Stiffening in gels containing whey protein isolate

Gels made only from whey protein isolate (WPI) stiffened over the first few days of storage, after which the textural properties remained nearly constant. However, protein gels containing WPI microparticles, at the same total protein content, stiffened over a longer period than those without microparticles. This stiffening was suggested to be the result of rearrangement of crosslinks in the gel. Addition of particles induces additional effects leading to water distribution between the protein particles and continuous phase. The stiffness change over time was different for gels made from a mixture of locust bean gum and xanthan gum containing microparticles. The stiffness of matrix gel and of gels containing 20% (w/w) microparticles was rather stable over time; microscopy analysis of these gels showed that particle size was constant after 72 h storage. Nevertheless, changes were observed in small deformation; this might be the consequence of slow rearrangements within the protein particles.     

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.     

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.     

Mathematical modeling of swelling in high moisture whey protein gels

Gels prepared from whey proteins can be used for controlled release of nutrients or active ingredients in food systems. The objective of this study was to characterize the water uptake by these hydrophilic gels to aid in the design of release systems. Whey protein isolate (WPI) gels (17% w/w protein) of different aspect ratios were submersed in aqueous solution at pH 7.0. Modeling of mass uptake is presented in terms of Case I (Fickian diffusion) and Case II (kinetic) models. Due to the extent of swelling, the Fickian diffusion with moving boundaries provided the most realistic reflection of the physics. An optimization routine provided the best fit values for the diffusivity. The average diffusivity for the smaller gels (with an initial radius of 6.7 mm) was 1.40 × 10⁻¹⁰ m²/s. The average diffusivity of the larger gels (with an initial radius of 8.5 mm) was 0.79 × 10⁻¹⁰ m²/s. The average diffusivities differed due to the slight variation in the composition of the gels. The model also yielded instantaneous values of the radius and sample length. The functionality of moisture uptake and total surface area was linear. The Fickian diffusion with moving boundary model can be extended to evaluate different geometries for controlled release systems.     

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.     

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     

Effects of thiol reagents and ethanol on strength of whey protein gels

Chemical reagents which interfere with secondary interactions affected gel network formation and the properties of gels made from 20% whey protein isolates. Thus, p-hydroxymercuribenzoate and N-ethylmaleimide 0–16 mmol/dm³ and dithiothreitol 0–32 mmol/dm³ reduced the hardness and cohesiveness of whey protein gels apparently by interfering with formation of disulfide bonds. The addition of ethanol (0–15%) increased the hardness of gels presumably by enhancing electrostatic interactions and hydrogen bonding.     

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.     

Characterization of cold-set gels produced from heated emulsions stabilized by whey protein

This paper reports the cold gelation of preheated emulsions stabilized by whey protein, in contrast to, in previous reports, the cold gelation of emulsions formed with preheated whey protein polymers. Emulsions formed with different concentrations of whey protein isolate (WPI) and milk fat were heated at 90 °C for 30 min at low ionic strength and neutral pH. The stable preheated emulsions formed gels through acidification or the addition of CaCl₂ at room temperature. The storage modulus (G′) of the acid-induced gels increased with increasing preheat temperature, decreasing size of the emulsion droplets and increasing fat content. The adsorbed protein denatures and aggregates at the surface of the emulsion droplets during heat treatment, providing the initial step for subsequent formation of the cold-set emulsion gels, suggesting that these preheated emulsion droplets coated by whey protein constitute the structural units responsible for the three-dimensional gel network.     

Properties of gels from whey protein concentrate and honey at different pHs

Structural and functional properties of heat-induced gels from whey protein concentrate (WPC)-honey prepared at pHs 3.75, 4.2 and 7.0 were analyzed. Gel structure was observed by scanning electron microscopy, and the apparent transition temperature for protein denaturation was determined by differential scanning calorimetry. The solubility of the protein components in different extraction media, and the water-holding capacity, firmness, elasticity, relaxation time, adhesivity, cohesiveness and color of gels were determined. Results show that disulfide interchange reactions are important in determining the elasticity, water-holding capacity, relaxation time and cohesiveness of WPC gels. Honey decreases the relaxation time of gels prepared at pHs 7.0 and 4.2, and increases the browning and the water-holding capacity of gels, the apparent transition temperature of WPC dispersions at the three pHs assayed, and the adhesivity of acidic gels. The solubility of the protein constituents of gels in a pH 8.0 buffer increases slightly at honey concentrations of 27.5% or more, which correlates with a decrease in the gel cohesiveness, having these gels a structure with smaller pores. The products obtained could be utilized in the formulation of different desserts, such as flans and cake and tart fillings.     

Characterisation of carrageenan and whey protein gels using NMR PGSTE diffusion experiments

NMR PGSTE diffusion experiments are used to characterise the gelling behaviour of carrageenan mixtures and whey protein mixtures. The structure of developing networks can be observed as a function of the composition of the mixture (concentration of hydrocolloids, sugars and salts), the measuring temperature, and the protein denaturation, depending on the denaturation temperature and duration. Besides the fundamental aspects of these preliminary studies, these data are helpful for modelling processes sufficiently precisely for industrial production, to guarantee specific properties concerning the flow-, gelling- and syneresis/storage-behaviour and the texture and sensorial behaviour of food products, and to realise a process and quality control of thermal protein denaturation and hydrocolloid gelling processes. Therefore, further quantitative correlations between macroscopic quantities and the available NMR parameters must be established to trigger industrial applications of NMR diffusion experiments.     

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.     

