Interfacial and foaming interactions between casein glycomacropeptide (CMP) and propylene glycol alginate.

Proteins and polysaccharides are widely used in food formulation. While most of the proteins are surface active, only few polysaccharides can adsorb at the air-water interface; this is the case of propylene glycol alginates (PGA). It is known that casein glycomacropeptide (CMP), a bioactive polypeptide derived from κ-casein by the action of chymosin, presents a great foaming capacity but provides unstable foams. So, the objective of this work was to analyze the impact of mixing CMP and a commercial variety of PGA, Kelcoloid O (KO), on the interfacial and foaming properties at pH 7.0. It was determined the surface pressure isotherm, the dynamics of adsorption and the foaming properties for CMP, KO and the mixed system CMP-KO. CMP dominated the surface pressure of CMP-KO mixed system. The presence of KO synergistically improved the viscoelastic properties of surface film. The foaming capacity of CMP was altered by KO. KO foam presented a higher stability than CMP foam and it controlled the stability against drainage and the initial collapse in the mixed foam.

Kinetics of adsorption of whey proteins and hydroxypropyl-methyl-cellulose mixtures at the air-water interface.

The aim of this research is to quantify the competitive adsorption of a whey protein concentrate (WPC) and hydroxypropyl-methyl-cellulose (HPMC so called E4M, E50LV and F4M) at the air-water interface by means of dynamic surface tensiometry and Brewster angle microscopy (BAM). These biopolymers are often used together in many food applications. The concentration of both protein and HPMC, and the WPC/HPMC ratio in the aqueous bulk phase were variables, while pH (7), the ionic strength (0.05 M) and temperature (20 degrees C) were kept constant. The differences observed between mixed systems were in accordance with the relative bulk concentration of these biopolymers (C(HPMC) and C(WPC)) and the molecular structure of HPMC. At short adsorption times, the results show that under conditions where both WPC and HPMC could saturate the air-water interface on their own or when C(HPMC) > or = C(WPC), the polysaccharide dominates the surface. At concentrations where none of the biopolymers was able to saturate the interface, a synergistic behavior was observed for HPMC with lower surface activity (E50LV and F4M), while a competitive adsorption was observed for E4M (the HPMC with the highest surface activity). At long-term adsorption the rate of penetration controls the adsorption of mixed components. The results reflect complex competitive/synergistic phenomena under conditions of thermodynamic compatibility or in the presence of a "depletion mechanism". Finally, the order in which the different components reach the interface will influence the surface composition and the film properties.

Bulk and interfacial behaviour of caseinoglycomacropeptide (GMP).

Caseinoglycomacropeptide (GMP) is a hydrophilic glycopeptide released from milk kappa-casein by chymosin hydrolysis during cheese making. GMP is thought to be a potential ingredient for specific dietary applications with several health benefits. In this study GMP was characterized at the air-water interface and its behaviour was related with the self-assembly of GMP in solution as affected by pH. This GMP self-assembly was investigated by dynamic light scattering and the interfacial properties were determined by tensiometry and surface dilatational measurements at pH 4, 5 and 7. At pH 5 GMP exhibited higher surface pressure at equilibrium than at pH 7. At pH 4 the behaviour was more complex due to self-assembly close to GMP pI. Dynamic measurement showed that the adsorption/penetration rate constant (K(ads)) is facilitated at higher GMP bulk concentrations, while the rate constant of rearrangement (K(r)) decreased at higher GMP concentrations which could be attributed to the existence of a steric restriction due to the higher GMP load at the interface. K(r) was higher at pH 5 because of lower electrostatic interactions close to the pI. The viscoelastic properties showed a complex behaviour due to the existence of protein-protein interactions depending on the GMP concentration, on the pH of the bulk and on the rates of diffusion, adsorption and rearrangement of GMP at the air-water interface.

Structural, topographical, and rheological characteristics of beta-casein-dioleoyl phosphatidylcholine (DOPC) mixed monolayers.

