Journal of Food Engineering 95 (2009) 423–431

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Journal of Food Engineering

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Ultrafiltration fouling of amylose solution: Behavior, characterizationand mechanism

Heru Susanto *, I Nyoman WidiasaDepartment of Chemical Engineering, Universitas Diponegoro, Semarang, Indonesia

a r t i c l e i n f o a b s t r a c t

Article history:Received 12 January 2009Received in revised form 4 April 2009Accepted 2 June 2009Available online 6 June 2009

Keywords:AdsorptionAmyloseFoulingStarchUltrafiltration

0260-8774/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.jfoodeng.2009.06.003

* Corresponding author. Tel./fax: +62 24 7460058.E-mail address: heru.susanto@undip.ac.id (H. Susa

Applications of ultrafiltration membrane often deal with feed streams containing amylose starch. Thispaper describes a detailed investigation of amylose fouling during ultrafiltration. Commercial mem-branes made of polysulfone and fluoro polymer were used. Both adsorptive and ultrafiltration foulingwere investigated. Experiments using different membrane characteristics, feed concentrations andtrans-membrane pressures were carried out. The resulting fouling was characterized by water flux andcontact angle measurements and was visualized by scanning electron microscopy (SEM). The results sug-gest that solute adsorption has occurred as noticed by significant water flux reductions as well as changesin membrane characteristics. Further, both reversible and irreversible fouling have occurred during ultra-filtration with irreversible fouling was more dominant. Apparently, cake layer formation initiated byeither adsorption due to hydrophobic–hydrophobic interactions or pore blocking is the dominant foulingmechanism. However, pore narrowing instead of pore blocking was also observed for the membrane hav-ing large and relative uniform pore structure or for the ultrafiltration using low trans-membrane pressureor low solute concentration. Membrane autopsy using SEM confirmed the formation of solute layer on themembrane surface.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

With an average pore diameter within the range 2–100 nm,ultrafiltration (UF) is usually used for purifying, concentratingand fractionating macromolecules or fine particle suspensions.UF is gradually emerging as promising separation tool in manyapplications across a range of food and beverage industries, includ-ing fluid milk, dairy products, juiced fruit and vegetables, wine, su-gar and other sweeteners, vegetable oils, and water andwastewater treatments (Cheryan, 1998; Girard and Fukumoto,2000; Jönsson and Trägårdh, 1990). Because fouling significantlyreduces the performance of UF membrane, efforts to overcomethe fouling problem have drawn more and more attention andcan be generally classified into: (i) foulant identification and char-acterization, (ii) investigation of fouling mechanisms, and (iii) min-imizing or control of fouling. Even though control of fouling seemsto be the most critical issue from a practical point of view, under-standing of fouling behavior and its mechanism is very important.By this knowledge, a proper method for control of fouling can befurther determined.

During its applications in food and biotechnological industries,UF often deals with macromolecule biopolymers such as polysac-charides and proteins. In UF fouling investigations, beside protein

ll rights reserved.

nto).

solution, which can easily be found in many previously reportedstudies (Becht et al., 2008; Huisman et al., 2000; Koehler et al.,1997; Petrus et al., 2008; Torres et al., 2002), polysaccharide solu-tion is nowadays of great interest being investigated. For examples,fouling of polysaccharide alginate as representative model foreither polysaccharides of extracellular polymeric substances(EPS) or natural organic matter (NOM) during water and wastewa-ter treatments using membrane processes has intensively beenstudied. The results showed that alginate could foul both UF andmicrofiltration (MF) membranes. Furthermore, several foulingmechanisms, which depended on the membrane structure as wellas operating condition, have been proposed (Drews et al., 2006; Le-Clech et al., 2007; Nagaoka et al., 1998; Ye et al., 2005). Backflu-shing for control of alginate fouling has recently been studied (Kat-soufiddou et al., 2008; van de Van et al., 2008). Dextran is anotherpolysaccharide that has often been used in UF fouling research forwater treatment and food applications (Garcia–Molina et al., 2006;Gekas et al., 1992; Kweon and Lawler, 2005; Susanto and Ulbricht,2005; Susanto et al., 2007). In addition to alginate and dextran,wine and juice polysaccharides have also been investigated duringUF/MF (Saha et al., 2006; Vernhet and Moutounet, 2002). In thisstudy, UF of amylose starch solution, which is one of polysaccha-rides, is investigated.

