Study on the rheological properties of heat-induced whey protein isolate-dextran conjugate gel

Effect of glycosylation on the rheological properties of whey protein isolate (WPI) during the heat-induced gelation process was evaluated. Significant changes in browning intensity, free amino groups content and SDS-PAGE profile showed that the conjugate of WPI and dextran (150 kDa) was successfully prepared using the traditional dry-heating treatment. For the conjugate, during the heating and cooling cycle, the curves of G′ and G″ were considerably shifted to lower values and their shapes varied comparing to the corresponding spectra of initial WPI and WPI + dextran mixture. After holding at 25 °C, G' reached a value of about 2200 Pa, only a tenth of the value that obtained in the initial WPI gel. Moreover, frequency sweep measurements revealed that the stiffness of gel was greatly reduced in the conjugate, although a typical elastic gel was still formed. All data showed that the rheological properties of thermal gelation could be modified upon the covalent attachment of dextran.     

Segregative interactions and competitive binding of Ca in gelling mixtures of whey protein isolate with Naκ-carrageenan

Gelling mixtures of Na⁺κ-carrageenan with whey protein isolate (WPI) at pH 7.0 have been studied rheologically and by differential scanning calorimetry (DSC), with comparative measurements for the individual constituents of the mixtures. The concentration of WPI was held fixed at 10.0 wt% and carrageenan concentration was varied in the range 0.05–3.0 wt%. Ca²⁺ cations, which have been shown previously to be particularly effective in inducing gelation of κ-carrageenan, were introduced as CaCl₂. The concentration of CaCl₂ used in most of the experiments was 8 mM, but other concentrations were also studied. Mixtures were prepared in the solution state at 45 °C, and showed no evidence of either phase separation or complex formation. Rheological changes were monitored by low-amplitude oscillatory measurements of storage modulus, G′, during (i) cooling (1 °C/min) and holding at 5 °C, to induce gelation of the carrageenan in the presence of non-gelled WPI; (ii) heating and holding at 80 °C to dissociate the carrageenan network and induce gelation of WPI; (iii) cooling and holding again at 5 °C, to give composite networks with both components gelled; and (iv) re-heating to 80 °C to dissociate the carrageenan network. Gel structure was characterised further by creep–recovery measurements at the end of each holding period, and by torsion measurements at 5 °C, before and after thermal gelation of WPI.     

Reducing the stiffness of concentrated whey protein isolate (WPI) gels by using WPI microparticles

Concentrated protein gels were prepared using native whey protein isolate (WPI) and WPI based microparticles. WPI microparticles were produced by making gel pieces from a concentrated WPI suspension (40% w/w), which were dried and milled. The protein within the microparticles was denatured and the protein concentration after drying was similar to the native WPI powder. WPI microparticles had irregular shape with an average size of about 70 μm. They absorbed water when dispersed in water, but the dispersion did not gel upon heating. Replacing part of the native WPI powder with WPI microparticles in the protein gel resulted in lower gel stiffness compared with a gel with the same overall protein concentration but without microparticles. However, microparticles also strengthened the continuous phase because they take up water from this phase. This might increase gel stiffness more than would be expected from an inert particle/filler. There was also good bonding between the microparticles and the WPI continuous phase in the gel, which contributed to gel stiffness.     

Effects of fluid shear and temperature on whey protein gels, pure or mixed with xanthan

The effects of shear on whey protein isolate (WPI) gels, pure or mixed with xanthan, have been investigated at pH 5.4 by dynamic oscillatory measurements and light microscopy (LM). The shear was performed on the suspensions under a constant stress of between 0.04 and 2.1 Pa. Various temperature conditions were chosen in order to describe the effects of shear at the different states of aggregation of the WPI. Shear-sensitive aggregation phenomena were already found around 40°C for the pure WPI samples. Continuous shearing during heating from 20 to 40°C, prior to heat treatment at 90°C, resulted in a gel with a storage modulus (G′) half that of the unsheared gel, independent of the shear stress. Continuous shearing during heating from 20 to 76°C resulted in a further decrease in G′. Inhomogeneities arose in networks formed from continuously sheared suspensions during heating from 20 to 50°C and above. Depending on the shear stress and on the heating range of the shear, the networks showed areas of varied compactness and different classes of pores, ranging from 10 to 200 μm. A higher G′, compared to that for the unsheared gel, was found for gels subjected to shear for short periods in the vicinity of the gel point. The presence of xanthan inhibited the aggregation and demixing of the WPI, described as a sterical phenomenon. Under static conditions, the presence of xanthan resulted in a more homogeneous WPI network. Exposing the mixed suspensions to shear generally increased the inhomogeneity of the network structure. Short periods of shearing in the vicinity of the gel point affected the kinetics of the gel formation and resulted in gels with higher G′ values than the unsheared gel. Continuous shearing under stresses below 0.09 Pa, during heating from 20 to 60°C and above, also resulted in gels with an increased G′. Continuous shear under stresses above 0.9 Pa resulted in gels with a decreased G′     

