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Protein Denaturation, Rheology, and Gelation Characteristics of Radio-Frequency Heated Egg White DispersionsJasim Ahmeda; Hosahalli S. Ramaswamya; Inteaz Allia; Vijaya G. S. Raghavanb

a Department of Food Science & Agricultural Chemistry, Macdonald Campus of McGill University, Ste.Anne de Bellevue, PQ, Canada b Department of Bioresource Engineering, Macdonald Campus ofMcGill University, Ste. Anne de Bellevue, PQ, Canada

To cite this Article Ahmed, Jasim , Ramaswamy, Hosahalli S. , Alli, Inteaz and Raghavan, Vijaya G. S.(2007) 'ProteinDenaturation, Rheology, and Gelation Characteristics of Radio-Frequency Heated Egg White Dispersions', InternationalJournal of Food Properties, 10: 1, 145 — 161To link to this Article: DOI: 10.1080/10942910600986970URL: http://dx.doi.org/10.1080/10942910600986970

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International Journal of Food Properties, 10: 145–161, 2007Copyright © Taylor & Francis Group, LLCISSN: 1094-2912 print / 1532-2386 onlineDOI: 10.1080/10942910600986970

145

PROTEIN DENATURATION, RHEOLOGY, AND GELATION CHARACTERISTICS OF RADIO-FREQUENCY HEATED EGG WHITE DISPERSIONS

Jasim Ahmed, Hosahalli S. Ramaswamy and Inteaz AlliDepartment of Food Science & Agricultural Chemistry, Macdonald Campus ofMcGill University, Ste. Anne de Bellevue, PQ, Canada

Vijaya G.S. RaghavanDepartment of Bioresource Engineering, Macdonald Campus of McGill University,Ste. Anne de Bellevue, PQ, Canada

Gel properties of radio frequency (RF) heated egg white dispersions at 27.12 MHz werestudied as function of concentration (2.5–2.5 kg/100 kg sample), pH (3–11) and heatingtime (60–180 s). Egg white dispersions demonstrated a gradual liquid-solid transformationas they denatured and gelled during RF treatment. The critical concentration and heatingperiod for egg white protein denaturation and gelation were found to be 7.5% (w/w) and150 seconds. The elastic modulus (G¢) of RF-heated samples increased with concentrationand heating period (temperature), whereas complex viscosity (h*) increased exponentiallywith concentration. In an alkaline condition, the egg white dispersion did not produce agel; however, in acidic condition it resulted in a strong gel with significantly (P < 0.05)higher G¢. This could be attributed to the high dielectric constant (ε¢) and loss factor (ε²)values of acidified samples as compared to the alkaline and control egg white dispersion.Effect of heating rate (1, 5, 10, and 20°C/min) in situ on rheometer plate significantlyaffected gel rigidity; the RF treated sample rigidity was comparable to samples heated at therate of 5 and 10°C/min. Differential scanning calorimetry, dielectric measurement, andsodium dodecyl sulfate (SDS) PAGE electrophoresis results were used to confirm gelationbehavior during both conventional and RF heating conditions.

Keywords: Egg white, Radio frequency, Elastic modulus, Complex viscosity, Proteindenaturation, Dielectric properties.

INTRODUCTION

Radio frequency (RF) heating is an innovative technique among the several recentelectro-technologies such as ohmic heating, microwave dielectric heating, inductive/ohmic combinations, inductive heating, and radiative magnetic heating. The RF generatorproduces an alternating electric field between two electrodes in radio frequency system.The material to be subjected to RF heating is conveyed between the electrodes, where the

Received 26 March 2006; accepted 1 September 2006.Address correspondence to Jasim Ahmed, Department of Food Science & Agricultural Chemistry,

Macdonald Campus of McGill University, Ste. Anne de Bellevue H9X 3V9, Canada. E-mail: jasim.ahmed@mcgill.ca or jahmed2k@yahoo.com