Starch – polysaccharide carbohydrate from glucose polymerjoined by glycosidic bonds – is an important dietary energy forman. Starch occurs in many sources of food such as fruits, seeds,

424 H. Susanto, I Nyoman Widiasa / Journal of Food Engineering 95 (2009) 423–431

cereals, roots and tubers and pulses. Demand for starch is nowa-days getting increase especially in food, pharmaceutical, textileand paper industries (Tester and Karkalas, 2005). For example,pure starch is hydrolyzed by either acid or enzyme to glucose con-taining products and further be treated to fructose. Starch is alsoused as raw material for the production of bio-alcohol. In foodindustry, because of its viscosity in aqueous media, starch is exten-sively used as a stabilizer or thickener. It is added to yoghurt as astabilizer and to large number of desserts, convenience foods andready-to-cook meals. Starch partially replaces oil in low caloriemayonnaise and contributes to the desired texture.

Starch mainly consists of amylose and amylopectin molecules(Boleon et al., 1998; Hoover, 2001). The proportion of both mole-cules is dependent on its source. Amylose (unbrunched) has lowermolar mass with a relatively extended shape, while amylopectinhas high molar mass bit compact molecules. Starch also containsother components such as lipid, protein and mineral (Tester andKarkalas, 2005). Amylose (Fig. 1) is an essentially linear a-glucancontaining around 99% a-(1–4) and 1% a-(1–6) bonds with molec-ular weight within the range 1 � 102–1 � 103 kg/mol within thenumber of repeated glucose subunits range 300–3000 (Boleonet al., 1998). Amylose can form an extended shape within theapproximate range of hydrodynamic radius 7–22 nm while amylo-pectin is within the range 21–75 nm (Parker and Ring, 2001). Thea-(1–4) bonds promote the formation of a helix structure. Variousmethods for the separation of amylose from amylopectin havebeen proposed based on their different structural behavior (Bald-win, 1930; Vorwerg et al., 2002).

Beside the above mentioned applications, membrane processeshave been proposed for refining raw starch syrups and filtration ofstarch hydrolysate (Singh and Cheryan, 1997, 1998a,b; Slominskaand Grzeskowiak-Przywecka, 2004). An interesting application ofUF membrane as reactor as well as separator during productionof cyclodextrins or maltose syrup by using enzymatic membranebioreactor has been studied (Aymard et al., 1997; Houng et al.,1992; Słominska et al., 1998, 2002). Furthermore, potential appli-cations for more complex systems containing starch have also beeninvestigated (Beolchini et al., 2006; Sakinah et al., 2007). Unfortu-nately, much less attention has been devoted to the fundamentalstudy using single solute amylose. Definitely, this study is veryimportant to determine the fouling mechanism and the responsi-ble foulant during applications for complex system. Thus, a propermethod for control of fouling can be engineered and process per-formance can be increased. This paper presents a systematic studytowards a better understanding of the fouling characteristics andmechanisms during UF of feed stream containing amylose starchobtained by studying membrane–solute and membrane–solute–solute interactions. For this study, commercial UF membraneswere used because the consistency of membrane properties ofmembranes manufactured industrially should be better than forlab-made membranes and the obtained data should therefore havea larger fundamental and (potentially) practical relevance. On thatbasis, this work provides fundamental information for a betterunderstanding of fouling for stream containing starch. It will fur-ther facilitate both detailed mechanistic investigations and thedevelopment of methods for controlling such fouling.

Fig. 1. Chemical struc

2. Materials and methods

2.1. Materials

Two commercial polysulfone (PS) UF membranes with nominalcut-off of 20 (GR61PP) and 100 (GR40PP) kg/mol (kDa) and one UFmembrane made from fluoro polymer with cut-off of 20 kg/mol(FS61PP) were used. All these membranes were supplied by AlfaLaval, Denmark, in sheet shape and were cut in order to be used.Prior to use, the membranes were washed by soaking overnightin water to remove impurities left from the manufacturing processor additives used for stabilization. Fresh membranes were used inall experiments. A commercial amylose (average molar mass of150 kg/mol) was purchased from Serva, Heidelberg. Dextran T-4,T-10, T-35, T-70, T-100 and T-200 (the numbers indicate averagemolar mass from manufacturer in kg/mol) were from Fluka. Phenoland sulfuric acid were purchased from Sigma and Merck, respec-tively. Pure water was used in all experiments.

2.2. Methods

2.2.1. Amylose and dextran analysesAmylose concentration was measured in terms of carbohydrate

by colorimetric test developed by Dubois using phenol–sulfuricacid method (cf. (Saha and Brewer, 1994)). The sample was firstlyreacted with phenol in acid medium forming an orange–yellowcolor and its absorbance at 490 nm was measured with UV-spec-troscopy. The molar mass distribution of amylose was measuredusing gel permeation chromatography (GPC) using dextran as stan-dard solution. The concentration of dextran (used during rejectionmeasurements) as well as its molar mass distribution in solutionwas analyzed by GPC. Calibrations were performed using differentdextran molar mass standards.

