Effect of protein concentration on whey protein gels obtained by a two-stage heating process

 The influence of protein concentration on the properties of gels obtained by a two-stage heating process was determined. In the first stage, whey protein dispersion (3–10%) was heated at pH 8.0, and in the second stage it was diluted to 3% protein, adjusted to pH 7.0 and heated again. Increased protein concentration in the first stage of polymerization resulted in the gels obtained in the second stage having a lower phase angle, increased storage modulus and increased hardness. Increased protein concentration also resulted in gels with an increased optical density, which suggests thathigher protein concentration leads to more and larger aggregates. Gels obtained from dispersions preheated at a higher protein concentration had higher permeability coefficient (B _gel) values. The increase in B _gel suggests that the higher protein concentration increased the size of the aggregates, which in a second stage of heating formed a gel matrix with a larger pore size. Received: 11 February 1999     

Utilizing whey protein isolate and polysaccharide complexes to stabilize aerated dairy gels

Heated soluble complexes of whey protein isolate (WPI) with polysaccharides may be used to modify the properties of aerated dairy gels, which could be formulated into novel-textured high-protein desserts. The objective of this study was to determine the effect of polysaccharide charge density and concentration within a WPI-polysaccharide complex on the physical properties of aerated gels. Three polysaccharides having different degrees of charge density were chosen: low-methoxyl pectin, high-methoxyl type D pectin, and guar gum. Heated complexes were prepared by heating the mixed dispersions (8% protein, 0 to 1% polysaccharide) at pH 7. To form aerated gels, 2% glucono-δ-lactone was added to the dispersions of skim milk powder and heated complex and foam was generated by whipping with a handheld frother. The foam set into a gel as the glucono-δ-lactone acidified to a final pH of 4.5. The aerated gels were evaluated for overrun, drainage, gel strength, and viscoelastic properties. Without heated complexes, stable aerated gels could not be formed. Overrun of aerated gel decreased (up to 73%) as polysaccharide concentration increased from 0.105 to 0.315% due to increased viscosity, which limited air incorporation. A negative relationship was found between percent drainage and dispersion viscosity. However, plotting of drainage against dispersion viscosity separated by polysaccharide type revealed that drainage decreased most in samples with high-charge-density, low-methoxyl pectin followed by those with low-charge-density, high-methoxyl type D pectin. Aerated gels with guar gum (no charge) did not show improvement to stability. Rheological results showed no significant difference in gelation time among samples; therefore, stronger interactions between WPI and high-charge-density polysaccharide were likely responsible for increased stability. Stable dairy aerated gels can be created from WPI-polysaccharide complexes. High-charge-density polysaccharides, at concentrations that provide adequate viscosity, are needed to achieve stability while also maintaining dispersion overrun capabilities.     

pH Induced Aggregation and Weak Gel Formation of Whey Protein Polymers

Whey protein polymers were formed by heating (80 °C) a 4% (w/v) whey protein (WP) isolate dispersion at pH 8.0 for 15, 25, 35, 45, or 53 min. Dispersions were adjusted to pH 6.0, 6.5, 7.0, 7.5, or 8.0 after heating and the rheological properties were determined. Viscosity increased with increased heating time and decreased pH. At pH 7.0 and 7.5, high-viscosity dispersions with pseudoplastic and thixotropic flow behavior were formed, while weak gels were formed at pH 6.0 and 6.5. The storage (elastic) and loss (viscous) moduli of pH-induced gels increased when temperature was increased from 7 °C to 25 °C, suggesting that hydrophobic forces are responsible for gelation. Key Words: weak-gels, whey proteins, polymers, gelation, functionality     

Ultrasonic generation of aerated gelatin gels stabilized by whey protein β-lactoglobulin