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alternating current causes the polar molecules in the material to continuously try to alignthemselves with the oscillating electrical field much like the way bar magnets behave inan alternating magnetic field. The friction resulting from molecular movement causes thematerial to rapidly heat throughout its entire mass. The amount of heat generated in theproduct is a function of frequency, the square of the applied voltage, dimensions of theproduct, and the dielectric properties (mostly the loss factor) of the material. The dielectricproperties of materials dictate, to a large extent, the behavior of the materials whensubjected to RF or microwave (MW) heating of the materials.[1]

RF heating (13.56, 27.12, and 40 MHz) differs from the higher frequency micro-wave heating (915 MHz and 2450 MHz) in which the wavelength is comparable orsmaller than the dimensions of the sample and heating occurs in a metal chamber with res-onant electromagnetic standing waves. Since RF waves have a longer wavelength thanMW, the electromagnetic waves in the RF system can penetrate deeper into the product sothere is less surface overheating, and hot or colds spots—a common problem with MWheating. The RF heating also offers simple uniform field patterns as opposed to the morecomplex and characteristically non-uniform standing wave patterns in a microwave oven.

One of the carliest applications of RF heating was in the baking of bread and there-after the technology has been extensively exploited by the bakery industry for post-bakedrying/finishing purposes.[2–3] Several review articles are available on the RF-heating andits applications.[4–7] Demeczky[8] reported that RF pasteurised fruit juices had bettermicrobiological and sensory qualities as compared to conventional thermal treatments.Some studies have been carried out on RF pasteurisation of meat products.[6,9] Since, mostof the liquid foods are now processed aseptically, development and optimization of RFheating will be attractive for processing and preservation of liquid foods.[10]

Zhong et al.[5] reported that a 30 kW, 40.68 MHz RF system heated water uniformly,while heating of carboxymethylcellulose (CMC) solution was non-uniform in both batchand continuous modes of operation. The difference was attributed to the dielectric propertiesof water and CMC samples. Luechapattanaporn et al.[11] studied the thermal resistance ofClostridium sporogenes (PA 3679) in phosphate buffer and mashed potatoes in aluminiumthermal-death-time tubes using 27.12 MHz RF heating and reported that the RF heating sys-tem could be used to produce safe and shelf stable packaged foods. Although, the RF heatingprocess has been experimented with pasteurization or sterilization, there has been no detailedstudy on the evaluation of protein denaturation or gelation behavior of RF heated foods.

Egg white protein (EWP) has the unique characteristics and is extensively used inmany food applications. Gel formation, water holding capacity, foaming, and emulsifyingproperties are some of the important functional characteristics of egg white. Heat inducedgelation of protein is considered to form a three-dimensional network assembled fromprotein molecules or aggregates, surrounded by water. The gel network is generally mea-sured in terms of rheological properties, water holding capacity, and/or micro-structuralchanges all of which are affected by protein concentration, pH, temperature, heatingperiod, and rate of heating.[12–13] Recently, some studies have focused on subjecting wheyand muscle proteins to dielectric heating and evaluating the denaturation behaviour byobserving changes in their dielectric properties.[14–15]

Food gels are aqueous dispersions or solutions of high molecular weight carbohy-drates or proteins, cross-linked so as to form an interconnected molecular network thatspans the volume of the liquid medium. Rheological techniques have been used to mea-sure the rigidity and deformation of gels using various techniques (small and large-strainrheology).[16–22] Gel network structure and interactions within the network strand control the

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rheological properties of protein gels. Most works conclude that gel rigidity is increased withan increase in protein concentration, temperature, and pH. However, for egg white, no infor-mation is available on denaturation and gel formation during radio frequency heating.

The objective of this work was to study application of radio frequency heating toliquid egg white and evaluate its effect on gelation and gel strength as function of proteinconcentration, pH, and heating time in comparison with conventionally heated product.Since, RF heating is directly dependent on dielectric properties; the second objective wasto study the dielectric properties of egg white dispersions as function of temperature andpH in the RF range.