The viscosity of amylose solution was measured using HAAKEViscotester 1 plus, Thermo electron GmbH, Karlsruhe, Germany.

2.2.2. Adsorptive fouling and rejection test proceduresThe experiments were carried out by using a dead-end stirred

cell filtration system (Amicon cell models 8010 from Millipore).To avoid the effects of compaction, each membrane was firstlycompacted by filtering pure water at 4 bar for at least 0.5 h. Purewater flux (J0) was then measured for each membrane sample.An amylose solution with certain concentration was added to thecell. Thereafter, the outer membrane surface was exposed for 3 hwithout any flux at a stirring rate of 300 rpm (our preliminaryexperiment indicated that 2.5–3 h of adsorption was sufficient toachieve saturation of the surface adsorption capacity for this amy-lose). Afterwards, the solution was removed, and the membranesurface was rinsed twice by filling the cell with pure water(5 mL) and shaking it for 30 s. Pure water flux (Ja) was again mea-sured. The extent of adsorptive fouling was expressed in term ofrelative water flux reduction (RFR; cf. Eq. (1)), which was calcu-lated from the water fluxes at the same pressure before and afteradsorptive fouling.

RFR ¼ J0 � Ja

J0ð1Þ

ture of amylose.

H. Susanto, I Nyoman Widiasa / Journal of Food Engineering 95 (2009) 423–431 425

In this experiment, the effect of membrane property on adsorp-tive fouling was investigated. Furthermore, because the extent offouling during UF is significantly influenced by solute concentra-tion, the effect of concentration on adsorptive fouling was theninvestigated. The study was performed by varying the amyloseconcentration within the range 0–10 g/L using the GR61PP andFS61PP membranes.

A six component mixture of dextrans within the molar massrange 5–200 kg/mol, i.e., T-5, T-10, T-35, T-70, T-100 and T-200(the number indicating molar mass in kg/mol) was used for thecharacterization of the rejection curves including the cut-off ofthe membranes. The total concentration of dextran mixture solu-tion was 1 g/L with the composition of each dextran was 10, 20,20, 20, 20 and 10 wt.% for T-5, T-10, T-35, T-70, T-100 and T-200,respectively. The concentrations of dextran in the feed, permeateand retentate mixtures were analyzed. Rejection for each molarmass was determined as:

R ¼ 1� Cdownstream

Cupstreamð2Þ

2.2.3. Cross-flow ultrafiltration experimentsA self home-made laboratory scale for cross-flow filtration test

was used in all ultrafiltration experiments. The set-up consisted ofa feed tank (3 L volume), a pump, a pressure indicator connected tofeed side of membrane to determine the trans-membrane pressureand a flat-sheet membrane cell. A simplified diagram of the set-upis given in Fig. 2. In each experiment, a new circular membranedisk with the effective area of 3.14 cm2 was used. In order to main-tain constant feed concentration, the retentate and permeate werereturned to the feed tank. All experiments were conducted at roomtemperature (28 ± 2 �C). As done in adsorption experiments, themembrane was firstly compacted by filtering the water for at least0.5 h at a pressure of 3 bar. During UF experiments, the flux profileover time was gravimetrically monitored. The flux was calculatedby using Eq. (3):

J ¼ QA

ð3Þ

where, Q and A are permeate flow rate and membrane area,respectively.

Because concentration polarization is known to be reversiblefouling (dynamic layer), changing the operation conditions suchas lowering the pressure should revert the system back to the

Fig. 2. Schematic diagram of the cross-flo

pressure controlled regime (Cheryan, 1998), therefore, to investi-gate the effect of concentration polarization, after UF, the pressurewas lowered to �0 bar for 5 min (in this way, no permeate flux wasproduced). Thereafter, permeate flux was again measured at simi-lar pressure with UF. During UF, �2 mL of the feed and permeatesamples were taken at certain times for concentration analysis.The apparent amylose rejection was calculated using Eq. (2).

2.2.4. Membrane morphologyThe top surface morphology of the membranes was visualized

by using a Supra 35 VP field emission scanning electron micro-scope (FESEM) with applied voltage was 20 kV. The outer surfaceof the sample was coated with gold/palladium and sputtered for1 min before analysis.

2.2.5. Contact angle (CA)Sessile drops static CA of fresh and fouled membranes was mea-

sured using an optical contact angle measurement system (OCA 15Plus; Dataphysics GmbH, Filderstadt, Germany). Five microliter ofwater was dropped on the membrane surface from a microsyringewith a stainless steel needle in room temperature. Multiple contactangle values were measured and average values were obtainedfrom at least five bubbles at different locations on the membranesurface.

