Cross¯ow micro®ltration of passion fruit juice after partial enzymaticliquefaction

F. Vaillant a,b, P. Millan b, G. OÕBrien b, M. Dornier a,c,*, M. Decloux c, M. Reynes a

a Centre International de Recherche Agronomique pour le D�eveloppement/FLHOR Dept., B.P. 5085, 34032 Montpellier Cedex 1, Franceb Universidad Del Valle, Depto. De Ciencia y Tecnologia de Alimentos, Sede Melendez, Cali, Colombia

c Tropical Food Engineering Dept., Ecole Nationale Sup�erieure des Industries Alimentaires, B.P. 5098, 34033 Montpellier Cedex 1, France

Received 1 June 1999; accepted 23 August 1999

Abstract

To obtain clari®ed passion fruit juice, cross¯ow micro®ltration after enzymatic liquefaction was studied using ceramic mem-

branes with 0.2 lm pore size. The e�ect of a high-rate enzymatic treatment for the degradation of suspended solids was assessed,

resulting in the selection of a commercial enzymatic preparation. Partial enzymatic liquefaction of cell-wall polysaccharides prior to

micro®ltration provided an unusual pattern of ¯ux increase after a short decline when cross¯ow velocity was high (7 m sÿ1). It was

found that a synergistic e�ect between pectinase and cellulase activities enhanced permeate ¯ux increase. With total recycling at

36°C, the combination of low transmembrane pressure (150 kPa) and high enzyme concentration (1 ml lÿ1) provided the highest ¯ux

(113 l hÿ1 mÿ2). These conditions were then assessed with concentration in order to verify industrial feasibility and evaluate physico-

chemical characteristics of ®nal products. A volumetric reduction ratio of 3 was maintained during 18 h without any decrease

in permeate ¯ux, which ¯uctuated around 40 l hÿ1 mÿ2. The quality of permeate was satisfying even its aromatic strength

was weakened. Retentate had similar characteristics of raw juice and could be recycled in order to use its residual enzyme

activity. Ó 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Micro®ltration; Passion fruit; Enzymatic liquefaction; Fruit juice

1. Introduction

Yellow passion fruit (Passi¯ora eduils var. ¯avicarpa)juice is marketed world-wide, principally because of itspleasant unique aroma and ¯avour. Nevertheless, the¯avour of passion fruit is extremely sensitive to changeas a result of heat processing (Whit®eld & Last, 1986).Conventional stabilisation methods, such as thermal

pasteurisation, induce general losses in ¯avour volatileconcentration, and speci®c changes in aromatic com-pounds, even when short-duration procedures areapplied (Casimir, Ke�ord & Whit®eld, 1981). Mem-brane-based techniques, on the other hand, do not in-volve the application of heat. Micro®ltration can beused to separate juices into a ®brous concentrated pulp,and a clari®ed fraction free of spoilage micro-organisms.The `commercially sterile' clari®ed fraction can thenundergo non-thermal membrane concentration, andeventually whole juice reconstitution by combinationwith the pasteurised pulp, in order to obtain a productwith improved sensorial properties (Ganlmann, 1993).Also, a superior quality clari®ed passion fruit juice couldmake a strong impact in new market areas, such as clearjuice blends, liqueurs and related products, carbonatedsoft drinks, and in all applications where suspendedsolids have a negative e�ect on ®nal product quality.

A major constraint in relation to the micro®ltrationof juices rich in pulp, is the formation of a highly swollenfouling layer on the membrane surface, which greatlyreduces performance. For fruit juice, the fouling mate-rials are mainly composed of cell-wall polysaccharides

Notation

AIS alcohol insoluble solids g kgÿ1

Cx endo-cellulase activity IU/ml

J permeate ¯ux l hÿ1 mÿ2

Pl pectin±lyase activity IU/ml

SS suspended solids g kgÿ1

TmP average transmembrane pressure kPa

U cross¯ow velocity m sÿ1

VRR volumetric reduction ratio ad

Journal of Food Engineering 42 (1999) 215±224

www.elsevier.com/locate/jfoodeng

* Corresponding author. Fax: +33-4-6761-44-33.

E-mail address: dornier@cirad.fr (M. Dornier)

0260-8774/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved.

