Whey Protein Isolate Solutions Research Articles

Physical properties of edible films made from mixtures of sodium caseinate and WPI

Abstract The physical properties of thin films (25–30 μm) made from mixtures of sodium caseinate (NaCas) and whey protein isolate (WPI) were investigated. Films were formed by mixing solutions of NaCas (2.5% w/w protein), plasticised with glycerol (NaCas–gly) at a glycerol:protein ratio of 0.32, with WPI solutions (2.3% w/w protein), plasticised (WPI-gly) at a glycerol:protein ratio 0.37. Tensile and water barrier properties of films formed from mixtures of NaCas–gly and WPI-gly were similar to films containing NaCas–gly only. Films containing only WPI-gly had higher maximum load and elastic modulus values than the mixed films. Increasing the NaCas–gly content of the films from 25 to 100% greatly increased solubility. This increased film solubility may increase the number of food applications for protein-based films.

Heat-induced changes in the ultrasonic properties of whey proteins.

The physical aggregation of commercial whey protein isolate (WPI) and purified beta-lactoglobulin was studied by ultrasound spectroscopy. Protein samples were dialyzed to achieve constant ionic strength backgrounds of 0.01 and 0.1 NaCl, and gelation was induced in situ at constant temperatures (from 50 to 75 degrees C) or with a temperature ramp from 20 to 85 degrees C. Changes in the ultrasonic properties were shown in the early stages of heating, at temperatures below those reported for protein denaturation. During heating, the relative ultrasound velocity (defined as the difference between sample velocity and reference velocity) decreased continuously with temperature, indicating a rearrangement of the hydration layer of the protein and an increase in compressibility of the protein shell. At temperatures <50 degrees C the ultrasonic attenuation decreased, and <65 degrees C both velocity and attenuation differentials showed increasing values. A sharp decrease in the relative velocity and an increase in the attenuation at 70 degrees C were indications of "classical" protein denaturation and the formation of a gel network. Values of attenuation were significantly different between samples prepared with 0.01 and 0.1 M NaCl, although no difference was shown in the overall ultrasonic behavior. WPI and beta-lactoglobulin showed similar ultrasonic properties during heating, but some differences were noted in the values of attenuation of WPI solutions, which may relate to a less homogeneous distribution of aggregates caused by the presence of alpha-lactalbumin and other minor proteins in WPI.

The antigenic response of ?-lactoglobulin is modulated by thermally induced aggregation

The antigenic response of thermally denatured and aggregated β-lactoglobulin was determined by an indirect competitive enzyme-linked immunosorbent assay using polyclonal antibodies from the egg yolk of chicken immunized with heat-denatured β-lactoglobulin as a measure for the potential antigenicity/allergenicity. The heat denaturation and the aggregation of heated whey protein isolate solutions were followed by reversed-phase high-performance liquid chromatography and photon correlation spectroscopy. Thermally modified whey proteins showed a remarkable increase of antigenicity when heated to 90 °C, possibly as a consequence of the exposure or formerly hidden epitopes. Above 90 °C, the antigenic response decreased owing to the loss of conformational epitopes and masking of sequential epitopes in the course of aggregation to particles. When large and compact particles were formed, the antigenicity was reduced remarkably. Depending the heating conditions applied, the structure and the size of whey protein particles and thus the potential allergenicity may be modulated in a wide range.

Effect of conductivity control on the separation of whey proteins by bipolar membrane electroacidification.

Since the limiting factor of the bipolar membrane electroacidification (BMEA) process at 20% WPI (whey protein isolate) was hypothesized to be the lack of mobile ion inherent to the protein solution at pH 5.0, the aim of the present work is to study the effect of the conductivity control on the precipitation behavior of whey protein. BMEA performances were evaluated by measuring electrodialytic parameters, protein kinetic precipitation, molecular profiles, and isolate chemical composition and purity. The highest protein precipitation with 10% WPI solution was obtained at pH 4.6 and at a conductivity level of 200 microS/cm maintained with many 0.4-mL additions of 1.0 M KCl (200 microS[+]), with a 46% precipitation of the total protein, beta-lg composing the main part of the precipitated protein. With a 20% WPI solution, it was possible to reach pH 4.65 with conductivity control at 350 microS/cm. However, the 27% protein precipitation was still low. The changes in viscosity as pH decreases observed at 20% WPI would decreased the final precipitation rate of beta-lg, since the viscosity of the 20% WPI dispersion was very different.