3. Results and discussion

3.1. Amylose molar mass, hydraulic permeability and membrane poresize characterizations

Separation mechanism in UF occurs via sieving or size exclusionby the membrane pores. Therefore, it is important to know theamylose and the membrane pore sizes. Fig. 3 gives molar mass dis-tribution of amylose obtained by GPC measurement. It is obviouslyseen that the molar mass of amylose is distributed within therange �6–200 kg/mol with weight average of molecular weight(Mw) is �64.8 kg/mol.

To know the characteristics of separation of the membraneused, hydraulic permeability and pore size as well as its distribu-tion were measured. Fig. 4 shows the water flux as function oftrans-membrane pressure. It is seen that the FS61PP and GR61PPmembranes with the same nominal cut-off (from manufacturer)show different water flux values. By contrast, the GR40PP

w filtration set-up used in this study.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

1000 10000 100000 1000000

Molar mass (g/mol)

Re

spo

nse

Fig. 3. Molar mass distribution of amylose (0.01 g/L) used in this study obtainedfrom GPC: Mn = 46.8 kg/mol, Mw = 64.8 kg/mol, polydispersity index (PDI) = 1.38.

y = 413.2x

y = 65.9x

y = 371.9x

0

200

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600

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1000

1200

1400

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2000

0 1 2 3 4 5Pressure (bar)

Flux

(L m

-2 h

-1)

FS61PPGR61PPGR40PP

Fig. 4. Water flux as a function of trans-membrane pressure for differentmembranes used.

0

10

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10 100 1000 10000 100000 1000000

Molar mass (g/mol)

Re

ject

ion

(%

)

GR41PP

FS61PP

GR61PP

cutoff line

Fig. 5. Rejection curve of membranes determined by using a dextran mixturesolution with total concentration of 1 g/L (as the feed) and at a pressure of 1 bar.

0.32

0.25

0.41

0

0.1

0.2

0.3

0.4

0.5

0.6

FS61PP GR61PP GR40PPMembranes

RFR

Fig. 6. Effects of membrane properties (pore size and material) on relative fluxreduction during adsorptive fouling (amylose concentration 10 g/L, adsorption time3 h). The error bars represent standard deviation.

426 H. Susanto, I Nyoman Widiasa / Journal of Food Engineering 95 (2009) 423–431

membrane shows almost similar flux with the FS61PP membrane.Further, the hydraulic permeabilities – the gradient of flux vs.trans-membrane pressure – are 413.2, 65.9 and371.9 Lm�2 h�1 bar�1 for the FS61PP, GR61PP and GR40PP mem-branes, respectively. Similar result was observed during rejectionmeasurement of polydisperse macromolecular test substances(dextran) to quantify the pore size and its distribution of selectivebarrier (Fig. 5). The FS61PP membrane shows similar pore size dis-tribution as well as nominal cut-off (the molar mass of a test sub-stance that would be retained >90% by the membrane) with theGR40PP membrane but significantly different with the GR61PPmembrane. In addition, the nominal cut-off data obtained fromthis experiment show different values from the manufacturer forboth GR61PP and FS61PP membranes. Only GR40PP membraneshows similar cut-off with the manufacturer value. In general, suchobservations are not fully surprising. Susanto and Ulbricht (2005)found similar phenomena and explained the reasons behind thosephenomena. Briefly, different test substances as well as their inter-action with the membrane and test conditions during measure-ment were the most probable reason.

3.2. Adsorptive fouling

Adsorptive fouling (membrane–solute interactions) was inves-tigated by exposing the outer membrane surface to a certain amy-lose solution. The relative water flux reduction (RFR) was used to

identify these interactions. In many experiments, a high concentra-tion of solute (10 g/L) was used because the concentration of soluteat the membrane surface will gradually increase during UF as con-sequence of solute rejection by the membrane.

Fig. 6 presents the effect of membrane property (cut-off andmaterial) on relative water flux reduction. It can be clearly seenthat the effect of adsorptive fouling of amylose on water flux isinfluenced by the membrane nominal cut-off (GR61PP vs. GR40PP).For the same material, the flux reduction of the membrane withlarger pore size (GR40PP) is greater than the smaller one indicatingthat membrane with larger pores is more susceptible to adsorptivefouling. This observation can be explained as follows: Consideringthe molar mass distribution of amylose (cf. Fig. 3), it is reasonablethat the possibility of amylose to access membrane pores is higherfor the GR40PP membrane than for the GR61PP membrane (cf. poresize distribution in Fig. 5). Consequently, the internal fouling forthe GR40PP membrane would be more significant. Such phenom-ena are in agreement with previously reported results (Ye et al.,2005; Susanto and Ulbricht, 2005). For membranes having similarcut-off (cf. GR61PP vs. FS61PP [based on manufacturer data] orGR40PP vs. FS61PP [based on rejection measurement]), it is ob-served that the more hydrophobic membrane (FS61PP) results inthe higher relative flux reduction (cf. contact angle measurementdata Table 1, below). Solute–membrane interactions are strongerfor hydrophobic material than for hydrophilic one. This is inaccordance with the results found in previous publications

Table 1Static contact angles of fresh and fouled membranes measured with sessile drop method (data are presented as contact angle ± standard deviation, in degrees).