PII: S 0 2 6 0 - 8 7 7 4 ( 9 9 ) 0 0 1 2 4 - 7

such as pectin, cellulose, lignin and hemicellulose (Yu,Chiang & Hwang, 1986). To enhance ®ltration perfor-mance, fruit juices are usually treated before ®ltrationwith enzyme preparations aimed at hydrolysing mainlysoluble polysaccharides responsible for high viscosity(Chiang & Yu, 1987; Cheryan & Alvarez, 1995). Thistreatment is e�ective on juices with relatively low pulpcontent. Nevertheless, when it comes to pulpy juices,micro®ltration processes undergo di�culties and poor¯ux generally result. Apparently, a more aggressive en-zymatic degradation is required, not only to reduceviscosity but also to liquefy insoluble cell-wall polysac-charides which are retained by the membrane.

The research described in this paper was undertakento investigate the e�ects of enzymatic liquefaction ofcell-wall polysaccharides prior to micro®ltration, in or-der to introduce inside a juice plant a processing line forthe production of clari®ed passion fruit juice withoutany new waste disposal.

2. Materials and methods

2.1. Passion fruit juice

All juice was processed at the juice plant of PassicolS.A. (Chinchin�a, Colombia) using fully ripe fruit fromplantations located in the department of Caldas, be-tween 1500 and 1800 m altitude. Juice was taken fromthe industrial processing line after peel removal, enzy-matic treatment of pulp (15 min) at ambient tempera-ture for the separation of the seeds from juice sacs (20 lllÿ1 of Ultrazym 100 Gâ, NOVO, Denmark), seed re-moval, juice ®nishing and centrifugation at 1000 g (1.8±2min) in order to remove heavy starch granules. The juicewas also gently ¯ash-pasteurised (40 s/80°C) in a plateheat exchanger. A 2000 l batch of this juice was homo-genised. Sub-samples were stored in 100 l bags and keptfrozen at ÿ20°C until required. All trials were carriedout using juice from this 2000 l batch, with the exceptionof the pre-industrial trials with concentration wherefresh juice from the same production line was used, butbefore ¯ash pasteurisation.

2.2. Chemical and physical analysis

Passion fruit juice, permeate and retentate were an-alysed for pH, titrable acidity and density by standardmethods (AOAC, 1990). Soluble solids was measuredwith an Abbe refractometer (ATAGO, Japan). Glucose,fructose and sucrose were analysed by high performanceliquid chromatography (Model D 7000, Merk-Hitachi,Germany) using an Aminex HPX-87P column (Bio-radlaboratories, USA) with water at 85°C as mobile phaseat ¯ow rate 0.6 ml/min (Voragen, Schols, Leeuwen,Beldman & Rombouts, 1986). Viscosities were measured

at 20°C with a glass capillary Cannon±Fenske routineviscometer for permeates and with a rotative Brook®eldLVT viscometer for whole juice and retentates. Sus-pended solids (SS) was determined by centrifugation(2200 g/15 min) of 10 ml of a pre-weighed homogenisedsub-sample. Settled solids weight was determined afterdraining under controlled conditions (5 min). Alcoholinsoluble solids (AIS) were determined by dispersing apreviously weighed homogenous aliquot of retentateinto boiling ethanol (80 vol%). Residue was washedextensively with ethanol (80 vol%) and dried using sol-vents (acetone and ether) to constant weight (Voragen,Heutink & Pilnik, 1980). Galacturonic acid representingpectin in raw juice or hydrolysed pectin in enzymetreated juice were determined by the m-hydroxy-diphe-nyl spectrophotometric method (Blumenkrantz & As-boe-Hansen, 1973) directly on clari®ed juice or onsupernatant of pulpy products after centrifugation at6000 g/10 min.

2.3. Enzymes and enzymatic treatment of juice

Commercial fungal enzyme preparations were ob-tained from GIST BROCADES (Seclin, France). Ac-cording to technical information provided by themanufacturer, Cytolase CL was obtained from Tricho-derma longibrachiatium; Rapidase Press was derivedfrom Aspergillus niger; Rapidase Pomaliq 2F and Rapi-dase Liq� were derived from a mixture of both strains.Assays of enzyme activity were carried out using pro-cedures described earlier (Vaillant et al., 1999) and ac-tivities are expressed in lmol of products released permin (IU). Residual enzyme activities were determineddirectly on the permeate and on the retentate superna-tant recovered after centrifugation at 6000 g/10 min.