MATERIALS AND METHODS

Material

Sample of albumin from chicken egg white was obtained from Sigma Chemicals(Sigma-Aldrich Inc., St Louis, MO). The product was a crude preparation of dried eggwhite, light yellow in color. The moisture content of the powder sample was 9.2% andcontained 62–88% ovalbumin with a minimum 12.5% nitrogen. The molecular weight ofthe sample was approximately 44.3 kDa (as per specifications).

Preparation of Sample

Samples of egg white dispersions were prepared (2.5, 5, 7.5, 10, and 12.5 kg/100 kgwater) by mixing the required amount egg white powder and deionized water (conductance:18 Ω, Milli-Q, Millipore, Bedford, CT) under continuous stirring after which it was held for2 hours at room temperature (20°C) for hydration. The different concentrations were chosento evaluate the critical level required for gel formation. The normal pH of the dispersion was6.89. The pH of the samples were adjusted to various levels (3, 5, 9, and 11) by addingrequired volume of hydrochloric acid or sodium hydroxide solution while the control sampleis considered to at pH 7 for comparison purposes. Each sample was prepared in triplicate.

Apparatus

A laboratory scale radio frequency dielectric heating system (Model IRF-01; iCorzTechnologies, Guelph, Ontario, Canada) was used in this work operating at 27.12 MHzand a nominal output power of 2 kW. Apart from the generator, the RF system wasequipped with a vertical applicator tube of 0.0254 m diameter and 0.762 m height.

The RF load applicator was adjusted with load and tune to match 50 Ω impedance net-work. The unit was designed to work either in batch or continuous mode with the connectionof the applicator tube to a pump. However, the present work was carried out in a batch modeby closing at the bottom of the applicator tube with a plug. Water was placed in the applica-tor, and the level was kept slightly above radio frequency zone (∼210 mL). A polypropylenevial (Nalge Nunc Int., Rochester, NY) of 2 mL capacity containing egg white sample wassuspended in the applicator radio frequency zone and held in place using a cotton thread duringthe heating period A fiber optic probe (FISO Technologies Inc., Quebec, Canada) was insertedinside the vial (Fig. 1). The sample temperature was recorded using the data logger and soft-ware provided by the fiber optic probe supplier. The top of the applicator tube was coveredwith aluminium foil padded wad to prevent leakage of RF waves and minimize heat loss.

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Rheological Measurements

The rheological measurements of RF heated egg white dispersions were carried outby using a controlled-stress rheometer (AR 2000, TA Instruments, New Castle, DE). Aparallel plate geometry with a 60-mm diameter of upper plate was used with a gap of 1000microns. The sample temperature was internally controlled by peltier system (−20 to200°C with an accuracy of ±0.1°C) attached with a water circulation unit. A platinumresistance thermometer sensor positioned at the centre of the plate ensured temperaturecontrol and measurement. For each test, a measured volume of thoroughly mixed sample(approximately 2 mL) was placed at the center of the rheometer plate for 5 minutes forstress relaxation and temperature equilibration before the actual measurements.

Small-amplitude oscillation (SAOS) measurements were performed within linearviscoelastic region over a frequency range between 0.1 and 10 Hz, which enables thematerial to preserve the structure. To study the effect of heating rate, the sample washeated from 30 to 92°C (1Hz frequency) at selected rates (1,5,10, and 20°C/min) and fol-lowed by immediate cooling to 20°C with the internal peltier heating system. Measure-ments were made after temperature equilibration to 20°C and each time new sample wasused for rheological measurement. All the rheological measurements were carried out intriplicate and rheological parameters (elastic modulus, G′; viscous modulus, G″; phaseangle, δ; and complex viscosity, η*) were directly obtained using the manufacturer sup-plied computer software (Rheology Advantage Data Analysis Program, TA Instruments,New Castle, DE).

Dielectric Properties Measurement

A vector network analyzer (Model: Agilent 8722ES, Agilent Technology, Palo Alto,CA) with an open-ended coaxial probe was used to measure dielectric properties of egg white

Figure 1 Schmetic diagram of the experimental set up in an applicator tube.

