Journal of Food Engineering 104 (2011) 323–331

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Journal of Food Engineering

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Effect of whey protein concentration on the fouling and cleaning of a heattransfer surface

Adel Fickak, Ali Al-Raisi, Xiao Dong Chen ⇑Biotechnology and Food Engineering Group, Department of Chemical Engineering, Monash University, Clayton Campus, Victoria 3800, Melbourne, Australia

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

Article history:Received 11 March 2010Received in revised form 4 November 2010Accepted 10 November 2010Available online 28 December 2010

Keywords:Cleaning-in-place (CIP)FoulingHeat induced gelsDissolutionDairy processing

0260-8774/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.jfoodeng.2010.11.004

⇑ Corresponding author. Tel.: +61 3 99059344; fax:E-mail address: dong.chen@eng.monash.edu.au (X

In the studies of fouling and cleaning of heat exchange surfaces in dairy plants, whey protein deposits andheat induced whey protein gels (HIWPG) are considered as suitable model material to simulate the pro-teinaceous based type ‘‘A’’ milk fouling. Protein concentration of the fouling solution may significantlyinfluence the formation of milk deposits on heat exchange surfaces, hence affecting the cleaning effi-ciency. In this study, a laboratory produced heat induced whey protein gels (HIWPG) and a pilot plantheat exchanger fouling/cleaning were used to investigate the effect of protein concentration on formationand cleaning of dairy fouling. Here, HIWPGs made from different protein concentrations were formed incapsules and then dissolved in aqueous sodium hydroxide (0.5 wt%). The dissolution rate calculationbased on the UV spectrophotometer analysis. In the pilot-scale plant study, whey protein fouling depositswere formed by recirculating whey protein solutions with different concentrations through the heatexchange section in different runs, respectively. The deposit layers were then removed by recirculatingaqueous sodium hydroxide (0.5 wt%) and the cleaning efficiency was monitored in the form of the recov-ery of heat transfer coefficient while both fluid electric conductivity and turbidity were monitored asindications of cleaning completion. It was found that increasing the protein concentration of the HIWPGsignificantly increased the gel hardness and the dissolution time. In addition, increasing the protein con-centration significantly increased both, the amount of the fouling on the pilot-scale plant and the timerequired to clean the fouling deposit.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

During the thermal processing of dairy products, deposit layersare often formed on the process surface of the heat exchangers. Thecleaning, or removal, of such deposits is crucial for quality andsafety issues. Cleaning using chemicals is costly, both economicallyand environmentally (Toyoda et al., 1994). The fouling and clean-ing of proteinaceous deposits have received considerable attentiondue to their importance in the dairy industries (Visser, 1997). Inthe last decades, the understanding of the fouling and cleaningprocesses in dairy plants has been considerably improved (Fryeret al., 1996a; Wilson et al., 1999, 2002; Chen et al., 2004). The ma-jor components in dairy fouling deposits are the heat-sensitivewhey proteins (Visser, 1997). In fact aggregated whey protein mol-ecules dominate the basic structure of the fouling deposits. Be-cause of this and also the complex nature of milk deposits, manyresearchers (Belmar-Beiny et al., 1993; Schreier et al., 1994; Del-place and Leutiet, 1995; Fryer et al., 1996b; Davies et al., 1997;

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+61 3 99055686..D. Chen).

Gillham et al., 1999; Chen et al., 2000, 2001; Xin et al., 2002a,b),found heat induced whey protein gels (HIWPG) to be a reliablemodel system for investigating milk fouling and cleaning.

