Pulsed Electric Field Assisted Pressing of Sugar Beet ... · atically redistributed and pressed at the same pressure a third time (3rd press). So, pressed juices after PEF treatment

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doi:10.1016/j.biosystemseng.2005.09.008PH—Postharvest Technology

Biosystems Engineering (2006) 93 (1), 57–68

Pulsed Electric Field Assisted Pressing of Sugar Beet Slices: towards a NovelProcess of Cold Juice Extraction

A.B. Jemai; E. Vorobiev

Equipe des Technologies Agro-Industrielles, Departement de Genie Chimique, Universite de Technologie de Compiegne,Centre de Recherche de Royallieu, B.P. 20529, 60205 Compiegne, France; e-mail of corresponding author: [email protected];

email: [email protected]

(Received 17 April 2004; accepted in revised form 14 September 2005; published online 28 November 2005)

The use of pulsed electric fields (PEF) as an intermediate treatment for the cold juice extraction from sugarbeet ‘cossettes’ (i.e. long grated particles) has been investigated using a pilot scale multi-plate and framepressing equipment (pressure of 5–15 bar; particles filling of 4�5–15 kg) and a pulse generator (1000V–1000A).It has been possible to validate laboratory-scale results (40 g of particles) by studying the feasibility andadvantages of a PEF-assisted cold pressing of sugar beet cossettes on a much larger scale (4�5–15 kg). A bestscheme scenario for an adequate PEF-assisted cold pressing of sugar beet cossettes consists of two initialpressing steps with an intermediate PEF treatment, followed by one or more washing steps and a final pulppressing. A yield of about 80% in juice per initial mass of cossettes has been achieved before washing. Byperforming some washing and final pressing operations, losses of sugar in pulp could be significantly reducedto about 3% of the initial sugar content. The purity of pressed juices was systematically higher following PEFtreatment compared to that of juices prior to PEF treatment (96–98% and 90–93%, respectively).Spectrophotometric colour measurements, reflecting impurities and fine suspended particles contents in juices,showed that purified PEF juices had colour values three to four times lower than those of purified factoryjuices, a difference confirmed by the colour of sugar crystals obtained from both types of juices. In addition,significant amounts of potassium, sodium and a-amino nitrogen were found to remain in PEF-treatedparticles, which explains why better purity juices are obtained following PEF treatment.r 2005 Silsoe Research Institute. All rights reserved

Published by Elsevier Ltd

1. Introduction

The use of pulsed electric fields (PEF) to assist in thenon-thermal sugar beet pressing process presents a realopportunity for the sugar industry. Since the late 1990s,the Industrial Agro-Food Technologies research unit(TAI) of the Compiegne University of Technology(France) has been conducting laboratory-scale researchwork for the industrial implementation of this novelsugar extraction process (Bouzrara & Vorobiev, 2000,2001; Bouzrara, 2001; Vorobiev et al., 2000). Otherwork dealing with PEF utilisation for sugar extractionhas also been reported (Eshtiaghi & Knorr, 2002). To befully accepted, this technology should be applicable to alarger scale with better process performances.

1537-5110/$32.00 57

Several laboratory-scale studies (i.e. 40 g of gratedparticles termed ‘cossettes’) demonstrated the efficiencyof applying a short duration PEF treatment during atwo step cold pressing of sugar beet cossettes (Bouzrara& Vorobiev, 2000, 2001; Bouzrara, 2001). In general, thePEF was applied after a first period of pressing whenextraction is significantly reduced due to cake compact-ness and blockage of drainage channels. Applying PEFat this particular instant increases the permeability ofmost of the remaining cell membranes leading toenhanced extraction of intra-cellular juice and betterjuice quality. Bouzrara and Vorobiev (2000, 2001) thusreported that up to around 82% of overall yield could beachieved by intermediately applying a PEF during a twostage pressing process at 10–20 bar. At a pressure of

r 2005 Silsoe Research Institute. All rights reserved

Published by Elsevier Ltd

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A.B. JEMAI; E. VOROBIEV58

5 bar, the yield was about 78%. In addition, the secondjuices (i.e. after PEF application) are systematicallymore concentrated in sugar and have lighter colour.Despite these encouraging results, questions as towhether or not the technology is transposable on anindustrial scale remained unanswered. For viability, thetechnique has to be applicable to a much larger scale(e.g. from the semi-industrial to the industrial scale). Inother words, it should fit into an overall extractionprocess, which at least quantitatively equals the con-ventional process and provides juices and pulps of bettercharacteristics.The objectives of the present work were, first, to

validate laboratory scale results on a larger scale(4�5–15 kg of cossettes) and then to demonstrate thebenefits of using a PEF on the quantitative andqualitative levels on such a scale.