Membrane Fresh Fouled by adsorptive Fouled by UF Fouled by UF long-term

GR61PP 65.9 ± 1.6 61.3 ± 3.6 56.7 ± 5.2 n.d.*

FS61PP 77.2 ± 2.3 69.9 ± 3.3 62.5 ± 4.1 57.3 ± 3.9

* n.d.: not done.

H. Susanto, I Nyoman Widiasa / Journal of Food Engineering 95 (2009) 423–431 427

(Matthiasson, 1983; Susanto and Ulbricht, 2005; Wei et al., 2006).The mechanism of interactions will be explained in more detail inSection 3.4.

As shown in Fig. 7, the RFR increases as the amylose concentra-tion is increased. However, the increase in RFR is more pronouncedat concentration less than 5 g/L. Slight increase is observed beyondthe concentration of 7.5 g/L. It is also seen that in all concentra-tions, the FS61PP membrane shows higher RFR than the GR61PPmembrane. This observation can be explained from pore size andhydrophobicity aspects (cf. above). To further interpret the data,available isotherm adsorption models were used. It is reasonableto assume that the RFR will correlate with the amount of solute ad-sorbed on the membrane surface and therefore, the RFR was ana-lyzed using the Langmuir and Freundlich models for adsorption(Adamson, 1982). Fits of the experimental data gave the solid(Langmuir) and dashed (Freundlich) curves. The experimental re-sults show significant deviation compared to the results calculatedfrom Langmuir model. This observation suggests that the adsorp-tion follows not only monolayer adsorption on the solid surfacebut also solute–solute interactions. A good agreement betweenexperimental results with Freundlich model (mod2) is observedfor both membranes. This result supports that the solute–soluteinteraction has occurred. In addition, diffusion of solutes into themembrane pores is also possible.

3.3. Fouling behavior during cross-flow ultrafiltration

To investigate dynamic fouling behavior, cross-flow UF withconstant trans-membrane pressure mode was performed. The ef-fects of membrane characteristic, concentration and trans-pressureon the resulting flux were investigated. In addition, long-term UFwas also performed to examine the membrane performance forlong operation. All cross-flow UF results are plotted as permeateflux relative to initial water flux vs. filtration time. Rejection ofamylose at certain times was also measured

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0 2 4 6 8 10 12

Solute concentration (g/L)

RF

R

FS61PP GR61PPFS61PP-mod1 GR61PP-mod1FS61PP-mod2 GR61PP-mod2

Fig. 7. Influence of amylose concentration on relative flux reduction after staticadsorption (3 h adsorption). Solid and dashed curves are RFR calculation using theLangmuir (mod1) and Freundlich (mod2) models, respectively.

3.3.1. Effect of membrane characteristicsCross-flow ultrafiltration using three different membranes were

performed to investigate the effect of membrane pore size as wellas membrane material. Fig. 8 gives the profile of flux (F) as well asrejection (R) as a function of filtration time performed at a mediumtrans-membrane pressure (1 bar) and amylose concentration of0.1 g/L.

Rapid flux decline was observed at the early filtration for theGR61PP and FS61PP membranes whereas gradually decrease influx was observed for the GR40PP membrane. This rapid flux de-cline suggests that concentration polarization has occurred. How-ever, measuring the permeate flux after stopping the filtration for5 min indicated that CP is not the dominant reason. Comparingthe membranes with the same material, it is clearly seen that theflux behavior is influenced by the membrane pore size (cf. GR61PPvs. GR40PP). The flux rapidly drops for the membrane with smallerpore size (GR61PP) and gradually decreases for the membrane hav-ing larger pore size (GR40PP). The reason for this observationwould be that the possibility of the solute accessing the pores be-comes greater for the membrane having larger pore size which hasa higher propensity to narrow the membrane pores rather thanblock the pores. The possibility of amylose to block the membranepores would be higher for the membrane having smaller pore size.It was reported that the flux reduction due to pore blockage wasmore significant than pore narrowing (Belfort et al., 1994). Eventhough this result shows different trend with the adsorptive foul-ing result (cf. Fig 5) but GR40PP membrane has not yet shown sta-ble flux until 2.5 h filtration (Fig. 8). This indicates that fouling,which results in further decrease in flux, will still occur with fur-ther filtration. (cf. Fig. 8).

