Whey Protein Isolate Aggregates Research Articles

Formation and functionality of whey protein isolate–(kappa-, iota-, and lambda-type) carrageenan electrostatic complexes

The formation of electrostatic complexes between whey protein isolate (WPI) and (κ-, ι-, λ-type) carrageenan (CG) was investigated by turbidimetric measurements as a function of pH (1.5–7.0), biopolymer weight-mixing ratio (1:1–75:1 WPI:CG) and NaCl addition (0–500 mM) to better elucidate underlying mechanisms of interaction. Emulsion stabilizing effects of formed complexes was also studied to assess their potential as emulsifiers. Complex formation followed two pH-dependent structure-forming events associated with the formation of soluble (pH c) and insoluble (pH ϕ1) complexes. For both the WPI–κ-CG and WPI–ι-CG mixtures, pH c and pH ϕ1 occurred at pH 5.5 and 5.3, respectively, whereas in the WPI–λ-CG mixture values were slightly higher (pH c = 5.7; pH ϕ1 = 5.5). In all mixtures, maximum turbidity was found to occur near pH 4.5, before declining at lower pHs. Biopolymer mixing ratios corresponding to maximum OD was found to occur at the 12:1 ratio for both the WPI–κ-CG and WPI–λ-CG mixtures, and 20:1 ratio for WPI–ι-CG mixture. The addition of NaCl disrupted complexation within WPI–κ-CG mixtures as levels were raised, whereas when ι-CG and λ-CG was present, complexation was enhanced up to a critical Na + concentration before declining. Adsorption of CG chains to the small WPI–WPI aggregates during complexation was proposed to be related to both the linear charge density and conformation of the CG molecules involved. Emulsion stability in the mixed systems (12:1 mixing ratio), regardless of the CG type (κ, ι, λ), was significantly higher than individual WPI solutions indicating enhanced ability to stabilize the oil-in-water interface.

β-Lactoglobulin and WPI aggregates: Formation, structure and applications

The literature is reviewed on the formation and the structure of β-lactoglobulin and whey protein isolate (WPI) aggregates in aqueous solution induced by heating. The focus is on the effects of the pH and added salt. The use of β-lactoglobulin and WPI aggregates in cold-set gels, foams and emulsions, encapsulation, and films and coatings is also reviewed.

Phase Behavior of Whey Protein Aggregates/κ-Carrageenan Mixtures: Experiment and Theory

The phase separation behavior of whey protein isolate (WPI) aggregates and κ-carrageenan (κ-car) mixtures was studied using the Vrij's theory and image analysis method. The intrinsic parameter (molecular mass and radius of gyration) for κ-car and the WPI aggregates was determined using intrinsic viscosity and reduced viscosity of each biopolymer. Confocal microscopy observations revealed the appearance of protein aggregate domains when phase separation occurred, with microgel droplets of WPI included in a continuous κ-car phase. The occurrence of aggregate droplet has not been reported before for the phase-separating WPI/κ-car mixtures. So far, network emulsion-like microstructures have been observed with WPI in a network structure. By using different WPI concentrations (4% or 6%), the microstructure of the systems changes while increasing the κ-car concentration. The size of the microgels (1–2.5 μm) depends on both κ-car and WPI concentration. Confocal microscopy combined with image analysis (method of the variance) was used effectively as objective means to determine the phase boundary of the phase-separating systems. Additional information on the depletion layer thickness, Δ, was obtained using self-consistent field theory. The results show that Δ has a constant value of 80.5 nm for $$ {{\hbox{c}}_{\kappa {\rm{ - car}}}} \prec {\hbox{2 g}}/{l} $$ , in agreement with ∆ ≈ R g (radius of gyration). Above this concentration, Δ decreases as a function of κ-car concentration. The experimental phase boundary was well predicted using Vrij's theory. This work showed a new approach to generate phase diagrams (e.g., under shear) of phase-separating systems.

