egg white

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Food Research International 40 (2007) 347–355

Effects of sucrose and sodium chloride on foaming propertiesof egg white proteins

Vassilios Raikos, Lydia Campbell, Stephen R. Euston *

School of Life Sciences, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, Scotland, UK

Received 10 April 2006; accepted 2 October 2006

Abstract

Egg white proteins are extensively utilised in the food industry as foaming agents. A number of factors, singly or in combination, canaffect the foaming characteristics of egg albumen. In this study, egg white protein solutions heated at various temperatures in the presenceof variable concentrations of sucrose and NaCl were whipped for different periods of time. All factors had a significant impact on thefoaming properties of egg albumen. Increasing NaCl content and whipping time enhanced protein adsorption at the air–water interface.The presence of sucrose delayed foam formation but contributed to the stability of the aerated system. Controlled denaturation of theprotein solutions induced by mild heat treatment enhanced the foaming properties of egg white proteins. This data indicates that thefoaming properties of egg white proteins can be manipulated by altering the effect of extrinsic factors in order to achieve optimal for-mulations for food industrial applications.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Egg white proteins; Air–water interface; Foam stability; Denaturation; Interfacial film; Overrun; Sucrose; NaCl

1. Introduction

Liquid foams are two phase systems which consist of adiscontinuous air phase dispersed in a continuous liquidlamellar phase. Globular proteins such as egg white pro-teins are extensively used in aerated systems to enhancedesirable characteristics and food applications include mer-ingues, nougat, bavarois, whipped cream and chocolatemousse among others. Protein foams are characterised bytwo factors, namely foaming power and foam stability(Kato, Takahashi, Matsudomi, & Kobayashi, 1983).Foaming power or foamability is related to the level ofair phase volume upon the introduction of a gas into theprotein solution and is determined by measuring theincrease in foam volume. Foam stability is determined bymeasuring the rate of liquid drainage from foam or the rateof decrease in foam volume with time. Foam stability is

0963-9969/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.foodres.2006.10.008

* Corresponding author. Tel.: +44 131 451 3640; fax: +44 131 451 3009.E-mail address: [email protected] (S.R. Euston).

important for the shelf-life and product appearance of foodfoams and must be maintained when subjected to a varietyof processes such as heating, mixing and cutting (Foege-ding, Luck, & Davis, 2006).

The physicochemical properties of individual proteinsdetermine to a large extent the characteristics of foam-con-taining food products. However, real food systems veryoften contain a mixture of proteins. The properties of eggwhite foams depend on the individual chemical properties(molecular weight, pI, glycosylation, phosphorylation, sulf-hydryl/disulfide content) of a wide range of proteins whichare allowed to interact during foam formation (Li-Chan &Nakai, 1989). Furthermore, the formulations of food prod-ucts may include other ingredients such as sugars, salt,small-molecule surfactants. The addition of such moleculesmay affect the functionality of egg white proteins, whichmay result in gain or loss of handing properties. In addi-tion, the foaming properties of proteins depend on extrinsicfactors such as heating conditions (temperature/time),equipment for foam formation and methods of foam pro-duction (e.g. whipping time).

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348 V. Raikos et al. / Food Research International 40 (2007) 347–355

In previous work (Campbell, Raikos, & Euston, 2003,2005; Raikos, Campbell, & Euston, 2007) we have foundthat combinations of sucrose and NaCl protect egg pro-teins against heat-induced aggregation and gelation. Thisallows egg white protein to be pasteurised for shorter timesand higher temperatures than the normal 58 �C for 1 min.This is advantageous for the egg producer. However, itmust be ensured that the product is at least as effective interms of its functional properties compared to the normallypasteurised product. In this study, we set out to investigatewhether processing regimes designed to speed up the pas-teurisation process for egg have any adverse effect on thefoaming properties of egg. The combined effect of sucroseand NaCl and varied heating conditions on the foamingproperties of egg albumin was studied. The foam formingability of pre-heated egg white proteins, as affected by thepresence of sucrose and NaCl, at the air–water interfacewas determined by overrun measurements. The impact ofheat-processing conditions and sucrose and NaCl contenton foam stability was also investigated.