Gelation of whey protein and xanthan mixture: Effect of heating rate on rheological properties

The effects of heating rate and xanthan addition on the gelation of a 15% w/w whey protein solution at pH 7 and in 0.1 M phosphate buffer were studied using small-amplitude oscillatory shear (SAOS) rheological measurements and uniaxial compression tests. WPI solutions were heated from 25 to 90 °C at five heating rates (0.1, 1, 5, 10 and 20 °C/min). Gelation temperature of WPI decreased with decreasing of heating rates and with xanthan addition. Under uniaxial compression, the WPI gels prepared with no more than 0.2% w/w xanthan exhibited distinct fracture point and were tougher (i.e. higher fracture stress and fracture strain) than the gels prepared with no less than 0.5% w/w xanthan. In general, the fracture strain of WPI gels increased with heating rate, though not significantly, at all xanthan contents investigated. However, the fracture stress of WPI gels, generally, decreased with heating rate when xanthan content was 0–0.2% and increased with heating rate when xanthan content was 0.5 and 1%.     

Rheological properties of rennet casein-whey protein gels prepared at different mixing speeds

The rheological properties at small (oscillatory shear) and large (uniaxial compression) deformations of heat-induced gels (80 °C for 20 min, pH 7.3) containing 25% rennet casein (RCN), 2.5% disodium phosphate and 0%, 2.3% or 6.3% of whey protein isolate (WPI) were measured for samples cooked in a torque-rheometer at mixing speeds within a range of 20–200 rpm (shear rates: ∼15–230 s⁻¹). In addition, microstructure analyses were performed, separately staining RCN and WPI, by Confocal Scanning Laser Microscopy (CSLM). Both small and large deformation tests indicated that increasing addition of WPI prior to the cooking process of RCN resulted in gels exhibiting higher storage and deformability moduli than WPI-free samples. Increasing shear rates during cooking also affected the rheological properties of RCN–WPI gels, and stronger gels were formed as the shear rate during cooking was increased. Despite the data dispersion among replicates, the effect of shear rate on gel strength were evident for RCN gels with 6.3% WPI and relatively clear for gels with 2.3% WPI; however, the trend was uncertain for WPI-free RCN gels. Possible explanations for this observation are that when increasing WPI levels in the presence of RCN and heat, disulfide-thiol exchange reactions between denatured WPI and κ-casein (κ-CN) are increased and possibly promoted by shear rate, resulting in stronger and more cross-linked gel structure. CSLM results were not conclusive to support this hypothesis.     

Enhanced functionalities of whey proteins treated with supercritical carbon dioxide

The functionality of whey proteins can be modified by many approaches; for example, via complexation with carbohydrates, enzymatic cross-linking, or hydrolysis, and the objective of this work was to research the effects of supercritical carbon dioxide (scCO₂) treatments on the functionalities of commercial whey protein products including whey protein isolates (WPI) and whey protein concentrates (WPC). The WPI and WPC powders and a 10% (wt/vol) WPI solution were treated with scCO₂. The WPI solution was treated at 40°C and 10 MPa for 1 h, whereas WPI and WPC powders were treated with scCO₂ at 65°C and 10 or 30 MPa for 1 h. Dynamic rheological tests were used to characterize gelation properties before and after processing. Compared with the unprocessed samples and samples processed with N₂ under similar conditions, scCO₂-treated WPI, whether dispersed in water or in the powder form during treatments, formed a gel with increased strength. The improvement in gelling properties was more significant for the scCO₂-treated WPC. In addition, the scCO₂-processed WPI and WPC powders appeared to be fine and free-flowing, in contrast to the clumps in the unprocessed samples. Proximate compositional and surface hydrophobicity analyses indicated that both compositional and structural changes may have contributed to enhanced whey protein functionalities. The results suggest that functionalities of whey proteins can be improved by scCO₂ treatment to produce novel ingredients.     

