Mechanical SeparationDivision

Westfalia SeparatorFood Tec

Separatorsfor the Dairy Industry

4 1. Dairy Technology Today 6 2. Factors Affecting Creaming of Whole Milk

6 2.1 FactorsAffectingMilk Production

6 2.1.1 Breedofcow,climate feedingconditions

6 2.1.2 Mechanicalstressinmilk production

9 2.2 FactorsAffectingMilk Processing

9 2.2.1 Transportingthewholemilk tothedairy

9 2.2.2 Milkreception,wholemilkstore

10 2.2.3 Ageofthemilk

11 2.2.4 Qualityofthewholemilk

15 2.2.5 Sizedistributionofthefatglobules

15 2.2.6 Effectofair inthemilkon separationefficiency

18 2.2.7 Separationtemperature

18 2.2.7.1 Warmmilkseparation

20 2.2.7.2 Coldmilkseparation

20 2.2.8 Fatcontentinthecream

21 2.2.9 Throughputoftheseparator

22 2.2.10 Designcriteriaforthemilk processingline

22 2.2.10.1Balancetank

23 2.2.10.2Pumps

23 2.2.10.3Pipelinesystem

23 2.2.10.4Plateheatexchangers

24 2.2.10.5Separatorsettings

25 2.2.10.6 Installationof coldmilkseparators

27 2.2.11 Cleaning-in-place(CIP)

Contents

3WestfaliaSeparatorFoodTec

28 3. Milk Separators

28 3.1 TypeofConstruction

28 3.1.1 Separatorswithsolid-wallbowl

29 3.1.2 Separatorswithself-cleaningbowl

29 3.2 WarmMilkSeparators

29 3.2.1 General

30 3.2.2 SkimmingseparatorwithHydroSoft feedsystem

30 3.3 ColdMilkSeparators

31 3.4 HydraulicSystemforAutomatic BowlEjections

33 3.5 SeparatorTypesand FeedCapacities34 4. Special Processes 34 4.1 ButtermilkSeparation

34 4.1.1 Theproductasacriterionforselecting therightseparatormodel

34 4.1.2 Processparameters

35 4.1.3 Separationefficiency

36 4.2 WheySeparation

36 4.2.1 Criteriafordesigningawhey separationline

36 4.3 WheyConcentrateSeparation

36 4.4 RetentateSeparation

37 4.5 CreamConcentration

37 4.5.1 Concentrationoflowcream contentto4050percent

37 4.5.2 Increasingtheconcentration of40percentcream38 5. Measuring Methods for Determining the Residual Fat Content of the Skim Milk 40 6. Automatic Fat Standardising Installations

Today,thereareapplicationsforseparators inallareasofmilkprocessingsuchas:

Warmmilkseparation Coldmilkseparation Wheyseparation Buttermilkseparation Milkandwheyclarification Milkstandardisation Andtheremovalofbacteriafrom

milkanddairyproducts

Theprocessingofcertainproductssuchas:

Quark(softcheese) Doublecreamcheese Butteroil Low-fatwheypowders,suchasWPCs Optimisationinproductionoflactose Recoveryofsinglefractionssuchasfatsandproteinsisnolongerpossiblewithouttheaidofspeciallydesignedseparators

All the separators represent the state-of-the-artincentrifugeconstruction.Theycanbeoperatedcontinuouslyandofferthehighestlevelofproductsafetyandefficiency.

Continuedrationalisationandautomationindairieshasmadeitnecessaryforengineerstomoderniseoldprocessesanddevelopnewones.Centrifugalseparatorsareplayingan importantpart in thisrethinkingprocess.

Theadvancedtechnologicalsophisticationofsepa-ratorsenablestheupdatingofprocessestomeetmodern economic demands. Todays separa-tor installations, incorporatingcleaning-in-placesystems,canbeoperated24hoursaday.

Westfalia Separator HyVOL PROPLUS separa-tors with integrated Protein-Plus-System

When it comes tomakingdecisions aboutnewinvestments,WestfaliaSeparatoroffersanewstand-ardinmilkprocessing.

WestfaliaSeparatorHyVOLPROPLUSbrands: thenewseparatorgenerationthatcombines theex-cellent featuresof theHyVOL separatorswithPROPLUS,theProtein-Plus-System.Thebenefitsaresubstantiallyincreasedproteinyieldandsignificantcostsavings.