2. Materials and methods

2.1. Materials

Fresh eggs were purchased from Safeway’s supermarket,UK. Eggs were broken manually and the yolk was sepa-rated from the albumen. Sodium chloride and sucrose wereobtained from Sigma Aldrich Co.

2.2. Preparation of protein samples

Protein samples included the control (egg suspensionswith no added sucrose or NaCl) and egg solutions contain-ing different sucrose and/or NaCl concentrations (sucrose/NaCl = 12/0, 0/12, 6/6, w/w). The specific sucrose andNaCl concentrations were chosen to investigate the syner-gistic effect of both ingredients on egg protein functionalityas well as to monitor any changes occurring when one ofthem (e.g. sucrose) is replaced with an equal amount ofthe other (e.g. NaCl).

2.3. Heat treatment of egg albumin

Suspensions of egg white were subjected to heat treat-ment (whilst stirred) in a CM4 mashing water bath (Can-ongate Technology Ltd.). The control samples wereheated at 58 �C for 2 min (normal egg pasteurisation con-ditions), whereas the samples containing sucrose and NaClwere heated at different temperatures. The heating temper-atures were varied to investigate the effect of heat-induceddenaturation of egg albumen on protein functionality. Eggwhite proteins are relatively heat labile, hence the low pas-teurisation temperatures used. Previous experience hasshown us that in the presence of sucrose and NaCl the pas-teurisation temperature can only be increased by a few �Cabove the normal pasteurisation temperature (58 �C)

although this was sufficient to give a significant reductionin heating time. Thus, we have limited our experimentalheating temperatures to the range 60–64 �C. Egg suspen-sions were placed in the water bath during the whole warm-ing up period to ensure they were treated at the requiredtemperature for the given period of time. The samples wereplaced on ice immediately after heating to prevent any fur-ther aggregation of the egg proteins.

2.4. Foaming properties

Foaming properties of egg albumen were determinedaccording to the method of Phillips, Haque, and Kinsella(1987). Foams were formed by whipping the pre-heatedprotein solutions in a household type mixer at ambienttemperature. Overrun and stability measurements weremade at certain intervals for a total of 15 min whippingusing 75 ml dispersions of egg white protein. The whippingtimes (10 min, 13 min and 15 min) were chosen to ensurethat all protein dispersions had adequate whipping timeto be incorporated into foam. The protein dispersions werewhipped in a double beater Breville SHM1 380watt mixer(COMET, UK). The mixer was calibrated by measuringthe beater rotational speed with a stroboscope. The rota-tional speed of the mixer bowl was set at the ‘‘high’’ settingand the rotational speed of the beaters was set 5 (maxi-mum). The rotational speed of the beaters was estimatedto correspond to 18 flashes/s of the stroboscope and thus,the rotational speed was calculated to be 1080 rpm.

2.4.1. Foaming ability

The pre-heated protein dispersions (75 ml) were pouredinto the bowl (2 l) and whipped for 15 min at ambient tem-perature. During foam formation, the mixer was stoppedmomentarily after 10 min and 13 min of whipping andthe mixer head was carefully lifted to minimise destructionof the foam structure. Samples of foam were gentlyscooped out with a plastic spatula and used to quickly filla pre-weighed weighing boat (100 ml) using small scoopsand avoiding the formation of entrapped air pockets. Theexcess foam was removed from the weighing boat using ametal spatula to level the top of the foam even with thetop of the weighing boat to obtain a constant volume ofsample for each measurement. The weight of the boat withthe foam was recorded and all foam was carefully returnedto the bowl to resume whipping. This phase of the proce-dure was limited to 2 min. All measurements were repli-cated three times. The % overrun was calculated by thefollowing equation:

Overrun ¼ ½ðW dÞ � ðW fÞ=ðW fÞ� � 100 ð1:1Þwhere Wd is the weight (g) of the unwhipped proteindispersion; Wf is the weight (g) of the whipped proteinfoam.

Fig. 1 is a plot of overrun vs time for three different tem-peratures at a sugar/salt concentration of 0/12. This showsthat the maximum in overrun occurs at 13 min.