Heat induced whey protein gel (HIWPG) also contain a smallquantity of minerals and have been found to have the same natureof type ‘A’ milk deposits described by Lyster (1965) and Burton(1968), where proteins represent more than 60 wt% of the depositmass. The formation of the HIWPG deposits results from the aggre-gation of whey proteins upon heating. At the time a HIWPG isformed, only a fraction of the whey proteins has aggregated (Ver-heul and Roef, 1998a,b). The magnitude of this fraction, as wellas the gel point, depends on many factors such as the gelationpH, temperature and protein concentration (Mulvihill et al.,1990; Langton and Hermansson, 1992; Renard and Lefebvre,1992; Verheul and Roef, 1998a). HIWPG with high protein concen-tration tend to form faster due to the increasing rate of aggregationand the decreasing coagulation time (Sharma and Hill, 1993). AStudy by Mleko (1999) has found that increasing the protein con-centration will increase the firmness and the aggregate size of WPCgels, accelerating the gelation process. Furthermore, a study byPuyol et al. (2001) found that WPI gels with high protein concen-tration tend to form at lower temperature.

Nomenclature

Q power requirement (W)A heating surface area (m�2)T temperature (�C)a slope of the curve of the heat transfer coefficient versus

timeR fouling resistanced fouling layer thicknessCIP cleaning-in-placeHIWPG heat induced whey protein gel

WPC whey protein concentrateHT heating timeDT temperature difference between the heater surface and

the bulk solution (�C)SubscriptsS heater surfaceB bulk

324 A. Fickak et al. / Journal of Food Engineering 104 (2011) 323–331

All of these studies clearly show that high protein concentrationcan greatly influence the formation mechanisms of whey proteingels; however, the effect of protein concentration on the foulingand cleaning of dairy heat exchange surface has not been demon-strated quantitatively.

The dissolution of HIWPG has been studied extensively by Xinet al. (2002a) and Mercadé-Prieto and Chen (2006) as mechanisticinvestigation of alkaline cleaning of proteinaceous deposits. Theypresented dissolution or ‘cleaning’ mechanisms based on thebreakdown of protein aggregates, diffusion of small oligomersthroughout the swollen layer, and disentanglement of large aggre-gates next to the gel–solvent boundary layer. However, little isknown about the effect of protein concentration within foulingand heat induced whey protein gel on their dissolution. Moreover,the cleaning of whey protein based deposits has also been studiedusing test rig experiments by a number of researchers (Bird andFryer, 1991; Gillham et al., 1999, 2000). In these studies, the clean-ing was achieved by circulating NaOH solution through the equip-ment. In most cleaning studies, a single fouling protocol is usedwhile the cleaning conditions are modified. It is therefore impor-tant to understand the implication of the fouling solution condi-tions (e.g. protein concentration) on cleaning.

In this study, the effect of whey protein concentration on theformation and dissolution behaviours of HIWPG has been investi-gated. Also, the effect of whey protein concentration on the foulingand cleaning behaviours of a pilot-scale heat exchanger has beenstudied using whey protein concentrate as the foulant solution.

Return line from UV-S

To UV-S

6

2. Experimental

2.1. Material

Whey protein concentrate (WPC 85) powder was obtained fromlocal suppliers. The approximate composition of the WPC is givenin Table 1. The cleaning reagent, 60 wt% sodium hydroxide solution(NaOH), was purchased from LabServ, Melbourne, Australia.

75 mm

21 5

4

2.2. Methods

2.2.1. Preparation of HIWPG in the lab and dissolution experimentsThe same sort of heat induced gels and the methodologies that

were previously employed by Xin et al. (2002a) and Mercadé-Pri-

Table 1Whey protein concentrate (WPC) powder composition.

Component Content (wt%)

Proteins 82.0Fat 6.2Moisture 3.5Ash 8.3

eto and Chen (2006) were used in the current study. All gels wereprepared in triplicate and the error bars are presented using excel.

An experimental apparatus (Fig. 1) was employed to dissolveHIWPG in the laboratory. The apparatus consisted of an analyticbalance, a digital magnetic stirrer plate with a stirring speed con-troller, a digital controller for the water bath, a dissolution cell (a600 ml shock bottle and a capsule holder) and a peristaltic pumpto recirculate the solutions through a UV spectroscopy that waslinked to a computer.

