


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Summary

SporesposethebiggestchallengetoUHTprocessing

anddeterminethesterilizationparameters. Absolutesterilitycanneverbereachedorguaranteed.

Eachlinkintheprocessingandpackagingchainmust

beequallystrong.

Acceptancequalitylevels(AQL)ormanintegralpart

oacompanysqualitypolicy.

Statisticalresultsareaectedonlybythesamplingsize

andnotbythetotalnumberounitsproduced.

Moresampleswillimprovethedetectionlevel.

Sterilizationeciencycanbeexpressedasthenumber

odecimalreductionsinmicrobialpopulation.

SporereductionandimprovedUHTtreatmentarethe

mosteectivemeansoincreasingsterilizingeciency.


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