Influence of muscle type on rheological properties of porcine myofibrillar protein during heat-induced gelation

The gelation characteristics of myofibrillar proteins are indicative of meat product texture. Defining the performance of myofibrillar proteins during gelation is beneficial in maintaining quality and developing processed meat products and processes. This study investigates the impact of pH on viscoelastic properties of porcine myofibrillar proteins prepared from different muscles (semimembranosus (SM), longissimus dorsi (LD) and psoas major (PM)) during heat-induced gelation. Dynamic rheological properties were measured while heating at 1 °C/min from 20 to 85 °C, followed by a holding phase at 85 °C for 3 min and a cooling phase from 85 to 5 °C at a rate of 5 °C/min. Storage modulus (G′, the elastic response of the gelling material) increased as gel formation occurred, but decreased after reaching the temperature of myosin denaturation (52 °C) until approximately 60 °C when the gel strength increased again. This resulted in a peak and depression in the thermogram. Following 60 °C, the treatments maintained observed trends in gel strength, showing SM myofibrils produced the strongest gels. Myofibrillar protein from SM and PM formed stronger gels at pH 6.0 than at pH 6.5. Differences may be attributed to subtle variations in their protein profile related to muscle type or postmortem metabolism. Significant correlations were determined between G′ at 57, 72, 85 and 5 °C, indicating that changes affecting gel strength took effect prior to 57 °C. Muscle type was found to influence water-holding capacity to a greater degree than pH.     

Casein glycomacropeptide pH-driven self-assembly and gelation upon heating

Casein glycomacropeptide (CMP) found in cheese whey is a C-terminal hydrophilic glycopeptide released from κ-casein by the action of chymosin during cheese making. In a previous work a self-assembly model for CMP at room temperature was proposed, involving a first step of hydrophobic assembly followed by a second step of electrostatic interactions which occurs below pH 4.5. The objective of the present work was to study, by dynamic light scattering (DLS), the effect of heating (35–85 °C) on the pH-driven CMP self-assembly and its impact on the dynamics of CMP gelation. The concentration of CMP was 3% w/w for DLS and 12% w/w for rheological measurements. The solutions at pH 4.5 and 6.5 did not show any change in the particle size distributions upon heating. In contrast the solutions at pH lower than 4.5 that showed electrostatic self-assembly at room temperature were affected by heating. The mean diameter of assembled CMP increased by decreasing pH. For all solutions with pH lower than 4.5, the particle size did not change on cooling, suggesting that the assembled CMP forms formed during heating were stable. The gel point determined as G′–G″ crossover, occurred in all systems at 70 °C, but at different times. The rate of self-assembly determined by DLS as well as the rate of gelation increased with increasing temperature and decreasing pH from 4 to 2. Increasing temperature and decreasing pH, the first step of CMP self-assembly by hydrophobic interactions is speed out. All the self-assembled structures and the gels formed at different temperatures were pH-reversible but did not revert to the initial size (monomer) but to associated forms that correspond mainly to CMP dimers.     

Rheology of whey protein isolate-xanthan mixed solutions and gels. Effect of pH and xanthan concentration

Flow properties at pH 5.5-7.5 of whey protein isolate (WPI)-xanthan solutions containing 0-0.5 w/w% xanthan were studied by viscosimetry, although rigidity and fracture properties of the corresponding heat-set gels (90°C, 30 min) were determined by uniaxial compression. All the studied solutions displayed generalized shearthinning flow behaviour. Synergistic WPI-xanthan interactions has been revealed by observing that rheological parameters [σ_msf, K, n, η (γ)] characterizing blends were larger than those calculated from the two separated solutions. Such a behaviour was attributed to segregative phase separation of whey proteins and xanthan. Effects of xanthan on WPI-xanthan gel properties both depended on pH and xanthan concentration. Simultaneous increased xanthan concentration and decreased pH inhibited gelation of WPI-xanthan blends. Regarding gel strength, synergistic WPI-xanthan interactions were observed at pH >7.0 and low xanthan concentration (0.05 or 0.1 w/w%). Antagonism between the two macromolecules occurred at low xanthan concentration and pH ≤6.5, and high xanthan concentration (0.2 or 0.5 w/w%) at all pH tested. Low xanthan concentration rendered mixed gels more brittle than protein gels, and high xanthan concentration decreased pH effects on gel stress-strain relationships. The balance between strong thermal aggregation of concentrated whey proteins - in presence of incompatible xanthan -, high viscosity of blends and repulsive surface forces of protein molecules was thought to be at the origin of WPI-xanthan gel mechanical properties.     

