Consumption of juices has increased significantly dur-ing recent years, and red plum juice is greatly acceptedamong consumers due to its pleasant colour andaroma. It contains high levels of phenolic compounds,such as flavonoids and phenolic acids; the antioxidantactivities of these components make it a nutritionaljuice (Heim et al., 2002; Kim et al., 2003). Filtration isone of the most important unit operations in industrialfruit juice processing. Membrane processing systemssuch as microfiltration (MF) and ultrafiltration (UF)are now used to clarify fruit juices due to their lesslabour requirements, higher efficiency, shorter processtime and therefore considerably lower operationalcosts in comparison with conventional juice-processingmethods (Cassano et al., 2010; Laorko et al., 2010;Yazdanshenas et al., 2010). The purpose of the clarifi-cation is to remove suspended solids as well as haze-inducing and turbidity-causing substances because the
obtaining juice needs to be clear without sediments,haze or turbidity during shelf life. Specifically, pheno-lic compounds via polymerisation and their interac-tion with other components are responsible for hazeformation and the undesirable colour in fruit juices(de Bruijn & Borquez, 2006). The introduction ofmembrane technologies in the manufacture of fruitjuices represents one of the technological answersto the problem of producing an additive-free juicethat has high quality and natural fresh taste (deOliveira et al., 2012). Several works have studied theuse of UF and MF in the clarification of fruit juicessuch as pomegranate (Mirsaeedghazi et al., 2010),mosambi (Rai et al., 2007), pineapple (Laorko et al.,2010), kiwifruit (Cassano et al., 2007) and apple(Yasan et al., 2007).Despite the several benefits of membrane clarifica-
tion, the performance of this operation is affected bythe declining permeate flux with time (Yazdanshenaset al., 2010). Accumulation and deposition of rejectedcompounds on membrane surfaces (concentrationpolarisation) or inside its pores result in membrane*Correspondent: Fax: +982612248804; e-mail: [email protected]
International Journal of Food Science and Technology 2013
fouling, which is a limiting factor in the industrialisationof membrane processes. Membrane fouling decreasespermeate flux and, potentially, membrane longevity;therefore, it is a key factor affecting the economic andcommercial viability of a membrane system (Hojjatpanahet al., 2011). Several efforts have been made to controlor eliminate membrane fouling, including the use ofshear-enhanced process, fabrication of antifoulingmembranes and pretreatment of feed juice (Luo et al.,2012; Zhu et al., 2013). Among these availableapproaches, filtration at the best operating conditionsis technically sound and economically attractive tomaximise permeate flux (or minimise membranefouling) as well as the quality of the permeate.
To minimise the fouling phenomena, parametersinfluencing declines in flux, and their contribution,must be studied. Therefore, in this work, foulingmechanisms were indentified during clarification of redplum juice at different processing parameters includingtransmembrane pressure (0.5, 1.3, 2.1 and 2.9 bar),pore size (0.025, 0.1 and 0.22 lm), membrane type[mixed cellulose ester (MCE) and polyvinylidene fluo-ride (PVDF)], velocity (0.2, 0.5 and 0.8 m s�1) andtemperature (20, 30, 40 °C). The effect of changes inprocessing conditions on membrane-fouling intensitywas also studied.
Theory
Hermia (1982) proposed four empirical models to pres-ent membrane-fouling mechanisms based on constantpressure: complete pore blocking, standard pore block-ing, intermediate pore blocking and cake filtration(Rai et al., 2007; Mirsaeedghazi et al., 2009; Raziet al., 2012). In complete pore blocking, the particlesize is larger than the membrane pore size; thus, poresare blocked completely. In the standard-blockingmechanism, particles are much less than the membranepore diameter so can enter the pores and deposit insidethe pore walls, which may lead to blocking of poresand reduce the pore volume. In the intermediate-block-ing mechanism, the particle size in the feed is the sameas the membrane pore size; however, the membranepore is not necessarily plugged by particles, and someparticles may deposit on each other. Both large andsmall particles can accumulate on membrane surface toform the cake layer in the cake-formation mechanism.This creates an additional barrier to the permeate flux.