2. Materials and methods

2.1. Sugar beet

Raw sugar beet roots were provided by a nearbysugar processing company (Beghin Say, Chevriere,France). For most of this study, beets were receivedclean from the reception and control centre of thefactory on a daily basis. During the final part of thestudy however, beets had to be stored unclean in acontrolled temperature environment (3–5 1C) in anearby vegetable storage company.

5

21

11 1 1 1 3 4

9

Fig. 1. Simplified schematic of filter-press equipment (Bouzrara, 20head; (5) fixed head; (6) guiding rail; (7) tighte

2.2. Equipment

2.2.1. Plate and frame filter-press

The filter-press (Choquenet, France) used throughoutthis study is illustrated by Fig. 1. It is a multi-plate andframe made of a number of membrane plates assembledone against the other, which are used to transmitpressure (5–15 bar) to the beet particles (4�5–15 kg). Thedimensions of the plates are 470mm by 470mm with across-sectional area of cake of about 1370 cm2.

In most runs, one or two chambers were manuallyfilled with grated cossettes (see Fig. 2 for shapes andcross-sections of two types of cossettes). In a few runs,up to six chambers were used at the same time. Eachchamber consists of a plate covered by a filter cloth anda flexible electrode (metallic grid) on one side and a rigidelectrode on the other. The pressure (compressed air) isapplied to the membrane of the plate, which in turnexerts and distributes the pressure over the cossettesplaced between the plate and the rigid electrode (Fig. 2).Juice is drained through channels leading to the outlet,where juice accumulation is monitored by a weighingbalance connected to a data acquisition system.

2.2.2. Electrical and data acquisition systems

The pilot scale generator (1000V–1000A) usedthroughout the present work was built by HazemeyerCo. (Gauchy, France), who scaled up the equipmentbased on laboratory results. The ratings of the squarepulse generator and the characteristics of the pulses aregiven in Table 1(a) and (b).

6 7

8

01): (1) membrane plates; (2) and (3), false plates; (4) movingning piston; (8) filter body; (9) liquid recipient

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Grated type

Industrial type

Filter cloth

Flexiblemembrane

Porous rigid electrode

Pressure

CossettesPorous flexible electrode

Juice evacuation channels

Pulse generator

Juice filtrate

Fig. 2. Schematic of a one chamber configuration of plate and frame filter press

Table 1Electrical equipment parameters and characteristics

(a) Rating of the pulse generator

Regulation control Maximumcurrent, A

Maximumvoltage, V

Nominalpower, kW

Voltage 100 1000 100

(b) Pulse parameters

Pulse type Period, ms Impulseduration, ms

Frequency,Hz

Uni-polar square 1–10 20–100 100–1000

PULSED ELECTRIC FIELD ASSISTED PRESSING OF SUGAR BEET SLICES 59

An acquisition programme written in the HPVEEenvironment (HP-VEE, V3�12, Hewlett-Packard Co.,USA) was used to control and command the generatorand the weighing system.

2.3. Experimental procedure

Throughout the present study, more than 100 experi-ments have been performed; these were organised so asto explore several objectives such as the effects ofpressure, the optimisation of washing operations,electrical power consumption, etc.

2.3.1. Cossette types and preparation

In most runs, grated cossettes (20mm in length and4mm in width) using a heavy-duty cutting equipment

(Robo-coupe, France) were obtained by first cuttingbeetroots into 20mm cubes, then grinding down thecubes using a grating disc of 4mm mesh. To study theeffect of cossette type, industrial cossettes (i.e. fine longparticles of V-shaped cross-section) were obtained usinga slicing equipment (provided by British Sugar), whilethe effect of varying the size was studied on grating typecossettes (e.g. 10mm by 1�5mm).

Initial dry weight content dW in % (wet basis), sugarconcentration SC in 1S (kg of sugar per 100 kg of solid),and soluble solids content SS in 1Brix (kg /100 kg juice)were systematically performed for each run. The rawbeets had a dry weight of about (22�270�7)%, a sugarconcentration of (17�2070�79) 1S, and soluble solidscontent of (18�8570�75) 1Brix. To estimate the dryweight, 20 g of freshly cut cossettes were placed in anoven at 105 1C for 24 h; sugar concentration in 1S,soluble solids content in 1Brix, spectrophotometriccolour in ICUMSA units (i.e. International Commissionfor Uniform Methods of Sugar Analysis), and juicehandling and analysis were conducted by a Beghin-Saytechnician on site throughout the entire study, usingsugar industry standard equipment and methods cour-tesy of Beghin-Say (France).