The surface pressure (pi)-area (A) isotherms and Brewster angle microscopy (BAM) images of beta-casein-dioleoyl phosphatidylcholine (DOPC) mixed films spread on buffered water at pH 7 and 9 and at 20 degrees C were determined as a function of the mass fraction of DOPC in the mixture (X(DOPC)). The structural characteristics, miscibility, and topography (morphology and reflectivity) of DOPC-beta-casein mixed films were very dependent on surface pressure and monolayer composition. The structure in DOPC-beta-casein mixed monolayers was liquid-expanded-like, as for pure components. The monolayer structure was more expanded as the pH and the DOPC concentration in the mixture were increased. From the concentration and surface pressure dependence on excess area and elasticity (E) it was deduced that DOPC and beta-casein form a practically immiscible monolayer at the air-water interface. The BAM images and the evolution with the surface pressure of the reflectivity of BAM images give complementary information on the interactions and structural characteristics of DOPC-beta-casein mixed monolayers, which corroborate the conclusions derived from the pi-A isotherm. After the spreading or just after the expansion at pi approximately 0 we have observed the presence of 2D-foams, typical topography of DOPC monolayers at low pi. The 2D-foams disappear after the compression of the monolayer and the topography is homogenous and isotropic. From the reflectivity of BAM images it is possible to distinguish between the coexistence of DOPC and beta-casein or beta-casein displacement by DOPC, depending on the surface pressure. The surface dilatational properties of the mixed films corroborate the coexistence of DOPC and beta-casein at pi lower than the equilibrium spreading pressure (pi(e)) of beta-casein and beta-casein displacement by DOPC at pi>pi(e) of beta-casein. The phenomena observed appear to be generic for protein and polar (monoglycerides) and ionizable (phospholipid) mixed monolayers.

Effect of sucrose on functional properties of soy globulins: adsorption and foam characteristics.

In this contribution, we have analyzed the effect of sucrose on dynamic interfacial (dynamic surface pressure and surface dilatational properties) and foaming (foam capacity and foam stability) characteristics of soy globulins (7S and 11S). The protein (at 1 x 10(-3), 1 x 10(-2), 0.1, and 1 wt %) and sucrose (at 0, 0.25, 0.5, and 1.0 M) concentrations in aqueous solution and the pH (at 5 and 7), and ionic strength (at 0.05 and 0.5 M) were analyzed as variables. The temperature was maintained constant at 20 degrees C. We have observed the following. (i) The dynamics of adsorption (presence of a lag period, diffusion, and penetration at the air-water interface) of soy globulins depend on the peculiar molecular features of proteins (7S or 11S soy globulin) and the level of association/dissociation of these proteins by varying the pH and ionic strength, as well as the effect of sucrose in the aqueous phase on the unfolding of the protein. The rate of adsorption increases with the protein concentration in solution, at pH 7 compared to pH 5, at high ionic strength, and in the absence of sucrose. (ii) The surface dilatational properties reflect the fact that soy globulin adsorbed films exhibit viscoelastic behavior. The surface dilatational modulus increases at pH 7 compared to pH 5, but decreases with the addition of sucrose into the aqueous phase. (iii) The rate of adsorption and surface dilatational properties (surface dilatational modulus and phase angle) during adsorption at the air-water interface play an important role in the formation of foams generated from aqueous solutions of soy globulins. (iv) The increased interfacial adsorption (at high surface pressures) and the combined effects of interfacial adsorption and interfacial interactions between adsorbed soy globulin molecules (at high surface dilatational modulus) can explain the higher stability of the foam, with few exceptions.

Implications of interfacial characteristics of food foaming agents in foam formulations.

The manufacture of food dispersions (emulsions and foams) with specific quality attributes depends on the selection of the most appropriate raw materials and processing conditions. These dispersions being thermodynamically unstable require the use of emulsifiers (proteins, lipids, phospholipids, surfactants etc.). Emulsifiers typically coexist in the interfacial layer with specific functions in the processing and properties of the final product. The optimum use of emulsifiers depends on our knowledge of their interfacial physico-chemical characteristics - such as surface activity, amount adsorbed, structure, thickness, topography, ability to desorb (stability), lateral mobility, interactions between adsorbed molecules, ability to change conformation, interfacial rheological properties, etc. -, the kinetics of film formation and other associated physico-chemical properties at fluid interfaces. These monolayers constitute well defined systems for the analysis of food colloids at the micro- and nano-scale level, with several advantages for fundamental studies. In the present review we are concerned with the analysis of physico-chemical properties of emulsifier films at fluid interfaces in relation to foaming. Information about the above properties would be very helpful in the prediction of optimised formulations for food foams. We concluded that at surface pressures lower than that of monolayer saturation the foaming capacity is low, or even zero. A close relationship was observed between foaming capacity and the rate of diffusion of the foaming agent to the air-water interface. However, the foam stability correlates with the properties of the film at long-term adsorption.