The HIWPGs were formed in capsules using the method devel-oped by Mercadé-Prieto et al. (2008). HIWPGs were formed insidetest tubes (diameter 12 mm; length 75 mm) using well mixedwhey protein solutions with different concentrations (14, 20, and26 wt%, respectively). The capsules were filled with the whey solu-tion, then sealed with plastic stoppers and covered with foil, andthen held vertically in a water bath kept at 80 �C for 1 h. The top2 mm of the gel was removed with a spatula to ensure an even sur-face before each dissolution experiment (Mercadé-Prieto et al.,2008). The gels were kept at 4 �C overnight before use. The gel cap-sules were then dissolved in batch mode and the dissolution rate ofeach gel was calculated based on the measured increase of the dis-solved protein concentration in the NaOH solution over time. Theconcentration of NaOH solution was verified by titration of an ali-quot with HCl (0.01 N). As shown in Fig. 1, the gel capsules wereheld vertically with stainless steel wire that was fixed to a holeat the top of the bottle lid and submerged inside a 600 ml test bot-

12mm 3

1. Dissolution solvent (0.5 wt %) NaOH2. Capsule containing heat induced whey protein gel 3. Magnetic stirrer 4. Water bath 5. Pump 6. Heater

Fig. 1. Apparatus for the dissolution of heat induced whey protein gels.

A. Fickak et al. / Journal of Food Engineering 104 (2011) 323–331 325

tle containing 500 ml of NaOH (0.5 wt%) solution, which wasplaced in a water bath at 60 ± 1 �C. The solution was stirred at200 rpm using a magnetic stirrer to ensure the reading of a wellmixed solution. A small fraction of the agitated solution was circu-lated through the cell of a UV spectrometer (HP Agilent 8453 Spec-trophotometer, model number: G1103A, Agilent Technologies,Melbourne, Australia) using a peristaltic pump. Using the methodof Mercadé-Prieto and Chen (2006), the absorbance of the dis-solved proteins was continuously recorded at 20 s intervals for atleast 8000 s at wavelength 280 nm.

The dissolution rate was calculated by measuring the increaseof the concentration of protein in the alkali solution over time.

2.2.2. Gel strength measurementThe mechanical properties, such as the hardness, of HIWPGs are

affected by the protein concentrations in the gels. A simple effec-tive technique to measure the change in gel strength and cohesive-ness is the depth sensing indentation hardness test, which hasbeen widely used for characterizing the consistency of fats andother materials (DeMan, 1983). The test involves the penetrationof a sample surface by a metal probe indenter with a known geom-etry (DeMan, 1969). A parameter referred to as the ‘yield value’ or‘hardness index’ of a sample can be calculated by monitoring thepenetration force of the probe and the time taken to achieve thatpenetration depth (DeMan, 1983; Narine and Marangoni, 2001).The sample gels were made in a 50 ml beaker using 10 ml of wellmixed whey protein solutions at concentrations of 14 and 26 wt%and held in a water bath at 80 �C for 2 h. The gels were kept at

1 Tank including coiled heater TI-1:2 Centrifugal pump TI-2:3 Flow meter TI-3:4 Fouling section (include a heater rod) TI-4:5 Conductivity meter TI-5:6 Turbidity meter TI-6:7 Computer for temperature, turbidity and TI-7:conductivity recording ( (TI

Fig. 2. Fouling and cle

4 �C overnight and brought to room temperature (23 ± 2 �C) beforebeing tested.

The texture analyses were performed at room temperature(23 ± 2 �C) using a EZ Graph texture analyser (‘‘EZ Graph 100 N’’,Shimadzu Scientific Instruments, Melbourne, Australia) equippedwith a 100-N load cell, and a 12.8 mm diameter cylindrical probe.The test procedure was developed using Shimadzu Instrumentstexture tests guidelines.