Fig. 8 also shows that the flux behavior is influenced by themembrane material (cf. FS61PP vs. GR40PP, these membraneshad similar pore size [cf. Fig. 5]). Although rapid flux decline inearly filtration for the FS61PP membrane might also be influencedby its pore structure (this membrane had small and big pore size),the effect of membrane material is clear. The more hydrophobicmembrane, the more flux reduction was observed (note thatFS61PP is the most hydrophobic, cf. Table 1). The membrane–sol-ute interactions are stronger for more hydrophobic membrane.

For all membranes, solute rejection increases with increasingfiltration time. In the beginning of filtration (when no solute wasdeposited), solute (especially for small size) can penetrate to per-meate side. This penetration decreases as the amount of particledeposited on (the membrane surface) and in (the membrane pores)is increased. Finally, the cake layer formed could act as a secondarymembrane.

3.3.2. Effect of the amylose concentrationThe effects of amylose concentration on permeate flux behavior

as well as solute rejection were investigated by varying the feedconcentration (0.01, 0.1 and 1 g/L). The moderate trans-membranepressure, i.e., 1 bar, was used in this experiment.

It is clearly observed that the permeate flux behavior is influ-enced by the feed concentration (Fig. 9). A sharp decrease in fluxis also observed. The feed solution containing higher concentrationof amylose results in higher permeate flux decline. Analogous tothe previous observation, CP is not the dominant reason for the flux

0

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1.1

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Time (min)

Flux

ratio

(J/J

o)

0.30

0.35

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0.45

0.50

0.55

0.60

Rej

ectio

n

F-C = 0.01 g/L F-C = 0.1 g/LF-C = 1 g/L R-C = 0.01 g/LR-C = 0.1 g/L R-C = 1 g/L

Relative water flux after stopping and

restarting the filtration

Fig. 9. Normalized flux (F) and rejection (R) profiles for different initial feedconcentrations at a pressure of 1 bar using the FS61PP membrane. Fluxes afterstopping and restarting the filtration relative to initial water flux are also included.

0

0.1

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Flux

ratio

(J/J

o)

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0.9

1

Rej

ectio

n

F-FS61PP F-GR61PPF-GR40PP R-FS61PPR-GR61PP R-GR40PP

Relative water flux after stopping and

restarting the filtration

Fig. 8. Normalized flux (F) and rejection (R) profiles for different membranes with0.1 g/L amylose in the feed stream and at a pressure of 1 bar. Fluxes after stoppingand restarting the filtration relative to initial water flux are also included.

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Time (min)

Flu

x ra

tio (

J/Jo

)

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0.60

Rej

ectio

n

F-P = 0.5 bar F-P = 1 barF-P = 1.5 bar R-P = 0.5 barR-P = 1 bar R-P = 1.5 bar

Relative water flux after stopping and

restarting the filtration

Fig. 10. Normalized flux (F) and rejection (R) profiles during ultrafiltration ofamylose solution (1 g/L) using the FS61PP membrane at various trans-membranepressures. Fluxes after stopping and restarting the filtration relative to initial waterflux are also included.

0

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1

1.1

0 10 20 30 40 50 60 70 80 90 100

Time (h)

Flux

ratio

(J/J

o)

0.40

0.50

0.60

0.70

0.80

Rej

ectio

n

Flux ratiorejection

Fig. 11. Normalized flux and rejection profiles during long-term ultrafiltration ofamylose solution (0.1 g/L) using the FS61PP membrane at a pressure of 1 bar.

428 H. Susanto, I Nyoman Widiasa / Journal of Food Engineering 95 (2009) 423–431

decline. Nevertheless, it is reasonable to predict that the concen-tration of solute at the membrane surface is greater for the higherfeed concentration. This would promote the adsorptive fouling andcake layer formation leading to more severe fouling. In addition,the increase in viscosity also had contribution. Our measurementshowed that the viscosity of amylose solution increased �5 timesas concentration was increased from 0.1 to 1 g/L.

In contrast with flux, rejection data show that the feed contain-ing higher amylose concentration had higher solute rejection. Thisresult suggests that a concentration polarization layer is formedand more pronounced at higher feed concentration. At higher sol-ute concentration the amount of rejected solute due to adsorptionand size exclusion by membrane pores would be more significant.

3.3.3. Effect of the trans-membrane pressureSeparation process in ultrafiltration is driven by trans-mem-

brane pressure (TMP). To investigate the effects of TMP on foulingbehavior and amylose rejection, UF experiment of solution con-taining 0.1 g/L of amylose was performed at different TMPs (0.5,1 and 1.5 bar) using the FS61PP membrane. Fig. 10 shows theexperimental results.