Formulation engineering can improve the interfacial and foaming properties of soy globulins.

In this contribution, we have analyzed the effect of different strategies, such as change of pH (5 or 7) or ionic strength (at 0.05 and 0.5 M), and addition of sucrose (at 1 M) and Tween 20 (at 1 x 10(-4) M) on interfacial characteristics (adsorption, structure, dynamics of adsorption, and surface dilatational properties) and foam properties (foam capacity and stability) of soy globulins (7S and 11S at 0.1 wt %). We have observed that (1) the adsorption (presence of a lag period, diffusion, and penetration at the air-water interface) of soy globulins depends on the modification in the 11S/7S ratio and on the level of association/dissociation of these proteins by varying the pH and ionic strength (I), the effect of sucrose on the unfolding of the protein, and the competitive adsorption between protein and Tween 20 in the aqueous phase. The rate of adsorption increases at pH 7, at high ionic strength, and in the presence of sucrose. (2) The surface dilatational properties reflect the fact that soy globulin adsorbed films exhibit viscoelastic behavior but do not have the capacity to form a gel-like elastic film. The surface dilatational modulus increases at pH 7 and at high ionic strength but decreases with the addition of sucrose or Tween 20 into the aqueous phase. (3) The rate of adsorption and surface dilatational properties (surface dilatational modulus and phase angle) during adsorption at the air-water interface plays an important role in the formation of foams generated from aqueous solutions of soy globulins. However, the dynamic surface pressure and dilatational modulus are not enough to explain the stability of the foam.

Structural and shear characteristics of adsorbed sodium caseinate and monoglyceride mixed monolayers at the air-water interface.

The structural and shear characteristics of mixed monolayers formed by an adsorbed Na-caseinate film and a spread monoglyceride (monopalmitin or monoolein) on the previously adsorbed protein film have been analyzed. Measurements of the surface pressure (pi)-area (A) isotherm and surface shear viscosity (eta(s)) were obtained at 20 degrees C and at pH 7 in a modified Wilhelmy-type film balance. The structural and shear characteristics of the mixed films depend on the surface pressure and on the composition of the mixed film. At surface pressures lower than the equilibrium surface pressure of Na-caseinate (at pipi(e)(CS) have important repercussions on the shear characteristics of the mixed films.

Monoglycerides and beta-lactoglobulin adsorbed films at the air-water interface. structure, microscopic imaging, and shear characteristics.

In this work we have analyzed the structural, topographical, and shear characteristics of mixed monolayers formed by adsorbed beta-lactoglobulin (beta-lg) and spread monoglyceride (monopalmitin or monoolein) on a previously adsorbed protein film. Measurements of the surface pressure (pi)-area (A) isotherm, Brewster angle microscopy (BAM), and surface shear characteristics were obtained at 20 degrees C and at pH 7 in a modified Wilhelmy-type film balance. The pi-A isotherm and BAM images deduced for adsorbed beta-lactoglobulin-monoglyceride mixed films at pi lower than the equilibrium surface pressure of beta-lactoglobulin (pi(e)(beta-lg)) indicate that beta-lactoglobulin and monoglyceride coexist at the interface. However, the interactions between protein and monoglyceride are somewhat weak. At higher surface pressures (at pi > or = pi(e)(beta-lg)) a protein displacement by the monoglyceride from the interface takes place. The surface shear viscosity (eta(s)) of mixed films is very sensitive to protein-monoglyceride interactions and displacement as a function of monolayer composition (protein/monoglyceride fraction) and surface pressure. Shear can induce change in the morphology of monoglyceride and beta-lactoglobulin domains, on the one hand, and segregation between domains of the film-forming components on the other hand. In addition, the displacement of beta-lactoglobulin by the monoglycerides is facilitated under shear conditions.

Limited enzymatic hydrolysis can improve the interfacial and foaming characteristics of beta-conglycinin.

