Cross-flow microfiltration of wine: Effect of colloids on critical fouling conditions

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DOI:10.1016/j.memsci.2011.08.008

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To cite this version :

El Rayess, Youssef and Albasi, Claire and Bacchin,

Patrice and Taillandier, Patricia and Mietton-Peuchot, Martine and Devatine,

Audrey Cross-flow microfiltration of wine: Effect of colloids on critical fouling

conditions. (2011) Journal of Membrane Science, vol. 382 (n° 1-2). pp. 1-19.

ISSN 0376-7388

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Cross-flow

microfiltration

of

wine:

Effect

of

colloids

on

critical

fouling

conditions

Y.

El

Rayess

_{, C.}

_Albasi

_{, P.}

_Bacchin

_{, P.}

_Taillandier

_,

_M.

_{Mietton-Peuchot}

_,

_A.

_Devatine

a_{UniversitédeToulouse;INPT,UPS;LaboratoiredeGénieChimique;4AlléeEmileMonso,F-31432Toulouse,France} b_{CNRS;LaboratoiredeGénieChimique;F-31432Toulouse,France}

c_{UniversitédeToulouse;INPT,UPS;LaboratoiredeGénieChimique;118RoutedeNarbonne,F-31062Toulouse,France} d_{CNRS;LaboratoiredeGénieChimique;F-31062Toulouse,cedex09,France}

e_{UniversitédeBordeaux,ISVV,EA4577,UnitéderechercheOENOLOGIE,33882Villenaved‘Ornon,France} f_{INRA,ISVV,USC1219OENOLOGIE,33882Villenaved‘Ornon,France}

Keywords: Cross-flowmicrofiltration Winecolloids Criticalflux Polysaccharides Tannins

a

b

s

t

r

a

c

t

Criticalfoulingconditionswerestudiedduringwinecross-flowmicrofiltrationusingamultichannel ceramicmembrane(0.2mm).Theaimwastodeterminecriticaloperatingconditionsinordertolimit foul-ingcausedbywinecolloids(tannins,pectinandmannoproteins)andenhanceprocessperformances.The methodusedisasquarewavefiltrationbasedonthedeterminationofthereversibilityandirreversibility offouling.Filtrationswereperformedwithfilteredredwine(FW)addedwithdifferentconcentrations ofcolloids.ConsideringFW,criticalfluxforirreversibilitywasbeyondthestudiedrangeofpressure (≥1.4×10−4_{m/s).Noclearcriticalfluxcouldbedeterminedforanyofthetestedmoleculesinthe}

stud-iedrangeofpressure.Ontheotherhand,anupperlimitoffluxesrangehasbeenidentified(belowwhich criticalfluxcouldbefound).Irreversiblefoulingalwaystakesplacefromthebeginningofthefiltrations andevenatlowpressures.ForFWcontaining0.2g/lmannoproteinand0.5g/lpectin,alossof aver-agefluxesisobservedbeyondagivenlimitoftransmembranepressure.Thisfactwasattributedtothe compactionofagellayer.Finally,acriterion(Rif/Rm≤1)hasbeensuggestedtodeterminetheso-called “thresholdflux”belowit,foulingremainsacceptable.

1. Introduction

Afteralcoholicandmalolactic fermentations,thecrude wine isacomplexmediumpresentingaturbidaspectthatisnotwell acceptedbytheconsumer;therefore,itneedstobeclarified.In ordertohavealimpidwine,thewinemakersimplementsuccessive solid–liquidseparationsusingtraditionaltechnologiessuchas cen-trifugation,dead-endfiltration(filterpresses,filtrationonsheets, diatomaceousearthfiltration)andtheuseofexogenicadditives.

Nowadays,diatomaceousearthisclassifiedasdangerous sub-stancesduetothepresenceofcrystallinesilica[1].Diatomaceous earthhasalsoanegativeimpactonenvironment;afteruses,it can-notbedisposedbutitmustbetransportedtowastedisposalsites tobetreated.So,environmentalandhealthrestrictionsforcethe oenologysectortosearchforalternativetechniquestotraditional filtrations. Cross-flow microfiltration could then represent this alternative.Indeed,thistechnologycansubstituteaone-step pro-ceduretotheconventionalprocesseswhichimplyseveralfiltration

∗ Correspondingauthorat:UniversitédeToulouse;INPT,UPS;Laboratoirede GénieChimique;4AlléeEmileMonso,F-31432Toulouse,France.

Tel.:+33534323626.

E-mailaddress:youssef.elrayess@ensiacet.fr(Y.E.Rayess).

stepsondiatomaceousearthprevioustothefinalmicrobial stabi-lizationobtainedbydeadendfiltrationonsheetsormembranes

[2].Inadditiontoagreatsimplificationofthewineprocessingline, cross-flowmicrofiltrationoffersanumberofadditionaladvantages suchaseliminationofearthuseanditsassociatedenvironmental problemsaswellasthecombinationofclarification,stabilization andsterilefiltrationinonesinglecontinuousoperation.