Cotton thread

Fiber optic probe

Vial

RF zone

Teflon applicator

Cotton thread

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dispersions. The built-in S-parameter test set provided a full range of magnitude and phasemeasurements in both the forward and reverse directions. The sample (approximately 40mL) was placed in a wide glass tube (50 mL), and the open co-axial probe (probe diameter≈ 20 mm) was set into the tube. The sample holder was dipped into a temperature con-trolled water bath with agitation. A thermocouple temperature sensor (BK Precision 390A,Yorba Linda, CA) was used to monitor the temperature during study and the temperaturevariation was minimal, ± 0.5°C. The network analyzer probe and the cable were fixedtogether so that there could be no movement during sample measurement and data acqui-sition. The instrument was calibrated using air, short, and water before each experiment.The dielectric properties of the samples were measured from 200 to 2500 MHz withintemperature range of 20 to 90°C. The dielectric spectra of the samples (dielectric constant∈′, and dielectric loss factor ∈″) were automatically computed and recorded with the man-ufacturer supplied computer software (Version 85070 D1.00, Agilent Technologies, PaloAlto, CA). All the measurements were carried out in duplicate and were reproducible ± 5%.

Differential Scanning Calorimetry

A differential scanning calorimenter DSC Q 500 (TA Instruments Newcastle, NJ)was used for gathering the thermograms. Control and RF heated egg white samples wereweighed (10–13 mg ± 0.12 mg) into an aluminum DSC sample pan (Part No 900793.901)and uniformly distributed across the bottom of the pan. The upper aluminium hermetic lid(Part No 900794.901) was placed and hermetically encapsulated samples for DSCmeasurements. All samples were analyzed within 3h of RF heating.

The DSC was calibrated with indium and sapphire for temperature and heat capac-ity. An empty pan was used as a reference to balance the heat capacity of the sample pan.In the experiment, sample pans were equilibrated to 20°C and held isothermal for another2 minutes. The heating rate of the DSC scans was 5°C/min over a range of 30 to 120°C.The DSC measurements were done in triplicate. The DSC data were analyzed with theUniversal Analysis Software (version 3.6C) for thermal analysis, which was providedwith the instrument (TA Instruments, Newcastle, NJ).

Electrophoresis

Sodium dodecyl sulfate (SDS)-PAGE was performed using the method of Fritzet al.[23] to demonstrate the effects of RF heating on the egg protein. Stacking gel andresolving gel of 4 and 12% acrylamide were used for SDS-PAGE. Slab gels (0.75-mmthickness) were run at a constant current of 15 mA/gel; the electrophoresis was performedwith a Bio-Rad Mini Trans blotting system as detailed in earlier studies.[24]

Data analysis

The Microsoft Excel software package (Microsoft Corporation, Redmond, WA) wasused to carry out the statistical analysis and linear regressions of experimental data.

RESULTS AND DISCUSSION

Temperature profiles of water (medium) and egg white dispersions at different con-centrations are shown in Fig. 2. Protein concentrations did not affect the heating pattern.

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Normally, in conventional heating, the heating pattern gradually changes from convectionto conduction with an increase in protein concentration in the dispersions thereby consid-erably slowing down the rate of heating. However, this was not the case with RF heating.This could be attributed to the dielectric heating in the RF system with a much deeper pen-etration power (uniform heating), which was independent of sample concentration. Thetemperature along wall side of the applicator was not significantly (P > 0.05) differentfrom that at the center of the Teflon tube (difference less than 2–3°C, data are not shown).Similar observation has been reported by Zhong et al.[5] during experiments with RF heat-ing of water and CMC dispersion.

Gel Formation and Gel Rigidity Measurement by SAOS

SAOS measurement indicated the possible gelation of egg white dispersions duringRF-heating. The gel characteristics were represented by dynamic rheological parameters:elastic modulus (G′), viscous modulus (G″), phase angle (δ), and complex viscosity (η*).A typical rheogram of RF-treated egg white dispersion is shown in Fig. 3. The elasticmodulus predominated significantly (P < 0.05) over viscous component in whole fre-quency range; however, with a slow change in G′ indicating a more elastic nature of theRF treated egg white dispersions.[25] Lower values of phase angle (which is a measure ofthe ratio of energy lost to energy stored in a cyclic deformation) data supported the visco-clastic nature of heated egg white sample.