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Whipping time (min)

% O

verr

un

Fig. 1. Overun vs whipping time for egg white samples with added sugar/salt in a 0/12 (w/w) ratio at three temperatures. r, 60 �C; j, 63 �C; m,64 �C.

V. Raikos et al. / Food Research International 40 (2007) 347–355 349

2.4.2. Foam stability

Foam stability was measured by monitoring drainage atambient temperature. To facilitate continuous measure-ment of drainage from the foams, the stainless steel bowlwas modified by drilling a 0.6 cm hole in the bottom ofthe bowl 5.0 cm from the centre. The hole was sealed dur-ing the whipping process by placing a tape over the hole onthe outside of the bowl. The pre-heated protein dispersion(75 ml) was poured into the bowl. The bowl, beaters andprotein dispersion were weighed and whipping started atthe calibrated setting for a predetermined time e.g.10 min, 13 min or 15 min. A timer was started immediatelyafter the whipping process was completed to estimate thedrainage period. The bowl, beaters and protein foam werequickly weighed to quantify the moisture loss during whip-ping and obtain an accurate weight of liquid in the foam.The tape was removed, the hole was cleared with a glassrod and the bowl was placed at a 30� angle above a pre-weighed weighing boat (to ensure that the hole was at thelowest point), standing on the balance pan of a Sartorius1207 MP2 electronic scale. The liquid was collected in theweighing boat on the balance pan and the increase inweight was recorded over time. Foam stability was deter-mined by measuring the time required to attain 50% drain-age (Halling, 1981). All measurements were replicated threetimes.

2.5. Confocal laser scanning microscopy

Microscopic images of the egg albumen foams wereobtained using a Leica DM-IRE2 confocal laser scan-ning microscope (Leica Microsystems, Heidelberg,Germany) equipped with an Ar/HeNe laser and 10 Xobjective lens (NPLAN 10 · 0.25 DRY). The fluorescencedye was excited at 50% of maximum adsorption at543 nm and the detection bandwidth was set from488 nm to 543 nm. Images were recorded at a resolutionof 512 · 512 pixels and analysed by the manufacturer’ssoftware (Leica Software Development Kit, DM SDK,version 4.2.1). The labelling dye was Fluorescein (iso-thiocyanate isomer I) and it was purchased from SigmaAldrich Co. The egg white samples heated at 64 �C(2 min) and whipped for 10 min and 13 min were randomly

chosen for microscopic observation. The foam structureof the control (plain egg white, heated at 58 �C, 2 min)was also observed for the same whipping times. Tracesof the dye were added to the pre-heated protein sam-ples prior to whipping in the Breville SHM1 mixer toensure an even dispersion throughout the foam struc-tures. Foam samples were placed on a circular plasticslide (observation area 30 mm) which is designed to fit inthe base plate (81 · 55.5 · 5.5 mm) of a POC ChamberSystem (aluminium, black anodised) to avoid any destruc-tion of the foam structure. Time dependent images weretaken at 30 min intervals for an experimental period of60 min.

2.6. Quantitative determination of sulfhydryl (SH) groups

Suspensions of egg white were subjected to heat treat-ment in a CM4 mashing water bath (Canongate Technol-ogy ltd.) The egg samples were heated in the presence ofglucose and/or sodium chloride. The concentrations ofNaCl and glucose were varied. Egg suspensions wereplaced in the water bath during the whole warming upperiod to ensure they were treated at the required temper-ature for the given period of time. Egg white samples wereheated at 58 �C, 60 �C, 62 �C and 64 �C for 2 min. Control(non-heated—25 �C) samples were also prepared for eggwhite. The samples were placed on ice immediately afterheating to prevent any further aggregation of the eggproteins.