It is obviously observed that ultrafiltration experiment withhigher trans-membrane pressure yields more significant foulingthan UF with lower TMP. Even though it has higher normalizedflux in the beginning of UF indicating less fouling, but after lessthan 30 min of filtration, UF at higher TMP shows lower flux ra-tio (cf. 1 bar vs. 1.5 bar). The amount of solute at the membranesurface as well as in the membrane pore increases with increas-ing trans-membrane pressure as a consequence of increasing fluxpermeates. This condition can further promote solute adsorptionon/in the membrane leading to adsorptive fouling. In addition,the increase in solute concentration at the membrane surface in-creases osmotic pressure effect, which will reduce the permeateflux. Rejection data support these explanations. At first, the sol-ute rejection was lower for higher TMP, but it gradually in-creases with filtration time and is finally higher compared tolower TMP.

3.3.4. Long-term ultrafiltrationIn order to obtain information the flux behavior in practical

application, long-term ultrafiltration was performed. Fig. 11 shows

H. Susanto, I Nyoman Widiasa / Journal of Food Engineering 95 (2009) 423–431 429

the permeate flux as well as rejection profile during long-termoperation.

Analogous flux and rejection profiles are observed, i.e., rapid de-crease at the beginning of filtration followed by gradual decrease influx and gradual increase in rejection. The resulting permeate fluxis �15% of initial water flux after 90 h of filtration, whereas therejection increases from �45% to �62%. This suggests that the foul-ing mechanism during long-term seems similar with short timeultrafiltration. Overall, flux behavior indicates that fouling obvi-ously occurred during ultrafiltration of amylose. This observationis supported by surface imaging and contact angle measurement(cf. below).

3.4. Characterization of amylose fouling on membranes

The adsorptive and ultrafiltration fouling and their effect onmembrane characteristics were characterized by measuring thecontact angle (CA) of virgin and fouled membranes. In addition,deposition of amylose on membrane surface was visualized byusing SEM. Table 1 and Fig. 12 show the contact angles of freshmembranes and the same membranes after static adsorption aswell as ultrafiltration and the visualization of change in CA,respectively.

First, it is observed that the polysulfone membrane, GR61PP, hadlower CA (�66�) than the fluoro polymer membrane, FS61PP,(�77�). This indicates that the GR61PP membrane is more hydro-philic than the FS61PP membrane. Even though surface porosityhas influence on the measured contact angle, but difference inmaterial chemistry would be the reason in this case (note that theFS61PP membrane should have higher surface porosity than theGR61PP membrane). In general, exposing the membranes to theamylose solution during adsorptive fouling and ultrafiltrationchanged the CA implying that a change in surface property has oc-curred. Further, the apparent surface hydrophilization of the mem-brane surface indicates that solute deposition after adsorption andultrafiltration has occurred. In addition, these results suggest thatthe amylose used is more hydrophilic than the membranes used.The more significant decrease in CA after ultrafiltration fouling indi-cates the more solute deposited on the membrane surface after UF.

As presented in Fig. 13, visualization of membrane surface mor-phology supports the preceding results. Even though both theGR61PP and FS61PP membranes have the same cut-off (from man-ufacturer) but they show different surface morphology. The FS61PPmembrane has much greater pore size compared to that of theGR61PP membrane. By contrast, the membrane GR40PP, whichhas larger membrane cut-off from manufacturer (100 kg/mol),shows similar pore size with the FS61PP membrane. Nevertheless,the pores of FS61PP membrane are less evenly distributed. Someparts appeared to be denser than the others and some large poreare also observed. This surface visualization of the virgin mem-branes agrees well with the hydraulic permeability measurementas well as rejection curve measurements, where the FS61PP mem-brane showed the highest water permeability and had similarrejection curve with the GR40PP membrane (cf. Section 3.1).

Surface visualization after static adsorption indicates that themembrane surface is apparently covered by a solute layer (cf.

Fig. 12. Visualization of a 5 ll water droplet during contact angle measur

Fig. 13, FS61PP membrane images for fresh and after adsorption).The surface, which has a denser appearance and pore size reduc-tion, is observed. This is consistent with the relative flux reductionafter static adsorption results (cf. Section 3.2). However, it is hardto identify the difference in surface morphology for GR61PP mem-brane. More significant membrane surface coverage as well as porenarrowing is observed after ultrafiltration. Again, the effect for theFS61PP membrane is more pronounced. Indeed, a cake layer of sol-ute, which covers the entire membrane surface, is observed fromthe surface of the FS61PP membrane after long-term ultrafiltration.