In this contribution, we have determined the effect of limited enzymatic hydrolysis on the interfacial (dynamics of adsorption and surface dilatational properties) and foaming (foam formation and stabilization) characteristics of a soy globulin (beta-conglycinin, fraction 7S). The degree of hydrolysis (DH = 0, 2, and 5%), the pH of the aqueous solution (pH = 5 and 7), and the protein concentration in solution (at 0.1, 0.5, and 1 wt %) were the variables studied. The temperature and the ionic strength were maintained constant at 20 degrees C and 0.05 M, respectively. The rate of adsorption and surface dilatational properties (surface dilatational modulus, E, and loss angle) of beta-conglycinin at the air-water interface depend on the pH and DH. The adsorption decreased drastically at pH 5.0, close to the isoelectric point of beta-conglycinin, because of the existence of a lag period and a low rate of diffusion. The interfacial characteristics of beta-conglycinin are much improved by enzymatic treatment, especially in the case of acidic aqueous solutions. Hydrolysates with a low DH have improved functional properties (mainly foaming capacity and foam stability), especially at pH values close to the isoelectric point (pI), because the protein is more difficult to convert into a film at fluid interfaces at pH approximately equal to pI.

Self-assembly of monoglycerides in beta-lactoglobulin adsorbed films at the air-water interface. Structural, topographical, and rheological consequences.

In this work, we have analyzed the structural, topographical, and surface dilatational characteristics of pure beta-lactoglobulin adsorbed films and the effect of the self-assembly of monoglycerides (monopalmitin or monoolein) in beta-lactoglobulin films at the air-water interface. Measurements were performed in a single device that incorporates a Wilhelmy-type film balance, Brewster angle microscopy, and interfacial dilatational rheology. The structural and topographical characteristics of beta-lactoglobulin adsorbed and spread films are similar. However, the surface dilatational modulus of beta-lactoglobulin films shows a complex behavior depending on film formation. The self-assembly of monoglyceride in a beta-lactoglobulin adsorbed film has an effect on the structural, topographical, and dilatational properties of the mixed films, depending on the interfacial composition and the surface pressure (pi). At low pi, a mixed film of monoglyceride and beta-lactoglobulin may exist. At high pi (after the collapse of beta-lactoglobulin), the mixed films are dominated by monoglyceride molecules. However, the small amounts of collapsed beta-lactoglobulin have a significant effect on the surface dilatational properties of the mixed films. Protein displacement by monoglyceride is higher for monopalmitin than for monoolein. However, some degree of interaction exists between proteins and monoglycerides, and these interactions are more evident in adsorbed films than in spread films.

Structural characteristics of dipalmitoylphosphatidylcholine and hydrolysates from sunflower protein isolate mixed monolayers spread at the air-water interface.

In this work, surface film balance and Brewster angle microscopy techniques have been used to analyze the structural characteristics (structure, topography, reflectivity, thickness, miscibility, and interactions) of hydrolysates from sunflower protein isolate (SPI) and dipalmitoylphosphatidylcholine (DPPC) mixed monolayers spread on the air-water interface. The degree of hydrolysis (DH) of SPI, low (5.62%), medium (23.5%), and high (46.3%), and the protein/DPPC mass fraction were analyzed as variables. The structural characteristics of the mixed monolayers deduced from the surface pressure (pi)-area (A) isotherms depend on the interfacial composition and degree of hydrolysis. At surface pressures lower than the equilibrium surface pressure of SPI hydrolysate (pi(e)(SPI hydrolysate)), both DPPC and protein are present in the mixed monolayer. At higher surface pressures (at pi > pi(e)(SPI hydrolysate)), collapsed protein residues may be displaced from the interface by DPPC molecules. The differences observed between pure SPI hydrolysates and DPPC in reflectivity (I) and monolayer thickness during monolayer compression have been used to analyze the topographical characteristics of SPI hydrolysates and DPPC mixed monolayers at the air-water interface. The topography, reflectivity, and thickness of mixed monolayers confirm at microscopic and nanoscopic levels the structural characteristics deduced from the pi-A isotherms.

Thermodynamic and dynamic characteristics of monoglyceride monolayers penetrated by beta-casein.

In this work, we have analyzed the dynamics of the penetration of beta-casein into monoglyceride monolayers (monopalmitin and monoolein) and the structural, dilatational, and topographical characteristics of mixed films formed by monoglyceride penetrated by beta-casein. Different complementary experimental techniques [dynamic tensiometry, surface film balance, Brewster angle microscopy (BAM), and surface dilatational rheology] have been used, maintaining the temperature constant at 20 degrees C and the pH at 7. The surface pressure of the monoglyceride monolayer at the beginning of the penetration process (at pi(i)MP and pi(i)MO for monopalmitin and monoolein, respectively) was the variable studied. beta-Casein can penetrate into a spread monoglyceride monolayer at every surface pressure. The penetration of beta-casein into the monoglyceride monolayer with a more condensed structure, at the collapse point of the monoglyceride, is a complex process that is facilitated by monoglyceride molecular loss by collapse and/or desorption. However, the structural, topographical, and dilatational characteristics of the monoglyceride penetrated by beta-casein mixed monolayers are essentially dominated by the presence of the monoglyceride (either monopalmitin or monoolein) in the mixed film.

