Food Hydrocolloids 23 (2009) Contents lists available at ScienceDirect. Food Hydrocolloids

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1 Food Hydrocolloids 23 (29) Contents lists available at ScienceDirect Food Hydrocolloids journal homepage: Interfacial and foaming properties of soy protein and their hydrolysates Karina D. Martínez a, Cecilio Carrera Sánchez b, Juan M. Rodríguez Patino b, Ana M.R. Pilosof a, * a Departamento de Industrias, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, 1428 Buenos Aires, Argentina b Departamento de Ingeniería Química, Facultad de Química, Universidad de Sevilla, C/Prof. García González, 1, 4112 Seville, Spain article info abstract Article history: Received 3 April 28 Accepted 26 March 29 Keywords: Foam Soy protein Hydrolysates Air water interface Surface pressure Dilatational rheology The objective of the work was to study the impact of soy protein hydrolysis on foaming and interfacial properties and to analyze the relationship between them. As starting material a sample of commercial soy protein isolate was used (SP) and hydrolysates were produced by an enzymatic reaction, giving hydrolysates from.4% to 5.35% degree of hydrolysis (DH). In this contribution we have determined foam overrun (FO), stability against liquid drainage and foam collapse, and the apparent viscosity of foams produced by a whipping method. The surface properties determined were the adsorption isotherm and surface dilatational properties of two hydrolysates (2 and 5.35% DH, H1 and H2 respectively). The hydrolysis of soy proteins increased the surface activity at bulk concentrations where SP adopts a condensed conformation at the monolayer. At concentrations where it adopts a more expanded conformation a very low degree of hydrolysis (H1) also promoted the enhancement of surface activity. However, at 5.35% degree of hydrolysis (H2) the surface activity decreased. Moreover, H2 presented lower surface activity than H1 at every bulk concentration. The hydrolysis increased the elastic component of the dilatational modulus and decreased phase angle of films at bulk concentrations below that corresponding to the collapse of SP monolayer ( bulk protein). SP hydrolysis increased foam overrun and the stability against drainage that could be related to increased surface activity of protein hydrolysates. However, the collapse of foams was promoted by hydrolysis and could be ascribed to a decrease of the relative viscoelasticity (higher phase angle) of surface films. The results point out that a low degree of hydrolysis (2 5%) would be enough to improve the surface activity of SP, decrease foam drainage and maintaining a considerable viscoelasticity of the surface films to retard foam collapse. Ó 29 Elsevier Ltd. All rights reserved. 1. Introduction Soybean proteins are widely used in many foods as functional and nutritional ingredients (Vohra & Kratzer, 1991). Native soy protein, because of its compact tertiary structure has limited foaming (Kinsella, 1979; Utsumi, Matsumura, & Mori, 1997; Wagner & Guéguen, 1999; Yu & Damodaran, 1991) and emulsifying (Kinsella, 1979; Liu, Lee, & Damodaran, 1999; German, O Neil, & Kinsella, 1985) properties. Structural modifications allowing greater conformational flexibility of protein may improve their ability to stabilize foams and emulsions. Many studies have demonstrated that the enzymatic hydrolysis of soy proteins improves its functional properties, including solubility, emulsifying and foaming characteristics (Pusky, 1975; Kim & Kinsella, 1985; Were, Hettiarachchy, & Kalapathy, 1997). * Corresponding author. Tel.: þ ; fax: þ address: apilosof@di.fcen.uba.ar (A.M.R. Pilosof). As the protein fraction with lower molecular mass increases at higher degrees of hydrolysis (Miñones Conde, Yust, Pedroche, Millán, & Rodríguez Patino, 25), foam and emulsion formation may be promoted due to the faster diffusion of molecules to fluid interfaces (air water and oil water) (Horne & Rodriguez Patino, 23; Rodriguez Niño, Wilde, Clark, Husband, & Rodríguez Patino, 1997a, 1997b; Rodriguez Niño & Rodríguez Patino, 22; Rodríguez Niño, Rodríguez Patino, Carrera, Cejudo, & Navarro, 