Effect of monoglycerides and diglycerol-esters on viscoelasticity of heat-set whey protein emulsion gels

Summary Effects of small-molecule surfactants (emulsifiers) on the small-deformation viscoelastic properties of heat-set whey protein emulsion gels have been investigated using a controlled stress rheometer. The surfactants used in this investigation were the water-soluble diglycerol monolaurate (DGML) and diglycerol monooleate (DGMO), and the oil-soluble glycerol monooleate (GMO). The elastic modulus of the emulsion gel was found to decrease in the presence of a small amount of surfactant, but then to recover at higher surfactant concentrations. The initial reduction in modulus correlates with protein displacement from the oil droplet surface. The recovery of the storage and loss moduli at higher surfactant concentrations of DGML or DGMO may be due to the depletion flocculation of the emulsion prior to heat-treatment. However, for systems containing high content of GMO in the oil phase, the recovery of the moduli is probably owing mainly to the smaller average particle size. Effects of surface monolayer composition, droplet aggregation and average particle size were discussed. The behaviour obtained here was compared with results for previously investigated whey protein emulsion gel systems containing different emulsifiers.     

不同因素对乳清蛋白乳化凝胶特性的影响

探讨了葡萄糖酸-δ-内酯（GDL）、中性盐（NaCl、CaCl₂）以及增稠剂对乳清分离蛋白乳化凝胶强度、弹性以及保水性的影响。结果表明:GDL为0.6%、NaCl为1%、CaCl₂为0.25%、增稠剂卡拉胶为0.2%以及黄原胶为0.1%制备的乳化凝胶强度显著提高。GDL为0.4%、NaCl为0.75%、CaCl₂为0.25%、增稠剂卡拉胶为0.05%以及黄原胶为0.15%制备的乳化凝胶弹性显著提高。增稠剂卡拉胶为0.15%、黄原胶为0.1%所制备的乳化凝胶保水性显著提高。      

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

A protein dispersion blend of β-lactoglobulin and α-lactalbumin was heat-denatured at pH 7.5, hydrolyzed by α-chymotrypsin and then acidified with glucono-δ-lactone to form gels at room temperature. Heat treatment induced the formation of whey protein polymers with high concentration of reactive thiol groups (37 μmol/g). The reactive thiol group concentration was reduced by half after 40 min enzymatic hydrolysis. It was further reduced after enzyme thermal deactivation. During acidification, the first sign of aggregation for hydrolyzed polymers occurred earlier than for non hydrolyzed polymers. Increasing the hydrolysis duration up to 30 min resulted in more turbid gels characterized by an open microstructure. Elastic and viscous moduli were both reduced, while the relaxation coefficient and the stress decay rate constants were increased by increasing the hydrolysis duration. After one week storage at 5 °C, the hardness of gels made from hydrolyzed polymers increased by more than 50%. The effect of polymer hydrolysis on acid-induced gelation is discussed in relation to the availability and reactivity of thiol groups during gel formation and storage.     

Kinetics of chemical marker formation in whey protein gels for studying microwave sterilization

The kinetics of 4-hydroxy-5-methy-3(2H)-furanone (M-2) formation in a model food system (20% whey protein gel) was determined for studying cumulative time–temperature effects in high-temperature-short-time processes. M-2 was formed from -ribose and amines through non-enzymatic browning reactions and enolization under low acid conditions (pH > 5). The order of the reaction for M-2 formation was determined by non-linear regression analysis and further confirmed by graphical method. M-2 formation followed a first-order kinetics and the rate constant temperature dependence was described using an Arrhenius relationship. The reaction rates and activation energy were determined using two-step, multi-linear and non-linear regression analyses. This study also demonstrated the use of M-2 formation in determining the cumulative heating effect in a model food system subjected to 915 MHz microwave heating.     

Effect of denatured whey protein concentrate and its fractions on rennet-induced milk gels

Denatured whey protein concentrate was fractionated by centrifugation to study the effect of its different components (sedimentable aggregates, non-sedimentable component, and diffusible component) on rennet-induced coagulation of milk and gel contraction capacity. Milk coagulation properties were characterized by optical density measurement and dynamic rheometry. The contraction kinetics of the gel during cooking was also characterized. The diffusible component of denatured whey protein concentrate showed no significant effect on coagulation or contraction parameters. Sedimentable aggregates negatively influenced the kinetics of rennet gel formation, as measured by rheology; these aggregates also reduced the contraction capacity of the gel. The non-sedimentable component negatively influenced milk coagulation properties, as measured with both optical and rheological methods, and decreased the contraction capacity of the gel. The results suggest that, beyond the effect of sedimentable whey protein aggregates, soluble proteinaceous complexes (non-sedimentable and non-diffusible) could interact with renneted casein micelles and limit gel formation and contraction.     

The breakdown properties of heat-set whey protein emulsion gels in the human mouth

Differently structured whey protein emulsion gels were formed by heating at different concentrations of NaCl. The formation of gels was monitored by oscillatory rheometry. The large deformation properties relevant to breakdown properties in the human mouth were measured by a uniaxial compression test and fracture wedge set test using a texture analyzer. A panel of 8 subjects was used to examine the in-mouth behaviours of gels including mastication parameters, degree of fragmentation and oil droplet release. The results showed that in general the gel hardness increased with increasing NaCl concentration. The gels containing 10/25 and 100/200 mM NaCl were characterized as being soft and hard, respectively. These soft and hard gels had different breakdown patterns in the mouth. On the other hand, sensory experiments showed the gel with 10 mM NaCl needed a significantly lower number of chewing cycles (19.4 ± 2.1) compared with gels with higher NaCl. The values of median size of particles in masticated gels containing 10, 25, 100 and 200 mM NaCl were about 4.00, 2.85, 1.05 and 0.95 mm, respectively, which suggested that higher hardness led to greater fragmentation in the human mouth. The fragmentation of the gel was highly correlated with functions of the mechanical properties. There was no obvious coalescence of the oil droplets during oral processing and only very few oil droplets were released from protein matrix during mastication.