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diffusion at the air/water interface and its adsorption to the same,especially due to an increase in exposed hydrophobicity andmolecular unfolding, improving the protein ability to form andstabilize foams (Medrano, Abirached, Panizzolo, Moyna, & An,2009; Schmitt et al., 2005). In this sense, the study of glycosyla-tion via the MR (glycation) ofb-Lg as a tool to improve its foamingand stabilizing capacity, particularly at pH values close to its pI,could be of interest.

Several authors have described a direct relationship between thefoam formation and stability and the interfacial properties of adsor-bed protein lms (Martin, Grolle, Bos, Cohen-Stuart, & van Vliet,2002;Murray, 2002;Rodrguez Patino et al., 2008;Rouimi, Schorsch,Valentini, & Vaslin, 2005). Among them, the dynamic of adsorptionandthe rheological propertiesof interfaciallmshavebeenshowntoinuence foam properties, depending on the mechanisms causingfoam destabilization (Baeza, Carrera, Rodrguez Patino, & Pilosof,2005; Martnez, Carrera, Rodrguez Patino, & Pilosof, 2009;Rodrguez Patino et al., 2008). To the best of our knowledge, studiesin the literature about the impact ofb-Lg glycation on the interfacialproperties and, consequently, on the foaming properties of thisprotein are very scarce. Schmitt et al. (2005) in b-Lg:acacia gumconjugated by Maillard reaction at pH 4.2, 5.3 and 7.0 observed

a higher capacityto formand stabilizefoams of glycoconjugates thanunglycatedb-Lg,especiallyatpH5.3.Theseauthorsneeded14daysat60 C to obtain the maximum level (15%) of NH2loss. Because of thereaction with polysaccharides needs strong conditions and longincubation periods, which would be more expensive from theindustrial standpoint, the use of monosaccharides such as galactose,might be of interest, since it allows obtainingmodied proteins witha high yield under milder reaction conditions (Corzo-Martinez,Moreno, Olano, & Villamiel, 2008).

Thus, the aim of this work was i) to study the effect of glycationwith galactose on the adsorption ofb-Lg at the air/water interfaceand to characterize the rheological properties of the interfaciallms; and ii) to evaluate foaming properties (foamability and foamstability) of b-Lg glycoconjugates in relation to their interfacial

behaviour.

2. Materials and methods

2.1. Materials

Galactose (Gal) and bovineb-lactoglobulin (b-Lg) (mixture of Aand B variants) were purchased from SigmaeAldrich (St. Louis, MO,USA). All other reagents were of analytical grade.

2.2. Preparation and purication ofb-lactoglobulinegalactose

conjugates

Gal and b-Lg in a weight ratio of 1:1 were dissolved in 0.1 M

sodium phosphate buffer, pH 7 (Merck, Darmstadt, Germany), andlyophilized. The b-LgeGal powders were kept at 40 and 50 Cfor24and 48 h, respectively (Corzo-Martinez et al., 2008), undera vacuum in a desiccator equilibrated at an awof 0.44, achievedwith a saturated K2CO3 solution (Merck). In addition, controlexperiments were performed with b-Lg stored at 40 and 50 Cwithout galactose during the same periods (control heated b-Lg).

After incubation, the products were reconstituted in distilledwater to a protein concentration of 1 mg/mL. To remove freecarbohydrate, 2 mL portions were ultraltered through hydrophilic3 kDa cut-off membranes (Centricon YM-3, Millipore Corp., Bedford,MA) by centrifugation at 1548 gfor 2 h. After removal of free Gal,samples were lyophilized and stored at20 C for further analysis.

Incubations were performed in duplicate, and all analytical

determinations were done at least in duplicate.

2.3. Solubility ofb-lactoglobulin conjugates

For solubility evaluation, solutions of native, control and gly-cated b-Lg in distilled water (1 mg/mL) were adjusted to pH 5 and 7using HCl or NaOH 1 N. After 30 min of stirring at room tempera-ture, the samples were centrifuged for 15 min at 4 C and 15000g.The protein content in the supernatants was determined bymeasuring the absorbance at 280 nm (A280) in a Beckman DU 70spectrophotometer (Beckman Instruments Inc., Fullerton, CA) andthe solubility was expressed as the percentage of the total proteincontent, considering as 100% the A280of native b-Lg.