23). However, peptides formed during hydrolysis may be too small to stabilize fluid interfaces, which is essential for the formation and stability of the dispersed system (Damodaran, 1997, chap. 3; Dickinson, 1992; Halling, 1981). At high degrees of hydrolysis, the decrease in molecular size can be expected to decrease the ability of the polypeptides at the interface to interact so that less viscoelastic films will cause a decrease in foam stability. Foam formation is influenced by the adsorption of the foaming agent at the air water interface and its ability to rapidly reduce surface tension. However, foam stabilization requires different surface properties such as the formation of a cohesive viscoelastic 268-5X/$ see front matter Ó 29 Elsevier Ltd. All rights reserved. doi:1.116/j.foodhyd

2 215 K.D. Martínez et al. / Food Hydrocolloids 23 (29) film via intermolecular interactions (Dickinson & McClements, 1995, chap 3). There are few studies focused on both interfacial and foaming properties of the same material, specially using protein concentrations relevant to the food industry (Fruhner, Wantke, & Lunkenheimer, 1999; Beneventi, Carrera, & Gandini, 21; Davis, Doucet, & Foegeding, 25; Rodríguez Patino et al., 27). In the present work we have studied the impact of soy protein hydrolysis on foaming and interfacial properties and the relationship between them. 2. Materials and methods 2.1. Materials A commercial soy protein isolate (SP) (9% protein) from Sanbra, Brazil was used as substrate for the hydrolysis with fungal protease (5B455) from Aspergillus oryzae with endopeptidase activity, provided by Quest International. The protein isolate was denatured as determined by differential scanning calorimetry (Metler Toledo DSC 822). (Perez & Pilosof, 24). Solubility of samples in distilled water and buffer Trizma [(CH 2 OH) 3 CNH 2 /(CH 2 OH) 3 CNH 3 Cl] (Sigma, St. Louis, MO > 99.5%), ph 7 and ionic strength of.5 M was calculated. The protein solutions at wt/wt (in distilled water or the buffer) were centrifuged for 3 min at room temperature. The supernatant with the soluble fraction was lyophilized with a Stokes equipment (Barber-Colman, Philadelphia, PA 1912, USA) and after 48 h, was weighted and solubility was calculated as: S% ¼ ðsoluble proteinðgþ=total proteinðgþþ 1 The solubility in distilled water was of 55% (total soluble fraction) and 4 in buffer Trizma. Surface hydrophobicity determined with the fluorescence probe 1-anilino-8naphatalene-sulphonate (ANS) was So ¼ 685 (Kato & Nakai, 198) Enzymatic hydrolysis SP isolate (72 g in 12 ml of water) was hydrolyzed according to Zylberman and Pilosof (22) batch-wise by treatment with fungal protease at ph 7, 5 C for 1 h, with enzyme/substrate (E/S) ratios:.5/1, 2/1 and 4/1 (three times). Hydrolysis was stopped by heating at 8 C for 1 min. The ph variation was adjusted back to the original value with diluted NaOH. Hydrolysates were lyophilized. The degree of hydrolysis (DH), defined as the percentage of peptide bonds cleaved, was calculated from the determination of free amino groups by reaction with o-phthaldialdehyde (OPA) according to Church, Swaisgood, Porter, and Catignani (1983). Protein hydrolysates with.4, 2 (H1), 5, 5.21 and 5.35% (H2) degree of hydrolysis were obtained. The solubility of hydrolysates H1 and H2 was respectively 35 and 48% (total soluble fraction) in distilled water and 46 and 71% in buffer Trizma. The solubility decrease in distilled water may be attributed to the heat treatment applied for enzyme inactivation. Nevertheless in buffer Trizma the interactions between the buffer and the ionic bonds of hydrolysates increase solubility by cleavage these bonds between aggregated peptides. This aggregation throughout ionic bonds would predominate in the hydrolysate with higher DH (H2). Surface hydrophobicity determined with the fluorescence probe 1-anilino-8naphatalene-sulphonate (ANS) (Kato & Nakai, 198) was So ¼ 53 and So ¼ 657 for hydrolysates H1 and H2 respectively Foaming properties Foam formation 3 ml of solutions at wt/wt were foamed at 25 C in a graduated tube (3 cm diameter) for 3 min with a Griffin & George stirrer at 25 