1. Dairy Technology Today

Anewgenerationmilk

separatorwith

SoftStreaminletsystem,

typeMSE500-01-777

5

WestfaliaSeparator

FoodTec

Fastpaybacktimesandadditionalprofitfromtherawmilkusedthroughouttheentirelifecyclearetheresultsyoucanexpectrightfromtheoutset.

WestfaliaSeparatorHyVOL PROPLUSseparatorsgiveyouthefollowingbenefits:

PROPLUS,theProtein-Plus-System Increasedproteinyield,reducedwatercon-

sumption,reductionofthesolidsvolume Absoluteavailability

Highthroughputs,highefficiency,high economy,auniversalsolution

Absoluteintegration Flexibleprocessing,highproductquality

Absoluteproductprotection Gentlefeedsystem,gentleproducttreatment

Absoluteintelligence Easyoperation,uniformsolidsdischarge, optimumyield

Absoluteeconomy Lowmaintenancecosts,lowoperatingcosts, lowwaterconsumption,lowenergyrequire-

ment Absoluterobustness

Problem-freeoperation,service-friendly, longservicelife

6WestfaliaSeparator

FoodTec

Nexttoprotein,milkfatisthemostvaluablecom-ponentofthemilk.Basedonthedrymattercon-tent,themilkfatconstitutesabout30percentofthemilk.

Thefollowinginterdependentcriteriaandvariablesareofgreatimportance:

Nutritivevalue Variables:breedofcow,lactationtime, climateandfeeding

Physicalproperties Variables:mechanicaland heattreatment

Chemicalproperties Variables:enzymereactions, bacterialinfluence

Economicimportance Variables:efficiencyofmechanical separationprocesses

2. Factors Affecting Creaming of Whole Milk

Inadditiontoexplainingthedesignpossibilitiesofcentrifugalseparationtechnologyasappliedtotheseparationofwholemilk,thisdocumentdeals inparticularwithanumberofimportanttechnicalandengineeringparametersthathaveadecisiveinflu-enceontheresidualfatcontentintheskimmilk.

2.1 Factors affecting milk production 2.1.1 Breed of cow, climate, feeding conditions

Apartfromanumberofindividualtestswhichdonotrepresentanacceptablecrosssection,insuffi-cienttestshavebeencarriedouttoprovideindis-putableevidenceoftheeffectoftherelationshipbetweenthementionedfactorsontheseparabilityof thewholemilk. Itcanbesaid,withcertainty,thatseasonaldifferencesintheseparabilityofthewholemilkdooccur.Theseseasonally-dependentdeviationscanvaryinintensitydependingonthedifferencesintheamountsofmilkdelivered.Devia-tionsoccurringinthenutritivevalueofthefeedtypecompositionandlittleornocontrolofthelactationperiodscanleadtovariationsintheamountofmilkdelivered.However,theeffectonseparabilityalsodependsondifferencesinthesizesofthefatglob-ulesdispersedinthewholemilk.

Figures3and4areextremeexamplesofdifferencesintheamountsdeliveredwhicharetypicalforthedairyindustryinNewZealand.

2.1.2 Mechanical strain in milk production

Thevarioussequentialprocessesthatarepartofmilkproductiongreatlyinfluencetheseparabilityofthewholemilk.

Fig.3

Proportionofdeliveredmilk

inanannualcycle

0 5 10 15 20 25 30 35 40 45 50

1000

ton

nes 260

234

208

182

156

130

104

78

52

26

0

Calendarweek

Wholemilkaverage

values:

Fat: 3.75.3%

Protein:3.34.3%

7WestfaliaSeparator

FoodTec

Theseinclude:

Preservingextractionofthemilk Transportofthemilkthroughthe

milkingplant.Caremustbetaken toseparatetheairnecessaryfor transportofthemilkasmuch

aspossiblefromthemilkitself

Necessarytoachievethisare:

Lowvacuumlevel Minimalinclinesinthepipelinesystem Avoidanceofleaksinthepipelinenetwork Adequatepipelinecross-sectionsforthe

capacityoftheplant

Ascanbeseenfromfigures1and2,theairusedtotransportthemilkisworkedintothemilkifpipelinecross-sectionsaresmall.

