Protein-stabilized foams and emulsions

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.

Salts soaking treatment for improving the textural and functional properties of trifoliate yam (Dioscorea dumetorum) hardened tubers

Processing of Dioscorea dumetorum hardened tubers into flour could be a means of adding a longer-term value to this tropical plant with a high nutritional potential, but which presents a postharvest hardening problem characterized by a hard-to-cook defect. In an attempt to investigate the changes leading to salts soaking treatment of hardened tubers, the central composite rotatable design for K= 2 was used to study the combined effect of NaCl concentration (0% to 6%) and pH (4 to 10) on tubers cooked hardness, and 6 kanwa alkaline salt concentrations (1:3 (w/v); 0%, 0.2%, 0.5%, 0.8%, 1%, and 1.5%; pH 11.3 +/- 0.2) were used to study the effect of kanwa treatment on tuber cooked hardness and functional properties of resulting flours. The results showed that salts soaking treatment significantly decreased (P</= 0.05) tubers cooked hardness, independently of the solution pH, but could not totally overcome the tubers hardening phenomenon, as a consequence of the multiple mechanisms of D. dumetorum tuber hardening. Nevertheless, kanwa concentrations of 0.8% to 1.5% could be used to tenderize hardened tubers prior to its transformation in flour. Except oil absorption capacity, initial soaking of hardened tubers in kanwa solutions significantly influenced (P</= 0.05) the functional properties of the resulting flours. Water absorption capacity increased with increasing of kanwa concentration while the other properties evaluated (water solubility index, hydrophilic-lipophilic index, bulk density, swelling capacity, least gel-forming concentration) decreased.

The investigation of protein adsorption behaviors on different functionalized polymers films

The adsorption of BSA and fibrinogen onto plasma-polymerized di-(ethylene glycol) vinyl ether, allylamine, and maleic anhydride films were investigated in detail by surface plasmon resonance spectroscopy (SPR). The chemical properties of the plasma polymers were initially determined by the plasma deposition conditions during the generation procedure. The analysis of the chemical structure of the films and the refractive index of plasma polymers in aqueous solution was carried out using Fourier transform infrared spectroscopy and waveguide mode spectroscopy, respectively. Using water contact angle measurement, the surface wettability of plasma polymers was also characterized. These properties have a critical influence on the behavior of protein adsorption on the surface of the plasma polymers. Protein adsorption was found to depend not only on the types of functionalized groups, but also on the plasma polymer thickness since the protein molecules penetrate into the plasma polymer network bulk. According to the size of protein molecules in aqueous solution and the amount of adsorbed proteins observed by SPR, the conformational changes of proteins could be deduced.

Implantable biodegradable sponges: effect of interpolymer complex formation of chitosan with gelatin on the release behavior of tramadol hydrochloride

The effect of interpolymer complex formation between positively charged chitosan and negatively charged gelatin (Type B) on the release behavior of tramadol hydrochloride from biodegradable chitosan-gelatin sponges was studied. Mixed sponges were prepared by freeze-drying the cross-linked homogenous stable foams produced from chitosan and gelatin solutions where gelatin acts as a foam builder. Generation of stable foams was optimized where concentration, pH of gelatin solution, temperature, speed and duration of whipping process, and, chitosan-gelatin ratio drastically affect the properties and the stability of the produced foams. The prepared sponges were evaluated for their morphology, drug content, and microstructure using scanning electron microscopy, mechanical properties, uptake capacity, drug release profile, and their pharmacodynamic activity in terms of the analgesic effect after implantation in Wistar rats. It was revealed that whipping 7% (w/w) gelatin solution, of pH 5.5, for 15 min at 25 degrees C with a stirring speed of 1000 rpm was the optimum conditions for stable gelatin foam generation. Moreover, homogenous, uniform chitosan-gelatin foam with small air bubbles were produced by mixing 2.5% w/w chitosan solution with 7% w/w gelatin solution in 1:5 ratio. Indeed, polyionic complexation between chitosan and gelatin overcame the drawbacks of chitosan sponge mechanical properties where, pliable, soft, and compressible sponge with high fluid uptake capacity was produced at 25 degrees C and 65% relative humidity without any added plasticizer. Drug release studies showed a successful retardation of the incorporated drug where the t50% values of the dissolution profiles were 0.55, 3.03, and 4.73 hr for cross-linked gelatin, un-cross-linked chitosan-gelatin, and cross-linked chitosan-gelatin sponges, respectively. All the release experiments followed Higuchi's diffusion mechanism over 12 hr. The achieved drug prolongation was a result of a combined effect of both cross-linking and polyelectrolyte complexation between chitosan and gelatin. The analgesic activity of the implanted tramadol hydrochloride mixed chitosan-gelatin sponge showed reasonable analgesic effect that was maintained for more than 8 hr. Therefore, the use of chitosan and gelatin together appears to allow the formulator to manipulate both the drug release profiles and the mechanical properties of the sponge that could be effectively implanted.