rpm. Overrun was calculated as: FOð%Þ ¼½ðfoam volume 3Þ=3Š1 (1) The data reported are means of at least two replicates. The error was less than 1% Foam drainage and collapse The volume of liquid drained to the bottom of the graduated tubes and foam height (collapse) were recorded over time. The following empirical mathematical model was applied to fit drainage over time (Carp, Bartholomai, & Pilosof, 1997): vðtþ ¼Vt n =c þ t n (2) where v(t) was the drained volume at time t; V is the maximum drained volume; n was a constant related to the sigmoid shape of the curves; and c was a constant related to drainage half time by c 1/ n. The rate constant for drainage (k dr ) was calculated as: k dr ¼ n=vc 1=n (3) The data reported are means of at least two replicates. The error in k dr was less than 1. The decrease of foam volume (V) over time (foam collapse) was described by the time required to decrease in 1 ml the height of the foam (collapse time). The data reported are means of at least two replicates. The error was less than Apparent viscosity The initial apparent viscosity of foams at 2 C was determined as described by Carp et al. (1997) with a Brookfield LV viscometer, with Dvloader and Wingather software. A T-C spindle was used. The error in apparent viscosity data was less than.5% Interfacial properties Preparation of solutions Solutions for interfacial studies were prepared by dissolving SP, H1 ( DH) and H2 (5.35% DH) in Milli-Q ultrapure water. The ph and ionic strength were kept constant at 7 and.5 M respectively by using a commercial buffer solution called Trizma (CH 2 OH) 3 CNH 2 /(CH 2 OH) 3 CNH 3 Cl (Sigma, >99.5%) Surface pressure isotherm Equilibrium surface tension (s eq ) was determined by the Wilhelmy plate method, using a roughened platinum plate attached to the Sigma 71 digital tensiometer (KSV, Finland). Protein and hydrolysates solutions in an increased range of concentrations wt/wt were allowed to age during 24 h at 4 C before measurements. The final protein bulk concentrations were within 1 7 and (wt). The reduction in surface tension, s, was recorder continuously and equilibrium was assumed when the pressure did not change by more than.1 mn/m in 3 min. The final surface pressure (p eq ) value was calculated as p eq ¼ s o s eq ; where s o is the bulk surface tension in the absence of surfactant. All experiments were replicated at least two times. It could be found that p eq could be reproduced to.5 mn/m.

3 K.D. Martínez et al. / Food Hydrocolloids 23 (29) Dynamic surface pressure and surface dilatational properties Dynamic surface pressure (p) and surface dilatational properties of adsorbed protein films at the air water interface were measured with an automatic drop tensiometer (TRACKER, IT Concept, Longessaine, France) as described elsewhere (Carrera, Rodríguez Niño, & Rodríguez Patino, 1999). The following mathematical model was applied to fit surface pressure over time in order to evaluate a global rate of adsorption (Kitabake & Doi, 1988). pðtþ ¼p m t n =C þ t n (4) where p(t) was the surface pressure at a time t, p m was the maximum surface pressure; n was a constant related to the sigmoid slope of the curves and C was a constant related to surface pressure half time by C 1/n. The global adsorption rate constant (K ads ) was calculated as: K ads ¼ n=p m C 1=n ½K ads Š¼ðmN=msÞ 1 (5) The method involved a periodic automatically controlled, sinusoidal interfacial compression and expansion performed by decreasing and increasing the drop volume, at the desired amplitude (DA/A) and angular frequency (u). The surface dilatational modulus (E) (Eq. (6)), its elastic (Ed) and viscous (Ev) components, and the phase angle (q) were derived from the change in surface pressure (p) resulting from a small change in surface area. The surface dilatational properties were measured as a function of time, t. The percentage area change was determined (data not shown) to be in the linear region. E ¼ ds=da=a ¼ dp=dlna (6) E ¼ðs =DA=A Þðcos q þ isin qþ ¼Ed þ Ev (7) where s and A are the strain and stress amplitudes, respectively, q is the phase angle between stress and strain, p ¼ s s is the surface pressure, and