The test was performed by allowing the probe at speed of50 mm min�1 to penetrate 10 mm into the gel, and the force at5 mm of penetration was taken as gel strength value.

2.2.3. Microstructure analysisImages of the microstructure of the gel samples were obtained

using Scanning Electron Microscope (SEM) (JEOL JSM-840A SEM,1986, Japan), located at Monash University, Clayton Campus, Mel-bourne, Australia.

Prior to image scanning, 2 mm�2 gel samples were sliced andplaced in a dry oven at 30 �C for 10 min. The gel samples were thensputter coated (approximately 1 mm) with gold palladium forcharge dissipations. The samples were then viewed with the SEMoperating at 15 KV using back scattering electrons.

2.2.4. Pilot-scale fouling and cleaning-in-place (CIP) experimentsA pilot-scale plant test rig was designed to generate and remove

whey protein fouling layers (Fig. 2). A 60 L reservoir was equippedwith heating coils to preheat the sample solutions. The solutionin the holding tank was stirred continuously by re-circulating

Tank temperature sensor Fouling section inlet temperature sensor Heater rod bottom surface temperature sensor Heater rod middle surface temperature sensor Heater rod top surface temperature sensor Bulk solution temperature sensorFouling section outlet temperature sensor

-3) + (TI-4) + (TI-5)) / 3: Surface temperature

aning test system.

Table 2Pilot plant operation conditions during fouling and CIP processes.

Operation variable Fouling CIP

Feed tank liquid volume 50 L 50 LFeed tank temp 70 ± 0.5 �C 60 ± 0.5 �CVelocitya 0.1 cm s�1 10.4 cm s�1

Heat flux 6.56 kW m�2 1.098 kW m�2

a Based on the cross-sectional area of the heater section (the annuli).

326 A. Fickak et al. / Journal of Food Engineering 104 (2011) 323–331

through the bypass valves. A centrifugal pump was used to ensurean easy circulation of sample solutions. The flowrate in the heatingsection was kept constant (Table 2) during the fouling and cleaningprocesses and monitored through a flowrate meter (Model 257-133’ from RS Components, Melbourne, Australia) installed at theinlet of the fouling section. The velocity was calculated using theflowrate and the heating section area.

The fouling section consisted of a heater rod (diameter =17 mm, length = 160 mm) fixed inside a sealed glass tube reactor(ID = 80 mm with length = 300 mm) with the bottom inlet ofID = 20 mm. Three outlets (ID = 20 mm) were uniformly distrib-uted (120� apart) at the top. Other devices such as a turbidity me-ter (Model TB750G) and conductivity meter (Model DC402G) fromYOKOGAWA Electric Corporation, Melbourne, Australia) were in-stalled downstream of the fouling section to monitor the cleaningprocess. In the current study, the rig operating conditions werekept constant for each run (see Table. 2). Fig. 3 shows a typical plotof the flow rate record. The velocity values indicate to the flow ofthe medium inside through the heating section. The very lowvelocity used for fouling was to exaggerate the fouling process asthe removal due to the shear is minimized. Also, the freshness ofthe concentrate in the tank can last longer as the fouling is under-stood to be primarily due to the action of whey protein (in fact thenative whey proteins). In the cleaning process, high velocity wasused to improve the shear removal in any case.

2.2.5. Generation of the fouling layerWhey proteins concentrate powder (WPC, 80 wt%) was recon-

stituted to protein concentrations of 2, 4 and 6 wt% solids, respec-tively in 50 L of RO water. The solution was then transferred to theholding tank and preheated to approximately 70 ± 0.5 �C and thenheld for 5 min before being pumped through the fouling sectionand back to the tank. The recirculation of the solution in the hold-ing tank through the bypass valve results in a continuous stirring ofthe solution, this help to sustain the required tank temperature.

0 500 1000 1500 2000 2500 3000 3500 4000 45000

0.1

0.2

0.3

0.4

0.5

Time (s)

Flo

wra

te (L

/min

)

Fig. 3. Velocity during the fouling process.