The following Hermia’s equation is used to deter-mine the fouling mechanisms during processing:
d2t
dV2¼ k
�dt
dV
�i
ð1Þ
where t is the operation time, V is the cumulativevolume of filtrate, and the parameters k and i are char-acteristics of the fouling models. According to
Hermia’s model, in membrane processing, the permeatevolume is affected by the fouling mechanism. When thecurve of t/V vs. t is linear, standard blocking is domi-nant; in contrast, cake formation is the dominantmechanism with a linear cure of t/V vs. V. However,when the curve of Ln (t) vs. V is linear, intermediatepore blocking is the dominant fouling mechanism.The permeate flux is computed using the following
equation:
J ¼ dV
Adtð2Þ
Taking derivation of dtdV
¼ 1AJ
with respect to t leads
to the expression of d2t
dV2¼ � 1
AJ3dJdt
. Substituting these
equations in eqn 1 leads to the governing equation offlux decline with time (Razi et al., 2012):
J ¼ dJ
Adt¼ �aJð3�nÞ ð3Þ
where a is a constant (Razi et al., 2012).Field et al. (1995) modified classical constant pres-
sure dead-end filtration equation for cross-flow filtra-tion, which is expressed as (Razi et al., 2012) follows:
� dJ
dtJð2�iÞ ¼ kðJ� J�Þ ð4Þ
where J is the permeate flux of juice, J* is the steadystart flux, and t is the time. i in both models is block-ing index, which depends on fouling mechanism. Thevalues for the blocking index for cake formation, inter-mediate pore blocking, standard pore blocking andcomplete pore blocking are 0, 1, 1.5 and 2, respec-tively. By substituting of i for each mechanism ineqn 3 and integrating, the variations of permeate fluxwith time of the experiment are represented as (Raziet al., 2012) follows:Cake formation (i = 0):
J�2 ¼ J�20 þ kct ð5Þ
Intermediate pore blocking (i = 1):
J�1 ¼ J�10 þ kit ð6Þ
Standard pore blocking (i = 1.5):
J�0:5 ¼ J�0:50 þ kst ð7Þ
Complete pore blocking (i = 2):
lnðJ�1Þ ¼ lnðJ�10 Þ þ kbt ð8Þ
where kc, ki, ks and kb are the slopes of straightlines of J�2, J�1, J�0.5 and ln (J�1) vs. t, respectively,and show the intensity of cake formation, intermediatepore blocking, standard pore blocking and completepore blocking, respectively (Nandi et al., 2009). Thesefouling intensities (slopes of straight lines) were calcu-lated for each processing parameter, and then, the
International Journal of Food Science and Technology 2013
Clarification of red plum juice H. Nourbakhsh et al.2
effect of changes in processing parameters on the inten-sity of each blocking phenomenon was evaluated.
Materials and methods
Juice extraction
Red plum fruit (vt. Vampire) was purchased from alocal market (Karaj, Iran). Mature and fresh fruitswere selected for juice extraction. The juice wasextracted from washed and peeled fruits using manualpressing. The extracted juices were prefiltered using amesh filter (No. 9), then divided into portions of 1 Land stored at �25 °C before further use.
Membrane setup
A cross-flow membrane unit with a flat sheet modulein batch mode was used at laboratory scale (Fig. S1).A hydrophilic PVDF flat membrane with a pore sizeof 0.22 lm and MCE flat membranes with pore sizesof 0.22, 0.1 and 0.025 lm and a total effective filtra-tion area of 0.0209 m2 were used in this study (Milli-pore, Billerica, MA, USA). Feed temperature wascontrolled using a two-layer jacket tank and a waterbath. An inverter (LS, Model sv015ic5-1f, Cheongju,South Korea) coupled with a transmitter (WIKA,Type ECO-1, Klingenberg, Germany) was used tohold the pressure of the rotary van pump (PROCON,Series 2, Milano, Italy) at the required level for differ-ent flow rates. Two separate pressure meters (WIKA,model 2 13.53.06 3) were used to measure pressure inthe feed and retentate sides. Permeate was collected ina permeate tank and was weighed to measure permeateflux. Retentate was recycled to the feed tank.
Scanning electron microscopy
Fouling-layer morphology of the membranes wasobserved with a scanning electron microscope (SEM,KYKY-EM3200, KYKY Technology DevelopmentLtd., China) operated at 25 kV. Surfaces and crosssections of the membranes were sputtered with gold,observed and photographed to evaluate the effect ofjuice treatment on membrane fouling.