2.3.2. Overall process steps

The flow diagram of the process steps is illustrated byFig. 3. In general, a PEF-assisted pressing processincludes a first pressing step (1st press), a short time PEFtreatment, a second pressing step (2nd press), andsubsequent washing and pressing of pulps (1st wash,2nd wash, etc.).

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PEF and 2nd press

1st juice

2nd juice

3rd juice

3rd Press

1st press

Slicing

Grinding

Filling by hand

Beets

Washing of cakeand final pressing

Final pulp

wash

Water

3 × 10%or 5 × 5%

of initial mass of

fresh cossettes

Pressed cake

Cossettes

washwash4th 3rd 2nd 1st

wash

(1 or more steps)

Fig. 3. Experimental procedure: PEF, pulsed electric field


It should be noted, however, that due to its actualconstruction, the plate and frame press used throughoutthis study did not allow complete pressing of the wholecake (outer cake ring not well pressed). Consequently,following the second pressing, the cake was system-atically redistributed and pressed at the same pressure athird time (3rd press). So, pressed juices after PEFtreatment include that of the 2nd and 3rd press.Following the 3rd press operation, the final cake was

washed and pressed once or more frequently with freshwater (10% of initial weight of fresh cossettes). Thelatter was gradually poured over the pressed cake whilemixing the particles to insure best possible watercontact. The washed cake was then redistributed inone chamber of the filter press and re-pressed to obtainthe wash liquor.

2.3.3. Optimisation methods

Ways to optimise the process were tested by (i)varying the electrical energy input of the PEF treatment,(ii) exploring the washing steps, and (iii) increasing theapplied pressure.

From a power consumption stand point, the optimi-sation procedure consisted of studying the effect ofreducing the frequency of the PEF (i.e. the number ofimpulses, which is proportional to the treatment time)on the juice yield. To do so, impulse numbers of 100,300, 500, 750, and 1000 have been used, while keepingthe applied potential difference constant (i.e. 1000V).

In a cold pressing process, such as the one concernedwith the present study, one or more washing steps wereinevitable. In fact, significant juice and sugar amountsare always trapped in the pulp following the pressingoperations. These substances may only be extracted bysubjecting the pulp to a concentration difference usingfresh water for example. The washed particles may thenbe re-pressed to obtain wash juices containing sugar.

Two washing scenarios were studied for this purpose:(i) the first consisted of performing three washing stepsusing an amount of pure water equivalent to 10% of theinitial weight of the cossettes in each step (30% overallwater quantity), and (ii) the second consisted of fourwashing steps using only 5% water in each step (20%overall water quantity).

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00 10 20 30 40 50 60 70 80

200

400

600

800

1000

1200

1400

1600

1800

Time, min

Mas

se o

f ex

trac

ted

juic

e, g

1st press 2nd press 3rd press

PEF

Fig. 4. Typical juice extraction kinetics during a 3-pressprocess; pulsed electric field (PEF) applied at the end of the

1st pressing stage

0.6

0.7

0.8

ll yi

eld

1st press 2nd press 3rd press


The effect of varying the pressure has been studied fortwo values of the pressure: 5 and 10 bar.

2.3.4. Calculations and data analysis

Following are the relationships used for calculatingthe different parameters. The juice yield Y may be givenin terms of the mass of juices in kg, or in a normalisedform Y*, by the following equations:

Y ¼X

mjuices (1a)

Y � ¼ 1�mcake

mcoss

¼

Pmjuices þ ‘

mcoss

(1b)

where:P

mjuices is the sum of juice masses in kg, beforeand after the PEF treatment, respectively; ‘ is the overallportions of juices lost in pipes and plates in kg; mcoss isthe initial mass of cossettes in kg; and mcake is the massof the cake after all pressing steps.The composition of fresh cossettes, intermediate cake,

and final pulp is customarily defined by the followingrelationships:

MC ¼ 100� dW (2)

S0S ¼SSMC

ð100� SSÞ(3)

NSS ¼ 100� ðMC þ S0SÞ (4)

where: dW is the dry weight in kg per 100 kg of freshcossettes in %; MC is the moisture content of thecossettes in %; SS is the soluble solids content in 1Brix,i.e. kg of soluble solids per 100 kg of juice; S0S is thesoluble solids content in material, i.e. kg of solublesolids per 100 kg of material (cossettes, cake or pulp);NSS represents the solid non-soluble substances in kgper 100 kg of material. The initial moisture content offresh cossettes is designated by MCo in %, and M�

Co isthe normalised form of M�

Co ¼MCo=100:The quality of a given juice is characterised by its

purity in % and is given by the following formula:

P ¼ 100SC

SS

(5)

which is the ratio of sugar content SC to the overallamount of soluble substances SS in the same juice.