Conventionally,thecross-flowmicrofiltrationdevelopmentin winefiltrationhaslongbeenhamperedbysignificantfoulingof themembrane.Poorperformances,highcosts,andriskof exces-siveretentionofsomecomponentsarethemainconsequencesof fouling,leadingsometimestoalossofsomeorganoleptic charac-ters.Membranefoulingduringfiltrationofcomplexfluidssuchas fermentedfoodproducts(wine,beer)istheresultofinterplayof severalmechanisms.Theselattercouldbedividedinto[3,4]:

Internalfoulingbysmallparticlesandcolloidsasadsorptionand porepluggingwithintheinternalstructureofpores;

Externalfoulingbyparticles,macromoleculesandmacromolecules aggregatesasporeblockingandcakeformation

Themaindrawbackinwine filtrationliesinthedifficultyto understandwinefilterabilityandthereproducibilityofthe filtra-tions.Manyadvanceshavebeenmadebyresearchersespecially

inidentificationofwinecomponentsresponsibleoffouling and inimpactofmembranematerials[5–10].But,therestillalackof knowledgeofthemechanismscausingfouling.Theinfluenceofthe operatingconditionsonthesemechanismsisthennon-elucidated andmasteringtheprocessisnoteasy.

Membranesurfacefoulingcouldbecharacterizedwithregardto itsreversibility.Areversibleaccumulationwillberemovedwhen thetransmembranepressureisdecreased.Anirreversible depo-sitionwill beremovedonly bya physicalor chemical cleaning andwillremainwhenthepressureisreleased.Thelimitbetween reversibleandirreversiblefoulingdependsonthefluxvalue.The thresholdvalueforwhichthereversiblefoulingturnsinto irre-versiblefoulingiscalledas“criticalflux”[11].

In foodindustry, there weremany attempts todetermine a critical flux.Gésan-Guiziou et al.[12] showed whilecross-flow microfiltrationofskimmedmilkthatacriticalratioofpermeate fluxoverwallshearstresscouldbedetermined(0.9l/h/m2_/bar)

aboveitanirreversibledepositionisobserved.Youravongetal.

[13]evaluatedthecriticalfluxinultrafiltrationofskimmedmilk byplottingcriticalfluxversustheratioofwallshearstressover proteinconcentration.Theyshowedalinearrelationshipbetween criticalandwallshearstress/proteinconcentration.DeBrujinand Borquez[14]showedthatnocriticalfluxwasreachedduring ultra-filtrationofapplejuicewiththeiroperatingconditions.But,their simulationshaveshowedthatnocakeformationwasobservedat a tangentialvelocityof 7.4m/s andtransmembranepressure of 150kPa(Jcritical=6.8×10−5m/s).Thefiltrationconditionsusedin

theseworks(veryhightemperature50–55◦_{C,veryhightangential}

velocityupto7m/s)cannotbeappliedtowinefiltrationbecauseit wouldhavehugelymodifiedwinequality.

Theobjectiveofthisstudyistoinvestigateandsearchforsome criticaloperatingconditionsinwinecross-flowmicrofiltration.It hasbeenidentifiedthatsubstanceslikepolysaccharides(pectinand mannoproteins),polyphenolsandproteinsareinvolvedin mem-branefouling[5–9]butlittleinformationconcerningtheindividual impactonfoulingandthemechanismsinvolvedareavailable.The aimofthisstudyistolimitfoulingduetowinecolloids(tannins, pectinand mannoproteins) and enhanceprocess performances. Thechosenstudyprotocolwastoevaluatethecriticalfouling con-ditionsforeachofthefoulingmolecules.Thissituationdoesnot representa realwinebut itis a firststeptowards understand-ingrealwinecriticalfiltrationconditions.Themethodusedwasa squarewavefiltrationwhichallowsthedeterminationoftheextent offoulingreversibilityduringwinecross-flowmicrofiltration.This methodenabledthedistinctionbetweenreversibleandirreversible

foulingaswellasthecalculationofamoreaccuratevalueofthe criticalflux.

2. Background

Thefirstdefinitionsofthecriticalfluxconceptappearedin1995. Thecriticalfluxconceptwasdefinedas“thefluxbelowwhichno foulingoccurs”[15],as“thefluxbelowwhichadeclineoffluxwith timedoesnotoccur;aboveitfoulingisobserved”[16]oras“aflux belowwhichthereisnofoulingbycolloidalparticles”[17].

Therearetwo forms ofcritical flux: strongand weakforms

[16,18].Thestrongformisthefluxatwhichthetransmembrane pressurestartstodeviatefromthepurewater line,whichis of courselinear.Thestrongformofcriticalfluxhasbeendevelopedto discriminatenofoulingconditions.Theweakformofcriticalflux ischaracterizedbyaveryrapidfoulingonthestart-upandthe flux-transmembranepressurerelationshipisbelowthatofthepure waterline.Theweakformofcriticalfluxisthepointatwhichthis linebecomesnon-linear.

In2006,Bacchinetal.[11]refinedthedefinitionsofcriticalflux andanothertermhasbeenaddedascriticalfluxforirreversibility

Jci.Thistermwasexplained bythetransitionbetween

concen-trationpolarizationlayerstothedepositlayer.Fig.1,asseenby authors,illustratestheconceptofthiscriticalfluxforirreversibility. Whenfilteringatafluxbelowthecriticalflux,thecolloidalsystem isusuallystableastheresultofrepulsiveinteractions (interparti-cleorparticle-membrane)overcomingthedragforce(inducedby themovementofsolventthroughthemembrane).Beyondagiven valueofpermeateflux (criticalflux),when therepulsiveforces areoverwhelmedbythedragforces,adepositappearsand cre-ateshydraulicresistance.Thisphenomenonofdepositformation isnotinstantaneousandmaytakeseveralminutestoseveralhours tosettledependingontheoperatingconditionsandthetypeof particles.