Dispersed air provides an additional phase within gel-type foods may accommodate new textural and functional demands. This paper addresses the effect of using whey protein β-lactoglobulin (β-lg), with different degrees of denaturation, as stabilizing agent in the formation of aerated gelatin gels using ultrasound as a novel method to incorporate bubbles in model foods. The heat denaturation, aggregate formation and surface properties of β-lg dispersions were studied at three pHs (6.0, 6.4 and 6.8) and at a heating temperature of 80 °C. β-Lg dispersions with four degrees of denaturation (0%, 20%, 40% and 60%) were used to stabilize bubbles generated by high intensity ultrasound in aerated gelatin gels. Experimental methods to determine gas hold-up, bubble size distributions and fracture properties of aerated gelatin gels stabilized by β-lg (AG), as well as control gels (CG), aerated gelatin gels without β-lg, are presented. Gas hold-up of AG peaked at a degree of denaturation of 40% when AG were fabricated using β-lg heated at pH 6.4 and 6.8, whereas using β-lg heated at pH 6.0 gas hold-up decreased constantly with increasing degree of denaturation. The use of β-lg as surfactant at pH 6.8 and 6.4 reduced the bubble sizes of AG compared with CG, but no effect was observed at pH 6.0. AG showed values of stress and strain at fracture lower than CG (5.86 kPa and 0.62), probably because of the lower gas hold-up of CG. However, both type of aerated gels were weaker and less ductile than non-aerated gels, with a decrease in stress and strain at fracture for AG between 56–71% and 33–43%, respectively. This study shows that the presence of bubbles in gel-based food products results in unique rheological properties conferred by the additional gaseous phase.     

Mixed gels of fine-stranded and particulate networks of gelatin and whey proteins

Mixed gels of gelatin and whey protein concentrate were investigated, as well as their pure systems, by tensile tests and by dynamic oscillatory measurements. The systems were studied for homogeneous particulate whey protein gels at pH 5.4 and for inhomogeneous particulate whey protein gels at pH 4.6. The influence on the systems of the Bloom number of the gelatin component has also been investigated. Results of the fracture properties, such as stress and strain at fracture, indicate a transition in rheological properties. Results of the elastic modulus, obtained by tensile measurements, as well as the storage modulus, obtained by dynamic oscillatory measurements, both agree with predictions for phase inversions from the Takayanagi models as modified by Clark, which are in agreement with the fracture properties. The transition points are different for the different mixed gel series but take place between 1 and 3 wt% gelatin and 8 wt% whey protein concentrate, depending on factors such as the microstructure of the whey protein concentrate. Dynamic oscillatory measurements showed that gel formation of whey protein concentrate is unaffected by the presence of gelatin, which is in agreement with light microscopy results. Light microscopy revealed that the mixed gel systems were bicontinuous and that the whey protein network structure was unaffected by the presence of gelatin. It is postulated that the predicted phase inversions of the mixed gels are due to a shift in rheological properties without any phase inversions in the microstructure.     

乳成分对酸奶凝胶微观结构和流变学性质的影响

酸奶凝胶的许多宏观的物理特性与其微观结构和流变学性质密切相关。从酸奶的微结构、流变学性质和质地等方面综述了乳脂肪、蛋白质及调节酪蛋白和乳清蛋白比例对酸奶凝胶的影响。     

Combined microscopic and dynamic rheological methods for studying the structural breakdown properties of whey protein gels and emulsion filled gels

The microstructural and large deformation rheological properties of model food gels were studied by performing notch propagation tensile testing on the gels using a tensile stage and observing changes in the microstructure of the gels during tensile testing using confocal laser scanning microscopy (CLSM). Heat-set whey protein (WP) gels containing either added sodium caseinate (NaCN) or sunflower oil droplets emulsified with WP or NaCN as the emulsifier protein were prepared in 0 or 50 mM NaCl. The WP gel structure strengthened in the presence of added NaCl and NaCN. The rheological properties of WP gels containing sunflower oil droplets emulsified with WP or NaCN were influenced by the NaCl concentration, oil concentration and extent of oil droplet aggregation in the gel or by the type of emulsifier protein used. During tensile testing, the notch length in all gels increased above a certain critical stress, leading to fracture of the gels through the notch. Also, the microstructural changes in the oil phase of emulsion filled gels subjected to tensile testing were influenced by the structural properties of the WP gel matrix and the proximity of the oil droplet to the fracture path.     