Effect of concentration. Both G′ and G″ of RF treated egg white dispersions areshown as function of concentration in Fig. 4. RF heating changed the physical state of eggwhite dispersions leading to gelation. Visually it was noticed that a concentration of 7.5%was required to form a gel. Both G′ and G″ for the 2.5% heat-treated sample were too low, andit was difficult to obtain data of significance within the range of oscillation stress. There-fore, this concentration was excluded from further study. Sample concentration of 5% pro-duced precipitation. Rheological data supported the visual observation with significantly

Figure 2 Effect of RF-heating on water medium and egg white dispersions at constant power level.

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RADIO-FREQUENCY HEATING OF EGG WHITE DISPERSION 151

different magnitudes of G′ and G″. Both these parameters increased with concentration andthe maximum value was found at the egg white concentration of 12.5% (Fig. 4). Higher G′values indicate the ability of gel to maintain an intact network structure.

Complex viscosity (η*) measures the true visco-elastic nature of gels. Complexviscosity versus angular frequency of egg white dispersions as a function of concentrationis shown in Fig. 5. The values of η* significantly (P < 0.05) increased with concentration,however systematically decreased with angular frequency. The increase in complexviscosity (η*) followed an exponential relationship with concentration (C) at a constantfrequency (1 Hz) (Eq. 1).

Figure 3 Typical rheogram of RF-heated egg white dispersion.

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Figure 4 Effect of concentration on elastic and viscous modulus of RF-heated egg white dispersion at 1 Hz and180 seconds heating (1: 2.5, 2: 5, 3: 7.5, 4: 10, 5: 12.5%).

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Effect of heating time. The RF heating time was varied between 60 and 180seconds, and it was noted that heating period significantly (P < 0.05) affected proteindenaturation process and gel formation of egg white dispersion (Fig. 6). Both G′ and G″

Figure 5 Effect of concentration on complex viscosity of RF-treated egg white dispersions at 20°C.

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Figure 6 Effect of heating time on elastic and viscous modulus of RF treated 7.5% egg white dispersion at pH 7.

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increased as a function of heating period, and the peak values were noted at 180 seconds.However, a minimum heating period of 150 seconds (critical gelling time, correspondingtemperature 78–80°C) was required to denature egg white dispersion by RF heating and,to form a gel. The gel formation is an integrated effect of time, temperature, and reactionrates.[26] Increase in heating period and even holding for an additional 180 seconds andcooling immediately did not enhance either elastic or viscous modulus of samples (Fig. 7).It is possible that gelation of egg white protein was complete with the 180 seconds RFheating treatment, and hence, no further increase in rheological parameters was observedduring the extended holding time. All the rheological parameters (elastic modulus, phaseangle, and complex viscosity) demonstrated superimposition with each another.

Effect of heating rate and comparison with conventional heating. Theheating rate during RF treatment was found to be 22°C/min. To compare gelation behaviorbetween RF heating and conventional heating, a control sample of 10% was heated in rhe-ometer plate from 30 to 92°C at different heating rates of 1, 5, 10, and 20°C/min (maxi-mum heating rate of rheometer and a near comparison to the RF heating rate of 22°C/min),and the thermorheological behavior was measured in terms of phase angle which is animportant parameter for visco-elastic fluid[26] (Fig. 8). A sharp decrease in phase angle wasobserved at temperature of about 70°C which indicated initiation of protein denaturation.Heating rate at 1 and 5°C/min exhibited almost similar patterns of changes in phase angle,while a large scattering of data were noticed with a more rapid heating rate. The signifi-cant decrease in phase angle indicated protein denaturation with visco-elastic texture.However, at rapid heating rates, the denaturation got initiated at a slightly higher tempera-ture due to short residence times. There are several hypotheses for protein gel formationand gel rigidity. Many researchers have reported that faster heating rates would allow lesstime at temperature above the denaturation temperature of the proteins for aggregation to

Figure 7 Comparison of rheological parameters of RF heating only with RF heating-holding.