Inthecaseoflargecapacitycross-sections3and4,theairhasadequatespaceabovethemilk.

Fig.5

Diagramofdifferentmilkline

diametersforthesamemilk

flow28/301

38/402

50/523

66/704

Fig.4

Sizedistributionofthefat

globulesinanannualcycle

12,000

8000

4000

0.5

0.75 1.0

1.25 1.5

1.75 2.0

2.25 2.5

2.75 3.0

3.25 3.5

3.75 4.0

4.25 4.5

Fatglobulediameter[mm]

Skimmilkaveragevalues:1April-Junex=0.04in%byvol.(Gerber*)2November-Februaryx=0.05in%byvol.(Gerber*)*Spun5timesintheskimmilkbutyrometer

Volumeun

itsper[

ml]

1

2

FoodTec8

WestfaliaSeparator

Ifoptimumtransportofthemilkistobeachieved,theabsolutevacuumlevelmustbeadjustedsothatit is commensuratewith the energy consumed.Fig.6givesthedifferentpermissiblevacuumrangesforpipelineandbucketmilkingplants,aswellastherelevantpipelinediameters.

Coolingof themilkon the farm isalsoofgreatimportance.

Here,attentionmustbepaid,amongotherthings,tothefollowing:

Avoidanceoffoaming,particularlywhen themilkfromthefirstmilkingisfedinto largerefrigeratedtanks

Fig.6

Diagramofthe

permissiblevacuum

levelsinmilkingplantsNotpermissible

70/66 50/4452/50

40/3440/38

1in 11/2in

Pipelinediameter

Vacuum

level[kPa]

50

48

46

42

40

permissible permissible

Pipelineplant Bucketmilkingplant

Fig.7

Qualitycurveofthefree

fattyacidcontentandfree

fattyacidsaftermechanical

strainofthemilkorcream

0 10 20 30 40 50 60

Freefattyacids

Centrifugableorextractablefreefatcontent

TemperatureinC

Freefatcon

tent

Freefattyacids

Largetemperaturefluctuationsbythermostaticcontrolwhichhaveswitchingintervalsofe.g.34C.

Incomparison,switchingintervalsofapprox.1Ccanbeachievedwithelectronicthermo-stats.Thishelpstoreducethedangeroficingofthemilk,particularlywiththemilkfromthefirstmilking,e.g.bysettingthecoolingtempe-ratureto3C

Enteringtheareaofcriticaltemperature

AccordingtoProf.Kessler,Weihenstephan,particu-larlyhighlevelsoffreefat(FF)canbeproducedbymechanicalstrain,e.g.bythestirringprocess, inthetemperaturerangeof2030C.Ascanbeseenfromthediagram,asmalloptimumareaoccursintheproductionoffreefattyacids(FFA)inthetem-peraturerangeof15C.

9WestfaliaSeparator

FoodTec

Fig.8

Coolingthemilk

onthefarm

Fig.8shows that in factonly in thecaseof thefirstmilking is there danger of damage to thefat,as thesecondandallothermilkingsdonotenterthecriticaltemperaturerangeowingtotheinstantaneously occurringmixing temperature.

2.2 Factors affecting milk processing

2.2.1 Transporting the whole milk to the dairy

Caremustbetakenwhentransportingthewholemilkthatthecoolingchainisnotbroken.Conse-quently themilk transporting vehiclesmust beequippedtosuitthetransportingtimesandclimaticconditions.

Themotionoftankvehiclesonlypartiallyfilledcaus-esturbulenceintheliquid,andthiscandamagethemilk.Correctcleaningofthetank(CIP)isessential.

2.2.2 Milk reception, whole milk store

Threeparticularlyimportantconditionsmustbemetwithrespecttotheintakeofthemilkatthedairyif theseparabilityof thewholemilk isnot tobeimpaired.

First of all, it is important to avoid asmuch aspossibletheentrainmentofextraneousairduringemptyingofthemilktruckandfillingofthetanks.Thetypeanddesignoftheagitatorcanalsoaffectthemilk.