s and s are the surface tension in the presence and in the absence of protein, respectively. The dilatational modulus is a complex quantity and is composed of real and imaginary parts (Eq. (7)). The real part of the dilatational modulus or storage component is the dilatational elasticity, Ed ¼ jejcos q. The imaginary part of the dilatational modulus or loss component is the surface dilatational viscosity, Ev ¼ jejsin q. The ratio (s /A ) is the absolute modulus, jej, a measure of the total unit material dilatational resistance to deformation (elastic þ viscous). For a perfectly elastic material the stress and strain are in phase (q ¼ ) and the imaginary term is zero. In the case of a perfectly viscous material (q ¼ 9 ) and the real part is zero. The experiments were carried out at 2 C. The bulk protein concentrations were, 1 3,1 2 and (wt). The temperature was maintained constant at 2 C within.1 C by circulating water from a thermostat. Protein solutions were prepared freshly and stirred for 3 min. The solutions were placed in the syringe and then in a compartment and allowed to stand for 3 min to reach the desired constant temperature. Then a drop of solution was delivered and allowed to stand for 18 min at 2 C to achieve protein adsorption at the air water interface. The surface rheological parameters, the surface dilatational modulus (E) and the phase angle (q) were measured as a function of time with an amplitude (DA/A) and angular frequency (u) constant at 15% and 1 mhz respectively. The sinusoidal oscillation for surface dilatational measurement was made with five oscillation cycles followed by a time of 5 cycles without any oscillation up to the time required to complete adsorption. The average standard accuracy of the surface pressure is roughly.1 mn/m. However, the reproducibility of the results (for at least two measurements) was better than.5%. 3. Results 3.1. Foaming properties Changes in the molecular structure of soy protein as a consequence of hydrolysis (molecular weight and surface hydrophobicity) would originate specific surface characteristics/functional properties. Fig. 1 shows foam overrun as a function of the degree of hydrolysis of soy protein. The overrun was increased by protein hydrolysis mainly at the lowest DH (.4%). This is in accordance with previous results showing that a limited hydrolysis of protein is sufficient to improve foamability (Panyam & Kilara, 1996; Ipsen et al., 21; Molina & Wagner, 22; Zylberman & Pilosof, 22; Martínez, Baeza, Millán, & Pilosof, 25). A similar tendency have been reported by Bernardi, Pilosof, and Bartholomai (1991), where hydrolysates of soy protein obtained by two types of enzymes, showed the best foaming properties at about 1% DH, but higher levels of hydrolysis, resulted detrimental for foaming properties. Nevertheless, because of the different methods used to evaluate DH, the comparison may be done only qualitatively. Liquid drainage from foams was fitted with Eq. (2), obtaining R 2 values between.996 and.997. The drainage rates of foams calculated are shown in Fig. 2 as a function of the degree of hydrolysis of soy protein. It can be observed that hydrolysis strongly improved drainage stability. Molina and Wagner (22) have shown that the hydrolysis of native soy protein resulted in a low increment in foam formation capacity when compared to the native protein but in high drainage stability, probably due to its high hydrophobicity that allowed the formation of a stable surface film. Rodriguez Patino et al. (27) found that foam stability of sunflower protein isolate was also increased by a limited hydrolysis. Foams were less stable at DH higher than 5.6. Fig. 3 shows the time required to decrease the height of the foams by 1 ml (collapse time) as a function of the degree of hydrolysis of SP. It can be seen that foam stability against collapse was strongly decreased (four times) by protein hydrolysis. These results point out that stability against foam collapse and the stability against liquid drainage would be governed by different factors. FO (%) DH(%) Fig. 1. Foam overrun (FO) as a function of degree of hydrolysis (DH) of soy protein.