Linear and non-linear viscoelastic behaviors of crosslinked tapioca starch/polysaccharide systems

Effects of polysaccharides on both the thickening performance and large deformation behavior of mixtures of various crosslinked tapioca starch and polysaccharide were investigated. It was found that the gelatinized crosslinked tapioca starch at 1-8% w/w in aqueous solutions behaved as swollen granules with the low-shear viscosity described by Krieger & Dougherty equation. To achieve thickening performance, 3.5% w/w starch was cooked with a range of 0.5% w/w polysaccharides, i.e. konjac glucomannan, guar gum, xanthan gum, and their 50:50 mixtures. It was shown that additions of polysaccharides greatly affected the rheological behavior of the suspensions, predominantly in the linear viscoelastic regime. The studied mixtures of starch and polysaccharides exhibited strong synergistic effects as evidenced from the increase in their consistency coefficients and the excess storage moduli calculated from the Palierne equation. Moreover, the mixtures containing xanthan gum showed weak-gel characteristics while the others demonstrated liquid-like behavior. In the non-linear viscoelastic analysis, arrangement of starch granules and specific interfacial interactions evidently affected the deformation behavior of the mixtures of crosslinked tapioca starch and polysaccharide. The mixture of konjac glucomannan and xanthan also largely improved stress susceptibility of the suspension and gave the unique shear stiffening under large amplitude oscillatory shear experiment.     

Comparative effect of thermal treatment on the physicochemical properties of whey and egg white protein foams

The optimization of the functionalities of commercial protein ingredients still constitutes a key objective of the food industry. Our aim was therefore to compare the effect of thermal treatments applied in typical industrial conditions on the foaming properties of whey protein isolate (WPI) and egg white proteins (EWP): EWP was pasteurized in dry state from 1 to 5 days and from 60 °C to 80 °C, while WPI was heat-treated between 80 °C and 100 °C under dynamic conditions using a tubular heat exchanger. Typical protein concentrations of the food industry were also used, 2% (w/v) WPI and 10% (w/v) EWP at pH 7, which provided solutions of similar viscosity. Consequently, WPI exhibited a higher foamability than EWP. For WPI, heat treatment induced a slight decrease of overrun when temperature was above 90 °C, i.e. when aggregation reduced too considerably the amount of monomers that played the key role on foam formation; conversely, it increased foamability for EWP due to the lower aggregation degree resulting from dry heating compared to heat-treated WPI solutions. As expected, thermal treatments improved significantly the stability of WPI and EWP foams, but stability always passed through a maximum as a function of the intensity of heat treatment. In both cases, optimum conditions for foam stability that did not impair foamability corresponded to about 20% soluble protein aggregates. A key discrepancy was finally that the dry heat treatment of EWP provided softer foams, despite more rigid than the WPI-based foams, whereas dynamically heat-treated WPI gave firmer foams than native proteins.     

Microemulsions as nanoreactors to produce whey protein nanoparticles with enhanced heat stability by thermal pretreatment

Native whey proteins (NWPs) may form gels or aggregates after thermal processing. The goal of this work was to improve heat stability of NWPs by incorporating protein solutions in nanoscalar micelles of water/oil microemulsions to form whey protein nanoparticles (WPNs) by thermal pretreatment at 90 °C for 20 min. The produced WPNs smaller than 100 nm corresponded to a transparent dispersion. The WPNs produced at NWP solution pH of 6.8 had a better heat stability than those produced at pH 3.5. The salt concentration (0–400 mM NaCl) in NWP solutions did not significantly change the size of corresponding WPNs. Compared to NWPs, the 5% (w/v) dispersion of WPNs at pH 6.8, 100 mM NaCl did not form a gel after heating at 80 °C for 20 min. The improved heat stability and reduced turbidity of WPNs may enable novel applications of whey proteins in beverages.     