Mathematically,theirreversibilityformofcriticalflux(Jci)can

bedefinedby: ForJ<Jci:J= 1P−1˘ (Rm) = 1P (Rm+Rrf) or 1P (Rm+Rads+Rrf) ,

ifadsorptiontakesplace (1)

whereJisthepermeateflux,1Pthetransmembranepressure(Pa), 1˘theosmoticpressure(Pa),thepermeateviscosity(Pas),Rm

themembranehydraulicresistance(m−1_),R

adsresistancedueto

adsorption(m−1_)andR

rfthereversibleresistance(m−1).

Fig.1.DiagramrepresentingthestateofcolloidalsystematdifferentfluxvalueswhereRm=membraneresistance,Rads=resistanceduetoadsorption,Rrf=reversible

Fig.2.Schematicrepresentation,asseenbyauthors,ofthecriticalfluxof irre-versibilityanditsrelationshipwiththestrongformandweakformofcriticalflux (Rf=foulingresistance).

Thereversibleaccumulationofmatter,afteradecreasein pres-sure,isrelatedtothepolarizationlayeranditsinducedosmotic pressure.Thislatteractasanoppositeforcetotheappliedpressure. Inthis case,thereversibleresistanceassociatedwiththe polar-ization concentrationlayercanbetreated asatermof osmotic pressure.

Abovethecriticalfluxfor irreversibility,multi-layers of irre-versiblefoulingaredetectedintheboundarylayerwhereasbelow itonlyaconcentrationofpolarizationlayerexistsinallcaseswith anadditionalmonolayerofadsorbedspeciesinsomecases[11,19].

ForJ>Jci:J= 1P−1˘ (Rm+Rif) = 1P (Rm+Rrf+Rif) or 1P (Rm+Rads+Rrf+Rif) ,

ifadsorptiontakesplace (2) whereRifistheirreversibleresistance(m−1).

Fig.2,asseenbyauthors,illustratestherelationshipofthe differ-entformsofcriticalfluxandtheresistancepresentedinEqs.(1)and (2).Thestrongformisthe“ideal”casewheretheirreversible resis-tancebelowthecriticalfluxisequaltozero.Beyondthevalueofthe criticalfluxforirreversibility,theirreversibleresistancebecomes detectable.Inotherhand,theweakformischaracterizedbyagiven valueofresistanceforlowfluxesduetotheadsorptionofmolecules onmembranematerial.Thecriticalfluxforirreversibilityis deter-minedwhentheirreversibleresistancebecomeshigherthanthat oftheadsorption.Simultaneouslytothesevariations,thereis gen-erallyadecreaseinthereversiblepartoffouling(Rrf)whentheflux

increases.

Recently, a newnotionhasappeared knownas“sustainable flux”whichincludeseconomicalfactors[11,20].Thisnotionwas foundbecauseforsomesystemsoperatingatazerofouling condi-tionissimplynotfeasible.Thisconceptwasespeciallyusedforthe treatmentofcomplexfluidsaswastewatertreatedbymembranes bioreactors[20–22].

3. Materialsandmethods

3.1. Redwine

Theredwineusedinthepresentstudywaselaboratedin2008 at thecooperative cellar of Rabastens(France) fromDuras, Fer Servadou and Syrah grape varieties.Thermovinification process wasusedtoelaboratethiswineinordertoincreasethe extrac-tionof polyphenolic compounds.After alcoholicand malolactic

fermentations,thewinewascentrifugedatthecellarinorderto removemicroorganismsandparticles.Afiltrationwasperformed withacross-flowmicrofiltrationpilotplantequippedwithorganic membranehavinganaverageporesizeof0.2mm.Thewineis ana-lyzedandmaintainedat4◦_{Cuntilusetopreventmicroorganisms’}

development.Priortoexperiments,asecondfiltrationisperformed withthefilterusedforthisstudy(cf.Section3.4)inorderto elimi-nateeventualpotassiumtartratecrystalsandprecipitates.Thisfinal stepallowsobtainingthefilteredwine(FW).Thefilteredwineused forallfiltrationshas12%asalcoholcontent,3.6aspH,0.6g/las sugars(glucose+fructose)and0.1g/lasmalicacid.

3.2. Chemicals

Tannins (Biotan®_{) were purchased from Laffort (Bordeaux,}

France).Thesetanninsareproanthocyanidictanninsextractedfrom grapeskinwithinstantaneousdissolving.Theywereaddedtothe wine(FW) withtheconcentrationsof1.25g/land2.5g/l.Pectin waspurchasedfromSigma–Aldrich(Lyon,France)andusedata concentrationof0.25g/land0.5g/l.Mannoproteins(Mannostab®₎

werepurchasedfromLaffortandaddedtothewineat concentra-tionsof0.1g/land0.2g/l.Theconcentrationsofaddedmolecules arechosenaccordingtothosefoundinwineandidentifiedinthe literature[24,25].