Normallyitisnotpossibletocompletelyavoidtheentrainmentofair inthemilk.Consequently,thewholemilkshouldbegivensufficienttimetode-gasbeforefurtherprocessing.Forthisreason,thewholemilkshouldnotbefeddirectlyfromthemilktruckintotheprocessingline.

It isalso importanttoensurethateachpumpingprocessemployedforconveyingthemilkhasopti-mumhydraulicefficiency. If the feedconditionsare subject to fluctuation, thepumpsshouldbeequippedwithavariable-speeddrive.Theycanthenbesettotheiroptimumspeedofoperationviacom-parisonofthereferenceandactualvalues,e.g.byinductivemeasurementofflow.

Thisgentlepumpingprocesshasnomeasurableeffectontheseparabilityofthewholemilk.How-ever,thereisanegativeeffectwithpumpsthatarethrottled,forexampleto6070percentoftheirmaximumspeed.Asaconsequence,anadditionalresidual fatcontentofasmuchas10percentormoreisobtainedintheskimmilk.

AsshowninFig.9,itshouldnotbepossibletoinflu-encetheoperationofthepumps.

upto10h

30

25

20

15

10

5

C

Criticalarea

1stmilking 1st+2ndmilking 1st+2nd+3rdmilking

max12C

P0.80.9P0.60.7

PrisingtendencyP=pyrovatecontent

~2h ~1h ~1hbis10hmax1h max1h

Time(h)

FoodTec10

WestfaliaSeparator

Fig.9

Differentpumpoperations

fortransportingtherawmilk

Example1)showstheproblemofairentrainment.Inexamples2)and3),incomparison,mutualimpair-mentofpumpoperationalsoleadstoentrainmentofairandmechanicalstrainofthemilk.Inexample3)thepumpconnectedinparallelimpairsseparationefficiencybyupto1/100percent.

2.2.3 Age of the milk

Becauseofthetwo-daycollectionofthemilkfromthe farmscustomary today,and theconsequentneed to keep themilk cold at temperatures of35C,theseparabilityofthewholemilkisreduced.Thereasonisthat,asthemilkisheldforsolongatalowtemperature,verysmallwaterdropletsbindwiththefatglobules.Withthewholemilksimul-taneouslysubjectedtomechanicalstrain,e.g.duetostirring,thefatglobulemembraneundergoesapartialstructuralchange.Anincreaseinspecificden-sityoccurs,ontheonehandduetotheexchangeoftheoriginalmembranecomponentswithproteinsfromtheserum(carrier liquid),oraspureproteinabsorptionofthemembrane.However,thegreaterportionremainsintheskimmilk,therebyincreasingtheresidualfatcontent.

1

1.5

2

2.5

20 30 40 50 60

Freshmilk

Refrigeratedandstoredmilk

Fatco

nten

tinskimm

ilkasamultip

leofthe

fatco

nten

tat55C

MilkseparationtemperatureC

Fig.10

Residualfatcontentintheskim

milkafterprocessingofcold-stored

andfreshwholemilk

PI

PI

Bad:TankfilledfromaboveHoselinetopump

Better:TankfilledfrombelowGravityfeedbyfixedpipelinePipelinediameteradequate

Bad:Tankfilledandemptiedviaasinglepipeline

Better:OnetankbeingfilledOnetankbeingemptied

Bad:Parallelswitchingofpumps

Better:Eachpumphasitsownsuctionline

1)

2)

3)

11

WestfaliaSeparator

FoodTec

Thisprocessistosomeextentreversible.Byincreas-ingtheseparationtemperaturethefatglobulemem-branescanbereturnedalmosttotheiroriginalstate.Forthisreason,higherseparationtemperaturesareusedtodaythanwerecustomaryinthepast.Fig.10illustratesthebehaviourofseparationefficiencyasafunctionoftheholdingtimeatlowtemperature.

2.2.4 Quality of the whole milk

Theseparabilityofrawmilkdepends,inadditiontototalbacterialcount,amongotherthingsonthe:

pHlevel Freefatvalues(FF) Freefattyacids(FFA) Sizedistributionofthefatglobules

The influenceofforeignparticles isnormallynotimportant.However,ifthereisahighlevelofimpu-rityinthemilk,centrifugalclarification,e.g.directlyafterdelivery,willimproveitsseparability.Iftheseinfluencing factorshavebeen largely takencareof,astatementcanbemadeastotheextentto

whichthewholemilkhasalreadybeensubjectedtomechanicalstrain,andtherebytopossibledamageofthemilkfat.