Physical properties and sensory evaluation of WPI films of varying thickness

The effect of varying thickness on the water barrier properties, tensile properties and sensory characteristics of glycerol-plasticised whey protein isolate (WPI) films was investigated. Thickness was varied by preparing films with increasingly dilute film forming WPI solutions in the range 9.5–2.3 g protein/100 g. All films had a glycerol to protein ratio of 0.37 (Gly: Pro). Tensile strength (TS), elastic modulus (EM) and film permeance were unaffected by film thickness but maximum load (ML) and % elongation (E) decreased ( P<0.05) with decreasing thickness. In a sensory test with crackers and melted cheese, panelists could readily detect the thickest films (79 μm) but not the thinnest films (23 μm) ( P<0.05). The results indicate that reducing the thickness of glycerol plasticised WPI films makes them less perceptible in a food system while maintaining moisture barrier and certain tensile properties.

Cross-linking and rheological changes of whey proteins treated with microbial transglutaminase.

Modification of the functionality of whey proteins using microbial transglutaminase (TGase) has been the subject of recent studies. However, changes in rheological properties of whey proteins as affected by extensive cross-linking with TGase are not well studied. The factors affecting cross-linking of whey protein isolate (WPI) using both soluble and immobilized TGase were examined, and the rheological properties of the modified proteins were characterized. The enzyme was immobilized on aminopropyl glass beads (CPG-3000) by selective adsorption of the biotinylated enzyme on avidin that had been previously immobilized. WPI (4 and 8% w/w) in deionized water, pH 7.5, containing 10 mM dithiothreitol was cross-linked using enzyme/substrate ratios of 0.12-10 units of activity/g WPI. The reaction was carried out in a jacketed bioreactor for 8 h at 40 degrees C with continuous circulation. The gel point temperature of WPI solutions treated with 0.12 unit of immobilized TGase/g was slightly decreased, but the gel strength was unaffected. However, increasing the enzyme/substrate ratio resulted in extensive cross-linking of WPI that was manifested by increases in apparent viscosity and changes in the gelation properties. For example, using 10 units of soluble TGase/g resulted in extensive cross-linking of alpha-lactalbumin and beta-lactoglobulin in WPI, as evidenced by SDS-PAGE and Western blotting results. Interestingly, the gelling point of WPI solutions increased from 68 to 94 degrees C after a 4-h reaction, and the gel strength was drastically decreased (lower storage modulus, G'). Thus, extensive intra- and interchain cross-linking probably caused formation of polymers that were too large for effective network development. These results suggest that a process could be developed to produce heat-stable whey proteins for various food applications.

Whey protein isolate coating on LDPE film as a novel oxygen barrier in the composite structure

AbstractTo examine the feasibility of whey protein isolate (WPI) coating as an alternative oxygen barrier for food packaging, heat‐denatured aqueous solutions of WPI with various levels of glycerol as a plasticizer were applied on corona‐discharge‐treated low‐density polyethylene (LDPE) films. The resulting WPI‐coated LDPE films showed good appearance, flexibility and adhesion between the coating and the base film, when an appropriate amount of plasticizer was added to the coating formulations. WPI‐coated LDPE films showed significant decrease in oxygen permeability (OP) at low to intermediate relative humidity, with an Arrhenius behaviour and an activation energy of 50.26 kJ/mol. The OP of the coated films increased significantly with increasing relative humidity, showing an exponential function. Although the coated films showed a tendency to have less oxygen barrier and more glossy surfaces with increasing plasticizer content, differences in the OP and gloss values were not significant. Haze index and colour of the coated films were also little influenced by WPI coating and plasticizer content. The results suggest that whey protein isolate coating could work successfully as an oxygen barrier and have potential for replacing synthetic plastic oxygen‐barrier layers in many laminated food packaging structures. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Influence of κ-carrageenan on the aggregation behaviour of proteins in heated whey protein isolate solutions

The influence of the addition of κ-carrageenan to 2% whey protein isolate (WPI) at pH 7 and heated at 75 °C for different times, has been investigated by size-exclusion chromatography with multi-angle laser light scattering and gel electrophoresis. Under the conditions used, the addition of κ-carrageenan to WPI did not influence the loss of native β-lactoglobulin or α-lactalbumin. However, κ-carrageenan addition affected the molecular weight ( M w) of the aggregates formed by the heat treatment. The M w of the whey protein aggregates formed in the early stages of heating in the presence of κ-carrageenan were lower than the aggregates formed in WPI solutions heated in the absence of κ-carrageenan. At longer heating times, the M w of the aggregates in the WPI/κ-carrageenan mixtures were similar to the M w of the aggregates in the WPI solutions heated alone.