2.4. Interfacial properties measurement

Interfacial properties (dynamic of surface pressure and surfacedilatational properties) of native, control heated and glycated b-Lgwere determined at pH 7 and 5. For this, samples were dissolved inTrizma-HCl buffer (0.05 M, pH 7.0) or acetic acid/acetate buffer(0.05 M, pH 5) (SigmaeAldrich, St. Louis, MO), the nal proteinconcentration being 5 mg/mL.

Time-dependent surface pressure and surface dilatationalmeasurements of native, control heated and glycatedb-Lg adsorbed

lms at the air/water interface were performed with an automaticpendant drop tensiometer (TRACKER, IT Concept, Longessaigne,France) as previously described (Rodrguez Nio & RodrguezPatino, 2002; Rodrguez Patino, Rodrguez Nio, & Carrera, 1999).The method involved a periodic automated-controlled, sinusoidalinterfacial compression and expansion performed by decreasingand increasing the drop volumeat a given desired amplitude(DA/A)and angular frequency (u), and the response of the surface pressure(p, mN m1) is monitored throughout the experiment, being:

p s0 s (1)

wheres0 is the surface tension of aqueous solution, in the absenceof protein (s0 72.5 mN m1), and s (mN m1) is the surfacetension in the presence of protein.

Since rate of increase ofp is initially controlled by the proteindiffusion from the bulk phase to the interface, in this work,dynamicof protein adsorption was evaluated considering the rst stage ofthe protein diffusion, by determinating the apparent diffusionconstant (Kdif). This was calculated as the slope of the line betweenthe origin (point 0.0) andrst point on the plot p vs. square root oftime (q).

Regarding surface rheological parameters, the surface dilata-tional modulus (E) derived from the change in interfacial tension(dilatational stress), s (Eq.(2)), resulting from a small change insurface area (dilatational strain),A (Eq. (3)), may be described byEq.(4)(Lucassen & van den Tempel, 1972):

s s0sinu$q f (2)

A A0sinu$q (3)

E dsdA=A

dpdlnA

jEjeif EdiEv (4)

wheres0 andA0 arethe stressand strainamplitudes,respectively, q isthe time,fis the phase angle between stress and strain, andjEj, theabsolute modulus, a measure of the total unit material dilatationalresistance to deformation (elasticviscous), is the ratio (s0/A0).

Surface dilatational modulus (E) is a complex quantity and it iscomposed of real and imaginary parts. The real part of the dilata-tional modulus (or storage component) is the dilatational elasticity,Ed jEj$cosf. The imaginary part of the dilatational modulus (or

loss component) is the surface dilatational viscosity, Ev jEj$

sinf.

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The phase angle (f) between stress and strain is a measure of therelative lm elasticity. For a perfectly elastic material stress andstrain are in phase (f 0) and the imaginary term is zero. In thecase of a perfectly viscous material,f 90 and the real part is zero.

Interfacial experiments were carried out at 20 0.3 C. Thetemperature was maintained constant by circulating water froma thermostat. Sample solutions were placed in the syringe andsubsequently in a compartment, and they were allowed to stand for30 min to reach the desired constant temperature. Then a drop wasdelivered and allowed to stand for 10,800 s to achieve proteinadsorption at the air/water interface. Surface rheological parameters(E, Ed, Ev andf) were measured as a function of adsorption time (q),at10% of deformation amplitude (DA/A) and at 0.1 Hz of angularfrequency (u). Sinusoidal oscillation for surface dilatationalmeasurementwasmadewith veoscillationcyclesfollowedbyatimeof50 cycles without any oscillation upto thetimerequiredto completeadsorption. Measurements were made at least twice. The averagestandard accuracy of the surface pressure was roughly 0.1 mN m1.The reproducibility of the results was better than 0.5% and 5.0% forsurface pressure and surface dilatational properties, respectively.