The Effect of monoglycerides on structural and topographical characteristics of adsorbed beta-casein films at the air-water interface.

The effect of monoglycerides (monopalmitin and monoolein) on the structural and topographical characteristics of beta-casein adsorbed film at the air-water interface has been analyzed by means of surface pressure (pi)-area (A) isotherms and Brewster angle microscopy (BAM). At surface pressures lower than that for the beta-casein collapse (pi(c)(beta-casein)), attractive interactions between beta-casein and monoglycerides were observed. At higher surface pressures, the collapsed beta-casein is partially displaced from the interface by monoglycerides. However, beta-casein displacement by monoglycerides is not quantitative at the monoglyceride concentrations studied in this work. From the results derived from these experiments, we have concluded that interactions, miscibility, and displacement of proteins by monoglycerides in adsorbed mixed monolayers at the air-water interface depend on the particular protein-monoglyceride system, the interactions between film-forming components being higher for adsorbed than for spread films. The adsorbed films are more segregated than spread films, and both collapsed protein domains and monoglyceride domains in adsorbed films are smaller than for spread films.

Thermodynamic and dynamic characteristics of hydroxypropylmethylcellulose adsorbed films at the air-water interface.

Surface pressure isotherms and structural and surface dilatational properties of three hydroxypropylmethycelluloses (HPMCs, called E4M, E50LV, and F4M) adsorbed films at the air-water interface were determined. In this work we present evidence that HPMC molecules are able to diffuse and saturate the air-water interface at very low concentrations in the bulk phase. As bulk concentration increased, structural changes at a molecular level occurred at the interface. These changes corresponded to transition from an expanded structure (structure I) to a condensed one (structure II). When the surface concentration of HPMC was high enough, the collapse of the monolayer was observed. The three HPMCs formed very elastic films at the air-water interface, even at low surface pressures. E4M showed features that make it unique. For instance it showed the highest surface activity, mainly at low bulk concentrations (<10(-4) wt %). The differences observed in surface activity may be attributed to differences in the hydroxypropyl molar substitution and molecular weight of HPMC. All three HPMCs formed films of similar viscoelasticity and elastic dilatational modulus, which can be accounted for by their similar degree of methyl substitution.

Spreading of monoglycerides onto beta-casein adsorbed film. Structural and dilatational characteristics.

The effect of monoglycerides (monopalmitin and monoolein) on the structural, topographical, and dilatational characteristics of betacasein adsorbed film at the air-water interface has been analyzed by means of surface pressure (pi)-area (A) isotherms, Brewster angle microscopy (BAM), and surface dilatational rheology. The static and dynamic characteristics of the mixed films depend on the interfacial composition and the surface pressure. At surface pressures lower than that for the beta-casein collapse (at the equilibrium surface pressure of the protein, pi(e)(beta-casein)) a mixed film of beta-casein and monoglyceride may exist. At higher surface pressures the collapsed beta-casein is partially displaced from the interface by monoglycerides. However, beta-casein displacement by monoglycerides is not quantitative at the monoglyceride concentrations studied in this work. The protein displacement by a monoglyceride is higher for monopalmitin than for monoolein and for spread than for adsorbed films. The viscoelastic characteristics of the mixed films were dominated by the presence of beta-casein in the mixture. Even at the higher surface pressures (at pi > pi(e)(beta-casein)) the small amounts of beta-casein collapsed residues at the interface have a significant effect on the surface dilatational properties of the mixed films. The structural, topographical, and viscoelastic characteristics of the mixed films corroborate the fact that protein displacement for monoglycerides is higher for spread than for adsorbed mixed films.

Structural characteristics of hydrolysates of proteins from extracted sunflower flour at the air-water interface.