3.3. Winecomponentsanalysis

Spectrophotometric analyseswerecarried out onanAgilent 8453UV/VISspectrophotometer.Totalpolyphenolsinwinewere estimatedbytheTotalPolyphenolIndex(TPI)usingtheabsorbance at 280nm and under1cm optical path.Colour Intensity(IC)is thesumofopticaldensitiesat420nm,520nmand620nmunder 1mmopticalpath.Totalpolysaccharidesweredeterminedusing themodified Usseglio-Tomassetmethodbasedonthe precipita-tionofthepolysaccharideswithethanol[23].Mannoproteinswere alsodeterminedusing themodifiedUsseglio-Tomasset method. TotalanthocyaninsweredeterminedaccordingtoRibéreau-Gayon methodusingthesodiumbisulphite[24].Totaltanninswerealso determinedaccordingtoRibéreau-Gayonmethodbytransforming theproanthocyanidinsintoanthocyanidins[24].pH,%ofalcohol, malicacid,glucoseandfructoseconcentrationweredeterminedon thewinebyFTIRspectroscopy(Fouriertransforminfra-red spec-troscopy).Wineviscosityisdeterminedwitha controlled-stress rheometer(AR-2000ex).Turbiditymeasurements(NTU)were per-formedwithaEutechTN-100turbidimeter.

Table1showstheanalyticalcompositionof thewine before addingmolecules(FW)andafteraddingmolecules.Anetincrease in TPI,IC and turbidityisobserved afteraddition oftannins. It shouldbenotedthat about80%of totaltanninsarefoundafter adding tanninspowder. This wasexplained bythe assays con-ductedontanninspowderwhichshowthatitcontains80%tannins andisfreeofpolysaccharides.Theothercompoundsareorganic acids, sugars,minerals and vitaminswhich are not involved in membranefoulingduringmicrofiltration.Ontheotherside,the determinationoftotalpolysaccharidesinwinesFW+pectinand FW+mannoproteinshowsthattheamountsofmeasured polysac-charidesareequivalenttothoseadded[24,25].

3.4. Experimentalapparatus

Thefiltrationswereperformedwithawinefiltrationpilot sys-tem(Fig.3)designedforthisstudyandprovidedbyPeraCompany (Florensac,France).Theequipmentconsistsofa10lstainlesssteel feedtank,acentrifugalpump(forabetterrespectofwinequality), anelectronicflow-metertomeasuretheaxialfeedflowrate,2 tem-peraturesensors(T1andT2)and3pressuresensorslocatedatthe

Table1

Analyticalcompositionofwineaddedwithtestedmolecules. TPI Totalanthocyanins

(mg/l)

Totaltannins(g/l) Totalpolysaccharides (mg/l) IC Turbidity(NTU) FW 44.9(±0.5) 349(±3.42) 2.45(±0.16) 60(±30) 0.88(±0.07) 0.1(±0.2) FW+1.25g/ltannins 59 355 3.49(±0.1) 64 0.93 38.7 FW+2.5g/ltannins 73 360 4.45(±0.15) 68 0.99 72.5 FW+0.25g/lpectin 45.1 n.d. 2.45 320 0.85 12.2 FW+0.5g/lpectin 44.7 n.d. 2.4 510 0.85 18 FW+0.1g/lmannoprotein 44.8 n.d. 2.46 165 n.d. 4.5 FW+0.2g/lmannoprotein 44.5 n.d. 2.45 275 0.84 8.5

feedtankentrance(P1),attheinlet(P2)andattheoutlet(P3)ofthe

membranemodule.Transmembranepressure(1P)wascalculated as1P=(P2+P3)/2.Thepressureinthesystemisobtainedwith

com-pressedairthatpressurizesthefeedtank.Thepressureisaccurately regulatedinthepilotbyacurrenttopressuretransducercontrolled withacomputersoftwareinterface.Adigitalbalance,connectedto thesystem,wasusedtodeterminetheevolutionofpermeatemass withinthetimeandtocalculatethepermeateflux.Adata acqui-sitionsystemwasconnectedtothemicrofiltrationpilot:itallows thecontinuousmonitoringof1P,temperaturesandpermeatemass alongthetime.

The microfiltration module contains a multi-channel (44) ceramic membrane (BK-Kompact, Novasep, France) shown in

Fig.3.Itsaverageporediameteris0.2mm.Thetotalactive mem-branesurfacewas0.118m2_{,withanexternaldiameterof25mm.}

The membrane is madeof ZrO2/TiO2 layers laid on monolithic

TiO2–Al2O3 supportlayer. The flow velocity is fixed at 2ms−1

whichisconventionallyusedinwinefiltration.

Aftereachexperiment,a6stepsprocedureofchemical clean-ingisperformedtoregeneratethemembrane.Thisprocedureis summarizedin Table2.Themembranepermeability ischecked withosmotic water afterchemical cleaning and mustbe equal

Table2

Chemicalcleaningprocedureafterfiltrationexperiment. Step Action

1 Rinsingmembranewithwater(5min)

2 Rinsingmembranewithwarmwater(45–50◦_C)+0.5%NaOH

(10min)

3 Rinsingmembranewithhotwater(75–80◦_{C)+2–4%NaOH}

(15min)/Filtration

4 Rinsingmembranewithwarmwater(45–50◦_{C)(10min)/filtration}

5 Rinsingmembranewithwater(20–25◦

C)+0.2%citricacid (10min)/filtration

6 Rinsingmembranewithwater(20◦_{C)(5min)/filtration}

orabove900l/h/m2_{/bar(at20–22}◦_{C).Ifthispermeabilityisnot}

reached,severalchemicalcleaningarethenneededtoregenerate themembraneandtoreachthedesiredreferencepermeabilityof themembrane.