00 20 40 60 80

0.1

0.2

0.3

0.4

0.5

Time, min

Nor

mal

ised

ove

ra

PEF

Fig. 5. Normalised accumulated mass of pressing

3. Results and discussion

3.1. Performance of pulsed electric field-assisted pressing

process

3.1.1. Kinetics of juice yield

A typical acquisition curve describing the kinetics ofjuice yield during pilot scale pressing is given by Fig. 4.

The normalised form of this curve is given in Fig. 5. Itshould be noted that this curve does not take intoaccount the amounts of juice losses that represent 5–7%of the initial weight of cossettes.

This result illustrates the successful pilot scale-up ofthe process with respect to the laboratory-scale resultobtained for the same pressing and treatment conditions(Bouzrara & Vorobiev, 2000). Table 2 summarises thescale-up performances based on the present pilot scalestudy, and shows that using 4�5 kg (i.e. 112�5 timeslaboratory-scale) or 15 kg (i.e. 375 times) of initialcossettes, similar juice yields were obtained compared tolaboratory scale yields from only 40 g of cossettes.

One should bear in mind, however, that to confirmthese results, the process must be scaled-up even further.In order to do so, continuous flow equipment, whichallow complex pressing effects of a few hundreds ofkilograms to a few tonnes of cossettes, are needed forfuture work.

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3.1.2. Juice yield following pulsed electric field-assisted

pressing at 5 bar

To illustrate the type of performance that can beobtained in a PEF-assisted pressing process, considerthe results from pressing at 5 bar given in Fig. 6. At thispressure, an average first pressing juice yield of about29% can be achieved. Applying the PEF allowed anadditional 48% (2nd and 3rd press) to be obtained.The characteristics of the different juices are sum-

marised in Table 3. It can be seen that, compared to the1st press, juices following the PEF treatment (2nd and3rd press) systematically have higher purity values

Table 2Scale-up performance of pilot scale results compared to

laboratory scale data�

Scaling factor 1st juice 2nd juice

Yield, % Purity, % Yield, % Purity, %

1 28�5 NA 49�5 NA112�5 29�0 90–93 47�0y 96–98375 27�0 93–94 46�0y 95–97

�Using 40 g of grated cossettes (Bouzrara, 2001).yCorresponding to 2nd+3rd juices with 3rd juice representing an

average of 10% yield.

00.050.1

0.150.2

0.250.3

0.350.4

0.45

Nor

mal

ised

juic

e yi

eld

Pressing Washing

1st juice 2nd juice 3rd juice 1st wash 2nd wash3rd wash

Fig. 6. Juice yield during a typical 3-press/3-wash process at5 bar (error bars are standard deviation values based on 13

experiments)

TablCharacteristics of different juices for

Stage of treatment Juice yield, % Soluble solids, 1Br

1st press, before PEF� 27–30 18�5–19�52nd+3rd press, after PEF 45–50 19–211st wash 9–11 10–132nd wash 9–11 6�5–7�53rd wash 9–11 4–5

�PEF, pulsed electric field.

(90–93% compared to 96–97%, respectively). Thecolour of juices following the PEF is systematicallythree to four times lighter than that of the juice beforethe PEF and factory juices. The colour was measured inICUMSA units (International Commission for UniformMethods of Sugar Analysis), which is based on spectro-photometer colour measurements, reflecting impuritiesand fine suspended particles contents in juices; thisobservation is confirmed by the colour difference ofsugar crystals obtained from juices following PEFtreatment compared to factory juice crystals (Table 4).This is why PEF juices are preferably handled separatelyin order to take advantage of this quality difference.Furthermore, maximising juice quantities after PEFshould accordingly be considered.

Other important criteria to consider in characterisingthe performance of the pressing process are thecharacteristics of the final pulp. For instance, significantamounts of non-sugars should have remained in thepulp following all pressing operations; this explains thehigh juice purity obtained following PEF treatment.Table 5 gives the residual pulp contents estimated basedon experimental data and extrapolated to 2% sugarcontent of pulp. The general tendency is that, comparedto industrial values, PEF pulp contained three to fivetimes more a-amino nitrogen and two to three timesmore sodium and potassium (Anonymous, 2002).

e 3a 3-press/3-wash process at 5 bar

ix Purity before purification, % Purity after purification, %

90–93 93–9595–97 96–9894–95 —91–94 —90–93 —

Table 4Colour of factory and pulsed electric field (PEF) juices and final

crystals in the international commission for uniform methods of

sugar analysis (ICUMSA) Units�

Juice Colour, ICUMSA units

Initial product beforeconcentration

Crystals

Thick juice fromfactoryy

1150–1740 280–470

PEF juice 350 100

�Summarised from SUBEEP report (Anonymous, 2002).yBeghin-Say factory at Chevriere, France.