The total free SH concentration of the egg samples wasdetermined using the standard colorimetric‘‘Ellman’s test’’(Ellman, 1959). The aim was to quantify the free –SHgroups as affected by heat treatment and thus, the sampleswere not exposed to denaturants. Fifty microliter of DTNBreagent (50 mM sodium acetate, 2 mM DTNB in dH2O)were mixed with 100 ll Tris solution (1 M Tris, pH 8)and 500 ll of egg sample and the volume was made up to1 ml with distilled H2O (350 ll). The solution was mixedcarefully and was allowed to stand at room temperature(25 �C) for 5 min. The sample solution was introduced intoa 1 ml cuvette and absorbance was recorded at 412 nmusing a UV–VIS spectrophotometer (Novaspec II, Amer-sham Pharmacia Biotech.). Measurements were carriedout in triplicate. A blank sample containing distilled H2Oinstead of egg sample was used to record backgroundreadings each time. The concentration of the SH group(lM) is calculated from A412 readings using Beer’s law(Eq. (1.2)):

CðlMÞSH ¼ DA412

e412

� �v—lL

vs—lL

� �U ð1:2Þ

where DA412 is the absorbance change corrected for the re-agent blank; v—lL is the total volume sample in the mea-surement cuvette; vs—lL is the original volume of the foodsample; U is the fraction of the assay volume poured intothe cuvette (U = 1); e412 is the extinction coefficient of thereagent (13,600 M�1 cm�1).


3. Results and discussion

Figs. 2–4 present results for the effect of NaCl on foam-ing ability. The addition of NaCl enhances foaming abilityand the samples exhibit significantly higher overrun com-pared to the control and the samples containing sucrose.Conversely, the % overrun decreases with increasingsucrose concentration. This is possibly attributed to an

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(%)

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Sugar/Salt = 0/12

Sugar/Salt = 6/6

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Fig. 2. Effect of sugar and salt content and whipping time on % overrun ofegg white aerated systems heated at 60 �C (2 min). Data are the mean ofthree replicates.

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increase in the medium viscosity, which allows less air tobe incorporated. The foaming ability is enhanced as whip-ping time increases, apart from the sample with the highestNaCl concentration. In this case, the maximum overrun isobtained after 13 min of whipping and ‘overbeating’ occursafter this stage. The same phenomenon is observed for thesample containing sucrose/NaCl in a 6/6 (w/w) ratio whenheated at 64 �C (Fig. 4). This improved functionality maybe attributed to the input of higher amounts of mechanicalenergy with increasing whipping time, which in turnenables more protein to adsorb and aggregate on the inter-facial lamellae resulting in decreased surface tension. Insome cases, a maximum in the overrun is observed afterseveral minutes of whipping and further input of mechan-ical energy results in overrun decline. For instance, thesamples containing sucrose/NaCl in a 0/12 weight ratioreach a maximum overrun after 13 min of whipping andlonger whipping times lead to lower overrun values. Thesame phenomenon (maximum overrun after 10 min ofwhipping) is observed for the sample heated at 64 �C(2 min) in the presence of sucrose and NaCl in a 6/6 weightratio. The mechanism responsible for the overrun declineafter a specified time of whipping, also known as ‘‘over-beating’’, has been known for a long time (Halling, 1981;Kinsella, 1981). According to the theory, ‘‘overbeating’’due to prolonged whipping periods leads to excessive coag-ulation of globular proteins at the air–water interface,which is usually associated with the formation of insolubleaggregates that exhibit little water-holding ability. Theimpaired water-holding ability of proteins at the interfaceis in turn responsible for the foam collapse, which isreflected by the overrun decline.

To explain the effects of the cosolutes on foaming, wecan speculate that NaCl counter ions may screen thecharged protein molecules, reduce the electrostatic repul-sion between adsorbed and non-adsorbed protein mole-cules and thus facilitate adsorption at the air–waterinterface. Davis, Foegeding, and Hansen (2004) reportedsimilar findings with respect to the effect of NaCl on theadsorption of whey protein isolate at the air–water inter-face. On the contrary, sucrose delays foam formation anddecreases the foaming power of egg white samples, espe-cially in the first part of the beating period (Lomakina &Mıkova, 2006). It has been documented that the presenceof sugar can affect the thermodynamic and functionalproperties of food proteins, especially the adsorption andaggregation behaviour (Dickinson & Matia-Merino,2002). With increasing sucrose concentration the amountof air incorporated decreases and the overrun values ofthe samples containing sucrose/NaCl in a 12/0 weight ratioare lower compared to the control. According to Lau andDickinson (2005), the addition of sugar results in increasedcontinuous phase viscosity, which in turn is disadvanta-geous for air incorporation and the rapid diffusion andunfolding of the protein in the vicinity of the interface.Moreover, Antipova, Semenova, and Belyakova (1999) sta-ted that the adsorption of ovalbumin decreases in the pres-

00.000020.000040.000060.000080.0001

0.000120.000140.000160.000180.0002

25 58 60 62 64

Temperature (˚C)

-SH

co

nc.