Alkali Cold Gelation of Whey Proteins. Part I: Sol−Gel−Sol(−Gel) Transitions

The cold gelation of preheated whey protein isolate (WPI) solutions at alkaline conditions (pH>10) has been studied to better understand the effect of NaOH in the formation and destruction of whey protein aggregates and gels. Oscillatory rheology has been used to follow the gelation process, resulting in novel and different gelation profiles with the gelation pH. At low alkaline pH, typical sol-gel transitions are observed, as in many other biopolymers. At pH>11.5, the system gels quickly, after approximately 300 s, followed by a slow degelation step that transforms the gel to a viscous solution. Finally, there is a second gelation step. This results in a surprising sol-gel-sol-gel transition in time at constant gelation conditions. At very high pH (>12.5), the degelation step is very severe, and the second gelation step is not observed, resulting in a sol-gel-sol transition. The first quick gelation step is related to the quick swelling of the WPI aggregates in alkali, as observed from light scattering, which enables the formation of new noncovalent interactions to form a gel network. These interactions are argued to be destroyed in the subsequent degelation step. Disulfide cross-linking is observed only in the second gelation step, not in the first step.

The Effect of Shear Rate on the Molecular Mass Distribution of Heat-Induced Aggregates of Mixtures Containing Whey Proteins and κ-Carrageenan

The present study attempts to characterize the effect of shear rate on the composition, size, and molecular weight of the protein aggregates present in the upper layer after phase separation of 5% whey protein isolate (WPI) mixed with 0.5% κ-carrageenan (κ-car) at pH 7.0. The mixtures were heated and sheared under different shearing rates. Size exclusion chromatography (SEC), dynamic light scattering, and static light scattering were employed to describe the effect of shear rate on the size and molecular mass of WPI aggregates. At the molecular level, the size of the aggregates increased with an increase in shear rate. Shear rate also caused a decrease in turbidity of the upper layer after centrifugation. SEC combined with multi-angle laser light scattering showed that the WPI aggregates molecular mass was between 106and 107 g/mol when the shear rate increased from 3.6 to 86.4 s−1. Two empirical models described well the effect of shear rate on the size of WPI aggregates, and both models gave comparable results. By varying process parameters such as flow behavior and temperature, it is possible to control WPI aggregation and, thus, obtain aggregates with a range of different characteristics (size).

Development and characterization of composite edible films based on whey protein isolate and mesquite gum

Physical properties and microstructure of films made of mixtures of whey protein isolate (WPI) and mesquite gum (MG) were investigated. A photoacoustic method was used to measure water vapor diffusivity. WPI and MG were totally compatible to make solutions and films. Results showed that film structure is based on a continuous network of WPI aggregates which enlarge their size as the MG content increases. Neither water vapor diffusivity nor permeability was affected significantly by MG content. However, the incorporation of increasing amounts of MG in the formulation resulted in films with improved flexibility, that is, with significantly lower tensile strength and higher elongation at break. This result suggests the use of MG to improve mechanical properties of WPI films as an alternative of using larger amounts of low molecular weight plasticizers.

EFFECT OF DROPLET–MATRIX INTERACTIONS ON LARGE DEFORMATION PROPERTIES OF EMULSION‐FILLED GELS

ABSTRACT The aim of this work was to identify the effect of droplet–matrix interactions on the large deformation properties of emulsion‐filled gels. A study was carried out on the behavior in compression of gelatin, whey protein isolate (WPI) and κ‐carrageenan gels containing emulsions stabilized by different emulsifying agents (WPI, WPI aggregates, lysozyme, polyoxyethylenesorbitan monolaurate [Tween 20]) to control droplet–matrix interactions. For gelatin gels, emulsions stabilized with WPI and lysozyme induced an increase of the modulus, while emulsions stabilized with Tween induced a decrease of the modulus. It was concluded that in the first two cases, the oil droplets were bound to the gel matrix and that in the latter case the oil droplets were unbound. For WPI gels, emulsions stabilized with WPI aggregates, lysozyme and low concentration of Tween induced an increase of the modulus (bound droplets). The modulus variations observed for κ‐carrageenan gels were mainly related to interactions between κ‐carrageenan and the emulsifying agent present in the aqueous phase. Theories based on the effect of the oil content on modulus satisfactorily fitted the trend of the experimental results, both for bound and unbound droplets. The fracture strain decreased with increasing oil concentration for droplets bound to the matrix and remained constant for unbound droplets, while the fracture stress was unaffected by bound droplets and decreased in the case of unbound droplets. Theories describing the effect of filler content on fracture strain and stress failed to predict the experimental results. This was attributed to the small size of the oil droplets of the emulsions used in this study, which was similar to the size of the inherent defects present in the gel network.PRACTICAL APPLICATIONSThis work deals with the mechanical properties of emulsion‐filled gels with different matrices and with different interactions between oil droplets and gel matrix. The effect of different interactions on the Young's modulus and the fracture properties of the gels is presented. Furthermore, the usefulness of several theories for the prediction of these properties is discussed. The gel systems studied resemble real food systems. Therefore, the obtained knowledge will enable the industry to engineer products with desired mechanical properties. This will make the process of new product development more efficient.