3.5. Criticalfluxdetermination:themethodofSWB

Thesquarewavebarovelocimetry(SWB) techniquehasbeen developed by Espinasse et al. [26] and detailed also by the

Fig.4. Squarewavebarovelocimetrymethodtomeasurethecriticalflux[24].

sameauthors[27].Theprincipleofthismethodis composedof alternative increasing and decreasing pressure steps as shown schematicallyinFig.4.Thepressurestepsarethealternationof pos-itiveandnegativevariations.TheUstepscorrespondtotheupper stepswhiletheLstepscorrespondtothelowersteps.Thepermeate fluxiscalculatedcontinuously.

Thistechniqueallowstheevaluationofthefluxlossbetween twostepsofpressure.Thefluxiscomparedbetweenstepshaving thesamepressure,forexampleU1/L2orUn−1/Ln.Ifthepermeate

fluxisthesameattheindicatedsteps(U1/L2),thefouling

associ-atedtothestepU2 isconsideredastotallyreversible.Iftheflux

decreasesfor2stepshavingthesamepressure(Un−1/Ln),the

foul-ingassociatedtothestepUn isconsideredaspartlyirreversible.

Thistechniqueofdatatreatmentallowshavingaccuratevaluesof thecriticalfluxandtherateofirreversibilityofthecreateddeposit onthemembrane.Thenumberofstepsandthecorresponding pres-suresusedinthisstudyaresummarizedinTable3corresponding tooperatingconditionsusedinwinefiltration.Eachsteplastedfor 4min.

3.6. Calculationmethodoftheirreversibleandreversible resistance

Therepresentationoffoulingresistance(R_f)versusfluxallows thedeterminationofadegreeofreversibilityofthefouling.The reversibleresistancetermisusedtodescribeorquantifythe por-tionofthefoulingresistancethatiseliminatedwithadecreasein pressure.IfRfatstepLnisequaltoRfatstepUn−1,itmeansthat

foulingistotallyreversibleatpressurestepn.

Theirreversibleresistancethatappearsforanupperpressure stepcanbereachedbycomparingthefoulingresistanceatstepsLn

andUn−1steps(Fig.4).Itcanbecalculatedasfollow:

rif,n Rm =









whererif,nistheirreversiblefoulingrelativetostepn.

Inordertocalculatethevalueofthetotalirreversiblefouling (Rif)atagivenpressurestep,allthemeasuredrifatprevioussteps

aresummedasfollow:

Rif=(rif,n+rif,n−1+rif,n−2+...) (4)

Fig.5. Permeatefluxand1PevolutionduringfiltrationofFWandtheirassociated stepsnumbers.

Thereversibleresistance(Rrf)canbecalculatedateachstepas

follows: Rrf Rm = Rf Rm − Rif Rm (5)

Sometimes,thecalculationof thereversibleresistancegives negativevalueswhichdonothaveaphysicalmeaning.Thatcould beexplainedby:(i)thefactthatthequasi-steadystateofpermeate fluxisnotreachedor(ii)thepresenceofanabnormalincreasein foulingresistance(depositcompactionorgelationofthedeposit) duringthepressuresteps.Inthiscase,thereversibleresistanceis takenatzeroandthevalueoftheirreversibleresistanceisthen consideredequaltothetotalfoulingresistance.

4. Results

4.1. Determinationofthecriticalfluxforfilteredwine(FW)

Beforeinvestigatingtheeffectoftanninsandpolysaccharides oncriticalflux,itisnecessarytoobservethepatternof perme-atefluxevolutionduringthefiltrationofthe“basicmatrix”(i.e. thefilteredwineFW).Fig.5showstheevolutionofpermeateflux and transmembranepressurein time whilefilteringFW. When comparingthevalueoffluxesfortwostepshavingthesame1P, fluxesremainedalmoststablesandidentical.Thispointisobserved throughoutthewholecycleof1Pstepping.Thismeansthatno sig-nificantfoulingtookplace.Thereversibleandirreversiblehydraulic resistancesdeducedfromtheSWBexperimentareplottedinFig.6. Whentheirreversibleresistanceisequaltozero,itmeansthatthe foulingcouldbeconsideredastotallyreversible.InthecaseofFW, theirreversibleresistanceisnotequaltozerofromthebeginning offiltrationbutitremainsalmoststableandthesame through-outthevariationsof1P.Thisobservationcouldbeexplainedby anadsorptionofsomewinecomponentsonthemembranesurface andinthepores.Itleadstoanincreaseintheirreversible resis-tanceanddecreaseinthereversibleresistance.Thesameconcept wasillustratedinFig.1anditcorrespondstotheweakformof crit-icalflux.InthecaseofFW,thecriticalfluxforirreversibilityisover therangeofthestudiedconditions(Jci≥1.4×10−4m/s).

Table3

Numberofstepsandtheassociatedpressure.

Step 1 2 3 4 5 6 7 8 9

Pressure(mbar) 200 250 200 300 250 350 300 400 350

Step 10 11 12 13 14 15 16 17 18

Fig.6.EvolutionofreversibleRrfandirreversibleRiffoulingresistanceduring

fil-trationofFWandtheassociatedtransmembranepressure(mbar).