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Table 5

Estimated residual contents of pulp (per 100 g) following pulsed

electric field (PEF) versus extraction process (extrapolated at

sugar content SC ¼ 2%)

Pulp Residual content�, g/100 g pulp

Potassium Sodium a-Aminonitrogen

Sugar

PEF pulpy �25 25–30 �10 2Factory pulpz �10 10–25 1–3 p2

�Summarised from SUBEEP report (Anonymous, 2002).yExtrapolated values from experimental data.zCommon industry values.

0.300.350.400.450.500.550.600.650.700.750.800.850.900.951.00

Nor

mal

ised

juic

e yi

eld

0 1 2 3 4 5 6 7 8 9 10 11 12Energy consumption, kJ/kg

Optical energyconsumption zone

Fig. 7. Juice yield (in normalised form) after the 3-press processas a function of energy input (optimisation experiments): —,

model; , experiment


3.2. Optimising the process

From the present study, it appeared that following thethree pressing steps, the pulp still contains significantamounts of juice and sugar; so pressing alone is notsufficient to obtain all the juice and sugar initiallycontained in the cossettes. Consequently, ways tooptimise this process were explored. For example, oneor more washing steps have been performed in order toextract further amounts of sugar remaining in the pulps.Optimising the level of treatment may also reveal thelimits of the PEF treatment in achieving adequate juiceyields. Finally, studying the impact of increasing theapplied pressure on the juice yield and characteristicswas explored.

3.2.1. Optimising the electric treatment

The juice yield versus energy consumption curve,given by Fig. 7, shows that the PEF effect is significantup to an energy input of about 2 kJ/kg; this effect levelsoff beyond 3�6 kJ/kg. An optimum energy consumptionrange between these two limits is thus recommended forobtaining adequate juice yield with minimal consump-tion. It should be noted that with more appropriatepressing equipment (e.g. with complex shearing effects),this optimal range might further be reduced; in addition,the yield could be further enhanced owing to thecomplex pressing effects with some equipment such asbelt or screw press.

3.2.2. Additional washing operations

Due to the limits of the equipment used, three or fourwashing steps (batch) were needed to approach actualindustry performances.From industry standards, wash is allowed using water

representing 10–40% of the initial weight of cossettes(van-der-Poel et al., 1998). Ideally, a single wash stepusing an amount of fresh water equivalent to 10% of theinitial weight of the cossettes is to be performed leading

to extracting almost all sugar content of the pulp (o2%of pulp weight). This operation is best performed usingcontinuous flow equipment. It is worth noting that toachieve actual industrial performance, it has beenprojected that at least 85% juice yield has to be obtainedduring the pressing steps in order to reduce the washingto only one step (see Section 3.3).

(i) Three step washing operations

Average results from a 3-press/3-wash process at 5 barare given in Table 6; about 29% juice yield is achievedafter the first pressing stage, while the second and thirdpress juices (2nd press+3rd press) account for from47% to 48% yield. Each subsequent wash step yieldsabout 10% liquor. Despite washing and pressinglimitations (i.e. simple pressing, short time washing,etc.), this illustrates that more than 10% of wash liquorcould easily be extracted from the washed cake usingmore appropriate equipment. The data described hereconcern just the juice yield; one should further look atsugar yield and the purity aspect of the process.

Characteristics of the different juices are shown inTable 6 and Fig. 8. The general tendency is that juicesafter PEF treatment have higher soluble solids contentand purity. In fact, juices of the first pressing (beforePEF treatment) have purity ranging from 90% to 93%,equivalent to that of factory juices; however, coldpressing juices are more concentrated in sugar:18�5–19�5 1Brix compared to 14�5–16 1Brix for factoryjuices. On the other hand, press juices obtainedfollowing PEF treatment have systematically highersoluble solids content and purity (96–97%) beforepurification. As shown in Fig. 9, the difference in puritybetween 1st pressing juice and that of 2nd pressing isbetween 3% and 3�5%, while the difference between 3rdpressing juice and 1st wash liquor is about 2�5% and2%, respectively.