Comparisons of the foaming and interfacial properties of whey protein isolate and egg white proteins

Whipped foams (10%, w/v protein, pH 7.0) were prepared from commercially available samples of whey protein isolate (WPI) and egg white protein (EWP), and subsequently compared based on yield stress (tau(0)), overrun and drainage stability. Adsorption rates and interfacial rheological measurements at a model air/water interface were quantified via pendant drop tensiometry to better understand foaming differences among the ingredients. The highest tau(0) and resistance to drainage were observed for standard EWP, followed by EWP with added 0.1% (w/w) sodium lauryl sulfate, and then WPI. Addition of 25% (w/w) sucrose increased tau(0) and drainage resistance of the EWP-based ingredients, whereas it decreased tau(0) of WPI foams and minimally affected their drainage rates. These differing sugar effects were reflected in the interfacial rheological measurements, as sucrose addition increased the dilatational elasticity for both EWP-based ingredients, while decreasing this parameter for WPI. Previously observed relationships between tau(0) and interfacial rheology did not hold across the protein types; however, these measurements did effectively differentiate foaming behaviors within EWP-based ingredients and within WPI. Interfacial data was also collected for purified beta-lactoglobulin (beta-lg) and ovalbumin, the primary proteins of WPI and EWP, respectively. The addition of 25% (w/w) sucrose increased the dilatational elasticity for adsorbed layers of beta-lg, while minimally affecting the interfacial rheology of adsorbed ovalbumin, in contrast to the response of WPI and EWP ingredients. These experiments underscore the importance of utilizing the same materials for interfacial measurements as used for foaming experiments, if one is to properly infer interfacial information/mechanisms and relate this information to bulk foaming measurements. The effects of protein concentration and measurement time on interfacial rheology were also considered as they relate to bulk foam properties. This data should be of practical assistance to those designing aerated food products, as it has not been previously reported that sucrose addition improves the foaming characteristics of EWP-based ingredients while negatively affecting the foaming behavior of WPI, as these types of protein isolates are common to the food industry.

Foaming and interfacial properties of hydrolyzed beta-lactoglobulin

beta-lactoglobulin (beta-lg) was hydrolyzed with three different proteases and subsequently evaluated for its foaming potential. Foam yield stress (tau0) was the primary variable of interest. Two heat treatments designed to inactivate the enzymes, 75 degrees C/30 min and 90 degrees C/15 min, were also investigated for their effects on foam tau0. Adsorption rates and dilatational rheological tests at a model air/water interface aided data interpretation. All unheated hydrolysates improved foam tau0 as compared to unhydrolyzed beta-lg, with those of pepsin and Alcalase 2.4L(R) being superior to trypsin. Heat inactivation negatively impacted foam tau0, although heating at 75 degrees C/30 min better preserved this parameter than heating at 90 degrees C/15 min. All hydrolysates adsorbed more rapidly at the air/water interface than unhydrolyzed beta-lg, as evidenced by their capacity to lower the interfacial tension. A previously observed relationship between interfacial dilatational elasticity (E') and tau0 was generally confirmed for these hydrolysates. Additionally, the three hydrolysates imparting the highest tau0 not only had high values of E' (approximately twice that of unhydrolyzed beta-lg), they also had very low phase angles (essentially zero). This highly elastic interfacial state is presumed to improve foam tau0 indirectly by improving foam stability and directly by imparting resistance to interfacial deformation.

Variation of saturated surface density of ovalbumin on bubble surface in continuous foam separation

The adsorption of ovalbumin (OA) onto the bubble surfaces was studied with various pHs (3.5, 4.6, 6.0 and 8.0) by a continuous foam separation technique. From the value of the saturated surface density of adsorbed OA, the variation of effective diameter (D) of an OA molecule on the bubble surface was estimated for various pHs (3.5, 4.6, 6.0 and 8.0) of the OA solutions, assuming that the cross section of the OA molecules be circular and that the OA molecules adsorb on the bubble surface in a closest packing structure. The estimated variation of D with pH was attempted to explain based on a model modified from that proposed by Pujar and Zydney. The modified model could well reproduce the variation of the effective diameter with pH; the values of D calculated on the basis of the modified model almost agreed with that estimated from the saturated surface density in the present experimental pH range. From these, conclusion was drawn that the modified model presented in this study can express the variation in the effective diameter with pH.