Manufacture of acid gels from skim milk using high-pressure homogenization

The effect of high-pressure homogenization (HPH) alone or in combination with a thermal treatment (TT) was investigated for the manufacture of acid gels from skim milk. Raw skim milk was subjected to HPH (0 to 350 MPa) or a TT (90°C, 5 min), or both, in the following processing combinations: 1) HPH, 2) HPH followed by TT, 3) TT followed by HPH, 4) TT, and 5) raw milk (control). After treatments, L* (lightness) values were measured, and then skim milk was acidified with 3% glucono-δ-lactone and rheological properties (G′ and gelation time), and whey holding capacity was evaluated. Treatments in which HPH and TT were combined showed greater L* values than those in which just HPH was applied. In all treatments, the L* values decreased as the pressure was increased up to 300 MPa with little change afterward. Gelation times were lower when HPH was combined with TT compared with the acid skim milk gels that were just pressure treated. The final G′ in gels obtained from skim milk subjected to the combined process (HPH and TT) was greater and pressure-dependent compared with all other gels. A maximum G′ (∼320 Pa) was observed with skim milk subjected to a combination of thermal processing before or after HPH at 350 MPa. Acid gels obtained from HPH milk at 350 MPa showed a linear decrease in whey holding capacity over time, retaining 20% more whey after centrifugation for 25 min compared with samples treated at lower pressures and all other treatments. Our results suggest that HPH in combination with TT can be used to improve the rheological properties and stability of yogurt, thus decreasing the need for additives.     

Comparative study of denaturation of whey protein isolate (WPI) in convective air drying and isothermal heat treatment processes

The extent and nature of denaturation of whey protein isolate (WPI) in convective air drying environments was measured and analysed using single droplet drying. A custom-built, single droplet drying instrument was used for this purpose. Single droplets having 5 ± 0.1 μl volume (initial droplet diameter 1.5 ± 0.1 mm) containing 10% (w/v) WPI were dried at air temperatures of 45, 65 and 80 °C for 600 s at constant air velocity of 0.5 m/s. The extent and nature of denaturation of WPI in isothermal heat treatment processes was measured at 65 and 80 °C for 600 s and compared with those obtained from convective air drying. The extent of denaturation of WPI in a high hydrostatic pressure environment (600 MPa for 600 s) was also determined. The results showed that at the end of 600 s of convective drying at 65 °C the denaturation of WPI was 68.3%, while it was only 10.8% during isothermal heat treatment at the same medium temperature. When the medium temperature was maintained at 80 °C, the denaturation loss of WPI was 90.0% and 68.7% during isothermal heat treatment and convective drying, respectively. The bovine serum albumin (BSA) fraction of WPI was found to be more stable in the convective drying conditions than β-lactoglobulin and α-lactalbumin, especially at longer drying times. The extent of denaturation of WPI in convective air drying (65 and 80 °C) and isotheral heat treatment (80 °C) for 600 s was found to be higher than its denaturation in a high hydrostatic pressure environment at ambient temperature (600 MPa for 600 s).     

The physicochemical parameters during dry heating strongly influence the gelling properties of whey proteins

In this study we determined the composition (proportion of native proteins, soluble and insoluble aggregates) and quantified the gelling properties (gel strength and water holding capacity) of pre-texturized whey proteins by dry heating under controlled physicochemical conditions. For this purpose, a commercial whey protein isolate was dry heated at 80 °C (up to 6 days), 100 °C (up to 24 h) and 120 °C (up to 3 h) under controlled pH (2.5, 4.5 or 6.5) and water activity (0.23, 0.32, or 0.52). Gelling properties were quantified on heat-set gels prepared from reconstituted pre-texturized proteins at 10% and pH 7.0. The formation of dry-heat soluble aggregates enhanced the gelling properties of whey proteins. The maximal gelling properties was achieved earlier by increasing pH and water activity of powders subjected to dry heating. An optimized combination of the dry heating parameters will help to achieve better gelling properties for dry heated whey proteins.     