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Table 6

Example of sugar loss reduction by one additional washing step using less wash water (three times 10% of initial weight of cossettes

compared to four times 5%); process 1 versus process 2 corresponding to three and four washing steps, respectively

Stage of treatment Process 1�: 3 press–3 wash Process 2y: 3 press–4 wash

Juiceyield, %

Sugar,%

Solublesolids, 1Brix

Purity,%

Juiceyield, %

Sugar,%

Solublesolids, 1Brix

Purity,%

1st press, before PEFz 29 5�07 19�00 92 29�00 5�03 18�80 92�02nd+3rd press, after PEF 48 9�22 20�00 96 50�00 9�53 19�83 95�01st wash 10 1�02 10�70 95 6�78 0�98 15�24 94�42nd wash 10 0�65 6�97 93 6�22 0�66 11�45 93�03rd wash 10 0�41 4�54 91 5�33 0�43 8�53 94�14th wash 6�00 0�35 6�44 91�0

Overall 107 16�37y 103�33 16�98y

MC, moisture content; SC, sugar content; SS, soluble solids content; NSS, non-soluble solids.�Initial composition (per 100 kg beets): MC ¼ 77�5%; SC ¼ 17�5%; SS ¼ 19 1Brix; NSS ¼ 4�32%:yInitial compositions (per 100 kg beets): MC ¼ 77�9%; SC ¼ 17�5%; SS ¼ 19�05 1Brix; NSS ¼ 3�77%:zPEF, pulsed electric field.ySugar extraction (overall sugar yield divided by initial sugar content) of 93�54% and 97�03%, respectively.

86

88

90

92

94

96

98

Juic

e pu

rity

, %

Pressing Washing

1st juice 1st wash 2nd wash 3rd wash 4th wash2nd juice 3rd juice

Fig. 8. Purity of different juices for a typical 3-press/4-wash process at 5 bar (error bars are standard deviation values based on 43experiments)


Based on the present work, typical performances for a3-press/3-wash procedure (at 5 bar) are summarised inTable 6; within the limits of the equipment used, thisresult demonstrates that a significant portion of sugarinitially contained in the cossettes can be extracted atambient temperature (about 93�5% sugar yield).(ii) Four step washing operations

With the equipment at hand, optimising the washingprocedure consisted of increasing the number of washsteps while using less water or using the same amount ofnew water while recycling the previous wash liquor. Forexample, 4-wash steps using only 5% of fresh water eachstep have been tested. Another scenario, consisting of 4-

wash steps using 10% fresh water, which includesrecycling of wash liquor from a previous run, has alsobeen tested.

Results from the optimisation of the number of washsteps are summarised by Table 6. It can be seen thatsugar loss is reduced from 6�5% to 3% by increasing thenumber of wash steps while reducing the amount ofwash water. Furthermore, recycling one wash liquor toincrease the number of wash steps while using 10% ofwater may lead to a further decrease in losses. The majordifference lies in the amount of water used in both cases(4� 5% compared to 3� 10% of the initial weight ofcossettes).

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3.2.3. Increasing the pressure

One way to optimise the pressing process was toincrease the pressure. As can be seen in Table 7, theoverall juice yield did not seem to vary; however, the sugaryield slightly increased from 93�5% to 96�3%. With theequipment at hand, the applied pressure seems to effectthe 1st press (i.e. from 29% to 33�5% for 5bar up to10bar, respectively). In other words, more first pressingjuice is obtained to the detriment of juice amount afterPEF treatment. So in order to take advantage of thisobservation (specific to the equipment used), the durationof the first pressing stage should be decreased whileincreasing that of the second stage. The overall juice yieldis not affected, but more juice having better quality isaccumulated (i.e. more sugar can be extracted). Applying

Tabl

Effect of the pressure on the overall process performances; procesrespect

Stage of treatment Pressure of 5 bar

Juiceyield, %

Sugar, % Solublesolids, 1Brix

P

1st press, beforePEF�

29 5�07 19�00

2nd+3rd press, after 48 9�22 20�00PEF1st wash 10 1�02 10�702nd wash 10 0�65 6�973rd wash 10 0�41 4�54

Overall 107 16�37y

MC, moisture content; SC, sugar content; SS, soluble solids content; NSS�PEF, pulsed electric field.yInitial composition: MC ¼ 77�5%; SC ¼ 17�5%; SS ¼ 19 1Brix; NSS ¼zInitial compositions (per 100 kg beets): MC ¼ 77�7%; SC ¼ 17�7%; SySugar extraction (overall sugar yield divided by initial sugar content)

00.5

11.5

22.5

33.5

44.5

Puri

ty d

iffe

renc

e, %

2nd juice 3rd juice 1st wash

Fig. 9. Purity difference between the juices from the 1st pressand that of the 2nd press, 3rd press, and 1st wash, respectively(error bars are standard deviation values based on 43 experi-

ments)

a higher pressure led to the final pressed pulp havinghigher dry weight and lower sugar loss (dW of 25% andsugar loss of 0�66kg per 100kg of cossettes compared todW of 22�5% and sugar loss of 1�14kg per 100 of cossettes,for 10 and 5bar, respectively).