2.5. Foaming properties

The foaming properties of native, control heated and glycated b-Lg solutions were characterized through their foam formation andstability measured in a commercial instrument (Foamscan ITConcept, Longessaigne, France),based on the ideas by Popineau andco-authors (Guillerme, Loisel, Bertrand, & Popineau, 1993; Loisel,Guguen, & Popineau, 1993). With this instrument the foamformation, the foam stability and the drainage of liquid from thefoam can be determined by conductimetric and optical measure-ments. The foam is generated by blowing gas (nitrogen) at a owof45 mL/min through a porous glass lter (pore diameter 0.2 mm) atthe bottom of a glass tube where 20 mL of sample solution underinvestigation is placed. The foam volume is determined by use ofa CCD camera. The drainage of water from the foam is followed via

conductivity measurements at different heights of the foamcolumn. A pair of electrodes at the bottom of the column was usedfor measuring the quantity of liquid that was not in the foam, whilethe volume of liquid in the foam was measured by conductimetry inthree pairs of electrodes located along the glass column. In allexperiments, the foam was allowed to reach a volume of 120 mL.The bubbling was then stopped and the evolution of the foam wasanalyzed. Foaming properties were measured at 20 C from proteinaqueous solutions (5 mg/mL) at pH 5 and 7 and at an ionic strengthof 0.05 M.

Four parameters were determined as a measure of foamingcapacity. The overall foaming capacity (OFC, mL/s) was determinedfrom the slope of foam volume curve till the end of the bubbling.The foam capacity (FC), a measure of gas retention in the foam, was

determined by Eq. (5). The foam maximum density (MD),a measure of the liquid retention in the foam, was determined byEq.(6). The relative foam conductivity (Cf, %) is a measure of thefoam density and was determined by Eq. (7).

FC Vfoamf

Vgasf (5)

MD

hVliqi Vliqf

iVfoamf

(6)

Cf

Cfoamf

Cliqf

! (7)

where Vfoam (f) is the nal foam volume, Vgas (f) is the nal gasvolume injected, Vliq(i) and Vliq(f) are the initial and nal liquidvolumes, and Cfoam (f) and Cliq (f) are the nal foam and liquidconductivity values, respectively.

The static foam stability was determined from the volume ofliquid drained from the foam over time (Rodrguez Patino, Naranjo,& Linares,1995; Rodrguez Patino, Rodrguez Nio, & lvarez,1997).For this, it was calculated the half-life time (q1/2), referring to thetime needed to drain the half the volume of liquid of foam.

2.6. Statistical analysis

Statistical analysis was performed using the StatgraphicCENTURION XV Program (Statistical Graphics Corporation, Rock-ville, MD, USA) for Windows. One-way analysis of variance(ANOVA) (least signicant difference, LSD, test) was used for thestatistical evaluation of results derived from interfacial and foamingdeterminations of the glycated and unglycated b-Lg. Differenceswere considered signicant whenP< 0.05.

3. Results and discussion

On the basis of a previous paper of our research group (Corzo-Martinez et al., 2008), two types of glycoconjugates wereprepared at different stages of the Maillard reaction, one of them, inearly stages of the MR (b-Lg:Gal [24 h, 40 C]), consisted primarilyof complexes with a high glycation degree and a low aggregationlevel, while the glycoconjugate obtained after incubation undermore severe conditions (b-Lg:Gal [48 h, 50 C]), in the advancedstages of the MR, exhibited, in addition of a high glycation degree,an elevated content of protein aggregates.

In that paper, the progress of the Maillard reaction was evalu-ated by different methods. Thus, MALDI-TOF-MS analyses revealedthat an average number of 14 and 22 molecules of Gal were cova-lently linked tob-Lg after incubation at 40 C for 24 h and at 50 Cfor 48 h, respectively. Isoelectric focusing (IEF) analysis also showed

a high glycation degree ofb-Lg, being observed a noticeable shift ofthe isoelectric point of b-Lg glycated especially at 50 C towardsmore acidic pH as a result of the loss of basicity and, consequently,the increase in negative charge of the b-Lg molecule due to theblocking of Lys and Arg residues with carbohydrates.