4.2. Effectoftheaddedmolecules

4.2.1. Impactoftanninsontheevolutionofthecriticalflux

TanninsimpactoncriticalfluxwasstudiedbyaddingtoFWtwo differentconcentrationsoftannins:1.25g/land2.5g/l.Resultsof theeffectof1PsteppingonpermeatefluxarereportedinFig.7.A netincidenceoftanninsonpermeatefluxesisobservedfromthe beginningoftheexperiment.Thefoulingoccurswithinthefirst minuteoffiltrationforthetwotestedconcentrationsoftannins. Thefluxesdecreaseforthestepshavingthesame1P(inexample steps2and5).Thismeansthatthefoulingispartiallyirreversible. Theobtainedpermeatefluxesarehigherfortheconcentrationof 1.25g/loftannins(average2.5×10−5_{m/sat1000mbar(step16))}

than2.5g/l(average1.9×10−5_{m/sat1000mbar).Thefluxesare}

5–6timeslowerthanthoseobtainedwithFW(1.4×10−4_m/s)at

1000mbar.

Fig.8showstheevolutionofreversibleandirreversible resis-tance of wines with added tannins. As canbe seen, reversible resistanceforbothtestedconcentrationsdecreasedwhile increas-ingpressure. From thebeginning of theexperiments, theratio ofirreversibleresistanceovermembraneresistanceisnotequal tozero.It is equalto 0.39for FW+1.25g/loftannins and0.43 forFW+2.5g/loftanninswhicharetwicetheirreversible resis-tanceof FW.Thismeansthatirreversiblefouling occurswithin thefirstminuteoffiltration.Ontheotherhand,irreversible resis-tanceincreasedwithpressureuntilreach,at1000mbar,5times the hydraulic resistance of the membrane for both concentra-tions.

So,itisobviousthatcriticalfoulingconditionsarereachedfrom thebeginningoftheexperiments.Therefore,itisonlypossibleto

Fig.7. Permeatefluxevolutionduring1PsteppingperformedwithFWcontaining 1.25g/land2.5g/ltannins.

Fig.8. ImpactoftanninsonreversibleRrf()andirreversibleRif(d)fouling.

determinetheupperlimitoftherange,wherecriticalfluxcould befound,intheseconditions.Thisupperlimit(nottoexceed)is equalto1.95×10−5_{m/sforwinescontaining1.25g/ltanninsand}

1.63×10−5_{m/sforthosecontaining2.5g/ltannins.}

4.2.2. Impactofpolysaccharidesontheevolutionofthecritical flux

The impact of polysaccharides on critical flux was studied bytesting two categories of polysaccharides. Thefirst category includespectinwhichcomesfromgrapeberries.Pectineffectwas studiedat2concentrations:0.25g/land0.5g/l.Thesecond cate-goryisformedbymannoproteinswhosepresenceinwineisdueto thereleasefromyeastcellwall.Theimpactofmannoproteinswas alsostudiedattwoconcentrations:0.1g/land0.2g/l.Fig.9shows thepermeatefluxevolutionofthefourfiltrationsofwineadded withpolysaccharides.Forthetestedcompounds,resultsshowthat foulingoccursfromthebeginningofthefiltration.Also,fluxesare notequalfortwostepshavingthesamepressureevenatfirststeps ofpressure.Thisfactmeansthatthedeterminationofacriticalflux foreachcompoundatadefinedconcentrationisnotfeasiblefor thestudiedrangeofpressure.Theamountoffoulingisdifferent dependingonthetype ofpolysaccharidesandonits concentra-tion.

ThefluxevolutionsofFW+0.2g/lmannoproteinandFW+0.5g/l pectinareimportanttobeconsideredastheyprovideveryspecial shapeoffluxversustimeandresistanceversusfluxcurves:the ini-tialfluxinstepping1obtainedwhenprocessingFW+0.5g/lpectin (J=1.18×10−5_{m/s) is dividedby 2comparing to thatobtained}

Fig.9.Permeatefluxevolutionduring1PsteppingperformedwithFW+0.1g/l mannoproteins,FW+0.2g/lmannoproteins,FW+0.25g/lpectinandFW+0.5g/l pectin.

Fig.10.Pectin’simpactonreversibleRrf()andirreversibleRif(d)fouling.

withFW(J=2.49×10−5_{m/s).Inaddition,alittlegainintermsof}

averagefluxisobservedbyincreasingthetransmembrane pres-suretoreachamaximumofaveragefluxat400mbar(step8and

J=1.44×10−5_{m/s).Byincreasingthepressurebeyond400mbar,a}

lossintermsofaveragefluxisobservedcomparedtothatobtained attheindicatedpressure.

As for FW+0.5g/l pectin, the same behaviour has been noticed to FW+0.2g/l mannoproteins. The initial permeate flux of FW+0.2g/l mannoproteins is higher compared to that obtainedwithFW+0.5g/lpectin.Themaximumofaverageflux (J=2.65×10−5_{m/s)isreachedat600mbar(step12)forFW+0.2g/l}

mannoprotein.Beyond600mbar,permeatefluxdecreasedwhen increasingthetransmembranepressure.