For all trials, commercial enzyme preparations wereused in order to treat 20±50 l batches of whole juice in atemperature-controlled tank with constant agitation,prior to micro®ltration. Previous organoleptic test, inwhich passion fruit juice was maintained at di�erenttemperatures during 2 h had shown that 36°C was themaximum temperature at which a trained jury was un-able to discern any change of global quality. A 1 hincubation period was chosen as it is considered to bethe maximum residence time acceptable in the fruitindustry.

2.4. Micro®ltration

The micro®ltration unit used (TIA, Boll�ene, France)features two tubular ceramic multichannel membranes(Membralox IP19-40, SCT, Bazet, France) with a totale�ective ®ltration area of 0.48 m2 and 0.2 lm averagepore diameter. Tangential ¯ow was ensured by a cen-trifugal pump fed by a volumetric pump. A tubular heatexchanger allowed to control temperature in ®ltration

216 F. Vaillant et al. / Journal of Food Engineering 42 (1999) 215±224

loop. The minimum hold-up volume of the pilot unitwas 12 l. Inlet and outlet pressures were measured bytwo pressure gauges (�2%), inlet temperature by amercury thermometer (�1°C), feed and permeate ¯owsby electromagnetic ¯owmeters (�0.5%).

The slow start-up procedure applied for micro®ltra-tion was de®ned by Dornier, Petermann and Decloux(1995). Tests were carried out either with total recyclingor with concentration. In the ®rst case, permeate wasrecombined with retentate and again fed through thesystem, maintaining the total volume constant. In thesecond case, the permeate stream was not returned tothe feed tank. The volumetric reduction ratio (VRR)was determined by measuring feed and permeate vol-umes (Vf , Vp) according to Eq. (1). In the pre-industrialtrials with concentration, as soon as the VRR set-pointwas reached, it was kept constant by maintaining thefeeding-tank level with juice and removing a volume ofretentate, calculated according to the measured perme-ate volume recovered during the same time interval.

VRR � Vf

Vf ÿ Vp

: �1�

A cleaning-in-place procedure was used in order to re-cover the original permeability (1500 l hÿ1 mÿ2 barÿ1)after each experiment. The pilot unit was rinsed with hottap water (50°C). Then a 2% sodium hydroxide solution

was recirculated (50°C/15 min without ®ltration/15 minwith ®ltration). After rinsing with hot tap water, P3Ultrasil 10 (Bonewitz, Burlington, IA, USA) diluted at1% at 80°C was used during 30 min as recommended byPunidadas (1990). Finally, the circuit was rinsed withdistilled and deionised water at 50°C. Sporadically, astep with nitric acid 1% at 50°C was added before usingP3 Ultrasil 10. The entire cleaning procedure was carriedat U� 5 m sÿ1 and 100 < TmP < 200 kPa.

3. Results and discussion

3.1. Selection of enzyme preparation

In order to investigate the in¯uence of varying en-zyme activity composition upon overall ®ltration per-formance, four di�erent commercial enzymepreparations with a wide variety of di�erent activitieswere used to pretreat juice prior to micro®ltration inpermeate recycling mode (Table 1). A strong positivee�ect on permeate ¯ux was observed in all cases wherethe juice was enzymatically treated, but the magnitudeof this e�ect depended on the preparation (Fig. 1).Preparations containing a substantial level only of pec-tinase or cellulase activity (Rapidase Press, Cytolase,CL) had a lower e�ect in comparison with those

Table 1

Glycohydrolase activities contained in enzymatic preparation useda

Activity (IU/ml) Pectin-lyase Endo-polyga-

lacturonase

Pectin-

esterase

Endo-

cellulase

Cellobio-

hydrolase

Exo-

Arabinase

Galacto-

manase

Cytolase CL nd. 52 nd. 1113 161.0 2.1 382.2

Rapidase Press 95 2240 741 15 0.9 12.3 nr.

Rapidase Liq+ 62 800 360 462 0.5 15.4 277.0

Rapidase Pomaliq 2F 48 254 466 484 26.0 4.7 501.5

Ultrazym 100 G 1.0 nr. nr. 0.07 0.08 0.14 nr.

a nd.� not detected; nr.� not realised.