3.5. Mechanism of interaction and fouling

Based on the results of adsorptive fouling, ultrafiltration andmembrane characterization, it is clearly observed that adsorptionof amylose has occurred on/in the surface of fluoro polymer andpolysulfone membranes. Interestingly, the extent of surface cover-age by solute deposition shows a good agreement with the fluxreduction, i.e., adsorptive fouling < short-term UF < long-term UF.These observations are confirmed by measuring the contact angleof the membrane and visualization of membrane surface. In addi-tion, the results suggest that both adsorptive and UF fouling aremore severe for the FS61PP membrane than the GR61PP mem-brane. The amylose binding to both membranes observed duringadsorptive fouling could be due to hydrophobic–hydrophobicinteractions. The hydrophobic character of amylose can come fromits single helix structure that has hydrogen bonding O2 and O6atoms on outside surface of the helix with only the ring oxygenpointing inwards (Imberty et al., 1988). The hydrogen bonding be-tween aligns chains causes retrogradation and releases some of thebound water. These possesses extensive inter-and intra-strandhydrogen bonding resulting in a fairly hydrophobic structure.Because both the GR61PP and FS611 membranes are hydrophobic(cf. contact angle data, Table 1), their interactions with water areweak and can easily be displaced by a solute. The adsorptionthen occurs via surface dehydration, i.e., replacement of watermolecules at the surface by adsorbed solute causing increase in en-tropy (Vogler, 1999). The membrane–amylose interactions arestrong enough since amylose also has hydrophobic character. Thisexplanation could also be used to explain the observed solute–sol-ute interactions during adsorptive fouling.

The rapid flux decline in initial stage of UF implies that adsorp-tive fouling and/or pore blocking is the dominant fouling mecha-nism for the GR61PP and the FS61PP membranes. Then theformation of solute layer as noticed by gradual decrease in flux oc-curs in further filtration. For the GR40PP membrane, pore narrow-ing followed by solute layer formation seems to be the dominantmechanism. Nevertheless, changes in UF conditions, e.g. low feedconcentration and low trans-membrane pressure could changethe fouling mechanism. Indeed, a solute cake layer is observed byvisualization of surface morphology (cf. Fig. 13). A large molarmass fraction of amylose (which has greater size than membranepores) would block the membrane surface forming a cake. Thiscake prevents further penetration of solute into the membranepores. Instead of that new solute deposited will be formed on topof the existing solute layer.

ement of FS61PP membranes: fresh (left side) and fouled (right side).

Fig. 13. SEM images of membrane surface morphology: from the top to the bottom panel: virgin membrane, after adsorptive fouling, after short-term UF and after long-termUF, respectively.

430 H. Susanto, I Nyoman Widiasa / Journal of Food Engineering 95 (2009) 423–431

4. Conclusions

Behavior, characteristic and mechanism of amylose fouling dur-ing ultrafiltration have been investigated. Significant water fluxreduction after adsorptive fouling, which is influenced by mem-brane property (pore size and material) and solute concentration,confirms that amylose adsorption on the membrane surface as wellas inside its pores. A correlation between RFR and the extent ofadsorptive fouling was determined by using isotherm adsorptionequation. The adsorptive fouling followed not only monolayeradsorption on the solid surface but also solute–solute interactions.UF results suggest that both reversible and irreversible fouling haveoccurred with irreversible fouling is much more dominant. The fluxbehavior is influenced by the membrane characteristic, solute con-centration and trans-membrane pressure. Correlations between

the RFR values and the flux during UF is found and implies thatRFR measured after static adsorption is a valuable tool for the inves-tigation of ultrafiltration fouling. Contact angle measurement andvisualization of surface morphology by SEM verify both adsorptiveand ultrafiltration fouling. In particular, the SEM images revealedthe solute layer formation on membrane surface. The possible mech-anism for adsorption fouling via hydrophobic–hydrophobic interac-tions is discussed. Cake layer formation initiated by eitheradsorption or pore blocking is the dominant fouling mechanism dur-ing ultrafiltration. However, pore narrowing instead of pore block-ing is also observed for the membrane having large and relativeuniform pore structure or for the ultrafiltration using low trans-membrane pressure or low solute concentration. Finally, it wasclearly shown that amylose solution could be a potential foulantfor UF membrane and therefore should be considered for further de-

H. Susanto, I Nyoman Widiasa / Journal of Food Engineering 95 (2009) 423–431 431

tailed mechanistic investigations and method development for foul-ing control of a complex feed system containing amylosepolysaccharide.

Acknowledgements

The authors would like to thank Professor Mathias Ulbricht(Universität Duisburg-Essen, Germany) for giving the possibilityto perform contact angle measurements. We also thank Alfa Laval,Denmark, for supplying the membranes. This research was finan-cially supported by the state ministry of research and technology,Indonesia (No. 66/RT/Insentif/PPK/II/2008).

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