Theevolutionofreversibleand irreversibleresistanceduring filtration wine added with pectin is illustrated in Fig. 10. The reversibleresistancefortheFW+0.25g/lpectindecreaseswhile increasingpressureandthusthepermeatefluxuntilitbecomes zeroat600mbar.Theirreversibleresistanceincreasesduringthe experimenttobecome5.5timesthemembraneresistanceathigher pressure.The curvesshapesof reversibleand irreversible resis-tanceofFW+0.5g/lpectinarenotcommon.Atthebeginningof thefiltration,araiseinpressureincreasesthepermeatefluxandthe irreversibleresistance.Beyond400mbar(J=1.44×10−5_m/s),aloss

inaveragepermeatefluxisobservedwhileirreversibleresistance continuetoincrease.Itreaches5.5timesthemembraneresistance athigherpressure(1000mbar).Thereversibleresistancedecreases whenincreasingthetransmembranepressureandbecomeszeroat 500mbar.Therefore,beyondthispressure,theirreversible resis-tancebecomesequaltothetotalresistance.

Fig.11.Mannoproteins’impactonreversibleRrf()andirreversibleRif(d)fouling.

Fig.11showstheimpactofaddedmannoproteinonthe evo-lution of reversible and irreversible resistance. For FW+0.1g/l mannoprotein,theevolutionofbothresistancesissimilartothat obtainedwithtanninsand0.25g/lpectin.Irreversibleresistance reachesabout4 timesthemembrane resistance. In thecase of FW+0.2g/l mannoprotein,thecurves shapesof both resistance looklikethoseobtainedwith0.5g/lpectin.Thedifferenceisthe inflexionpointwhichisobtainedat600mbar(J=2.55×10−5_m/s).

Irreversibleresistancereachesin this case7.5 timesmembrane resistance.

5. Discussion

In thepresent study, experimentswere realizedin orderto determine the critical flux for irreversibility Jci in wine

cross-flowmicrofiltration.Forcolloidalfiltration,thistermisthemore appropriatebecauseitdiscriminatesbetweenreversibleand irre-versiblefouling.Whenfilteringcolloidaldispersion,foulingcannot betotallyavoided duetothephenomenonoftheconcentration polarizationand insomecasesadsorption onmembrane mate-rial.But,thecoagulationofdispersedphaseclosetothemembrane surface,followedbydepositionuponit,canbeavoided.

In wine, tannins,pectin and mannoproteinpresent colloidal behaviours.Intheory,whenfilteringbelowthecriticalfluxfor irre-versibility,irreversiblefoulingbywinecolloidscanbeprevented. Inourexperiments,nocriticalfluxforirreversibilitycouldbe deter-minedforthewinecolloidsforthestudiedrangeofpressureswhich arethesameusedinwinefiltration.Alltestedmolecules, what-evertheconcentration,exhibitirreversiblefoulingevenatverylow pressures.

For wine tannins,itwasdemonstrated thattheiradsorption on membrane surface occurs in static conditions [9,10]. Under dynamic conditions, tannins tend to accumulate at the pore entranceonthemembranefeedside[9].Itseemsalso,accordingto

Fig.8,thattherateoffoulinganditstypeisstronglyinfluencedby thetransmembranepressure.Theincreaseinirreversiblefouling canbeexplainedbythetransitionbetweenthestateofdispersed moleculestoaggregates.Thisfactispromotedbytheincreaseof thetransmembranepressurewhichforcesthemoleculestobenear themembranesurfaceandpromotesmembrane/tanninsand tan-nins/tanninsinteractions.

Winepolysaccharideshavebeenidentifiedtoplayamajorrole in membrane fouling during cross-flow microfiltration of wine

[4–10].AccordingtotheresultspresentedinSection4.2.2, foul-ingbypolysaccharidescannotbeavoided.Itwasshownthatunder staticconditions polysaccharidesadsorptionis negligible[7].In dynamicconditions,polysaccharidesadsorptiontendstobe gov-ernedbythehydrophobic/hydrophiliccharacterofthemembrane

[10]aswellasbymembranepolarity[7].Inmostfruitjuices,pectin (awell-knowngellingagent)formsagel-layeronthemembrane surface[28–31].Theformation ofthis layeris enhancedbythe process.Kirket al.[32]as wellas Szaniawskiand Spencer[33]

showedabellshapedprofilewhenplottingpermeatefluxversus TMPduringthefiltrationofsolutionsrichinpectin.These observa-tionswereexplainedasfollowing:anincreaseinpressurewould causethemacromoleculeswhicharealreadyonthemembrane sur-facetopackmoretightly;Athighpressures,thedenselypacked pectinmoleculesformabarrieracrossthemembraneandprevent theflowofpermeateflux.Thisbarrierwasidentifiedasgel-layer

[34].Duringfiltrationofsolutionscontainingpectin,Raietal.[35]

showedthatanincreaseinpressureleadtoanincreaseinfluxtilla limitwhereagel-typelayergrowsrapidlybecauseiftheenhanced forcedconvectionofthesolutestowardsmembrane.Pectin hydrol-ysisbypectinasesleadstoanimprovementofthepermeateflux

Fig.12.(A)Pecticgellayeratmembranesurface,(B)pecticgellayercompaction,and(C)unstructuredpecticgellayer.