Concerning conformational characterization of glycoconjugates,Corzo-Martinez et al. (2008) also observed a slight shift of thetryptophan (Trp) emission maximum at 50 C, whilst no shift of theTrp emission maximum was detected after glycation of b-Lg at40 C, suggesting that important structural changes in the threedimensional conguration of the protein occurred at 50 C.However, glycation at 40 C, although partially affected the sidechains of the protein in the tertiary structure, did not cause a greatdisruption of the native structure. According to this, a great

decrease in surface hydrophobicity (S0) of b-Lg:Gal [48 h, 50

C]was found, while glycation at 40 C only lead to a slight increase inb-Lg surface hydrophobicity, probably due to the exposition ofhydrophobic patches on the protein surface, as a consequence of itspartial denaturation. Likewise, results from size exclusion chro-matography (SEC) showed that, unlike b-Lg:Gal conjugate at 40 Cthat eluted predominantly as a protein dimer, SEC prole ofconjugate at 50 C displayed trimeric and oligomeric forms, indi-cating that glycation under these reaction conditions of b-Lgpromoted its polymerization.

3.1. Solubility

Since the solubility of a protein is a determining factor of its

dynamic of adsorption at the interface and, consequently, of its

M. Corzo-Martnez et al. / Food Hydrocolloids 27 (2012) 438e447440


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to the high solubility at pH 5 of this conjugate as compared to thatof control heated protein (Fig. 1).

The most remarkable result was obtained with b-Lg glycated at50 C (Fig. 2 (D)), which showed a diffusion rate signicantly(P< 0.05) higher than that of control b-Lg heated at 50 C. Inaddition, a clear increase in its diffusion rate at pH 5 with respect topH 7 was also observed, in agreement with the high solubility ofthis conjugate at pH 5 (Fig. 1).

3.2.2. Surface dilatational properties

With the purpose of studying the rheological properties of

adsorbed lms of native, control heated and glycated b-Lg, theirsurface dilatational modulus (E) was plotted versus time (q)(Fig. 3(A) andFig. 4(A)) and versus surface pressure (p)(Fig. 3(B) andFig. 4(B)), this second type of representation providing additionalinformation on the extent of interactions between components ofthe adsorbed lm.

In general, at pH 7, Eep plots (Fig. 3 (B)) of all the systemsstudied were above the behaviour of an ideal uid, not viscous(dashed line), suggesting the existence of relatively large interac-tions between components of the adsorbed lm (Lucassen-Reynders, Lucassen, Garrett, & Hollway, 1975). According toseveral authors, this could be due to the partial denaturation ofb-Lg, once adsorbed at the air/water interface, allowing the inter-molecular interaction via thiol-disulde exchange, that increase the

rigidity and cohesion of the interfacial

lm.

Control b-Lg heated at both 40 and 50 C gave rise to theformation of a lm with higher Evalues than that of native b-Lg(Fig. 3 (A)), probably due to its higher efciency of adsorption at theinterface (higherKdif) (Fig. 2(C)) (Bos & van Vliet, 2001; RodrguezPatino et al., 2008).

Likewise, whereas glycation at 40 C hardly altered rheologicalcharacteristics of adsorbed lm of b-Lg (Fig. 3 (A)), being onlyobserved a slight decrease in the dilatational modulus at long termadsorption (Eat10,800s, E10,800) with respect to native b-Lg, proteinglycated at 50 C led to the formation of a lm with the lowest Evaluesforagiventimeascomparedto lmsof native, control heated

and glycated (24 h, 40

C) protein.Wooster and Augustin (2007)obtained similar results in a study on the rheological properties ofthe adsorbed lms formed by WPI glycated with dextrans ofdifferent molecular weights. In agreement with these authors andtaking into account the results of intrinsic uorescence obtained inapreviouswork(Corzo-Martinezetal.,2008),thedecreaseobservedin thedilatational modulus (E)oftheb-Lg:Gal[48h,50C] adsorbedlmmightbeduetostructuralchangesundergonebyproteinduringthe advanced stages of the MR, since alteration of the conforma-tional state of protein is responsible for the loss of its structuralrigidity and, consequently, the loss ofrmness of the adsorbedlm.