Light-Scattering Study of the Structure of Aggregates and Gels Formed by Heat-Denatured Whey Protein Isolate and β-Lactoglobulin at Neutral pH

The structure of aggregates and gels formed by heat-denatured whey protein isolate (WPI) has been studied at pH 7 and different ionic strengths using light scattering and turbidimetry. The results were compared with those obtained for pure beta-lactoglobulin (beta-Lg). WPI aggregates were found to have the same self-similar structure as pure beta-Lg aggregates. WPI formed gels above a critical concentration that varied from close to 100 g/L in the absence of added salt to about 10 g/L at 0.2 M NaCl. At low ionic strength (<0.05 M NaCl) homogeneous transparent gels were formed, while at higher ionic strength the gels became turbid but had the same self-similar structure as reported earlier for pure beta-Lg. The length scale characterizing the heterogeneity of the gels increased exponentially with increasing NaCl concentration for both WPI and pure beta-Lg, but the increase was steeper for the former.

Large enhancements in thermogelation of whey protein isolate by incorporation of very low concentrations of guar gum

The effect of the non-gelling polysaccharide guar gum on thermal gelation of whey protein isolate (WPI) was investigated by compression testing, oscillatory rheology and differential scanning calorimetry (DSC). All samples were prepared in 100 mM NaCl, with the concentration of WPI held constant at 3.0 wt%. Under these conditions, DSC heating scans (Setaram microcalorimeter; sample mass ≈ 850 mg; 1 °C/min) for WPI in the absence of guar gum showed an endothermic denaturation process with maximum heat flow at ∼76 °C and an exothermic aggregation process at ∼87 °C. On incorporation of increasing amounts of guar gum (0.0–0.5 wt%) the exotherm moved to progressively lower temperature, until eventually, at 0.5 wt% guar gum, it was obscured by the more intense denaturation endotherm. This is attributed to segregative interactions (“thermodynamic incompatibility”) with guar gum promoting aggregation of thermally denatured whey protein. After heating to 80 °C and cooling to induce gelation of the whey protein, Young's modulus ( E) at 20 °C rose to a maximum at ∼0.05 wt% added guar gum and decreased at higher concentrations, until at 0.5 wt% guar gum the mixture was liquid-like. A similar initial increase and subsequent decrease with increasing concentration of guar gum was observed by oscillatory measurements (1 rad s −1; 0.5% strain) of storage modulus ( G′), but the maximum occurred at ∼0.1 wt% guar gum. The difference in optimum concentration is ascribed to use of a different time–temperature regime in preparing the samples for compression testing. The reduction in gel strength observed by both techniques at high concentrations of guar gum is attributed to excessive aggregation of WPI in response to segregative interactions, with consequent collapse of the crosslinked network. However, at the very low optimum concentration of ∼0.1 wt% guar gum a large (∼12-fold) enhancement in gel strength ( G′) was evident in comparison to WPI alone.

Matrix properties affect the sensory perception of emulsion-filled gels

The breakdown properties and sensory perception of emulsion-filled gels with different matrices were studied at varying emulsion concentrations. The gel matrices used were cold-set whey protein isolate (WPI), gelatin, κ-carrageenan and a mixture of κ-carrageenan and ι-carrageenan. The oil-in-water emulsions added to the gels were either stabilised by native WPI or WPI aggregates. The emulsion droplets were homogeneously distributed in WPI gels, slightly aggregated in mixed κ/ ι-carrageenan and extensively aggregated in κ-carrageenan gels. For gelatin, gels with equal composition but with either non-aggregated or aggregated emulsion droplets could be prepared by changes in processing conditions. The gel modulus as determined by large deformation experiments increased at increasing emulsion concentration for WPI and gelatin gels, but decreased for κ-carrageenan gels. For mixed κ/ ι-carrageenan no effect of emulsion concentration on gel modulus was observed. Based on these observations, we concluded that the emulsion droplets were bound to the matrix in WPI and gelatin gels and unbound in carrageenan gels. Most of the mouthfeel attributes generated by a quantitative descriptive analysis (QDA) sensory panel discriminated between emulsion concentrations and different types of gel matrix. At increasing emulsion concentration the perceived creaminess of the samples increased. For gelatin, κ-carrageenan and mixed κ/ ι-carrageenan gels, i.e., for gels melting at the oral processing temperature or containing unbound oil droplets, the creaminess scores were consistently higher than for WPI gels, which were perceived as more rough. For gelatin gels, no effect of oil droplet aggregation on sensory perception was found.