Statistical analysis
Results were expressed as the mean of triplicate deter-minations. Statistical analysis of data was performedusing one-way analysis of variance (ANOVA). The meancomparisons were carried out using Duncan’s multiplerange tests. Probability value of P < 0.05 was selectedas the criteria for statistically significant difference(SPSS for Windows, version 16.0, SPSS Inc., Chicago,USA).
Results and discussion
The effect of membrane type and pore size on foulingmechanisms
To evaluate the effect of membrane type and pore sizeon fouling mechanisms, the juice was treated usingMCE membranes with pore sizes of 0.22, 0.1 and0.025 lm and PVDF membrane with pore size of0.22 lm at the same pressure and velocity. The resultsindicated that cake filtration at all treatments was thedominant mechanism, as the curve of t/V vs. V hadthe best fitness with the linear curve (R2 ≥ 0.95)(Table 1). It has been similarly reported for ultrafiltra-tion of stevia extract with 5-, 10- and 30-kDa molecu-lar weight cut-off membranes (Mondal et al., 2011).According to the blocking index, cake formation
was the first fouling mechanism in all membrane pro-cesses (Fig. 1a). In an overall comparison betweenMCE 0.22-lm and PVDF 0.22-lm membranes as dif-ferent types, it could be found that standard and com-plete blocking occurred earlier at MCE membrane.The hydrophobic characteristics of PVDF membraneprevent fluid movement into the membrane pores.Figure 1a shows that standard blocking and completepore blocking did not occur when pore size wasreduced from 0.22 to 0.025 lm, so that cake formationwas the dominant mechanism until the end of the pro-cess. It should be noted that the components responsi-ble for MF (0.22 lm and 0.1 lm) membrane foulingand UF (0.025 lm) membrane fouling are not reallysimilar. Due to the small size of the pores (0.025 lm),the particles could not fit in the membrane pores;instead, they increasingly accumulated on the mem-brane surface and made a thick cake layer. Also,Mirsaeedghazi et al. (2009) found that the blockingindex in processing with a MCE membrane with a poresize of 0.025 lm was close to zero during treatment ofpomegranate juice. Similar results were obtained byGirard & Fukumoto (2000), but Hojjatpanah et al.(2011) found that standard blocking was the dominantfouling mechanism in filtration of black mulberryjuice with MCE 0.22 lm and MCE 0.1 lm, whileintermediate blocking was the dominant blockingmechanism with MCE 0.025 lm.
The effect of transmembrane pressure on foulingmechanisms
Plum juice was treated using MCE membrane with apore size of 0.22 lm at 0.5, 1.3, 2.1 and 2.9 bar at con-stant velocity and temperature to evaluate the effect oftransmembrane pressure on fouling phenomena. Theresults showed that the most important mechanism infouling was cake formation (Table 1). Also, other foul-ing mechanisms play an important role in fouling
International Journal of Food Science and Technology 2013
Clarification of red plum juice H. Nourbakhsh et al. 3
phenomena. Therefore, eqn 1 was used to plot thecurve of blocking index vs. t (Fig. 1b). It was able todetermine the dominant mechanism at each moment ofmembrane processing. The results indicated that thecake layer formed at the beginning of the process; thiswas followed by, in order, intermediate, standard andcomplete blocking at all applied pressures. However,complete blocking occurred earlier at higher pressuresdue to the effect of greater driving force, whichincreased the entry of fine particles into membranepores. These results are in agreement with the study ofpomegranate juice done by Mirsaeedghazi et al. (2009).
The effect of feed temperature on fouling phenomena
Red plum juice was treated using an MCE membranewith a pore size of 0.22 lm at different temperatures (20,30 and 40 °C) and constant pressure (0.5 bar) and veloc-
ity (0.2 m s�1) to evaluate the effect of feed temperatureon fouling phenomena. A comparison among the curvesof changes in permeate volume at different times showedthat cake formation was the main blocking mechanism(Table 1). The results showed that cake formation andintermediate, standard and complete pore blockingoccurred at all temperatures. Also, standard and com-plete pore blocking occurred earlier with an increase intemperature from 20 to 40 °C (Fig. 1c). This can beexplained by the fact that the increase in temperatureincreases the diffusion coefficient of macromolecules (forexample pectin) that can penetrate into the pores anddeposit along the pore walls (Wang et al., 2005).