At the start of the experiment, the heater rod surface tempera-ture (the average of the temperature around the outside diameterof the heater rod upper, middle and bottom surface) was obtainedat approximately 81 ± 1 �C by applying the heat flux described inTable 2 using a 5A variac autotransformer. The fouling layerformed in each run was found to be reasonably evenly distributedalong the heater rod surface.

2.2.6. Monitoring of foulingThe fouling process was monitored through the change in the

heat transfer coefficient. A drop in the heat transfer coefficientindicates the formation of a fouling layer on the heater surface.The heat transfer coefficient was calculated using the followingequation:

U � QAðTS � TBÞ

ð1Þ

where U = overall heat transfer coefficient (W m�2 K�1); Q = powerinput to the heater rod (W); A = heater rod surface area (m�2);TS = heater rod surface temperature (�C) (the average of the temper-ature around the outside diameter of the heater rod upper, middleand bottom surface); TB = bulk fluid temperature (�C) measured inthe fouling section.

2.2.7. Monitoring the temperatureAs mentioned above the average temperature measurement of

the heater rod surface was obtained by attaching three thermocou-ples around the outside diameter of the upper, middle and bottomsurface of the heater rod. The bulk fluid temperature was measuredthrough another thermocouple that was placed inside the bulksolution in the heating reactor. The inlet and outlet temperaturesof the fouling section were monitored using thermocouples in-serted at the inlet and outlet of the fouling section. The tank tem-perature was monitored using a thermocouple located at thecentre of the tank. All measured temperatures (surface, bulk, inlet,outlet and tank) were logged to a computer.

2.2.8. Cleaning of the fouled surfaceThe fouled surface was cleaned using a three-stage cleaning

method. In the first stage, after the fouling layer was formed, thewhey protein solution was drained and the system was rinsed withwater at a velocity of 10.423 cm s�1 (for approximately 10 min),until there were no protein traces left in the rinsing water. Therinsing efficiency was indicated using the turbidity meter (seeFig. 4 for one example). The rinsing process was stopped once

0 200 400 600 800 1000 1200 1400 1600 1800 20000

5

10

15

20

25

30

Time (s)

Tur

bidi

dty

(NT

U)

Fig. 4. Turbidity measurements of the rinsing water during the CIP process.

A. Fickak et al. / Journal of Food Engineering 104 (2011) 323–331 327

the standard turbidity of drinking water (0.5–1 NTU) was reached(USEPA, 2001).

In the second stage, a cleaning solution (50 L of NaOH at0.5 wt%) was used. During cleaning, the cleaning solution temper-ature was kept constant at 60 ± 0.5 �C. In the cleaning-in-place(CIP) process, first the cleaning solution was recirculated throughthe system. The heater rod surface temperature (approximately66 ± 0.5 �C) was obtained by applying the heat flux described in Ta-ble 2 using an enclosed 5A variac autotransformer. The CIP processwas monitored visually by observing the change in the heat trans-fer coefficient and by observing the complete removal of the foul-ing layer through the glass wall of the fouling section (see Fig. 9b).An increase of the heat transfer coefficient corresponds to the re-moval of the fouling layer on the heater surface. The CIP solutionwas drained when the fouling layer was seen to be completelyremoved.

0 200 400 600 800 1000 12000

200

400

600

800

1000

1200

Time (s)

Con

duct

ivity

(µS

)

Fig. 5. Conductivity measurements of the rinsing water during the CIP process.

Fig. 6. SEM images of the microstructure of (a

In the third stage, the system was continuously rinsed withwater for approximately 10 min until there were no NaOH tracesleft in the rinsing water. The rinsing efficiency at this stage wasmonitored using the conductivity meter (see Fig. 5 for one exam-ple). The rinsing was stopped when the typical conductivity ofdrinking water (<500 lS/cm) (DeZuane, 1990) was reached. Theturbidity and conductivity measurements were both used to indi-cate the efficiency of the cleaning process.