Fig. 1. E�ect of enzymatic preparation added at 1 ml lÿ1 on permeate ¯ux (J) with total recycling (TmP� 150 kPa, U� 7 m sÿ1).

F. Vaillant et al. / Journal of Food Engineering 42 (1999) 215±224 217

preparations which featured substantial levels of bothactivities (Rapidase Pomaliq 2F, Rapidase Liq+). In-terestingly, an unusual pattern of increased ¯ux after theinitial decrease was observed with all preparations, al-beit that this e�ect was much more notable with treat-ments using Rapidase Pomaliq 2F and Rapidase Liq+.Pectinolytic activities combined with cellulase activitieswere seen to considerably enhance permeate ¯ux, indi-cating that the principal foulants were composed of in-soluble pectic and cellulosic materials. Unaccompaniedpectinolytic activity is able to hydrolyse all solublepectin present in the juice serum, but hydrolyses only avery small part of the insoluble pectin fraction, which isclosely bound to cellulose inside cell-wall particles. Thee�ect of the preparations featuring substantial levels ofboth cellulase and pectinase activities indicates a synergybetween these two enzymes. The explanation would bethe combination of the two activities results in a greaterability to depolymerise insoluble cell-wall polysaccha-rides which largely constitute the suspended solids afterjuice extraction. Long chains of pectin and cellulosewere simultaneously broken down and each substratewas more available for enzymatic reactions. This syn-ergistic e�ect involving pectinases and cellulases hasbeen observed previously during the liquefaction ofpassion fruit juice (Vaillant et al., 1999) or other fruits(Hart, Hanni & Huxsoll, 1994; Ben-Shalom 1986).

With pulpy juice such as passion fruit, the major re-sistance to permeate ¯ow is created by the accumulationof cell-wall particles, composed mainly of insolublematerials such as protopectin, cellulose, hemicelluloseand lignin. Consequently, for better ®ltration perfor-mance, any enzymatic preparation used must be adaptedto these speci®c substrates and contain at least su�cientpectinolytic and cellulolytic activities, as in the case ofRapidase Pomaliq 2F and Rapidase Liq+. Secondaryactivities such as exo-arabinase and galactomananase,which constitute the main di�erences between RapidasePomaliq 2F and Rapidase Liq+, do not seem to have

in¯uenced ®ltration performance in the case of passionfruit juice. Taking availability into account, RapidasePomaliq 2F was chosen for use in all subsequent ex-periments. Moreover, Rapidase Pomaliq 2F at 1 ml lÿ1

arises the nearest combination available to the optimumcombination for liquefaction of passion fruit juice(Vaillant et al., 1999).

3.2. Optimal ®ltration conditions with total recycling

3.2.1. Cross¯ow velocity and enzyme concentrationPermeate ¯ux obtained with raw juice or with en-

zymed at U� 5 m sÿ1, markedly di�ered from otherresults (Fig. 2). A low, relatively steady ¯ux was ob-served. With enzymed juice at U� 7 m sÿ1, the ¯uxfollowed the unusual pattern observed previously. Aninitial drop of permeate ¯ux during the ®rst min(probably due to the formation of the fouling layer) wasfollowed by an increase. These phenomena varied ac-cording to enzyme concentration. Similar ¯ux patternshave been observed with apricot puree pre-treated withcellulase and pectinase before being micro®ltrated insimilar operating conditions (Hart & Huxsoll, 1989;Hart et al., 1994).

The ¯ux increase is not only due to the decrease inapparent viscosity brought about by enzymatic action.Most of the apparent viscosity decrease took placeduring the 60 min incubation period before micro®l-tration. During micro®ltration, the apparent viscosity ofthe juice remained unchanged, regardless of enzymeconcentration (Fig. 3).

Analyses of retentate revealed the suspended solidsconcentration decreased soon during ®ltration andreached 100 g kgÿ1 after 100 min (Fig. 4). This result wasnot observed with the control juice which was enzymedbut not micro®ltrated. A further analysis of the retentatepolysaccharide content revealed the dry colloid fractionexpressed as alcohol insoluble solids (AIS) remainedconstant during ®ltration just like the control juice. As

Fig. 2. E�ect of Rapidase Pomaliq 2F concentration and cross¯ow velocity on permeate ¯ux (J ) with total recycling (TmP� 150 kPa).