4 2152 K.D. Martínez et al. / Food Hydrocolloids 23 (29) K dr (mil.min) π (mn/m) H1 π cr SP H DH(%) Fig. 2. Liquid drainage rate (k dr ) as a function of degree of hydrolysis (DH) of soy protein. The stability of foam is affected by surface tension, bulk viscosity, surface rheological properties, and surface forces (Langevin, 2; Stubenrauch & Miller, 24; Prins, 1999; Waltermo et al., 1996; Wang & Yoon, 26; Monteux, Fuller, & Bergeron, 24). High bulk viscosity mainly controls liquid drainage and hence foam collapse. However, in less viscous foams, foam collapse is mainly controlled by rheological properties of interfacial films Interfacial properties Surface pressure isotherm The surface behaviour of the three samples (SP, H1 and H2) at the air water interface was first studied by means of tensiometry. Fig. 4 shows the effect of SP, H1 and H2 bulk concentration on the equilibrium surface pressure. The observed behaviour was sigmoidal, which is typical for biopolymers and surfactants (Tornberg, 1978; Graham & Phillips, 1979). The surface pressure increased with protein concentration and tended to pseudo-equilibrium in the case of SP, while the surface pressure of hydrolysates (H1 and H2) continuously increased over the range of concentrations studied. At bulk concentrations below 1 5 % (wt) SP, H1 and H2 did not present surface activity. At about % (wt) SP showed surface pressure, while at 1 3 % there was an inflexion point col.time(min) DH(%) Fig. 3. Time to decrease the height of the foam by 1 ml as a function of degree of hydrolysis (DH) of soy protein log C (% wt) Fig. 4. Surface pressure isotherm for SP, H1 and H2 at the air water interface. Temperature 2 C, ph 7and I ¼.5 M. C: soluble protein (% wt). corresponding to 22 mn/m. When the bulk concentration was between 1 3 and 1 1 % (wt), SP displayed a plateau at 23 mn/m after which the surface pressure increased again with protein concentration up to 27 mn/m. Protein monolayers adopt different structures as the bulk concentration increase. These structural patterns in the monolayer are the consequence of the conformation of protein molecules adsorbed at the air water interface that appear as trains (structure I) and loops and tails (structure II) (Nahringbauer, 1995). The transition from structure I to structure II represents the rearrangement of an expanded monolayer to a more condensed one. The collapse of the monolayer film is associated with formation of multiple interfacial layers (multilayers) (Wollenweber, Makievski, & Daniels, 2). The transition from structure I to structure II occurs at a defined value of surface pressure called the critical pressure (p cr ). The collapse of the film occurs at the collapse surface pressure (p c ). The changes observed in the p C isotherm, may be related to structural transitions of protein at the air water interface. It have been reported for 7S, 11S, and 11S reduced soy globulin monolayers, structural changes determined from the surface pressure through area isotherms (p A) (Carrera, Rodríguez Niño, Molina, Añón, & Rodríguez Patino, 24). In that work, surface pressure of spread SP globulins were studied in a Langmuir type balance. The results indicated that 7S globulin monolayers adopt two different stages of condensation and a collapse phase as most globular proteins (Phillips, Evans, Graham, & Oldani, 1975; Graham & Phillips, 1979; Benjamins, 2; Rodríguez Patino, Carrera, Rodríguez Niño, & Cejudo, 21). At low surface pressure, 7S globulin monolayer adopts an expanded conformation at the air water interface. This conformation becomes more condensed at higher surface pressures and the protein is displaced to bulk solution at the collapse point. The transition to a more condensed structure was observed at a surface pressure of 19 mn/m (p cr ), deduced from the p A isotherm (Carrera, Rodríguez Niño, Molina, Añón, & Rodríguez Patino, 24). Moreover, the monolayer collapse was observed at 27 mn/m (p c ). The 11S globulin displayed similar results. The p A isotherm also presented two monolayer condensation stages and a collapse point, at 2 and 27.6 mn/m (p cr and p c respectively). The surface behaviour of the reduced 11S fraction (DTT treatment), in which the A B subunit was splitted to A and B polypeptides was also studied by Carrera et al. (24). This sample is comparable with the commercial SP used in this work that was denatured as