(μM

) 10% glucose

7.5% glucose + 2.5%salt

5% glucose + 5% salt

Fig. 5. Effect of temperature and sugar and salt content on theconcentration of free SH groups of egg white. Data are the mean ofthree replicates.


ence of sucrose possibly because ovalbumin forms hydro-gen bonds with the sugar molecules, which results inincreased hydrophilicity and decreased surface activity.Thereby the ovalbumin molecules which participate inhydrogen bond formation with sucrose preferentiallyremain in bulk rather than adsorb to the interface. Weshould note that egg white is not a pure ovalbumin suspen-sion; rather it is a complex mixture of many proteins, themost abundant of which is ovalbumin. The functionalproperties of most of these proteins have not been studiedin the presence of sugar and salt. The effects of sugars onprotein functional properties is, however, a general prop-erty which we would expect to apply to all of the compo-nent proteins in the egg white, and thus a comparisonwith ovalbumin is pertinent.

The effect of heat treatment in the presence of sucroseand NaCl was, in most of the cases, beneficial for the foam-ing capacity of the egg white samples. This effect is moreevident for the samples that contain high sucrose content.The % overrun of the samples containing sucrose/NaClin a 12/0 weight ratio clearly improves when heated at64 �C. The negative effect imposed by the high sucrose con-centration is less severe when the samples are subjected toheat treatment at elevated temperatures. Hagolle, Launay,and Relkin (1998) stated that preheating treatmentincreased the ability of ovalbumin to adsorb to the air–water interface and thereby decrease the surface tension,provided that the formation of large aggregates was pre-vented. This enhanced foaming ability induced by heattreatment was attributed to higher surface hydrophobicityand increased chain flexibility (Kato, Komatsu, Fujimoto,& Kobayashi, 1985). Furthermore, Relkin, Hagolle, Dal-gleish, and Launay (1999) suggested that mild heat treat-ments of ovalbumin solutions may result in molecularspecies with partially-denatured structures which demon-strate enhanced foaming properties. The extent of denatur-ation of the native structures of protein molecules undercontrolled heating conditions could explain the enhancedadsorption capacity of egg white proteins at 64 �C. It hasbeen established that globular proteins, under certain con-ditions, may exist in stable conformations which are inter-mediate between the native state and the highly disorderedstate (Kuwajima, Hiraoka, Ikeguchi, & Sugai, 1985). Thisconfigurational state, also known as the ‘‘molten globule’’state, exhibits a native-like secondary structure and a disor-dered tertiary structure (Ptitsyn, 1992). Globular proteinswhen adsorbed at the air–water interface unfold andundergo various structural rearrangements in order todecrease the surface tension. Thus, it can be speculated thatthe configurational state of adsorbed protein moleculesresembles the ‘‘molten globule’’ state. Nevertheless, theavailable time for the compact globular proteins to unfoldproperly at the interface is rather limited because they arerapidly surrounded by other protein molecules, producinga close-packed structure in which there is little scope forfurther configurational adjustments (Matsumura, Mitsui,Dickinson, & Mori, 1994). Therefore, one may speculate