Moreover, as observed in Fig. 3 (B), b-Lg:Gal [48 h, 50 C]conjugate showed the lowest and closest values to the idealbehaviour Eep values, indicating the existence of weak interactions

between components of the adsorbed

lm.

Fig. 2. Surface pressure (p) as a function of time (q) of adsorbed protein lms (A and B) and kinetic behaviour during the diffusion stage (C and D) of native b-Lg; control heated

b-Lg D 24 h at 40 C and, 48 h at 50 C; and glycated b-L gB 24 h at 40 C and> 48 h at 50 C at pH 7 (A and C) and pH 5 (B and D). Error bars indicate the standard deviation ofthe mean. aec Different case letters indicate statistically signicant (P< 0.05) differences.

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values over the time (Fig. 4(C)) and pressure (Fig. 4(D)), which isindicative of the formation of a more elastic and resistant lm thanthat of native, control heated and glycated (at 40 C) protein, inagreement with the high Evalues observed for this system.

3.3. Foaming properties

3.3.1. Foaming capacity

The values of the overall foaming capacity (OFC, mL/s), the foamcapacity (FC), the foam maximum density (MD), and the relativefoam conductivity (Cf, %) obtained with each of the systems assayedat pH 7 and 5 are shown inFig. 5.

At pH 7, native, control heated (40 and 50 C) and glycated(40 C) b-Lg showed the same foaming properties (no signicant

differences between valuesof OFC, FC and MD), only differing in thevalue ofCf.These results indicate that the increase produced in theprotein diffusion rate (Kdif) as a result of the heat treatment orglycation at 40 C (Fig. 2(C)) has no signicant effect on its foamingcapacity, probably due to that the protein diffusion rate is alreadygood enough for the system foams.This same behaviourcan best beseen inFig. 6(A), where a higher Kdifvalue did no result in signif-icant increase (P< 0.05) in the OFC value.

Glycation at 50 C, however, had a negative effect on b-Lgfoaming capacity at pH 7, observing values for the formationparameters OFC and FC signicantly (P


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3.3.2. Foam stabilityTo evaluate the capacity to stabilize foams of b-Lg glyco-

conjugates, the half-life time (q1/2, s) of foams formed with all thesystems assayed was determined (Fig. 7).

As observed inFig. 7(A), at pH 7, stability of foam formed withnative b-Lg was higher than that of foams with control heated andglycated protein, particularly at 50 C. This is consistent with theworse surface dilatational properties of adsorbed lms formed bythese systems (Fig. 3).

At pH 5 (Fig. 7(B)), the half-life time of foam with native b-Lg(569 26.87 s) did not substantially changed with respect to thatobtainedat pH 7 (575 0.00 s), a factthat isrelatedto the stability ofsurface dilatational modulus (E) of lm of this protein againstchanges in pH. Likewise, the worse interfacial characteristics(dynamicof adsorption andsurfacedilatationalproperties) observedfor the lms formed by control heated b-Lg at pH 5 as compared tothose of native and glycated protein resulted in a lower stability offoams containing control heated protein as foaming agent.

Fig. 5. Values obtained for the parameters of overall foaming capacity (OFC, mL/s), foam capacity (FC), foam maximum density (MD), and relative foam conductivity (Cf, %) with

native, control heated and glycated b-Lg at 40 and 50 C during 24 and 48 h, respectively, at pH 7 (solid bars) and pH 5 (hatched bars). Error bars indicate the standard deviation of

the mean. aec Different case letters indicate statistically signicant (P< 0.05) differences.

Fig. 6. Relationship betweenthe rate ofdiffusion(Kdif) atthe air/waterinterfaceand theoverallfoaming capacity(OFC)of native,controlheatedandglycatedb-Lgat40and50Cduring

24and48 h,respectively,at pH7 (A) and pH5 (B). Nativeb-Lg; D control heated b-Lg 24h,40

C;B

b-Lg:Gal 24h, 40C;,

control heatedb-Lg 48h,50

C;>

b-Lg:Gal 48 h, 50

C.



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On the other hand, unlike at pH 7, glycoconjugates were foundto be the best stabilizing agents at pH 5. Thus, the half-life time (q1/2, s) of foam with b-Lg glycated, particularly at 50 C, was notably(P


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