The structural and topographical characteristics of a sunflower protein isolate (SPI) and its hydrolysates at different degrees of hydrolysis (DH = 5.62%, 23.5%, and 46.3%) spread at the air-water interface at pH 7 and 20 degrees C were determined from pi-A isotherms coupled with Brewster angle microscopy (BAM). The structural characteristics of SP hydrolysate spread monolayers depend on the degree of hydrolysis. We observed a significant shift of the pi-A(APPARENT) isotherms toward lower molecular areas as the degree of hydrolysis (DH) increased. This phenomenon was attributed to spreading of the protein at the interface, especially at DH 46.3%. A change in the monolayer structure was observed at a surface pressure of 12-15 mN/m. At a microscopic level, the heterogeneous monolayer structures visualized near the monolayer collapse and during the monolayer expansion proved the existence of large regions of protein aggregates. Reflectivity increased with surface pressure and was a maximum at the monolayer collapse. The monolayer thickness decreased as the degree of hydrolysis increased. These phenomena explain the poor functional properties for the formation and stabilization of a dispersion (emulsion or foam) of protein hydrolysates at high degrees of hydrolysis.

Effect of enzymatic treatment of extracted sunflower proteins on solubility, amino acid composition, and surface activity.

Industrial proteins from agriculture of either animal or vegetable origin, including their peptide derivatives, are of great importance, from the qualitative and quantitative point of view, in food formulations (emulsions and foams). A fundamental understanding of the physical, chemical, and functional properties of these proteins is essential if the performance of proteins in foods is to be improved and if underutilized proteins, such as plant proteins (and their hydrolysates and peptides derivatives), are to be increasingly used in traditional and new processed food products (safe, high-quality, health foods with good nutritional value). In this contribution we have determined the main physicochemical characteristics (solubility, composition, and analysis of amino acids) of a sunflower protein isolate (SPI) and its hydrolysates with low (5.62%), medium (23.5%), and high (46.3%) degrees of hydrolysis. The hydrolysates were obtained by enzymatic treatment with Alcalase 2.4 L for DH 5.62 and 23.5% and with Alcalase 2.4 L and Flavorzyme 1000 MG sequentially for DH 46.3%. The protein concentration dependence on surface pressure (surface pressure isotherm), a measure of the surface activity of the products (SPI and its hydrolysates), was obtained by tensiometry. We have observed that the degree of hydrolysis has an effect on solubility, composition, and content of the amino acids of the SPI and its hydrolysates. The superficial activity and the adsorption efficiency were also affected by the degree of hydrolysis.

Shear characteristics, miscibility, and topography of sodium caseinate-monoglyceride mixed films at the air-water interface.

In this contribution, we are concerned with the study of structure, topography, and surface rheological characteristics (under shear conditions) of mixed sodium caseinate and monoglycerides (monopalmitin and monoolein) at the air/water interface. Combined surface chemistry (surface film balance and surface shear rheometry) and microscopy (Brewster angle microscopy, BAM) techniques have been applied in this study to mixtures of insoluble lipids and sodium caseinate spread at the air-water interface. At a macroscopic level, sodium caseinate and monoglycerides form an heterogeneous and practically immiscible monolayer at the air-water interface. The images from BAM show segregated protein and monoglyceride domains that have different topography. At surface pressures higher than that for the sodium caseinate collapse, this protein is displaced from the interface by monoglycerides. These results and those derived from interfacial shear rheology (at a macroscopic level) appear to support the idea that immiscibility and heterogeneity of these emulsifiers at the interface have important repercussions on the shear characteristics of the mixed films, with the alternating flow of segregated monoglyceride domains (of low surface shear viscosity, etas) and protein domains (of high etas) across the canal.

Surface shear rheology of WPI-monoglyceride mixed films spread at the air-water interface.

Surface shear viscosity of food emulsifiers may contribute appreciably to the long-term stability of food dispersions (emulsions and foams). In this work we have analyzed the structural, topographical, and shear characteristics of a whey protein isolate (WPI) and monoglyceride (monopalmitin and monoolein) mixed films spread on the air-water interface at pH 7 and at 20 degrees C. The surface shear viscosity (etas) depend on the surface pressure and on the composition of the mixed film. The surface shear viscosity varies greatly with the surface pressure. In general, the greater the surface pressure, the greater are the values of etas. The values of etas for the mixed WPI-monoolein monolayer were more than one order of magnitude lower than those for a WPI-monopalmitin mixed film, especially at the higher surface pressures. At higher surface pressures, collapsed WPI residues may be displaced from the interface by monoglyceride molecules with important repercussions on the shear characteristics of the mixed films. A shear-induced change in the topography and a segregation between domains of the film forming components were also observed. The displacement of the WPI by the monoglycerides is facilitates under shear conditions, especially for WPI-monoolein mixed films.