that the ability of proteins to unfold rapidly at the interfaceand obtain a molecular conformation which is thermody-namically favourable can be enhanced if they exist in the‘‘molten globule’’ state prior to adsorption. The lattermay be achieved by various means including heating andit has been stated that the optimum conditions for convert-ing ovalbumin in the monomeric ‘‘molten globule’’ state isby heating at neutral pH (Matsumura et al., 1994).Although the results obtained on single-protein systemsare not easily extrapolable to complex systems (Lechevalieret al., 2005), we could hypothesise that the enhancedadsorption capacity of the egg white proteins followingheat treatment observed in this study may be attributedto the conversion of the protein structures from the nativestate into the ‘‘molten globule’’ state. Additional evidencefor this hypothesis may come from the fact that, as shownin Fig. 5, the concentration of free sulfhydryl groupsincreases when egg white samples containing various con-centrations of glucose and NaCl are heated at temperaturesabove 60 �C (2 min). The concentration of SH groups isnot affected to any great extent by the heat treatment whenthe samples are thermally processed at temperatures ashigh as 60 �C (Fig. 5). Nevertheless, at 62 �C and abovethe reactive SH group concentration increases rapidly. At62 �C, the denaturation of egg white proteins is initiatedin the presence of sugar and salt, which results in the expo-sure of SH groups that remained hidden in the interior coreof the protein molecule. Again, the samples containingNaCl appear to be more resistant to heat denaturationcompared to the samples heated in the presence of sugar.The increased concentration of free –SH groups followingheat treatment indicates protein conformational changesassociated with transformation from the native state tothe denatured state. –SH groups which were previouslyburied in the interior of the protein molecules are nowexposed and free to interact with other neighbouring mol-ecules through free thiol/disulfide interchange reactions.According to Doi, Kitabatake, Hatta, and Koseki (1989)the significance of disulfide bond formation between dena-tured ovalbumin molecules at the air–water interface withrespect to the foaming properties is limited. However, the


increased hydrophobicity and surface activity as indicatedby the increase in the –SH group concentration followingheat treatment at temperatures above 60 �C may be associ-ated with molecular conformations that enable the ease ofprotein unfolding at the interface, thereby exhibitingenhanced adsorption capacity and efficiently decreasingthe surface tension.

Figs. 6 and 7 illustrate the effect of sucrose and NaClcontent, whipping time and heating temperature on the sta-bility of the aerated systems. When the samples are sub-jected to whipping for 10 min, the sample containingsucrose/NaCl in a 0/12 weight ratio demonstrates clearlythe highest stability compared to all the other samples.Although there is no evidence of direct correlation betweenfoaming ability and stability, it may be hypothesised thatthe remarkable difference in foam stability exhibitedbetween the sample with high NaCl content and the restof the samples derives from their relative ability to rapidlyadsorb at the air–water interface. As described above thesamples containing high NaCl concentration are able torapidly adsorb at the air–water interface and lower the sur-face tension even when the whipping period is limited to10 min. Conversely, the samples containing sucrose andthe control exhibit a lag phase with respect to protein dif-fusion and adsorption, which possibly means that after aperiod of whipping time of 10 min, the protein concentra-tion at the interface is rather limited leading to low levels

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Fig. 6. Effect of sugar and salt content and whipping time on half-life ofegg white aerated systems heated at 60 �C (2 min). Data are the mean ofthree replicates.

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Fig. 7. Effect of sugar and salt content and whipping time on half-life ofegg white aerated systems heated at 64 �C (2 min). Data are the mean ofthree replicates.

of stability compared to the samples with high NaCl con-tent. This difference in the foam stability between the sam-ple containing high NaCl concentration and the rest of thesamples minimises under experimental conditions (e.g.increasing heating temperature) which enhance proteinadsorption and result in higher overrun values. Further-more, when the samples are subjected to whipping forlonger periods of time (13 min and 15 min), the situationwith respect to foam stability changes. The samples con-taining high sucrose content are far more stable comparedto all other samples. This increased foam stability demon-strated by the samples with high levels of sucrose may beattributed to the effect of sugars on the viscosity of the bulkphase. Sugars contribute to foam stability by increasing theviscosity of lamella water and thereby retarding drainage(Lau & Dickinson, 2005).