5 K.D. Martínez et al. / Food Hydrocolloids 23 (29) detected by differential scanning calorimetry. The p A isotherm of reduced 11S globulin also showed the transition to the second condensation stage at p cr ¼ 2 mn/m. As the inflexion point of surface pressure isotherms (p A) corresponds to the inflexion point and plateau in p C isotherms (Graham & Phillips, 1976; Perez, Carrera, Rodriguez Patino, & Pilosof, 26), the surface pressure of 22 mn/m (Fig. 4) would represent the p cr for SP. The surface pressure at 27 mn/m would correspond to the collapse point of the protein (p c ) occurring at (wt) bulk concentration, due to the plateau observed in the p C isotherm (Fig. 4). H1 showed an increase of surface pressure at lower bulk concentrations when compared to SP, which implies higher surface activity. This behaviour was also reported for hydrolysates of sunflower protein (Miñones Conde et al., 25). However, H1 did not display a defined plateau as SP sample did when increasing the surface pressure at higher concentrations. H1 showed a higher surface pressure than SP at low and at bulk concentrations higher than 1 2 % (wt), indicating the increase of surface activity for H1 (low DH). This behaviour can be attributed to a higher molecular flexibility as a result of the enzymatic hydrolysis of soy proteins. Moreover, the smaller size of peptides would favour the penetration of peptides at the interface, without structural changes (i.e. inflexion points in the p C isotherm). Hydrolysates had a broad molecular size range, but smaller than 2 kda as determined by SDS-PAGE electrophoresis (data not shown) (Martínez, Carrera, Pizones, Rodríguez Patino, & Pilosof, 27a). This fact would result in a broad size range of adsorbed peptides at the interface. Similar results were reported for hydrolysates of sunflower protein. Sunflower protein hydrolysates with 5.6 DH, presented higher surface pressures than the native protein (Miñones Conde & Rodríguez Patino, 26). However spread monolayers of hydrolyzed sunflower protein adopted two different condensation states and a collapse phase. The bulk concentration of H2 necessary to observe a surface pressure increase was similar to that observed for the native SP (>1 4 % wt); nevertheless H2 showed lower surface pressure than SP up to 1 2 % (wt). At higher concentrations (i.e. higher than p cr for SP) H2 exhibited higher p than SP. However, in comparison to H1, H2 showed lower surface activity at every bulk concentration, except above 1 2 % wt where no significant differences were found. The results point out that soy protein hydrolysates do not present structural changes as SP does, at least at the studied bulk concentrations. Probably, after monolayer saturation, the monolayer would became collapsed, without a second condensed state, or the monolayer structural transitions would occur at higher bulk concentrations, which are not studied in the present work Surface dilatational properties Proteins act as polymeric emulsifiers with multiple anchoring sites at the interface that, together with the unfolding process of the adsorbing protein molecule, stabilize the interfacial layer kinetically. This behaviour contributes significantly to the interfacial rheological properties and immobilizes proteins in the adsorbed layer (Bos, Nylander, Arnebrant, & Clark, 1997). On the other hand, emulsification and foaming involve interfacial deformation and the response of the adsorbed layer to such deformations (measured by the surface dilatational properties) is crucial for understanding the role of proteins in food systems (Benjamins, 2). Fig. 5a c shows the phase angle (q), which is a measure of relative viscoelasticity of adsorbed films, as a function of adsorption time for SP, H1 and H2 at 1 3,1 2 and wt bulk concentrations. All types of inter-and intramolecular bonds occurring in macromolecules such as proteins influence the film rheology a Phase angle (degree) b Phase angle (degree) c Phase angle (degree) time(s.) % time(s.) 1-2 % 1-3 % 1-2 % 1-3 % 1-2 % time(s.) Fig. 5. Time-dependent phase angle, for adsorbed (a) SP, (b) H1, (c) H2 at 1 3 %, 1 2 % and (wt) bulk concentration. (Martin, Bos, & van Vliet, 22). Chemical modification has been used largely as a way of changing the hydrophobicity or net charge of proteins as a function of ph (Krause, Kragel, & Schewenke, 1997; Krause, Wustneck, Seifert, & Schewenke, 1998). Most of the factors that affect bonding between molecules at the interface also affect the susceptibility of proteins to unfolding. The balance of protein protein interactions versus adsorption can also be changed by fragmenting a protein by enzymatic hydrolysis, which changes the balance of hydrophobic versus hydrophilic residues within the remaining fragments (Girardet et al., 21; Ipsen et al., 21).