Role of Covalent and Noncovalent Interactions in the Formation of Films from Unheated Whey Protein Solutions Following pH Adjustment

ABSTRACT: Films were formed from heated whey protein isolate (WPI) solutions (heated [H] films) and from unheated WPI solutions following adjustment to pH 11, with subsequent readjustment to pH 7 (unheated, readjusted [UR] films) or without readjustment to pH 7 (unheated, unadjusted films [UU] films). UU and UR films had significantly lower % elongation, tensile strength, and Young's modulus than H films. Film solubility and dispersion in water were in the order: H films &lt; UU films &lt; UR films. Free sulphydryl groups were lower and disulphide‐mediated polymerization was higher in heated than in unheated WPI solutions whereas solubility of H films increased in the presence of dithiothreitol.

Firmness of pressure-induced casein and whey protein gels modulated by holding time and rate of pressure release

The main protein fractions of milk, casein and whey protein, may react differently to processing due to their amino acid sequence and conformation and their ability to build up intermolecular covalent and non-covalent bonds. Gels made from solutions of micellar casein and whey protein isolate were induced by applying ultra-high pressure (600 MPa/30 °C). The pressure built up was held constant at 200 MPa min −1, while the holding time of 0.15 and 30 min was varied as well as the pressure release rate (20, 200, 600 MPa min −1). The firmness of the pressure-induced whey protein gel was essentially influenced by the degree of whey protein denaturation and determined by the amount of disulfide bonds stabilizing the gel microstructure. In contrast, the firmness of pressure-induced casein-based gels was mainly influenced by pressure release rate. A slow pressure release produced a rough and weak gel. A high release rate, however, caused a homogeneous microstructure to be formed that created a high level of firmness.

Use of applied air pressure to improve the baking properties of whey protein isolates in angel food cakes

Whey protein isolate (WPI) possesses limited application in angel food cake baking compared to liquid egg white (LEW). This study was conducted to determine whether applying air pressure in the oven during baking would improve the baking properties of WPI in angel food cakes. A special oven was designed for baking at oven air pressures up to 1.5 bar. Control angel food cakes were formulated with LEW (100/0) as its protein source and WPI-containing cakes were formulated with a mixture of 75 mL/100 mL LEW and 25 mL/100 mL WPI solution (75/25) or a mixture of 50 mL/100 mL LEW and 50 mL/100 mL WPI solution (50/50). Cakes were baked at atmospheric air pressure (AP) and at constant applied air pressure (CAP) or variable applied air pressure vs. baking time (VAP) to prevent overexpansion and collapse of WPI-containing cake batter. Cakes 75/25 and 50/50 baked at VAP exhibited improved physical, textural and sensory properties compared to those baked at AP or CAP conditions. Cakes 75/25 baked at VAP compared well with control angel food cakes baked at AP. Although 50/50 cakes baked at VAP were improved slightly over those baked at AP, none of them exhibited satisfactory properties. Therefore, additional research is needed to optimize baking conditions for cakes formulated with less than 75 mL/100 mL LEW.

Oxygen Barrier Properties of Whey Protein Isolate Coatings on Polypropylene Films

ABSTRACTOxygen permeation characteristics of whey protein isolate (WPI) coatings on polypropylene (PP) films were investigated to examine the feasibility of WPI coating as a novel biopolymer oxygen barrier for food packaging applications. Heat‐denatured aqueous solutions of WPI with several plasticizers including glycerol, sorbitol, sucrose, propylene glycol, and polyethylene glycol were applied on the surfaces of PP films previously treated with corona discharge. Among plasticizers used, sucrose conferred the best oxygen barrier property to the WPI‐coated films. Oxygen permeability (OP) of the resulting WPI‐coated films increased significantly with temperature, showing very good agreement with the Arrhenius model. OP of the coated films also increased exponentially with relative humidity.