Increasing the heating temperature of the egg white dis-persions increases the stability of the aerated systems. Theonly exception was the sample containing high NaCl levelswhen whipped for 10 min. It has been stated that mild heattreatments of protein solutions, under certain conditions(pH, ionic strength) are correlated with enhanced foamingproperties. This may be attributed to the unfolding of pro-tein structure, which may lead to the formation of solubleaggregates through inter-molecular linking. According toVardhanabhuti and Foegeding (1999), whey protein poly-mers resulting from mild heat treatments under controlledconditions have higher intrinsic viscosity than the nativeglobular protein. Furthermore, Davis and Foegeding(2004) stated that heat-induced whey protein polymers insolution decreased the air phase volume but made thefoams more stable compared to native whey protein sus-pensions. The increase in stability was directly associatedwith an increase in bulk viscosity, thereby retarding therate of drainage. These results are in agreement with thefindings in the present study. As suggested by the determi-nation of the free –SH concentration of the protein mole-cules, heating at 64 �C (2 min) in the presence of sugarand NaCl induces the unfolding to some extent of the pro-tein structure. These denatured protein molecules mayinteract inter-molecularly to form soluble protein–proteinaggregates, which remain in bulk increasing the solutionviscosity and contributing to the stability of the foammicrostructure.

Finally, longer whipping times have a negative impacton foam stability for the samples containing NaCl whereasthe samples containing sucrose and the control appear tobe fairly unaffected. It has been stated that prolonged whip-ping times are associated with liquid film thinning, mechan-ical deformation and bubble-wall rupture (Gassmann,Kroll, & Cifuentes, 1987). All these mechanisms, singlyor in combination, may contribute to foam collapse andcause instability.

The microstructures of the newly-formed foams asrevealed by confocal microscopy (Figs. 8–11) complementthe overrun and foam stability measurements and providefurther experimental evidence on the effect of sucrose and

Fig. 8. Confocal microscopy of aerated egg white systems containing sugar/salt in a 12/0 ratio (w/w) during heating at 64 �C (2 min) and whipped for10 min and 13 min. Evolution of microstructure was monitored by scanning at 30 min intervals. The scale bar is 300 lm.



NaCl on the adsorption capacity of egg white proteins. Theevolution of the microstructure of control samples (plainegg white, heated at 58 �C, 2 min) and samples heated for2 min at 64 �C and whipped for 10 min and 13 min areillustrated in Figs. 8–11. The sample containing sucrose/NaCl in a 12/0 ratio (w/w) and the control exhibit reducedfoaming capacity, especially when the whipping time waslimited to 10 min. This conclusion can be drawn from thenumber and size of air bubbles immediately after foam for-mation (0 min). When the whipping time increases to

13 min, the adsorption of egg white proteins at the air–water interface improves, resulting in enhanced foamingpower. These findings are consistent with % overruns ofthe specific samples under the given conditions (Fig. 4).On the other hand, the microstructures of the aerated sam-ples containing NaCl during the heating process suggestenhanced adsorption at the air–water interface after awhipping period of 10 min. This is revealed by the densityand the size of the air bubbles of the microscopic picturestaken immediately after foam formation (Figs. 9 and 10).


Fig. 11. Confocal microscopy of aerated plain egg white (control) systems heated at 58 �C (2 min) and whipped for 10 min and 13 min. Evolution ofmicrostructure was monitored by scanning at 30 min intervals. The scale bar is 300 lm.


With respect to foam stability, the microscopic observationof the aerated systems indicates that the samples containingsucrose show a lower rate of increase of the bubble sizeduring the standing period of 60 min compared to the sam-ples containing only NaCl and the control.

4. Conclusions

All the factors involved in this study (amount of sucroseand NaCl present during the heating process, heating tem-

perature, whipping time) have a significant impact on thefoaming characteristics of egg white protein dispersions.Increasing NaCl concentration, heating temperature andwhipping time enhances foam formation, whereas increas-ing the amount of sucrose confers foam stability for pro-longed periods of whipping. The degree of heat-induceddenaturation of protein molecules with respect to theirmolecular configuration in bulk may be crucial for proteinadsorption and rearrangement at the air–water interface.The increase in the viscosity of the medium due to the pres-


ence of sucrose and the controlled formation of proteinpolymers induced by mild heat treatments, are correlatedwith enhanced stability. Studies on the heat-inducedchanges of the secondary structure of egg white pro-teins would be beneficial to elucidate the structure–functionrelationship regarding the foaming properties of egg albu-men. Further investigations on the interfacial film compo-sition are required to verify the findings of the presentstudy.

Acknowledgement

The authors would like to thank Dr. Hershell Ball, MI-CHAEL FOODS INC, Minnesota, USA for fundingthis work and his permission to publish is gratefullyacknowledged.

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