6 2154 K.D. Martínez et al. / Food Hydrocolloids 23 (29) The evolution of dilatational properties as a function of time should be ascribed to adsorption and rearrangements of proteins as well as protein protein interactions, which sometimes lead to interfacial gel formation. The increased adsorption of SP at the air water interface, with time (Rodriguez Patino, Carrera, Molina, Rodriguez Niño, & Añón, 23, 24) resulted in the increase of the dilatational modulus (E) (data not shown) and the decrease of phase angle (q) at long adsorption time (Fig. 5a). The surface dilatational modulus and its elastic components were lower than those reported for native soybean 7S and 11S globulins (Rodriguez Niño et al., 23) (data not shown) but the phase angle exhibited similar values indicating that this rheological parameter is less affected by commercial processing denaturation (Martínez et al., 27a). The decrease of phase angle with time for the majority adsorbed films of H1 and H2 in Fig. 5b and c should be ascribed to adsorption of polypeptides resulting from the hydrolysis (Rodriguez Patino et al., 23) and the low phase angle value indicate that the protein films showed a gel structure, which involved the association between molecules (Rodriguez Patino et al., 23). The decrease of phase angle with time arises from the time evolution of Ed and Ev shown in Figs. 6 and 7. The continuous increase of phase angle with time (H2 at 1 3 %) show the increasing preeminence of the viscous character on the elastic one with adsorption time. As can be seen in Figs. 6c and 7c, both the viscous component (Ev) and the elastic one (Ed) increased with time. The more hydrolyzed soy protein film (H2) at protein was more viscoelastic (lower phase angle) than that formed by the less hydrolyzed preparation (H1). Increased surface hydrophobicity of hydrolysate H2 may account for by the increased film viscoelasticity, as peptides aggregation at the interface would be favoured (Martínez, Carrera, Pizones, Rodriguez Patino, & Pilosof, 27b). The more viscoelastic monolayers (lower q) were obtained at the lower bulk concentrations studied (1 3 %) suggesting that at low bulk protein concentrations of SP, H1 and H2, better protein protein interactions could be established to form more viscoelastic films (Girardet et al., 21; Ipsen et al., 21; Martin, Bos, & van Vliet, 22; Rodriguez Patino et al., 23, 24). Fig. 6a c shows that the decrease of film viscoelasticity (higher phase angle) at high protein concentration is mainly due to the decrease of the elastic component (Ed) at higher concentrations. This behaviour indicates that with decreasing surface concentration better protein protein interactions may be established (higher Ed). Fig. 8a c compares the elastic (Ed) and viscous (Ev) components and the phase angle (q) of films SP, H1 and H2 at long adsorption time (1, s) as a function of the soluble protein concentration. It can be seen that when soluble protein was lower than 1% (wt), the hydrolysis improved the monolayer dilatational elasticity (Fig. 8a) but decreased dilatational viscosity (Ev) (Fig. 8b). At 1% (wt) protein, Ed and Ev did not show dependence with molecular structure. Fig. 8c shows that hydrolysis decreased phase angle at bulk protein concentrations below 1% (wt), which means an improvement of film properties. However, at 1% (wt) soluble protein, SP film exhibited better viscoelastic properties (lower q) than the hydrolysates. The behaviour observed at 1% protein could be ascribed to the collapse of SP monolayer at this bulk concentration Relationship between foaming and interfacial properties SP hydrolysis increased foam overrun and the stability against drainage, but increased the collapse of foams (Figs. 1 3). The increase of foam overrun can be related with the increase of surface activity (p eq ) of hydrolysates in comparison to SP observed in p C a Ed (mn/m) b Ed (mn/m) c Ed (mn/m) % 1-2 % % 1-2 % isotherms (Fig. 4), at high bulk concentrations. It has also been reported a relationship between the foam overrun and the global rate of protein adsorption at the interface (K ads )(Kitabake & Doi, 1988; Martin et al., 22). Nevertheless, in this work we did not find any correlation at (wt) bulk concentration (K ads for SP, H1 and H2 were ; and mn/m min 1 respectively according to Eq. (5)). In those works, the differences found between adsorption rates were larger than in the present work because of the different origin of the proteins studied. As a consequence a good correlation with foam overrun could be observed in that case. 1-3 % 1-2 % Fig. 6. Time-dependent dilatational elasticity (Ed), for adsorbed (a) SP, (b) H1, (c) H2 at 1 3 %, 1 2 % and (wt) bulk concentration.