An unexpected phenomenon occurred when filtering FW+0.5g/l pectin and FW+0.2g/l mannoproteins (cf. Fig. 9). Afteragivenlevelofpressure,theaveragepermeatefluxbeginto decreasewithincreasingpressure.Infact,Kirketal.[32]showed thatpectinformsanelasticpecticgellayerwhichisevidencedby thepartialrestorationof permeateflux upongradualrelease of thetransmembranepressure.Thishighlights thatthepecticgel couldbe compressible.In our study,the following explanation canbe proposed. Thegel layerwascompressible tilla limit of pressure.Beyondthislimit,hydrogenbondbridges,thattransform thepecticchainsintogelaggregates(Fig.12A),collapseleadingto theclosureofinterstitialspacesbetweenthechains(Fig.12B).The gellayercanalsobeunstructuredunderpressureandfillpartially theporesleadingtoanadditionalfoulingincrease(Fig.12C).So, thecompactedpecticgellayeractsas asecondmembrane and mayretainothersolutes.Thisexplainstheresultsobtainedwith FW+0.5g/lpectin.

Mannoproteinsimpactonmembranefoulingwaslittlestudied intheliterature.Itwasshownthatmannoproteinsmightcausethe strongestdecreaseinwinefilterability[8].Mannoproteinsarenot agellingagentanddonotformagel-likestructureoverthe mem-brane.Mannoproteinsseemtoformadepositatthemembrane surface.Theresultsobtainedwhen filteringFW+0.2g/l manno-proteinscouldbealsoexplainedbyacompressiblecakewherethe spacesbetweenmoleculesarereducedwiththepressureincrease. Theseresultshighlightthatthecriticalfluxconceptis inappro-priatetobeindustriallyappliedtowinecross-flowmicrofiltration

withintheclassicalrangeofoperation.Therefore,otherconcepts shouldbetakenintoconsiderationlike“thresholdflux”term.Itis definedasthefluxatorbelowwhichmembranesystemwill gen-eratealowrateoffoulingandfluxesremainacceptablebutabove whichtherateoffoulingincreasesmarkedly[22].Thisdefinition wasusedfirstlyfor“sustainableflux”buttheconceptofthe lat-terismodifiedandincludeseconomicfactors.So,thisconceptis usefultodefineregionsoflowandhighfouling.Thecriteriaand aspectstodefinethisthresholdfluxmayvarydependingonwhat theresearchersareseeking.Thecriteriondefinedinthisstudyis basedontheratiobetweenirreversibleresistanceandthehydraulic membraneresistanceanditisdefinedas:“thefluxatwhichthe ratioRif/Rmisinferiorto1”.

Fig.13showedthecriterion(Rif/Rm≤1)usedtodeterminethe thresholdfluxandcomparisonoftheobtainedfluxwiththecritical fluxofirreversibility(Jci).ThedatashowninFig.13exceptforFW

representthehigherlimitoftherangethatcouldbeobtainedunder thedefinedconditions.

The threshold fluxes for all the filtrations are higher than those obtainedwith thecritical flux concept, even witha cer-taindegreeof fouling.Thegain influxes mayreach34%inthe case of FW+ 0.1g/l mannoprotein. In the cases of FW+0.5g/l pectinandFW+0.2g/lmannoprotein,criticalfluxfor irreversibil-itycouldnotbedeterminedbutathresholdfluxcouldbeobtained. In other hand, the critical and threshold fluxes are still much lower than those obtained with FW which are higher than 1.4×10−4_m/s.

Fig.13.Comparisonofcriticalfluxforirreversibility(thebargivesherethelowervalueofcriticalfluxforFWandtheuppervalueofcriticalfluxforFWwithaddedmolecules) andthethresholdflux.

6. Conclusion

In this study, critical operating conditions during wine cross-flowmicrofiltrationwerestudied.Thesquarewave barov-elocimetry(SWB)wasusedtoassesstheevolutionofreversible andirreversibleresistancewithpermeateflux.Itallowsthe deter-mination of critical flux for irreversibility (J_ci). For all tested macromolecules (tannins,pectinand mannoproteins) and asso-ciateconcentrations,noclearcriticalfluxforirreversibilitycanbe reallydeterminedintherangeoftestedpressures:thisstudy deter-minesthentheuppervalueofthecriticalflux.Infact,membrane foulinginpresenceofthesewinemoleculesoccursfromthefirst minuteoffiltration.Thisworkshowstheimportanceoftannins, pectinandmannoproteinonthemembranefoulingfor concentra-tionrangesclassicallyfoundinwine.Themainfoulingmechanisms areadsorptiononmembranematerialasshowedwithFW solu-tionand formationof depositlayeras proposedfor FW+0.2g/l mannoproteins.Agellayercompactionordeformationunderhigh pressuresisproposedtoexplainthephenomenonobservedwith FW+0.5g/lpectin.Newcriteriawereusedinordertodeterminea “thresholdflux”whereacertaindegreeoffoulingisacceptable.It leadstoaconvenientsetofoperatingconditionscompatiblewith industrialconstraints.

Acknowledgements

Theauthorsgratefullyacknowledge“PERA”societyand Cen-treNationaldeRechercheScientifique(CNRS)fortheirfinancial support.

Nomenclature

1P transmembranepressure(Pa) 1˘ osmoticpressure(Pa)

 permeateviscosity(Pas) FW filteredwine

J permeateflux(m/s)

Jci criticalfluxforirreversibility(m/s)

IC colourintensity

Rm membranehydraulicresistance(m−1)

Rads resistanceduetoadsorption(m−1)

Rrf reversibleresistance(m−1)

Rif irreversibleresistance(m−1)

SWB squarewavebarovelocimetry TPI totalpolyphenolindex

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