3. Results and discussion

3.1. Influence of protein concentration on HIWPG formation

From the SEM images of the microstructure of HIWPG (seeFig. 6), it is clear that HIWPG with high protein concentration‘‘26 wt%’’ (Fig. 6a) contain larger aggregate sizes than HIWPG withlower protein concentration ‘‘17 wt%’’ (see Fig. 6b). These observa-tions are consistent with the observations of Mleko (1999) who re-ported that increasing the protein concentration results inincreasing the aggregate size of the whey protein gel. These resultscan be explained as follows. Whey protein solution contains anumber of cysteine residues and upon heating the free – SH groupsof cysteine residues get oxidized and form disulphide bonds. Thesebonds are involved in cross-linking proteins to form large aggre-gates. However, increasing the protein concentration in the solu-tion will ultimately increase the number of cysteine residues inthe solution. According to Skudder et al. (1981), increasing the le-vel of free – SH group, accelerates the rate of the gel formation byincreasing the number of disulphide cross links between the pro-tein molecules. This in turn leads to an increased aggregation rate(Sharma and Hill, 1993) aggregate size (see Fig. 6a) and number ofaggregates (Mleko, 1999), resulting in the formation of a hard andrigid gel. In the current study, such hard and rigid gel was obtained.The texture analysis results (Fig. 7) show that a higher force wasrequired (approximately 48 N) to penetrate the HIWPG with highprotein concentration compared to the HIWPG with lower proteinconcentration. This suggests that HIWPG with high protein con-

) 26 wt% HIWPG and (b) 17 wt% HIWPG.

5 10 15 20 25 300

10

20

30

40

50

Time (s)

For

ce (

N)

(a)

(b)

Fig. 7. Texture analysis of (a) 26 wt% HIWPG and (b) 17 wt% HIWPG.

328 A. Fickak et al. / Journal of Food Engineering 104 (2011) 323–331

centration is harder and more rigid than HIWPG with low proteinconcentration.

3.2. Influence of protein concentration on HIWPG dissolution

The dissolution experiments (Fig. 8) showed that HIWPG withhigh protein content HIWPG had the lowest dissolution rate(0.12 g m�2 s�1). HIWPG with high protein concentration haveshown to contain large aggregates size (Fig. 6a), these aggregatesare cross-linked via disulphide bonds and as a result, hard(Fig. 7) and large clusters are formed (Fig. 6a). In the dissolutionprocess of HIWPG, these clusters are expected to be released anddisengaged very slowly from the HIWPG matrix hence loweringthe dissolution rate.

In the dissolution studies of polymers (Devotta et al., 1995;Narasimhan and Peppas, 1996), it was predicted that the largeclusters or the large chains of molecules are expected to disengagevery slowly from the polymer matrix into the solvent. In the samestudy, it was proposed that the rate at which these large chains ofmolecules disengage themselves from the gel–liquid interface con-trols the dissolution rate in large polymeric systems. However, forwhey protein gels, it is likely that due to the cluster chain size var-ies in a wider range as a result of the mixture, smaller and easier to

14 16 18 20 22 24 26 280.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Whey Protein Concentration (wt %)

Dis

solu

tion

Rat

e (g

m -2

s-1

)

Fig. 8. Dissolution rates of heat induced whey protein gels with different proteinconcentration in (0.5 wt%) NaOH at 60 �C.

be broken clusters can disengage from the surface and interior ofthe gel assembly. The disengaged species can leave the interior ofthe gel body and then leave the gel–solvent interface as proposedin previous works (Mercadé-Prieto and Chen, 2006; Mercadé-Pri-eto et al., 2006a,b; Mercadé-Prieto et al., 2008).