Rheological properties and structure of inulin–whey protein gels

The rheological properties, structure and synergistic interactions of whey proteins (1–7%) and inulin (20% and 35%) were studied. Gelation of whey proteins was induced with Na⁺. Inulin was dissolved in preheated whey protein solutions (80 °C, 30 min). Inulin gel formation was strongly affected by whey proteins. The presence of whey proteins at a level allowing for protein gel network formation (7%) significantly increased the G′ and G″ values of the gels. Scanning electron micrographs showed a thick structure for the mixed gel. Whey proteins at low concentrations (1–4%) were not able to form a gel; further, these low concentrations partly or wholly impaired formation of a firm inulin gel. Although interactions between inulin and whey proteins may be concluded from hydrophobicity measurements, the use of an electrophoretic technique did not show any inulin–whey protein complexes.     

ELECTROSTATIC EFFECTS ON PHYSICAL PROPERTIES OF PARTICULATE WHEY PROTEIN ISOLATE GELS

Physical properties of particulate whey protein isolate gels formed under varying electrostatic conditions were investigated using large strain rheological and microstructural techniques. The two treatment ranges evaluated were adjusting pH (5.2‐5.8) with no added NaCl and adjusting the NaCl (0.2‐0.6 M) at pH 7. Gels (10% protein w/v) were formed by heating at 80C for 30 min. The large strain properties of fracture strain (γ_f), fracture stress (σ_f), and a measure of strain hardening (R_0.3) were determined using a torsion method. Gel microstructure was evaluated using scanning electron microscopy (SEM) and gel permeability (B_gel). Overlaying σ_f and γ_f curves for pH and NaCl treatments demonstrated an overlap where gels of equal σ_f and γ_f could be formed by adjusting pH or NaCl concentration. The high fracture stress (σ_f～ 23 kPa and γ_f～ 1.86) pair conditions were pH 5.47 and 0.25 M NaCl, pH 7.0. The low fracture stress (σ_f～ 13 kPa and γ_f～ 1.90) pair conditions were pH 5.68 and 0.6 M NaCl, pH 7.0. The 0.25 M NaCl, pH 7 treatment demonstrated higher R_0.3 values than the pH 5.47 treatment. When the sulfhydryl blocker n‐ethylmaleimide was added at 2 mM to the 0.25 M NaCl, pH 7 gel treatment, its rheological behavior was NSD (p>0.05) to the pH 5.47 gel treatment, indicating disulfide bond formation regulated strain hardening. Altering surface charge or counterions, and disulfide bonding, was required to produce gels with similar large strain rheological properties. An increase in gel permeability coincided with an increase in pore size as observed by SEM, independent of rheological properties. This demonstrated that at the length scales investigated, microstructure was not linked to changes in large strain rheological properties.     

Combining protein micro-phase separation and protein–polysaccharide segregative phase separation to produce gel structures

The ability of protein micro-phase separation and protein–polysaccharide segregative phase separation to generate a range of gel structures and textures was evaluated. Whey protein isolate/κ-carrageenan mixed gels were prepared with 13% (w/v) whey protein isolate, 0–0.6% (w/w) κ-carrageenan and 50, 100 or 250 mM NaCl. The microstructure of gels, determined by confocal laser scanning microscopy, varied from homogenous to protein continuous, bicontinuous, coarse stranded or κ-carrageenan continuous, depending on the κ-carrageenan concentration. Microstructure also varied from stranded to particulate (micro-phase separated) depending on the salt concentration. The rheological behavior of mixed gels corresponded to the shift in the continuous phase from protein to κ-carrageenan. At small concentrations of κ-carrageenan, where carrageenan-rich droplets were dispersed in a continuous protein-rich matrix, gel strength (fracture stress) and firmness (G′) increased due to increased local concentration of proteins caused by phase separation. At higher κ-carrageenan concentrations, gels were substantially less firm, weaker and less deformable (fracture strain). The change in the continuous phase from protein continuous to carrageenan continuous explained the major change in mechanical properties and water-holding properties. The shift in microstructure occurred at lower concentrations of κ-carrageenan when whey proteins were under micro-phase separation conditions. The results demonstrated how the combined mechanisms of ion-induced micro-phase separation of proteins and protein–polysaccharide phase separation and inversion can be used to alter gel structure and texture.     