Statements and their limitations through detection of FFA (free fatty acids by the BDI-Method)

Thevalues forFFAatdeliveryof thewholemilktothedairiesobtainedfromtheliteratureandourownmeasurementsattheendofthe1960swas0.40.5milliequiv./kg.Ifthedataobtainedsincethemiddleofthe1980sisusedasabase,then,inthemilkproducingcountrieswitharelativelyhighlevelofindustrialisationofmilkproduction,theFFAvaluesarebetween0.81.0milliequiv./kgasaruletoday.Fig.11shows,amongotherthings,thecurveoftheFFAtakenoverthecourseofayearinSwitzerlandduringthesecondhalfofthe1980s.KnowledgeoftheFFAvaluehasacertain impor-tancetodayinjudgingcold,unpasteurisedwholemilk,butterandcheese-makingprocesses.

Fig.11

Contentoffreefattyacidsand

lipolysablefatinwholemilk

ondeliverytothedairyduring

theyear(Zurich)

Freefattyacids

Lipo

lysablefat(m

illiequ

iv./l)

1.3

1.2

1.1

1.0

0.9

0.8

0.7

0.6Jan. Feb. March Apr. May June July Aug. Sept. Oct. Nov. Dec.

LF

FFS

TanktruckrouteI

TanktruckrouteK

FoodTec12

WestfaliaSeparator

Fig. 12 shows clearly that the results obtaineddependtoagreatextentontheleveloftheinitialvalue.Thetestresultsaretherebynotreproducible.ItisquiteclearevenwithalowFFAvaluethattheincubationtimehardlyplaysapart.Toobtainreli-ablevalues,theincubationtimeshouldbeatleast3hoursat36C.

Anadditionalfactorthatnegativelyaffectsthereli-abilityoftheFFAvalueisthattheproductionofFFAistimedependent,becauseofthepresenceofthelipaseenzymeandfreefat(FF).

Thespeedofthisreactiondepends,amongotherthings,onthequalityofthefree,orextractablefat,theamountofactivelipaseenzymepresentandtheproducttemperature.Consequently,theincreaseinFFAisnotproportionaltotheincreaseintheeffectontheseparabilityofthewholemilk.

Fig.12

FFAvalueasafunctionof

theshelflifeandtheinitial

FFAvalue

Asalreadymentioned,theproducttemperaturehasadirectinfluenceonthereliabilityoftheFFAvaluesobtained.Ithasalreadybeenshowninfig.8(Milkcoolingonthefarm),thattherearetwooptimumtemperatures(15Cand40C).Moreover,thelipaseenzymeisalmostinactivebelow10Candistheo-reticallykilledoffattemperaturesL60C.Practicaltests,however,stillshownegligiblelipaseactivityaboveatemperatureofabout50C.

Free fat

Methods formeasuring the levelof free fat (FF)whichprovidereliableandreproducibleresultshavebeenavailable forsometime.Twomethods, thecentrifugalandextractionmethodscanbeused,however,onlytheextractionmethodgivesreliableinformation.Thedifferenceisthatthecentrifugalmethodmeasuresonlythefatoutsidethefatglob-ulemembrane.With theextractionmethod thefatenvelopedbyaporous, i.e.partiallydamagedmembrane, isalsomeasured.The latterdamagehasanoticeableeffect,particularlywithlongstor-agetimes,asthefatturnstooilwithtime.Ifactivelipase ispresent,there isalsotheriskof lipolyticattackintheeventofdamagedmembranes.

FFA 1.5

1.3

1.1

1.0

0.8

0.6

0.5

0 3

milliquiv./l

1.4

1.2

0.9

0.7

0.4

12h

+35.2%

xx

xx

xx

+31.5%

+19.3%

+13.3%

13

WestfaliaSeparator

FoodTec

Fig.13

RelationshipbetweenFFA

valuesandresidualfatcontent

intheskimmilk

Thecurveof theFFAvalue, ( ) inFig.14,wouldleadtowrongconclusions.Testseries1=milkedbyhand,showsclearlythehighqualityoffreshhand-milkedwholemilk( ).Atthesametime,however,there is thequitenegativeeffectofcoldstorage( , ).