The Effect of Monoglycerides on Structural and Topographical Characteristics of Adsorbed β-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.

Evolution of liquid holdup profile in a standing protein stabilized foam

Evolution of liquid holdup profile in a standing foam formed by whipping and stabilized by sodium caseinate in the presence of xanthan gum when subjected to 16 and 29g centrifugal force fields was measured using magnetic resonance imaging for different pH, ionic strength, protein and xanthan gum concentrations. Drainage resulted in the formation of a separate liquid layer at the bottom at longer times. Foam drainage was slowest at pH 7, lower ionic strength, higher protein and gum concentrations. Foam was found to be most stable at pH 5.1 near the isoelectric point of protein, lower ionic strength and higher protein and xanthan gum concentrations. A predicted equilibrium liquid holdup profile based on a previous model (G. Narsimhan, J. Food Eng. 14 (1991) 139) agreed well with experimental values at sufficiently long times. A proposed model for velocity of drainage of a power law fluid in a Plateau border for two different simplified geometries was incorporated in a previously developed model for foam drainage (G. Narsimhan, J. Food Eng. 14 (1991) 139) to predict the evolution of liquid holdup profiles. The model predictions for simplified circular geometry of Plateau border compared well with the experimental data of liquid holdup profiles at small times. At longer times, however, the predicted liquid holdup profile was larger than the observed, this discrepancy being due to coarsening of bubble size and decrease in foam height not accounted for in the model. A Newtonian model for foam drainage under predicted drainage rates did not agree with the experimental data.

Enhanced stabilization of aerosol-OT surfactant monolayer upon interaction with small amounts of bovine serum albumin at the air–water interface

An investigation is made of the influence from small amounts of the protein bovine serum albumin (BSA) on the lateral organization of low molecular weight surfactant sodium bis-2-ethylhexyl sulfosuccinate (AOT) at the air-water interface. Surface pressure (pi - A), surface potential (deltaV - A) and Brewster angle microscopy (BAM) experiments were carried out, with particular emphasis on the monolayer stability under successive compression-expansion cycles. AOT monolayer is not stable at the air-water interface, which means that the majority of AOT molecules go into the aqueous subphase as monomers and/or normal micelles. When a waiting time elapses between spreading and compression, the surfactant monolayer tends to reorganize partially at the air-water interface, with a monolayer expansion being observed for waiting times as large as 12 h. The incorporation of very small amount of BSA (10(-9)M) at the interface, also inferred from BAM, increases the monolayer stability as revealed by pi - A and deltaV - A results. For a waiting time of circa 3 h, the mixed monolayer reaches its maximum stability. This must be related to protein (and/or protein-surfactant complexes) adsorbed onto the AOT monolayer, thus altering the BSA conformation to accommodate its hydrophobic/hydrophilic residues. Furthermore, the effects from such small amounts of BSA in the monolayer formation and stabilization mean that the AOT monolayer responds cooperatively to BSA.

Proteins and emulsifiers at liquid interfaces

The interfacial properties of proteins and emulsifiers have been studied extensively in the field of food colloid research. Emulsions form the basis of a huge range of food products and are generally stabilised by either protein and/or emulsifiers. Proteins have been shown to stabilise emulsions by forming a viscoelastic, adsorbed layer on the oil droplets, which form a physical barrier to coalescence. Emulsifiers can be oil or water soluble, forming a fluid, close-packed layer at the interface with a low interfacial tension. This results in an emulsion with a small droplet size distribution, stabilised by the fluid Gibbs-Marangoni mechanism or weak electrostatic repulsion. In real food emulsions, there is usually a mixture of proteins and emulsifiers competing for the interfacial area. This can produce a finer emulsion, however, the emulsifiers break down the viscoelastic protein-adsorbed layer, resulting in an emulsion with reduced stability. We present a review recent work that aims to characterise the composition, structure and physical properties of mixed protein-emulsifier interfaces, in an effort to understand the mechanisms behind the stability behaviour of food emulsion systems.