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occur and, therefore, result in a weak gel formation.[27–28] This is also very much consis-tent with the concept of high temperature short time (HTST) processes in which betterquality retention has been traditionally observed indicating smaller thermal effects onquality changes such as starch gelatinization, protein denaturation or enzyme inactivation(except for microbial kill which gets accelerated).

Gel rigidity of RF and conventional thermal treated samples (at different heatingrate) were compared after heating at 92°C, followed by immediate cooling to 20°C.

Interestingly, the gel rigidity produced by heating at 1°C/min of egg white samplewas found to be the highest (65 minutes duration of heating-cooling), while gels producedby heating rate of 5°C/min (15 minutes duration of heating-cooling) and 10°C/min(7.5 min) were comparable with radio frequency heated sample (Fig. 9a). The gel rigidityof RF-heated sample was significantly higher than sample heated at rate of 20°C/min (4minutes). We observed similar results for complex viscosity of heated egg white disper-sions (Fig. 9b). This observation indicated that in addition to temperature effect, RF heat-ing could have some complementary effect on protein molecules to enhance denaturationand gel formation in the alternating current electric field. This could be attributed to thetotal thermal input (combination of time and temperature), rather than only the heatingrate, is the determining factor in heat-induced gel formation by egg white proteins.Recently, Riemann et al.[26] advocated that rapid heating followed by brief holding resultedsimilar gel texture of meat, which were cooked by slow heating and cooled immediately.

Effect of pH. Egg white dispersions at various pH levels showed significant dif-ferences in gel formation. Egg white dispersions adjusted to lower pH and subjected to RFheating formed a stronger gel as compared to control or higher pH sample (Fig. 10). Thisobservation is contradictory to earlier work on water bath heating of egg white disper-sions.[17] The previous authors reported that alkaline gels exhibited a fine ordered networkand the maximum hardness was found at pH 11. On the other hand, Renkema[21] observedhighest G′ values at lower pH (3.2) as compared to 5.2 and 7.6, respectively. In our study,the acidification resulted stronger gel, while alkaline dispersions (pH 9 and 11) did not

Figure 8 Effect of heating rate on phase angle of 10% egg white dispersions.

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form gel during RF heating. Elastic modulus of acidified samples was stronger than con-trol pH sample (6.87). The reason is not clear. The possible reason for non-gel formationof egg white in the alkaline pH region could be ionic interference in the radio frequencyzone.

Dielectric Properties

Effect of temperature on dielectric parameters of egg white dispersion is shown inFig. 11a. The dielectric constant (ε′) was independent of frequency; however, the dielec-tric loss factor (ε″) decreased up to 1200 MHz and remained constant at higher frequen-cies. Both the dielectric constant (ε′) and dielectric loss factor (ε″) increased with anincrease in temperature. However, no significant differences (P > 0.05) were observed

Figure 9a Comparison of conventional heating rate with RF heating on elastic modulus.

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Figure 9b Effect of conventional heating rate and time over RF heating on complex viscosity of 10% egg whiteat 20°C.

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156 AHMED ET AL.

in dielectric parameters between 70 and 80°C, while the parameters became significant(P < 0.05) at 85°C. Effect of temperature at selected frequencies (200, 915, and 2450MHz) is illustrated in Fig. 11b. It further confirmed the frequency independency ofdielectric constant, whereas loss factor decreased with an increase in frequency within thetemperature range employed. Temperature did not significantly affect dielectric parameters

Figure 10 Effect of pH on clastic and viscous modulus of 10% egg white dispersions heated for 150 seconds.

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Figure 11a Effect of temperature on dielectric properties of 10% egg white dispersion.

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in the temperature range between 50 and 80°C. A sharp increase in both ε′ and ε″ valuesobserved between 85 and 90°C, which finally decreased at 92°C. These temperatureranges correspond to egg white denaturation. Similar observation was earlier reportedby Bircan et al.[14] for whey protein, and the authors reported that the temperature wherean increase or decrease in dielectric parameters observed corresponded to whey proteindenaturation temperature. Further, Bircan and Barringer[15] found that both dielectricparameters increased with temperature for muscle proteins, and a sudden increase indielectric parameters represented the muscle protein denaturation temperature.