218 F. Vaillant et al. / Journal of Food Engineering 42 (1999) 215±224

the polysaccharide content did not decrease but thesuspended solids did this phenomena is probably relatedto a decrease of the swelling capacity of these com-pounds. Consequently the potential to form a swollenlayer at the membrane surface during micro®ltrationwas similarly a�ected.

After a 1 h incubation with the high enzyme con-centration used (1 ml lÿ1), residual cell-wall fragmentsresistant to enzyme attack may be essentially composedof cellulose, hemicellulose and lignin. These polysac-charides are known to be chemically quite inert and tohave very poor swelling capacity in contrast to pectinmaterial which is mainly responsible for cohesion ofaggregates. Cell-wall particles degraded by enzymeswould be then much more sensitive to high shear stressat the membrane wall, leading to a thinner fouling layer.

3.2.2. Transmembrane pressure and enzyme concentrationU was ®xed at the highest value acceptable with the

equipment used (7 m sÿ1), given the positive e�ect ob-

served on ¯ux. Only the transmembrane pressure (TmP)and Rapidase Pomaliq 2F concentration were variedaccording to a central composite experimental design(Box, Hunter & Hunder, 1978). The value range of thevariables was de®ned during preliminary tests. Repro-ducibility evaluated on 3 replicates at the centre pointwas satisfying �rnÿ1 < 5%).

Among the enzyme-treated combinations, threedistinct groups of curves can be observed, corre-sponding to low (<0.5 ml lÿ1), medium (about 0.5 mllÿ1) and high (>0.5 ml lÿ1) levels of enzyme concen-tration (Fig. 5). With medium and high concentra-tion, permeate ¯ux were enhanced apparentlyaccording to enzyme concentration. Flux values re-corded after 10 min were higher by 25% and 50%,respectively, in comparison with the untreated juice.Likewise, the initial decline in ¯ux ended earlier, withpermeate ¯ux increasing from around 40 min of ®l-tration onward. This contrasts markedly with a de-cline period of almost 100 min observed with the low

Fig. 4. Evolution of SS and AIS in passion fruit juice and retentate during the process after addition of Rapidase Pomaliq 2F at 1 ml lÿ1 (TmP� 150

kPa, U� 7 m sÿ1).

Fig. 3. Decrease of passion fruit juice viscosity during the process after addition of Rapidase Pomaliq 2F (TmP� 150 kPa, U� 7 m sÿ1).

F. Vaillant et al. / Journal of Food Engineering 42 (1999) 215±224 219

concentration treatments. 3D plot showed that ¯uxdepended mainly on enzyme concentration, and littleon TmP (Fig. 6).

However, it is noted that TmP had a slight positivee�ect on the total volume ®ltered at low enzyme con-centration. This e�ect became less perceptible with in-creasing enzyme concentration. The same tendency isobserved with membrane hydraulic resistance calculatedaccording to DarcyÕs law. The e�ect of TmP on thisresistance was very notable at low enzyme concentrationand become almost negligible for high enzyme concen-tration. This indicates that the nature of the foulinglayer changed with increased enzymatic degradation,becoming less compressible and less sensitive to internalfouling. Fouling material for juices without enzyme orwith an excessively low enzyme concentration is prob-ably composed mainly of pectic-cellulose substanceswhich do not undergo degradation, and forms a com-

pressible swollen gel-like matrix of low water solubility(Chiang & Yu, 1987). The fouling material with thehigh-dose treatment is composed of cell-wall fragmentsresistant to enzyme degradation which should be prob-ably unable to form a swollen structure. These resultscon®rm the precedent hypothesis.

Based on these observations, the best initial set ofoperating conditions were found to be low TmP (150kPa) and high enzyme concentration (1 ml lÿ1), yieldingboth the highest ¯ux and the lowest membrane resis-tance.

3.3. Experiments with concentration

3.3.1. E�ect of volumetric reduction ratioTrials were carried out in order to observe permeate

¯ux behaviour closer to an industrial context. Permeate¯ux were much lower than those obtained with totalrecycling, even during the initial phase (Fig. 7). Thebuild-up of retained solids has a strong e�ect uponpermeate ¯ux from the very beginning of the ®ltrationprocess. Permeate ¯ux decreased during the ®rst 30 minthen reached a steady value, which was maintained untila VRR of 3 was obtained at 100 min. As VRR contin-ued to rise, permeate ¯ux decreased considerably.