7 K.D. Martínez et al. / Food Hydrocolloids 23 (29) a a 75 Ev (mn/m) Ed (mn/m) 5 25 H1 SP H log C (% wt) b b 1. SP Ev (mn/m) 5. Ev (mn/m) 5. H H log C (% wt) c 1. c 3 Ev (mn/m) Phase angle (degree) 2 1 SP H2 H Fig. 7. Time-dependent dilatational viscosity (Ev) for adsorbed (a) SP, (b) H1, (c) H2 at 1 3 %, 1 2 % and (wt) bulk concentration. Rodriguez Patino et al. (27) found a close relationship between the foaming and the rate of diffusion of sunflower protein isolate to the air water interface. The foam stability quantified by the relaxation times of drainage and disproportionation/collapse correlated with the surface pressure at long-term adsorption and depended on DH. The foaming characteristics were poor for sunflower protein isolate, but they were improved for their hydrolysates. The foam capacity and stability were optimum for a low DH (3 5%). The increase of foam overrun by hydrolysis may be related to the apparent viscosity of foams (SP ¼ 2916; H1 ¼ 63,736 and H2 ¼ 55,769 Cp). Foam viscosity has been inversely related to log C (% wt) Fig. 8. Rheological parameters at 1, s of adsorption time as a function of soluble protein concentration for SP, H1 and H2 (a) dilatational elasticity (Ed), (b) dilatational viscosity (Ev), (c) phase angle. liquid volume fraction in foams (Nutt & Burley, 1989, chap. 8; Britten & Lavoie, 1992). The increase of liquid between bubbles, leads to an increment of fluidity and determines a decrease of foam viscosity. Therefore, as higher is the foam overrun, lower is the liquid volume fraction and higher would be the apparent viscosity of the foams. The increase of foam viscosity may also explain with the increase of drainage stability. The lower liquid volume fraction may be related with the lower drainage rate observed (Nutt & Burley, 1989; Britten & Lavoie, 1992).

8 2156 K.D. Martínez et al. / Food Hydrocolloids 23 (29) The mechanic properties of films should strongly influence the collapse rate of foams. The enhancement of foam collapse by hydrolysis may be ascribed to the increase of the phase angle (q) of interfacial films that is a measure of the decrease of relative viscoelasticity (Figs. 5 and 8). Baeza, Carrera, Rodriguez Patino, and Pilosof (25) found a correlation between the dilatational elasticity of b-lactoglobulin films in the presence of polysaccharides and the collapse time of foams. Kloek, van Vliet, and Meinders (21) reported that viscous and elastic moduli (relative viscoelasticity) are adequate as stability criteria. However, Ipsen et al. (21) found that hydrolysates of b-lactoglobulin, genetic variant A, hydrolyzed with Bacillus licheniformis to degrees ranging from 19 to 86%, exhibited an improvement of foam overrun. A limited hydrolysis (19 26%) resulted in an increased affinity for adsorption to the interface, whereas more extensive hydrolysis caused a delayed adsorption. All hydrolyzed samples exhibited lower maximum values of the interfacial viscoelastic properties compared to the intact sample, but the best foam stability was obtained with the most extensively hydrolyzed sample thus demonstrating that foam stability was not directly related to interfacial rheology. 4. Conclusions The hydrolysis of soy proteins increased surface activity at bulk concentrations where SP adopts a condensed conformation at the monolayer. At concentrations where it adopts a more expanded conformation a low degree of hydrolysis () also promoted the enhancement of surface activity. However, higher degrees of hydrolysis (5.35%) decreased surface activity. Moreover, H2 (5.35% DH) presented lower surface activity than H1 at every bulk concentration. The effect of protein hydrolysis can be attributed to the smaller size of peptides which allows more peptides to be adsorbed at the interface without undergoing structural changes and to increased molecular flexibility as a consequence of hydrolysis. The hydrolysis also improved the dilatational elasticity and viscoelasticity of the films at bulk concentrations below that corresponding to the collapse of SP monolayer ( bulk protein). The relationship between properties of surface films and foaming showed that very low degrees of hydrolysis (2 5.35%) are required to increase foaming capacity, maintaining the interfacial gelation capacity needed to form viscoelastic interfacial films, with high stability against drainage. In fact, soy protein hydrolysis increased foam overrun and decreased liquid drainage. However, promoted foam collapse, due to a decrease of film viscoelasticity. The required molecular characteristics to obtain high foam overrun and strong viscoelastic films are in general, opposite. To achieve a good foam overrun, the protein should be small and flexible, whereas, to form strong viscoelastic films, the protein should be larger, and able to interact at the air water interface to form a gel-like film. As a result, a low degree of hydrolysis reflects a balance of the required molecular properties of protein. Acknowledgments This research was supported by CYTED through project 15PI274 The authors also acknowledge the support from CYCYT through grant AGL27-645, Junta de Andalucía through grant PO6-AGR-1535, and Universidad de Buenos Aires, Agencia Nacional de Promoción Científica y Tecnológica and Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina. References Baeza, R. I., Carrera, C., Rodríguez Patino, J. M., & Pilosof, A. M. R. (25). 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