The effect of different velocities on fouling mechanisms
MCE membrane with a pore size of 0.22 lm wasselected to evaluate the effect of cross-flow velocity on
International Journal of Food Science and Technology 2013
Clarification of red plum juice H. Nourbakhsh et al.4
fouling mechanisms. Table 1 indicates that cake for-mation has the most important role in the fouling ofmembrane pores. Furthermore, results showed thatstandard and complete pore blocking occurred fasterat a velocity of 0.8 m s�1 than at 0.2 m s�1 (Fig. 1d).At higher velocities, the cake layer is thinner due tothe increase in shear stress on the membrane surface,which reduces concentration polarisation (Cassanoet al., 2007; Laorko et al., 2010). Therefore, because oflow resistance, the particles can penetrate more quicklyinto the pores.
The effect of changes in processing parameters on theintensity of blocking phenomena
Decrease in cake formationThe effect of processing condition on cake formationis shown in Table 2. The results showed that increas-ing the temperature from 30 to 40 °C was the mosteffective factor in reducing cake-layer fouling. Thiscan be related to a decrease in feed viscosity and anincreasing Reynolds number for the fluid at a certainspeed (Cassano et al., 2007). In other words, flowturbulence is increased and the deposition of particleson the membrane surface reduced. Increasing velocityhad less effect in reducing the cake-layer intensity; infact, an increase in speed from 0.5 to 0.8 m s�1 cre-ated no changes in the intensity of cake formation.This could be due to the effect of cross-flow velocity,which could explain the fact that the thinnest cakelayer was generated at a velocity of 0.5 m s�1, andhigher velocities made no significant difference incake formation. As reported in Table 2, decreasingthe pressure from 2.9 to 2.1 bar and from 2.1 to1.3 bar reduced cake-layer intensity by 33.3% and25%, respectively. Therefore, the reduction in trans-membrane pressure decreased the formation of cakelayer since higher pressure during membrane filtra-
tion; this, in turn, led to thicker and more concen-trated cake build-up. However, this effect becamegreater as the pressure value was decreased. Theseresults also showed that changes in both membranetype (from PVDF to MCE) and pore size (from 0.1to 0.22 lm) decreased cake-layer intensity by 22%.This may be due to the causing a severe cake by thelarger particles when they do pass through themembrane. As PVDF membrane is more hydrophobicthan MCE, it rejected more particles that would havemade a denser cake layer (Mirsaeedghazi et al.,2009).
The effect of changes in processing parameters on theintermediate-blocking intensity
Table 2 also shows the effect of different conditions onthe severity of intermediate fouling. It was observedthat increasing the temperature from 20 to 30 °Creduced the blocking intensity by 51.5%; however,higher temperatures made no significant difference.The increase in feed velocity to 0.5 m s�1 caused an86.1% reduction in this type of blocking. In addition,increasing the velocity from 0.5 to 0.8 m s�1 had onlya slight influence on intermediate fouling. Further-more, these results indicated that changing membranetype, decreasing transmembrane pressure and increas-ing pore size had highly desirable influences on reduc-ing intermediate-fouling intensity.
The effect of changes in processing parameters onstandard-blocking intensity
The influence of processing conditions on standard-blocking intensity is demonstrated in Table 3. Chang-ing temperature from 30 to 40 °C decreased stan-dard-fouling intensity by 16%. As reported in severalworks (Wang et al., 2005; Magerramov et al., 2007a,
Table 2 Effect of changes in processing
parameters on the intensity of cake-forma-
tion and intermediate-blocking mechanismsType of change in processing parameter
Decrease in
cake-formation
intensity (%)
Decrease in
intermediate-blocking
intensity (%)
Increase in temperature From 20 to 30 °C 14.3e* � 1.12 51.5e � 2.8
From 30 to 40 °C 66.7a � 3.18 1.4 g � 0.28
Increase in feed
velocity
From 0.2 m s�1 to 0.5 m s�1 14.3e � 0.98 86.1a � 3.5
From 0.5 m s�1 to 0.8 m s�1 0.003f � 0.001 2.6f � 0.42
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Clarification of red plum juice H. Nourbakhsh et al. 5
b), feed viscosity is reduced by increasing tempera-ture; therefore, permeate flux increased during mem-brane treatment with the increase in temperature.Mathematical modelling of fluid behaviour in micro-filtration systems by Mirsaeedghazi et al. (2011)showed that a higher flow rate of permeate results inthe passage of more particles into the permeate flux,thus preventing the precipitation of particles on thepore wall.