Fig. 8 shows the e�ects of the concentration processon SS and AIS. Clearly, contents of both variables in-creased in the retentate while VRR increased. None-theless, it is observed that the SS/VRR ratio actuallydecreased slightly while the AIS/VRR remained almostconstant. As noted previously in the total recycling tri-als, the explanation could be the decrease of swellingcapacity of polysaccharides. The SS content, from aminimum of 150 g kgÿ1, increased to less than 300 g kgÿ1

when VRR 3 was reached. Taking into account theamount of AIS remained relatively constant throughoutthe micro®ltration process a loss of almost 33% in

Fig. 6. E�ect of Rapidase Pomaliq 2F concentration (Cenz) and

transmembrane pressure (TmP) on permeate ¯ux at 240 min (J240min)

with total recycling according to the experimental design (U� 7 m sÿ1).

Fig. 5. E�ect of Rapidase Pomaliq 2F concentration and transmembrane pressure on permeate ¯ux (J) with total recycling according to the ex-

perimental design (U� 7 m sÿ1).

220 F. Vaillant et al. / Journal of Food Engineering 42 (1999) 215±224

swelling ability on the part of the suspended solids issuspected.

It would seem that, until VRR� 3, the increase inpolysaccharide concentration in the retentate was beingo�set by the loss in swelling ability mentioned above.This indicates that the way in which SS content ismeasured (taking into account the swollen ability ofsuspended solids) is a relevant factor which governedmostly micro®ltration performance. As the SS contentof the retentate passed beyond 300 g kgÿ1 and the VRRcontinued to increase beyond 3, the permeate ¯ux againdecreased notably.

Considering, for economic reasons, that the permeate¯ux must be as high as possible, the pre-set value forVRR should therefore be 3, which implies that theoverall process will yield 66% (by volume) of clari®edjuice. It is also important to point out that for fruit juiceindustry, retentate should not be considered as waste, asit may again be recombined with clari®ed juice afterthermal pasteurisation or mixed with raw juice.

3.3.2. Pre-industrial trialTo complete the investigation, long-duration trials

were carried out with concentration up to VRR 3, andmaintained at this value thereafter by constant feedingof enzyme-treated juice and periodic removal of volumesof retentate.

The enzymatic incubation of all feed juice was or-ganised in a series of batches, ensuring a 1±2 h residencetime. Previous trials had shown that variation withinthis time range did not result in signi®cant changes torelevant juice characteristics. After the usual abruptdecrease in permeate ¯ux during the ®rst 40 min and anear steady state until VRR 3 (100 min), ¯ux then in-creased, ¯uctuating around 40 l hÿ1 mÿ2 during most ofthe remainder of the trial. It may also be observed that,once VRR 3 was attained, the SS content of the reten-tate was almost constant, remaining at approximately300 g kgÿ1.

During at least 20 h of the experiment, the ¯ux wasalmost constant with a slight upward trend, implying

Fig. 8. Evolution of SS and AIS in retentate during incubation and concentration up to VRR� 3 after addition of Rapidase Pomaliq 2F at 1 ml lÿ1

(TmP� 150 kPa, U� 7 m sÿ1).

Fig. 7. E�ect of VRR on permeate ¯ux (J ) after enzymatic treatment with Rapidase Pomaliq 2F at 1 ml lÿ1 (TmP� 150 kPa, U� 7 m sÿ1).

F. Vaillant et al. / Journal of Food Engineering 42 (1999) 215±224 221

that the fouling layer maintained virtually the same or ahigher permeability throughout that period. After 20 hof continuous operation, the trial was stopped, although¯ux behaviour indicates that the trial could have con-tinued for considerably longer: membrane resistancemeasured immediately after rinsing with clean tap watersurprisingly had the same value as in other far shortertrials in which VRR 3 was reached after 100 min of®ltration. The full membrane cleaning procedure alsoproceeded as rapidly as with those previous trials. Theseresults show fouling is stabilised and high concentratedalkaline solutions used at high temperature are welladapted to e�ciently hydrolyse fouling polysaccharides.