Monitoring Entering and Spreading of Emulsion Droplets at an Expanding Air/Water Interface: A Novel Technique

The entering and spreading of emulsion droplets at quiescent and expanding air/water interfaces was studied using a new apparatus consisting of a modified Langmuir trough in which the air/water interface can be continuously expanded by means of rollers in the place of traditional barriers. When sodium caseinate and whey protein isolate-stabilized emulsion droplets were injected under the surface of sodium caseinate and whey protein isolate solutions, respectively, it appeared that the droplets entered the air/water interface only if the air/water surface pressure did not exceed a threshold value of ∼15 mN/m. This condition was satisfied either under quiescent conditions for low protein concentrations or by continuous expansion of the interface at higher protein concentrations. According to equilibrium thermodynamics, entering of the droplets and the formation of lenses should occur for all the systems investigated, but this was not observed. At surface pressures higher than ∼15 mN/m, immersed emulsion droplets were metastable. This is probably due to a kinetic barrier caused by the formation of a thin water film bounded by protein adsorption layers between the emulsion droplet and the air/water interface.

Effects of sucrose and sorbitol on the gel formation of a whey protein isolate

Thermal analysis by differential scanning calorimetry and dynamic rheology were used to evaluate possible effects of sucrose and sorbitol at various concentrations on the thermogelation of a 150 g kg −1 whey protein isolate solution at pH 6.0 and 8.5. At both pH values, addition of these solutes caused an increase in the thermal transition temperature of the protein denaturation/aggregation process T tr, with a linear relationship between solute concentration (on a w/w basis) and T tr. Dynamic oscillation tests showed that the gelation temperature T gel was raised accordingly. The transition enthalpy Δ H tr was only affected slightly and non-specific (changes within 1 J g −1 protein). Calculation of the apparent activation energy of the gelsetting E a app based on the temperature dependence of the gelation time t gel, as well as the G ∗ value, obtained after slow heating of the solutions, revealed effects of both sucrose and sorbitol at pH 6.0 but not at pH 8.5, especially at high concentrations of these solutes. This lead to the conclusion that the interactions between the polyhydric solutes and the whey proteins (mainly β-lactoglobulin) were of a very weak nature. Furthermore, based on differences in the gelation mechanism of whey proteins at pH 6.0 and 8.5, it was suggested that sucrose and sorbitol affect protein–protein interactions in gels through enhancement of hydrophobic interactions.

Influence of NaCl on optical properties, large-strain rheology and water holding capacity of heat-induced whey protein isolate gels

The influence of NaCl (0–1000 mM) on the physical properties of gels formed by heating 10 wt% whey protein isolate (WPI) solutions (pH 6.9) at 80 °C for 15 min has been studied. Elastic modulus, elastic recoverability and water loss of gels were measured during uniaxial compression/decompression. Optical properties of gels were characterized by spectral reflectance and colorimetry measurements. Below 60 mM NaCl, no gels were formed and the WPI solutions remained optically transparent. From 60 to 150 mM NaCl, gel elastic modulus increased, elastic recoverability remained high, water loss remained low and lightness remained low. From 150 to 200 mM NaCl, gel elastic modulus decreased, elastic recoverability decreased, water loss increased, and gel lightness increased. Above 200 mM NaCl, the elastic modulus, elastic recoverability, water loss and lightness of the gels were relatively insensitive to salt concentration. Theoretical predictions indicated that the lightness ( L) of dyed gels had a maximum value at protein particle radius of 100 nm and that L increased with protein particle concentration (especially from 0 to 1 wt%). Differences in gel physicochemical properties are interpreted in terms of differences in the microstructure and interactions of the protein molecules within the gel network.

Influence of glycerol on optical properties and large-strain rheology of heat-induced whey protein isolate gels

The influence of glycerol (0–50 wt%) on the properties of gels formed by heating 10 wt% whey protein isolate (WPI) solutions (pH 7.0, 100 mM NaCl) at 90 °C has been studied. The elastic modulus, breaking stress and breaking strain of the gels were measured using uniaxial compression. The optical properties of protein solutions were characterized by turbidity or colorimetry measurements. The onset of gelation at 90 °C was delayed in the presence of glycerol, presumably because the cosolvent increased the thermal stability of the globular proteins and reduced the protein–protein collision frequency. On the other hand, the elastic modulus, breaking stress and breaking strain of gels heated at 90 °C for 60 min increased with glycerol concentration, presumably because the glycerol increased the strength of the intermolecular attraction between fully denatured proteins. Glycerol decreased the lightness of the gels because it reduced the scattering efficiency of the protein aggregates. This study shows that glycerol can be used to modulate the physicochemical properties of whey protein gels.