3.3. Influence of protein concentration on whey protein solutionfouling

The results in Fig. 9 show: (a) the heat transfer coefficient pro-file during the fouling process; and (b) the temperature difference(DT) between the heat exchange surface (average) and the bulksolution during the fouling process of the 4 wt% whey protein con-centrate solution (as an example). The decreasing heat transfercoefficient is an indication of the fouling layer formation on theheat transfer surface.

As the fouling process proceeds the amount of deposit formedon the heat exchange surface also increases, leading the heat trans-fer coefficient to drop. Fig. 10 shows an example of fouled andcleaned heater rod, and it can be seen that higher protein concen-tration resulted in significantly more fouling. The heat transfercoefficient plots however, seem not very different especially to-wards the end of each experiment (Fig. 9a). The fouling behaviourof deposits formed from 4 to 6 wt% solutions seems even moresimilar to each other.

As it can be observed from the description of the results above,more highly concentrated (4–6 wt%) whey protein solutions fouledmore quickly than the less concentrated (2 wt%) whey proteinsolution. The possible explanation for this is that increasing thewhey protein concentration of the solution results in an increaseof the number of b-lactoglobulin molecules in the solution andincreasing the driving force of mass transfer of the fouling speciestowards the wall. Jun and Puri (2005) provided a more detailedsummary of the factors influence fouling. It is well known thatheating above 65 �C, these b-lactoglobulin molecules undergodenaturation. The denaturation of b-lactoglobulin is consideredby Grijspeerdt et al. (2004) to be significant in most dairy fouling.The denaturation reaction of b-lactoglobulin consists of a numberof steps in which the monomer to dimer dissociation is the firstessential process. This dissociation occurs at temperatures muchlower than those at which modifications to the tertiary structureare observed (Cairoli et al., 1994; Iametti et al., 1996). This is fol-lowed by the rearrangement of the native b-lactoglobulin confor-mation to a state in which the free thiol is exposed and initiatessulfhydryl/disulphide (SH/S–S) interchange reactions, leading toirreversible aggregation/polymerization (De Wit, 1990). The irre-versible aggregation/polymerisation may happen at the metal sur-face to form fouling that is not soluble in worm water.

The rate for the aggregation is expected to be strongly influ-enced by the protein concentration. At high b-lactoglobulin con-centrations, the rate for the aggregation is observed to increasedue to the more frequent interactions between protein intermedi-ate species (Roefs and de Kruif, 1994; Hoffmann and van Mil,1997). Indeed, the kinetics of whey protein, and especially b-lacto-globulin, thermal denaturation has been extensively studied(Lyster, 1970; Sawyer et al., 1971; Hillier and Lyster, 1979; Harwal-kar, 1986; Manji and Kakuda, 1987; Dannenberg and Kessler, 1988;Roefs and de Kruif, 1994; Anema and McKenna, 1996; Galani andApenten, 1997; Chen et al., 1998; Verheul et al., 1998). Accordingto Arnebrant et al. (1987) partly denatured proteins are more sur-face active than native proteins, so it would be expected that afouling layer forms very rapidly with increased native protein con-centration. Although there is an established link between proteindenaturation and fouling, the relative impact of the denaturedand aggregated proteins on fouling is not clear (Bansal and Chen,2006).

Fig. 10. (a) Fouled rod heater with deposit layer from (6 wt%) WPC solution before and (b) after cleaning.

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500100

200

300

400

500

600

700

Hea

t Tra

nsfe

r C

oeffi

cien

t(W

.m2 .K

-1)

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 55000

20

40

60

Time (sec)

ΔT (

o C) ΔT (Heater surface and bulk solution) for the 4 wt % run

2 wt % WPC

4 wt % WPC6 wt % WPC

(a)

(b)

Fig. 9. (a) Fouling behaviour of whey protein concentrates (WPC) solution with various protein concentrations and (b) heater surface and bulk solution temperaturedifference (DT) during the fouling of 4 wt% WPC.