Effect of pH on dielectric parameters of egg white dispersion is shown in Fig. 12.Change in pH increased both dielectric parameters of egg white dispersions while thehighly acidic pH (pH=3) exhibited significantly (P < 0.05) higher values. It has been wellestablished that salt concentration is an influencing factor on the dielectric properties of

Figure 11b Changes in the dielectric constants at 50–92°C of 10% egg white dispersion at 200, 915, and 2450 MHz.

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'e

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Figure 12 Effect of pH on dielectric parameters of 10% egg white dispersion at 80°C.

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foods.[29] The additional hydrogen ions affect the parameters to a large extent as comparedto excess sodium ions in the alkaline zone (pH=11) and, therefore result in a higherdielectric constant for acidified egg white dispersions. It infers that egg proteins in acidiccondition denatures faster in electric field than alkaline or neutral conditions due to morerapid dielectric heating.

Protein Denaturation

Protein denaturation during RF-heating was monitored by DSC (Fig. 13) and elec-trophoresis (Fig. 14), respectively. The protein denaturation temperature (Td) of eggwhite protein (mainly ovalbumin) was found to be 84.1°C. Some RF heating conditions(60, 90, and 120 seconds) did not denature ovalbumin (Figs. 11b–f). Effects of pH(3 and 11) on egg protein denaturation are compared in Figures 11d and 11e, respec-tively. The aggregation of protein occurred at both pH after heating for 120 seconds at110°C. The shift of protein thermogram could be the effect of RF heating. The sampleheated for 180 seconds and longer irrespective of pH denatured and no ovalbumin peakwas observed (Fig. 11f).

SDS PAGE (Fig. 14) compares between RF heating and conventional heating of eggwhite dispersions at similar conditions. It shows that RF heating time affected the eggwhite protein subunits. The ovalbumin band intensity decreased with heating time periodand major changes noticed in samples heated for 150 seconds and longer. The ovalbuminintensity was found to be higher for conventional heat treatments (column 8) as comparedto RF treatment (column 7). RF heating indicated a systematic realignment of proteinstructure with strong gel rigidity. Alzagtat[30] observed that SDS-PAGE detected molecularchanges of ovalbumin as a result of interaction with various lipids. The SDS-PAGE resultssupported protein denaturation behavior from rheological and calorimetric observation ofRF treated ovalbumin.

CONCLUSIONS

Radio frequency heating denatured egg protein dispersions and produced a strongergel. Elastic modulus of RF-treated samples increased with concentration and heatingperiod where as complex viscosity increased exponentially with sample concentration.The critical concentration and optimum time for the gelation was found to be 7.5% (w/w)and 150 seconds, respectively. Acidic dispersions produced stronger gel where as alkalinesolution of egg white did not produce a gel. Higher dielectric properties of acidifiedegg white dispersion made faster RF heating and consequently produced a stronger gelcompared to neutral and alkaline dispersions.

ACKNOWLEDGMENTS

The authors thank Muhammad Alu'datt, Department of Food Science & Agricultural Chemistry, andYvan Gariepy, Department of Bioresource Engineering, McGill University for performing the elec-trophoresis experiment and helping to set up the network analyzer, respectively. The authors wouldalso like to thank the Natural Sciences and Engineering Research Council of Canada for provingpartial funding to carryout the research.

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RADIO-FREQUENCY HEATING OF EGG WHITE DISPERSION 159

Figure 13 DSC thermograms of egg white dispersions: a. Control sample (12.5% at pH 7); b. 10%, pH 9 and 60seconds; c. 5%, pH 5 and 90 seconds; d. 7.5%, pH 11 and 120 seconds; e. 7.5%, pH 3 and 120 seconds; f. 7.5%,pH 7 and 150 seconds.

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160 AHMED ET AL.

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