Transport properties of chitosan and whey blended with poly(ε-caprolactone) assessed by standard permeability measurements and microcalorimetry

Blends of poly(ε-caprolactone) (PCL) with chitosan and a whey-protein-isolate (WPI) were prepared by solution mixing and film casting. The purpose was to increase the water vapour resistivity of chitosan and whey by blending them with a hydrophobic biodegradable polymer. The water vapour transmission rate was determined by a standard technique and by a new technique based on microcalorimetry. The blends were characterised by scanning electron microscopy (SEM), density measurements and thermogravimetry. Oxygen permeability was measured on the pure components and on some of the blends. The incorporation of PCL yielded a pronounced decrease in water vapour transmission rate of both chitosan and the WPI measured at a relative humidity gradient of 11 to 0%. A volume content of 17–18% of PCL lowered the water vapour transmission rate by 70–90%. It was found that the majority of the PCL particles were ellipsoidal in chitosan and fibrous in the WPI and the data indicated that the particle shape had an important influence on the water vapour transmission rate. The large decrease in water vapour transmission rate was also due to a reduction in water solubility because of limited swelling of the constrained chitosan or WPI matrix in the presence of PCL. SEM revealed that the miscibility/compatibility between PCL and the matrices was good. The water vapour transmission rate of the films decreased with increasing vacuum-drying time of the chitosan and WPI solutions. Microcalorimetry provided accurate estimates of water vapour transmission rate. Furthermore, this technique proved to be very flexible and the water vapour transmission rate could be determined over a broad range of relative humidities in a single experiment.

High pressure effect on foaming behaviour of whey protein isolate

The effect of pressure-processing at 150–450 MPa for 5–25 mm on foaming behaviour of whey protein isolate (WPI) have been investigated in pH range of 5.0–7.0. Assessment of foaming properties has been based on the total foam volume (FV) produced and the foam stability (FS), and the time for loss of half the initial FV. Response surface methodology has been applied to find the regression equations for predicting the FV and FS in terms of independent variables (the intensity of pressure, the treatment time and the pH). Protein solutions (1% w/v, 50 mM phosphate buffer) were pressure-processed and foams were produced from dilute solutions of WPI (0.005% w/v) by sparging with nitrogen gas at constant flow rate (0.2±0.01 dm 3/min) and constant temperature (20°C). It has been found that pH is significant ( P⩽0.01) on both FV and FS; FV is influenced significantly ( P⩽0.05) by pressure-intensity whereas the effect of pressure-treatment time on either FV or FS is not significant ( P⩽0.05). Foaming properties of WPI were reduced as the pH approached to the isoelectric point of proteins due to enhanced aggregation. Pressurisation at high-protein concentration (2% w/v) or in high-buffer molarity (100 mM) has been observed to diminish the foaming properties of WPI.

Characterization of food colloids by phase analysis light scattering

Recently a novel technique, phase analysis light scattering (PALS), has been developed to measure the electrophoretic mobility of charged particles that offers enhanced sensitivity at the low mobilities, typical of many food colloids. The principles of this technique are discussed in comparison to conventional laser Doppler electrophoresis (LDE). A sample of whey protein isolate solution (10 wt%) was titrated with HCl (0.1 M) and the electrophoretic mobility measured by PALS and LDE. Both methods showed an increase in electrophoretic mobility with pH, but PALS yielded better reproducibility. In addition, PALS could be used to measure the isoelectric point of the protein mixture as the pH where the mobility was zero.

Influence of sucrose on the thermal denaturation, gelation, and emulsion stabilization of whey proteins.

The influence of sucrose (0-40 wt %) on the thermal denaturation and functionality of whey protein isolate (WPI) solutions has been studied. The effect of sucrose on the heat denaturation of 0.2 wt % WPI solutions (pH 7.0) was measured using differential scanning calorimetry. Sucrose increased the temperature at which protein denaturation occurred, for example, by 6-8 degrees C for 40 wt % sucrose. The dynamic shear rheology of 10 wt % WPI solutions (pH 7.0, 100 mM NaCl) was monitored as they were heated from 30 to 90 degrees C and then cooled to 30 degrees C. Sucrose increased the gelation temperature and the final rigidity of the cooled gels. The degree of flocculation in 10 wt % oil-in-water emulsions stabilized by 1 wt % WPI (pH 7.0, 100 mM NaCl) was measured using a light scattering technique after they were heated at fixed temperatures from 30 to 90 degrees C for 15 min and then cooled to 30 degrees C. Sucrose increased the temperature at which maximum flocculation was observed and increased the extent of droplet flocculation. These results are interpreted in terms of the influence of sucrose on the thermal unfolding and aggregation of protein molecules.