A. Fickak et al. / Journal of Food Engineering 104 (2011) 323–331 329

3.4. The influence of protein concentration during fouling on thesubsequent cleaning

The results in Fig. 11 show: (a) the heat transfer coefficient pro-file during the cleaning process; and (b) the temperature difference(DT) between the heat exchange surface (average) and the bulksolution during the cleaning process of the fouling layer from4 wt% whey protein concentrate solution (as an example). The heattransfer coefficient recovery time indicates to the cleaning time ofthe fouling layer from the heat transfer surface.

The slope of the cleaning curve in Fig. 11a shows the speed ofthe recovery of the heat transfer coefficient, which is an increasingparameter. The initial slopes were not so different, which indicate asimilar nature of the deposits that were formed at the final stagesof fouling where the temperature at the fouling layer surfacewould be partly close to the bulk solution temperature. The finalslopes (a1 > a2 > a3) in Fig. 11a were distinctly different, whichindicate that the higher the protein concentration of the fouledsolution the harder is to remove the fouling formed at the begin-ning of the fouling process. These slopes would be a good indicator

0 500 1000 1500 2000 2500 30000

100

200

300

400

500

600

700

0 500 1000 1500 2000 2500 30000

5

10

Time (sec)

ΔT (Heater surface and bulk solution) for cleaning 4 wt % WPC fouling

α2 α3α1

(a)

(b)

2 wt % WPC 4 wt % WPC 6wt % WPC

Hea

t Tra

nsfe

r C

oeffi

cien

t(W

.m2 .K

-1)

ΔT (

o C)

Fig. 11. (a) Cleaning behaviour of fouling deposits from whey protein concentrates (WPC) solution with various protein concentrations. a1, a2 & a3 show the significant slopestowards the finish of the cleaning runs. (b) Heater surface and bulk solution temperature difference (DT) during the cleaning of 4 wt% WPC fouling.

330 A. Fickak et al. / Journal of Food Engineering 104 (2011) 323–331

of the effect of the deposit aging process for any single run, wherephysical and chemical changes might have occurred at the metalsurface temperature. Though the final heat transfer coefficient inthe fouling stage seems not so dissimilar, the time taken to cleanwas drastically different, with high protein concentration foulingtaking much longer to clean. This implies that the nature of thefouling that was formed under higher whey protein concentrationis quite different.

Considering the fouling resistances among the three concentra-tion levels similar, it means that the additional resistance d/Rfouling

(where, Rfouling = the thermal conductivity of the fouling layer and;d = the thickness of the fouling layer) would be similar. Consideringthat the fouling is mainly consisted of proteins and water, thenRprotein � 0.2 W m�2 K�1 and Rwater � 0.6 W m�2 K�1 (Rahman,1995), more protein in the layer leads to smaller thermal conduc-tivity of the layer.

Visualization of the fouled layers showed that the fouling layersformed at higher protein concentration appear denser which maysuggest the fouling layer has more proteins. The much longercleaning time indicates the fouling is stronger (most likely becauseof the higher protein concentration in the fouling layer hence low-er thermal conductivity hence greater thermal resistance) whichseems to be mirrored by the gel strength tests and the gel dissolu-tion experiments.

4. Conclusions

The formation and dissolution rate of HIWPG is influenced bythe whey protein concentration. It was found that the structureof HIWPG became more rigid with the increasing protein concen-tration. The dissolution rate of HIWPG decreased with the increas-ing protein concentration, almost linearly against concentration (inthe gel) increase.

In the pilot-scale fouling test, it was found that whey proteinsolution with increasing fluid bulk whey protein concentrationleads to faster build up of fouling. The fouling under higher proteinconcentration took much longer time to clean. The slope of theheat transfer coefficient increase versus time in the cleaning runsappears to be a good indicator of the effect of protein concentrationon the final and early stages of fouling.

Acknowledgement

The authors are grateful to the financial support provided byDairy Australia, Melbourne, Australia.

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