Increasing the pressure allows to decrease the numberof wash steps and leads to more wash liquor than theamount of wash water; so, with more appropriatepressing equipment significant quantities of wash liquorsare obtained leading to a high value of dW for the pulp.This avoids the need for subsequent drying of the pulp.

3.3. Best possible scheme for pulsed electric field-assisted

cold pressing

In view of the results from the present work, a bestpossible scheme for a PEF-assisted cold pressing ofsugar beet cossettes is given by Fig. 10. According to thisflow diagram, an overall pressing juice yield of at least85% is needed so that only one pressing step is requiredto ensure sugar extraction yield of at least 98�5%. This,in fact, corresponds to a loss of pulp of about 0�26 kgper 100 kg cossettes [i.e. equivalent to industry require-ment (van-der-Poel et al., 1998)].

Some interesting aspects of the process described bythe flow diagram in Fig. 10 should be pointed out; forinstance, significant quantities of high-quality juices areobtained following PEF application. Consequently,handling the different juices separately may be advanta-geous. Furthermore, intermediate pressed cossettes (i.e.

following pressing stages) have a value for dW of about40%. Attaining this high value is made possible by the

e 7

s 1y versus process 2

zcorresponding to 5 and 10 bar pressures,

ively

Pressure of 10 bar

urity, % Juiceyield, %

Sugar, % Solublesolids, 1Brix

Purity, %

92 33�50 5�79 18�50 93�5

96 46�50 8�86 19�80 96�5

95 11�00 1�21 12�00 94�793 10�00 0�70 7�30 94�791 11�50 0�47 4�40 92�0

112�50 17�03y

, non-soluble solids.

4�32%:

S ¼ 19�2 1Brix; NSS ¼ 3�05%:of 93�54% and 96�30%, respectively.

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1st juice : 35 kg

SS

= 18.5°BrixSugar = 5.96 kg

Purity = 92%

2nd juice : 50 kg

SS


Purity = 97%

SS

= 18.23°Brix

Sugar = 16.8 kg (98.5%)

Sugar = 0 .26 kg (1.5%)

Purity = 95%

SS


Purity = 95%

12 kg of Wash juice:

Press I

Press II

Washing

100 kg of fresh cossettes :

(Composition: MC

= 77.8%; S ′ = 18.85%; NSS

= 3.35%; SS

= 19.5°Brix); sugar = 17.05 kg

PEF

10 kg of fresh water

15 kg of pressed cossettes:

dW

= 39.8%

SC

= 10.9%

dW

= 34.8%

SC

= 1.96%

97 kg of overall juice:

1st juice 2nd juice Wash juice13 kg of exhausted pulp:

S

Fig. 10. Best scheme scenario for a pulsed electric field (PEF)-assisted cold pressing of sugar beet cossettes: MC, moisture content;NSS, non-soluble solid content; MC, moisture content; SC, sugar content; SS, soluble solids content; dW, dry weight


application of PEF during pressing with complexpressing equipment. In addition, despite the additionof wash water to the pressed cossettes, the final pulp alsohas a high value for dW (i.e. in the order of 35%). Thiseliminates the need to dry the pulp as customarilyneeded in industry (Jones, 1988).A correlation between the normalised juice yield Y*

and the intermediate pressed cake dry weight dW hasbeen established based on experimental data. It is givenby the following equation:

dW

100¼ 1�

ðM�Co � aY �Þ

ð1� Y �Þ(6)

where: M�Co is the normalised moisture content of fresh

cossettes (0�7870�01); and a is a correlation constant(0�81070�001). The values of M�

Co and a were obtainedusing a fitting software (TableCurve 2D WindowsV2�03, 1989–1994, by Jandel Scientific).As illustrated by Fig. 11, the semi-empirical model

and experimental data fit well (coefficient of determina-

tion r2 ¼ 0�914). It can be seen that a 0% juice yieldcorresponds to initial raw cossettes, while at 85% juiceyield, the value for dW is about 40%. Despite thelimitations of the equipment used, a value for dW ofaround 34% has been attained; so, with more complexpressing effects equipment (i.e. shearing), it would bepossible to attain 40% dry weight following the pressingsteps.