Inspectionoftestseries(2)and(3)showsanidenti-caltendencyintheslopeofthecurve.ThisallowsthefollowingconclusionstobedrawnwithrespecttotheFFvalues:

Inthecaseofmachinemilked,cold-storedmilkthesamplesmustbestoredforatleast

24hoursat46Ctoallowreliabletesting oftheFF

AnAGFvalueof4percentisgivenasthemaximumforastillwell-separatedwholemilk,

asfrom4percentthereisasteepriseinthecurvetoanuppermeasurement(case2)as

earlyasthefirstbasicoperation(Inthetestthemechanicalstrainspecificallyexceededthe

normalmeasurement) At23percentAGFintheinitialsamplethefirstmechanicalstrain(case3)raisestheFF

value,butonlythesecondtakesthevaluefromabout4percenttothehighupperlevel

Fig.14

InterrelationoftheFFvalues

andmechanicalstrainfor

differentqualitiesofwholemilk

' Mechanicalbasicactionsuperposedonthemilk'' Thesamemechanicalbasicactionsuperposedtwice onthemilk

1 Handmilked2 Every2daysfromfarmersi.e.4milkings3 Wholemilkfrombalancetankindairy freshtested 24h56Cstoragetemp. 36h56Cstoragetemp.

1.5

1.3

1 4

1.4

1.2

1.1

1.0

Samples

1.5

1.3

1.4

1.2

1.1

1.0

2 3 5 6 7 8 9

FFA1 FFAcontentindeliveredmilkFFA2 FFcontentinmilkimmediatly beforeseparation

FSK measuredby gravimetricanalysis

Freefattyacids,F

F[ FFA

2]

FFA1

Residu

alfatcon

tet,F

MM[F SK

measured

] F S

Km

in

1 2'

2

1'' 2 2'' 3 3' 3''1'

4

6

8

10

12

14

16

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Effectofco

ld

clarificatio

n

AGF[%

]

FFA[m

illiquiv./l]

FoodTec14

WestfaliaSeparator

Fig.16

Sizedistributionofthefat

globulesintheskimmilk

Fig.15

Sizedistributionofthefat

globulesinthewholemilk.

ValuesIandIIareforthedif-

ferentcatchmentareas.

1.03

0

1.09

1.10

1.23

1.30

1.38

1.47

1.56

1.65

1.75

1.86

1.97

2.10

2.22

2.36

2.50

2.66

2.82

2.99

3.18

3.37

3.58

3.80

4.03

4.27

4.54

4.81

5.11

5.42

5.75

6.10

6.48

6.57

7.30

7.74

8.22

8.72

9.25

9.82

10.42

11.06

11.73

25

50

I

II

Fatglobulediameterinm

5242

8810

4857

6

Volumeun

itsin

l

Wholemilk:FRGFatcontent:4%Periodtested:2/87

75

1572

864

0.70

0

0.79

0.89

1.00

1.12

1.26

1.42

1.60

1.81

2.03

2.29

2.58

2.90

3.27

3.68

4.15

4.67

5.26

5.93

6.67

7.52

25

50

I

II

Fatglobulediameterinm

4096

8192

Volumeun

itsin

l

Periodtested:2/'87

7512

288

0

15

WestfaliaSeparator

FoodTec

Fig.17

Separationcurves(whole

milktoskimmilk)

2.2.5 Size distribution of the fat globules

Therelativelyslightdifferencesinthesizedistribu-tionofthefatglobulesinthewholemilk(seeFig.15)onlyallowtrendstobeidentifiedtoalimitedextent.ValuesIandIIareforthedifferentcatch-mentareas.Fig.16showsthesizedistributionofthefatglobulesintheskimmilk(SK).CurvesIandIIillustratetheeffectivenessoftheinstallationcon-ceptsandprocessparameters.However,itispossi-bletooptimisethevaluesininstallationII.Asalreadymentioned,theabsoluteresidualfatcontentintheskimmilk thatwouldbeattainable,canonlybedeterminedasatrend.Bymeansofaparticlecount,thediagraminFig.17showsevenmoreclearlythedifficultyofprovidingareliableassessmentofthequalityoftheseparabilityofwholemilk.Here,thesizedistributionofthefatglobulesinthewholemilkandskimmilkarecompared. ItcanbeseenthattheseparationcurvesIandIIarealmostcongruentintheirbehaviour.IncomparisonwithFig.16theskimmilkIIhasadistinctlyhigherresidualfatcon-tent.TheresidualfatlevelsinSKIandSKII,testedinthelaboratorybytheGerbermethod,were0.045(I)and0.054(II).