3.3.3. Quality of proceeded juicesThe most relevant physico-chemical properties of

initial juice after centrifugation, permeate and retentateat VRR 3 are reported in Table 2. Surprisingly, thesoluble solids concentration is higher in the retentatethan in the recovered permeate, by approximately 20 gkgÿ1. Similar ®ndings have already been reported in fruitjuice micro®ltration (Padilla-Zakour & Mclellan, 1993;Todisco, Drioli & Tallarico, 1996). Nonetheless, con-centration of the three main sugar present in yellowpassion fruit juice (glucose, fructose and saccharose) inpermeate and retentate were not signi®cantly di�erentwhich means that refractometer analysis may be prob-ably a�ected by the presence of a high concentration ofsuspended solids at VRR 3. Also, at the end of theprocess, a higher acidity was observed in the retentatethan in the permeate, indicating a global retention ofabout 2.5±5%.

Residual enzyme activity in both the retentate and thepermeate was assessed only in terms of endo-cellulase(Cx) and endo-pectin-lyase (Pl) during the pre-industrialtrial. A global retention of enzyme activity in the re-tentate is indicated, which was almost constant for Pl(around 55%) throughout the trial. Cx rejection in-creased from 15% at VRR 1 to 25% at VRR 3. Reten-

tion of Pl activity by the membrane was apparentlygreater than that of Cx activity, as was observed previ-ously by Hart et al. (1994) during the micro®ltration ofenzyme-treated apricot puree. Size exclusion cannot beresponsible for these retentions, as the molecularweights of both enzymes are very low in comparison tothe membrane cut-o� (Van Houdenhoven, 1975; Peter-son, Fagerstam, Bhikhabhai & Leandoer, 1981). Onepossible explanation for their partial retention is thatsome enzyme molecules are adsorbed onto suspendedsolids, and consequently retained by the membrane.Moreover, this hypothesis may be reinforced by the factthat global enzyme recovery in the retentate and per-meate was only 60% for Pl against 93% for Cx activities,respect to the initial dosage of enzyme. Enzymatic ac-tivity recovery was respectively 27% Pl and 54% Cx inthe permeate and 33% Pl and 39% Cx in the retentate.

The physico-chemical properties of the VRR 3 re-tentate did not di�er substantially from those of the rawjuice. Suspended solids is very similar to that of rawjuice immediately after extraction before centrifugation(300±350 g kgÿ1). Organoleptic tests carried out afterpasteurisation of the retentate shows that it was notsigni®cantly di�erent from classically prepared juice.Then, at VRR 3 retentate was mixed with newly-ex-tracted raw juice and then processed normally intowhole passion fruit juice, pasteurised or concentrated ifrequired. Also, only 1 ml of retentate for 1 l of freshjuice when held for 15 min at ambient temperature,produced results in the raw juice which were very similarto those obtained during the light maceration step whenUltrazym 100G treatment was routinely used to facili-tate the separation of juice sacs from the seed (Fig. 9).Also, part of the rententate should be added to raw juiceafter centrifugation during the enzymatic liquefactionstep, but the optimum rate of retentate added withouta�ecting permeate ¯ow should be investigated. By theseways, 33% of Pl activity and 39% of Cx activity, respectto the initial dosage of enzyme prior to micro®ltration,

Table 2

Main chemical and physical characteristics of passion fruit juices (in parenthesis: standard deviation from 3 or 4 experiments)

Raw juicea Enzyme treated juiceb Clari®ed juice Retentate at VRR� 3

Soluble solids (g kgÿ1) 140 (8) 148 (10) 130 (8) 150 (10)

Suspended solids (g kgÿ1) 254 (10) 148 (5) <1 294 (10)

Titrable acidity (g citric acid kgÿ1) 42 (1) 42 (1) 42 (1) 43 (1)

pH 2.9 (0.1) 2.9 (0.1) 3.0 (0.1) 3.0 (0.1)

Glucose (g lÿ1) 26 (2) 34 (2) 31 (2) 33 (2)

Fructose (g lÿ1) 14 (1) 19 (1) 18 (1) 19 (1)

Sucrose (g lÿ1) 5 (1) 6 (1) 6 (1) 5 (1)

Galacturonic acid (g lÿ1) 1.4 (0.1) 1.4 (0.1) 1.4 (0.1) 1.7 (0.1)

Alcohol insoluble solids (g kgÿ1) 0.85 (0.05) 0.06 (0.05) <0.01 1.70 (0.06)

Density (kg mÿ3) 1027 (2) 1027 (2) 1026 (2) 1027 (2)

Viscosity at 20°C (mPa s) 15 (1) 8 (1) 1 (0.1) 6 (1.5)

a After centrifugation.b Treated with 1 ml lÿ1 Pomaliq 2F/1 h/30°C.