4. Conclusion

The present work conducted on a frame and plate-pressing equipment (4�5–15 kg of cossettes) validatedlaboratory scale results (40 g of cossettes) obtained usinga filter-press cell.

This successful scale-up confirmed that applying anintermediate pulsed electric field (PEF) treatment(following an initial pressing stage) significantly en-hanced juice extraction yield from about 29% (1st press)

ARTICLE IN PRESS

0.20.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90.85 10

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

Normalised juice yield Y *

Dry

wei

ght o

f pr

esse

d ca

ke d

W /1

00

dW /100 = 1− (0.78 − 0.81Y *) / (1−Y *)

r 2=0 .914

Fig. 11. Correlation between juice yield and cake dry weight after a 3-press process: —, model; , experiment; r2, coefficient ofdetermination


to about 80% (overall yield per cent of initial weight ofcossettes). It has been demonstrated that following thepressing operation at ambient temperature, sugarremaining in the pulp could be further extracted byperforming a number of washing steps. Indeed, despitesome equipment limitations and washing difficulties, ithas been shown that losses can be significantly reducedto about 3% of initial sugar content.The juices extracted after PEF application (2nd and

3rd press) systematically have high purity (ranging from95% to 97% before purification). While the purity of 1stpress juice and that of 3rd wash liquor is equivalent tothat of industrial juices, it is three to five points lowerthan PEF juices. The purity of the 1st step wash liquor isonly one to two points lower than PEF juice. Thepurified overall juices obtained during the presentprocess have ICUMSA units (International Commissionfor Uniform Methods of Sugar Analysis) three to fourtimes lower compared to purified factory juices (e.g.

400–700 units compared to 1500–1700 units). Thiscolour difference has been confirmed following thecrystallisation of PEF and factory juices to find thatPEF crystals seem to be of bigger size and have lowerICUMSA colour (100 compared to 280 and 470 forfactory juices).In addition to better juice quality, analysis of pressed

and washed pulps concluded that after PEF treatment,including appropriate washing, substantially higheramounts of potassium, sodium and a-amino nitrogenremain in the pulp (i.e. extrapolation values at 2%sucrose content of pulp). In other words, less of thesecomponents are transferred to the pressed juice.

From the results of the present work, furtherexploration is needed before any industrial implementa-tion of the new process. Decreasing sugar losses by usingmore appropriate pressing equipment, which combinescomplex shearing effects and continuous juice produc-tion, should be considered. Further optimisation of thePEF treatment should be explored and fitted into atechnical economic study.

Acknowledgement

The authors wish to thank SUBEEP programmepartners for their financial and technical support inconducting this work.

References

Anonymous (2002). Pulsed electric field assisted pressing ofsugar beet cossettes: a process of cold juice extraction.SUBEEP Programme Report. Compiegne University ofTechnology, Compiegne, France, GC-TAI

Bouzrara H (2001). Amelioration du pressage de produitsvegetaux par Champ Electrique Pulse: cas de la betterave asucre. [Enhancing pressing of vegetable products by pulsedelectric fields: case of sugar beet.] PhD thesis, Universite deTechnologie de Compiegne, France

Bouzrara H; Vorobiev E (2000). Beet juice extraction bypressing and pulsed electric fields. International SugarJournal, 102(1216), 194–200

Bouzrara H; Vorobiev E (2001). Non-thermal pressing andwashing of fresh sugarbeet cossettes combined with a pulsedelectrical field. Zuckerindustrie, 126(6), 463–466

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Eshtiaghi M N; Knorr D (2002). High electric field pulsepretreatment: potential for sugar beet processing. Journal ofFood Engineering, 52, 265–272

Jones G C (1988). Cossette pre-treatment and pressing.International Sugar Journal, 90(1077), 157–167

van-der-Poel P W; Schiweck H; Shwartz T (1998). SugarTechnology. KG, Verlag Dr. Albert Bartens, Berlin

Vorobiev E; Andre A; Bouzrara H; Bazhal M (2000). Proceded’extraction de liquide d’un materiau cellulaire, et dispositifsde mise en oeuvre du dit procede. [Process of cellularmaterial liquid extraction and implementation setup of sucha process] French Patent. No. 0002159 of 22/02/2000,France

Documents

Pulsed Electric Field Assisted Pressing of Sugar Beet ... · atically redistributed and pressed at the same pressure a third time (3rd press). So, pressed juices after PEF treatment