The effect of pH on gel structures produced using protein–polysaccharide phase separation and network inversion

Forming heat-induced gels through combined effects of micro-phase separation of whey protein isolate (WPI; 5%, w/v, 100 mm NaCl) by pH change (5.5, 6.0, and 6.5), and addition of κ-carrageenan (0–0.3%, w/w), were evaluated. The microstructure of WPI gels was homogeneous at pH 6.0 and 6.5 and micro-phase separated at pH 5.5. Addition of 0.075% κ-carrageenan to WPI solutions caused the microstructure of the gel to switch from homogeneous (pH 6.0 and 6.5) to micro-phase separated; and higher concentrations led to inversion of the continuous network from protein to κ-carrageenan. Protein solutions containing 0.075% (w/w) κ-carrageenan produced gels with increased storage modulus (G′) at pH 6.5 and decreased G′ at pH 5.5. All gels containing 0.3% (w/w) κ-carrageenan had κ-carrageenan-continuous networks. It was shown that microstructural and rheological changes were different in WPI and κ-carrageenan mixed gels when micro-phase separation was caused by pH rather than ionic strength.     

Effects of added whey protein aggregates on textural and microstructural properties of acidified milk model systems

Non-fat milk model systems containing 5% total protein were investigated with addition of micro- or nanoparticulated whey protein at two levels of casein (2.5% and 3.5%, w/w). The systems were subjected to homogenisation (20 MPa), heat treatment (90 °C for 5 min) and chemical (glucono-delta-lactone) acidification to pH 4.6 and characterised in terms of denaturation degree of whey protein, particle size, textural properties, rheology and microstructure. The model systems with nanoparticulated whey protein exhibited significant larger particle size after heating and provided acid gels with higher firmness and viscosity, faster gelation and lower syneresis and a denser microstructure. In contrast, microparticulated whey protein appeared to only weakly interact with other proteins present and resulted in a protein network with low connectivity in the resulting gels. Increasing the casein/whey protein ratio did not decrease the gel strength in the acidified milk model systems with added whey protein aggregates.     

Effects of pH and ionic strength on the rheology and microstructure of a pressure-induced whey protein gel

The effects of pH and ionic strength (I) on the properties of a pressure-induced gel from a whey protein isolate (0.2 g mL^?1) were studied using a series of buffers with different pH and ionic strength (I). The rheological properties and water-holding capacity (WHC) of the gel changed in a complex pattern with pH and I but could be successfully modelled using an artificial neural network (ANN) model. The degree of cross-linking of S–S bonds was higher at pH 8.0 than at pH 5.0; consequently, a rigid gel was formed at alkaline pH values. The geometric dimensions of the microstructure decreased with increasing pH and decreasing ionic strength, and the WHC mainly depended on the geometric properties of the microstructure. These results indicate that pH and ionic strength influenced the gel properties by controlling the cross-linking reaction and phase separation that occur during gelation. They also confirmed the good potential application of ANN in studies of gelation.     

Effect of oxidoreduction potential and of gas bubbling on rheological properties and microstructure of acid skim milk gels acidified with glucono-δ-lactone

Milk oxidoreduction potential was modified using gases during the production of a model dairy product and its effect on gel setting was studied. Acidification by glucono-δ-lactone was used to examine the physicochemistry of gelation and to avoid variations due to microorganisms sensitive to oxidoreduction potential. Four conditions of oxidoreduction potential were applied to milk: milk was gassed with air, nongassed, gassed with N₂, or gassed with N₂H₂. The rheological properties and microstructure of these gels were determined using viscoelasticimetry, measurement of whey separation, and confocal laser scanning microscopy. It appeared that a reducing environment led to less-aggregated proteins within the matrix and consequently decreased whey separation significantly. The use of gas to modify oxidoreduction potential is a possible way to improve the quality of dairy products.     

Heat-induced Gelation Properties of Salt-Soluble Muscle Proteins as Affected by Non-Meat Proteins

Addition of whey protein concentrate (WPC), whey protein isolate (WPI) or soy protein isolate (SPI) to salt-soluble muscle proteins (SSP) decreased the gel strength. WPI:SSP gels had higher water-holding capacity than SSP, SSP:WPC or SSP:SPI gels. Myosin heavy chain was a principal contributor to gel network formation in SSP, SSP:WPC, SSP:WPI and SSP:SPI systems. The characteristic fibrous network formed by SSP was the dominant feature of the microstructure of SSP:WPC and SSP:WPI gels. SSP:SPI gels had a more aggregated appearance due to the occurrence of clusters of SPI throughout the gel matrix.     