222 F. Vaillant et al. / Journal of Food Engineering 42 (1999) 215±224

should be reused in the global context of a processingplant producing both whole juice and clear juice.

Sugar and acid composition of permeate were closedto the initial juice. The product was very clear and ar-omatic. However, primary organoleptic tests showed alittle decrease of aromatic strength compared to theinitial juice. Few volatil compounds were probably re-tained by the membrane. Itoua-Gassaye, Davin, MietonPeuchot and Ben Aim (1991) indicated aroma lossesobserved with a similar membrane were mainly due toretention by the membrane of pectic material in nonenzyme-treated juices. More over, some authors noticedthat the ¯avor intensity of many fruits like apple juice(Jenniskens, Voragen, Pilnik & Posthumus, 1991) orpassion fruit (Winterhalter 1990; Drider, Chemardin,Arnaud & Galzy, 1994) are frequently enhanced duringenzymatic liquefaction, as commercial preparationscontain also a lot of side activities such as a-glucosidase,which released terpenols bound to polysaccharides(Wrolstad, Wightman & Durst, 1994).

4. Conclusion

Passion fruit juice has a high content of suspendedsolids which causes rapid fouling of the membraneduring micro®ltration. With total recycling (VRR� 1),

the e�ect of partial enzymatic liquefaction of insolublecell-wall polysaccharides of the juice prior to micro®l-tration produced an unusual ¯ux pattern, characterisedby an increase of ¯ux after an abrupt decrease. Thisincrease is not only due to a viscosity decrease but to animportant decrease of suspended solids concentration inthe retentate during micro®ltration. As AIS remainedalmost constant, it indicates the swelling capacity of thesuspended solids had changed. Permeate ¯ux increasewas enhanced with high cross¯ow velocity and with highconcentration of enzymatic preparation containingpectinase and cellulase activities. A synergistic e�ect wasobserved in ¯ux when both activities were used, con-®rming the assumption that the foulant materials inpassion fruit juice have an essentially cellulo-pecticstructure. Pre-industrial trials carried out at 36°C, U�7 m sÿ1, TmP� 150 kPa and VRR� 3, produced apermeate ¯ux which ¯uctuated around 40 l hÿ1 mÿ2

during 18 h without any sign of decline. The physico-chemical properties of the retentate and the permeateindicated that not only the clari®ed juice is of economicvalue but the retentate may also be marketed as wholejuice. One possible drawback of the process, is that whilepart of residual enzyme activities in the retentate can bereused in a plant producing whole juice and clear juice,nonetheless the high level of enzymatic preparation in-volved tends to increase costs. The economic viability of

Fig. 9. Proposal of ¯ow sheet for simultaneous production of clari®ed and whole passion fruit juices.

F. Vaillant et al. / Journal of Food Engineering 42 (1999) 215±224 223

the operation depends on the balance between reduced®ltration costs and cost of enzyme treatment. Conse-quently, special attention should be paid to the formu-lation of enzymatic preparations tailored to ®t better theliquefaction of fruit cell-wall polysaccharides prior tomicro®ltration. Also, an interesting alternative would bethe use of enzymatic reactors with immobilised pecticand cellulolytic activities to replace the batch enzymaticliquefaction. Both strategies which ought to increasee�ciency reducing the amount of enzyme requiredshould be investigated.

Acknowledgements

The authors wish to thank Passicol S.A. (Chinchin�a,Colombia), Colciencias (project n° 1106-07-283-95), theFrench Foreign A�airs Ministry (PRI project), for the®nancial support and especially Gorge A. Martinez,Victor M. Amu, Anne Leblanc for the valuable helpprovided to this work.

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