Acid-induced gelation of whey protein polymers: effects of pH and calcium concentration during polymerization

Heating whey protein dispersions (90°C for 15 min) at low ionic strength and pH values far from isoelectric point (pH>6.5) induced the formation of soluble polymers. The effect of mineral environment during heating on the hydrodynamic characteristics and acid-induced gelation properties of polymers was studied. Whey protein dispersions (80 g/l) were denatured at different pH (6.5–8.5) and calcium concentrations (0–4 mm) according to a factorial design. At pH 6.5, the hydrodynamic radius of protein polymers increased with increasing calcium concentration, while the opposite trend was observed at pH 8.5. Intrinsic viscosity results suggested that heating conditions altered the shape of protein polymers. Whey protein polymers were acidified to pH 4.6 with glucono-δ-lactone and formed opaque particulate gels. The storage modulus and firmness of gels were both affected by conditions used to prepare protein polymers. As a general trend, polymers with high intrinsic viscosity produced stronger gels, suggesting a relationship between polymer shape and gel strength.Acid gelation properties of whey protein polymers makes them suitable ingredients for yoghurt applications. Using whey protein polymers to standardize protein content increased yoghurt viscosity to 813 Pa.s while using skim milk powder at same protein concentration increased yoghurt viscosity to 393 Pa.s. Water holding capacity of protein polymers in yoghurt was 19.8 ml/g compared to 7.2 ml/g for skim milk powder protein. Acid gelation properties of whey protein polymers are modulated by calcium concentration and heating pH and offers new alternatives to control the texture of fermented dairy products.     

Modification of rheological properties of whey protein isolates by limited proteolysis

Whey protein isolate (WPI) was subjected to limited tryptic hydrolysis and the effect of the limited hydrolysis on the rheological properties of WPI was examined and compared with those of untreated WPI. At 10% concentration (w/v in 50 mM TES buffer, pH 7.0, containing 50 mM NaCl), both WPI and the enzyme-treated WPI (EWPI) formed heat-induced viscoelastic gels. However, EWPI formed weaker gels (lower storage modulus) than WPI gels. Moreover, a lower gelation point (77 °C) was obtained for EWPI as compared with that of WPI which gelled at 80 °C only after holding 1.4 min. Thermal analysis and aggregation studies indicated that limited proteolysis resulted in changes in the denaturation and aggregation properties. As a consequenece, EWPI formed particulated gels, while WPI formed fine-stranded gels. In keeping with the formation of a particulate gel, Texture Profile Analysis (TPA) of the heat-induced gels (at 80 °C for 30 min) revealed that EWPI gels possessed significantly higher (p < 0.05) cohesiveness, hardness, gumminess, and chewiness but did not fracture at 75% deformation. The results suggest that the domain peptides, especially β-lactoglobulin domains released by the limited proteolysis, were responsible for the altered gelation properties.     

Effect of protein composition on the rheological properties of acid-induced whey protein gels

Protein dispersions with different ratios of α-lactalbumin to β-lactoglobulin were heat-denatured at pH 7.5 and then acidified with glucono-δ-lactone to form gels at room temperature. Heat treatment induced the formation of whey protein polymers with reactive thiol group concentrations ranging from 1 to 50 μmol/g, depending on protein composition. During acidification, the first sign of aggregation occurred when the zeta potential reached −18.2 mV. Increasing the proportion of α-lactalbumin in the polymer dispersions resulted in more turbid gels characterized by an open microstructure. Elastic and viscous moduli were reduced, while the relaxation coefficient and the stress decay rate constants were increased by raising the proportion of α-lactalbumin in the gel. After one week of storage at 5 °C, gel hardness increased by 12%. The effect of protein composition on acid-induced gelation of whey protein is discussed in relation to the availability and reactivity of thiol groups during gel formation and storage.     

Factors Influencing Whey Protein Gel Rheology: Dialysis and Calcium Chelation

Influence of dialyzable compounds on the Theological properties (shear stress and shear strain at failure) of heat-induced whey protein concentrate (WPC) and whey protein isolate (WPI) gels was examined. Dialyzing WPC and WPI suspensions prior to gelation increased the stress of two of three WPC gels and a WPI gel. Dialysis also significantly increased the strain of the same two WPC gels, normalizing all strain values. Replacement of calcium lost through dialysis did not significantly change gel rheology. However, chelating calcium caused a significant decrease in the stress of all gels: a minimum amount of calcium and/or a calcium complex appears to have a major role in whey protein gelation.