Thuscountingthefatglobulesinthewholemilkorskimmilkhasonly limitedvalidity. IfFig.15istaken forcomparison, thenwithaclear shiftoftheoptimumforthesizedistributiontowardssmalldiameters,an impairmentof separability canbeexpected.Ontheotherhand,thesizedistributionintheresidualfatcontentoftheSK(Fig.16)doesnotfollowtheusualcourse.CurveIIinFig.16,how-ever,justifiestheclaimthatthisseparationlinecanbeoptimised.

2.2.6 Effect of air in the product onseparation efficiency

Freeaircarriedbytheproductcanhaveadirecteffectontheseparationefficiencyofthemilksepa-rator,inadditiontothenegativeeffectonthequal-ityoftheendproduct.Inevaluatingtheseeffectsthefollowingshouldbeborneinmind:wholemilkgenerallycontainsacertainamountofcombinedgas;thegassaturationpointofthemilkdecreaseswithincreasingtemperature;gascanthereforebeliberatedbyheattreatment.

0.70

0

0.79

0.89

1.00

1.12

1.26

1.42

1.60

1.81

2.03

2.29

2.58

2.90

3.27

3.68

4.15

4.67

5.26

5.93

6.67

7.52

25

50

III

Fatglobulediameterinm

3264 Sepa

ratio

nin%

EI=98.875%EII=98.65%Producttemperature53CCreamfatcontent42%Periodtested:2/'87

75

960

100

FoodTec16

WestfaliaSeparator

Ifthemilkistestedtoascertaintheactualvolumesofliberatedgas,thefollowingshouldbeborneinmind:

Inthecaseofachangeintemperature,thegasabsorptionintothemilkvaries

Ifthemilkisexposedtovacuum,thesatura-tionpointisloweredconsiderably

Inthecaseofoverpressure,thesaturationpointisraisedonlyveryslightly

ItcanbeseenfromthecurveinFig.18thatwithheattreatmentofthewholemilkinthemilksepara-tionline,gas(air)isautomaticallyliberated.Inordertobeabletoevaluatetheaircontentinthewholemilkupstreamoftheseparatorundernormalcondi-tions.

Inaccordancewiththediagram,approx.5mgO2/lgasisliberatedwhenthetemperatureofthewholemilk is increased from10C to55C. It canbeassumedthatthesaturationbehaviourofnitrogen(N2)issimilartothatofoxygen(O2).Thiscontentoffreeairisnotyetsufficienttoimpairseparationefficiency.

Afurtherincreaseingascontentisnormallyattributabletothefollowingcauses:

Theproductcontainsforeigngaswhenproces-singcommences

Thebalancetanksaretoosmall,orflowcon-ditionsinthebalancetanksareunfavourableduringfillingandemptying

Partsoftheinstallationarenotair-tight Inasystemunderpressure,aircanbedrawninwithoutlossofproductduetothe

injectoreffect Theseparatordischargepressureshavenotbeencorrectlyadjusted

Thepressurestagesinthesystemaretoolarge(asuddendropinpressurecausesgastobereleased)

Ifthevolumeoffreeairincreasestovalues>2.5per-cent(approx.55C),itwillhaveanegativeeffectontheseparabilityoftheproductintheseparator.

Fig.18

Saturationpointofgas

inliquidsasafunctionof

temperature

Oxygensaturationinwaterasafunctionofthe

temperature

xx Oxygensaturationinthemilk

ismeasuredasanincreaseinfreeaircontent20

15

10

5

0 10 20 30 40 50 C

Saturatio

npo

intmgO

2/l

x

xx

5mgO2=ca.25mgair 2510-6kgair

Thisgivesanairvolumeof:

Vair =GL

L

=2510

-6

1.29310-3