COLD GELATION OF WHEY PROTEIN EMULSIONS

ABSTRACT Stable and homogeneous emulsion‐filled gels were prepared by cold gelation of whey protein isolate (WPI) emulsions. A suspension of heat‐denatured WPI (soluble WPI aggregates) was mixed with a 40% (w/w) oil‐in‐water emulsion to obtain gels with varying concentrations of WPI aggregates and oil. For emulsions stabilized with native WPI, creaming was observed upon mixing of the emulsion with a suspension of WPI aggregates, likely as a result of depletion flocculation induced by the differences in size between the droplets and aggregates. For emulsions stabilized with soluble WPI aggregates, the obtained filled suspension was stable against creaming, and homogeneous emulsion‐filled gels with varying protein and oil concentrations were obtained. Large deformation properties of the emulsion‐filled cold‐set WPI gels were determined by uniaxial compression. With increasing oil concentration, the fracture stress increases slightly, whereas the fracture strain decreases slightly. Small deformation properties were determined by oscillatory rheology. The storage modulus after 16 h of acidification was taken as a measure of the gel stiffness. Experimental results were in good agreement with predictions according to van der Poel's theory for the effect of oil concentration on the stiffness of filled gels. Especially, the influence of the modulus of the matrix on the effect of the oil droplets was in good agreement with van der Poel's theory.

Effects of flow behaviour on the aggregation of whey protein suspensions, pure or mixed with xanthan

The effects of the flow behaviour at different temperatures on whey protein isolate (WPI) suspensions, pure or mixed with xanthan, have been investigated at pH 5.4. The experiments were performed in a continuous rotor/stator device, on the pilot-scale, using different rotational speeds and rotor geometries. The effects of a defined shear flow in a concentric cylinder system were compared with the shear and elongational flow resulting in a geometry with two scraping blades. The aggregation was studied by means of a phase inverse light microscope and quantified by particle laser diffraction measurements. The suspensions were rheologically analysed by a steady stress sweep. A decrease in aggregate size was found with an increased rotational speed, independent of temperature and rotor geometry. For the pure suspensions, the increase in rotational speed from 100 to 500 rpm, in the absence of scrapers, was accompanied by Taylor vortices. These changes in flow behaviour resulted in a dramatic decrease in aggregate size for the suspensions close to denaturation at 70°C. The rotor geometry also influenced the aggregate size. At 70°C, smaller aggregates were formed in the presence of scrapers compared to in the absence of scrapers, while below denaturation at 50°C, the opposite behaviour was found. The viscosity of the processed suspensions was lower compared to that for an unprocessed sample. The processed sample showed shear thickening behaviour. In the presence of xanthan, the effects of temperature and flow behaviour on the WPI aggregation were similar but less pronounced than those in the pure suspensions. This indicates that xanthan inhibits the effects on the WPI aggregation. When scrapers were attached to the rotor, WPI aggregates with a tendency to a long and rod-like shape were formed. These shaped aggregates were explained in terms of the flow behaviour and the pseudoplastic nature of xanthan. All mixtures showed this pseudoplastic behaviour, independent of process history.

Effects of Calcium and Magnesium Ions on Aggregation of Whey Protein Isolate and Its Effect on Foaming Properties

The effects of calcium and magnesium ions on time-dependent aggregation of whey protein isolate (WPI) and the influence of the aggregates on the foaming properties of WPI have been studied. Both Mg 2+ and Ca 2+ induced formation of stable colloidal aggregates; the rate of aggregation was slow, and the rate and extent of aggregation were maximum at 0.02-0.04 M concentration. Both foamability and foam stability of WPI foams were affected by the concentration of the divalent ions, as well as incubation time in salt solutions before foaming: Maximum foamability and foam stability were observed when WPI was foamed immediately after the addition of the divalent salts, and they decreased progressively with incubation time, indicating that solution-phase aggregation of WPI adversely affected its foaming properties