Also, decreasing feed velocity from 0.5 to0.2 m s�1 resulted in no standard fouling. This maybe due to the fact that the reduction in flow rateincreases the cake-layer thickness, and thus, fewercompounds can penetrate into the pores, reducingstandard blocking. As mentioned in the previous sec-tions, an increase in velocity from 0.5 to 0.8 m s�1
had no influence on cake-formation intensity. Increas-ing pressure also increased cake-layer intensity, andthe cake layer became thicker. This could preventfurther entrance of particles into the pores, reducingstandard blocking. The results showed that membranepore size used in this limit (0.1–0.22 lm) had noeffect on standard-blocking intensity; however, thestandard-fouling mechanism did not happen with theMCE membrane.
The change in complete-blocking intensity duringdifferent conditions
Table 3 also presents the influence of different process-ing parameters on the intensity of complete plugging.It was observed that a lower temperature was the fac-tor most likely to improve complete blocking. Thismay be due to the fact that an increase in temperatureis associated with more diffusion of macromolecules,
which cause more clogging. Consideration of the block-ing index at 40 °C also showed that complete foulingat this temperature occurred earlier in membrane pro-cessing. Increasing the flow rate up to 0.8 m s�1
reduced the blocking intensity by 25%, as higher veloc-ity could remove considerable numbers of particles onthe pore entrances. In addition, the increase in pressuredecreased complete-blocking intensity and eliminated itentirely at a pressure of 2.9 bar. This can be explainedby the fact that the thicker cake layer created by higherpressure inhibits larger particles from settling on pores.Finally, changing the membrane type and pore sizereduced the severity of this fouling, because a densercake layer was formed at these conditions.
Analysis of fouling by SEM
The microstructure of the fouled membranes ispresented in Fig. S2. Analysis of the SEM imageryconfirms that cake formation was the dominant block-ing mechanism in all treated membranes. The resultsalso showed that the cake layers formed in variousmembrane pore sizes were different with each other interm of particles size (Fig. S2a–c). The size of particleswithin the cake was higher in the larger membranepore size. In an overall comparison between 0.2 and0.5 m s�1 velocities, it cloud be found that at thehigher velocity, the cake layer was thinner (Fig. S2c,d).As discussed, it is due to more tangential force createdon the membrane surface. The SEM images at differenttransmembrane pressures also showed that thickness ofcake layer at pressure 2.9 bar was significantly higherthan lower pressure (Fig. S2e,f). These results agreedwith the findings regarding analysis of fouling mecha-nisms described at previous sections.
Type of change in processing parameter
Decrease in
standard-blocking
intensity (%)
Decrease in
complete-blocking
intensity (%)
Change in temperature From 20 to 30°C nd –
From 30 to 40°C 16c* � 0.7 –
From 40 to 30°C – 67.9d � 1.82
From 30 to 20°C – 61.9e � 2.1
Change in feed velocity From 0.8 m s�1 to 0.5 m s�1 0.005d � 0.002 –
From 0.5 m s�1 to 0.2 m s�1 99.9a � 0.1 –
From 0.2 m s�1 to 0.8 m s�1 – 25 g � 1.06
Increase in
transmembrane
pressure
From 1.3 bar to 2.1 bar 73.4b � 2.43 89b � 2.8
From 2.1 bar to 2.9 bar 99.95a � 0.03 99.96a � 0.01
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Clarification of red plum juice H. Nourbakhsh et al.6
Conclusion
In general, the analysis of fouling results showed thatcake formation was the dominant mechanism responsi-ble for membrane blocking. As the cake layer formsoutside the membrane, it may be controlled andremoved with the correct choice of operating condi-tions, such as high velocity (high shear force) and lowpressure. The evaluation of operating conditions’ influ-ence on the fouling-decline mechanism is very impor-tant for any membrane process. Results of this studyshowed the extent to which changes in each operationparameter can reduce or increase membrane-blockingphenomena. The results indicated that increasing thetemperature was the most effective factor in the reduc-tion in cake-layer fouling. Recognition of the mainfouling mechanism, its formation process and the mostsuitable operating conditions can help us to determinethe optimum process time and the methods of themembrane recovery.
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Supporting Information
Additional Supporting Information may be found inthe online version of this article:Figure S1. Plate and frame membrane unit: (1)