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.     

Small and large deformation rheology and microstructure of κ-carrageenan gels containing commercial milk protein products

The rheological behaviour of commercial milk protein/κ-carrageenan mixtures in aqueous solutions was studied at neutral pH. Four milk protein ingredients; skim milk powder, milk protein concentrate, sodium caseinate, and whey protein isolate were considered. As seen by confocal laser microscopy, mixtures of κ-carrageenan with skim milk powder, milk protein concentrate, and sodium caseinate showed phase separation, but no phase separation was observed in mixtures containing whey protein isolate. For κ-carrageenan concentrations up to 0.5 wt%, the viscosity of the mixtures at low shear rates increased markedly in the case of skim milk powder and milk protein concentrate addition, but did not change by the addition of sodium caseinate or whey protein isolate. For κ-carrageenan concentrations from 1 to 2.5 wt%, small and large deformation rheological measurements, performed on the milk protein/κ-carrageenan gels, showed that skim milk powder, milk protein concentrate or sodium caseinate markedly improved the strength of the resulting gels, but whey protein isolate had no effect on the gel stength.     

Microstructure of Whey Protein Isolate Gels Containing Emulsified Butterfat Droplets

The effects of concentration and droplet size of anhydrous butterfat globules on the microstructure of heat-induced whey protein isolate gels (pH 4.60) were studied by scanning and transmission electron microscopy (TEM). All fat globules were emulsified with whey protein isolate and incorporated into the system prior to gelation. Protein aggregates became more closely packed as whey protein concentration was increased from 8 to 15% by weight in gels without added fat. There was no notable change in overall gel microstructure upon addition of fat globules, up to 25% by weight, when viewed by scanning electron microscopy. However, it appeared fat globules were intimately associated with the gel protein matrix. A twofold difference in fat globule size was obvious by TEM. Clusters of droplets became more predominant as butterfat content increased.     

Gelation and Emulsification Properties of Partially Insolubilized Whey Protein Concentrates

Ultrafiltered retentate of whey was heat-processed to prepare whey protein concentrates (WPC) with protein solubilities ranging from 27.5% to 98.1% in 0.1M NaCl, pH 7.0. Proximate and protein compositions of each WPC were determined. Properties of 20% (w/w) protein WPC gels in 0.1M and 0.6 M NaCl, pH 7.0, on heating to 60, 70, 80, and 90°C and emulsification properties of WPC (0.5% (w/w) protein) in 0.1M NaCl at pH 6.0, 7.0 and 8.0 were analyzed. Gel apparent stress and strain at failure decreased and expressible moisture increased as solubility decreased from 98.1% to 41.0% at all heating temperatures. Emulsification Activity Index was highest at pH 7.0, emulsions were most stable at pH 8.0.     

Gelation of casein-whey protein mixtures

Heated milk consists of a mixture of whey protein-coated casein micelles and soluble whey protein aggregates. The acid-induced gelation properties of heated milk are consistently different from those of unheated milk—i.e., a shift in gelation pH, stronger gels, and a different microstructure of the gels. In this study we investigated the role of the different fractions of denatured whey proteins on the acid-induced gelation, the gel hardness, and the microstructure. Both whey protein fractions contribute to the observed shift in gelation pH, although by a different mechanism. Obtaining gels with high gel hardness occurs most effectively when all denatured whey proteins are present as whey protein aggregates. It was observed that disulfide bridge exchange reactions during the acid-induced gelation at ambient temperature play an important role for both whey protein fractions. Additionally, disulfide interactions seem to occur between the aggregates and the casein micelles during the gel state. In this study, we show the development of a new approach for confocal scanning laser microscopy measurements—i.e., separate staining of the proteins in milk. By using this method, we were able to determine that, although whey protein aggregates are not linked to the casein micelles, they nevertheless gel at the same moment. This work adds to a better understanding of the role of denatured whey proteins during acid-induced gelation and could improve the effective use of whey proteins.