Associative and segregative interactions between gelatin and low-methoxy pectin. Part 3. Quantitative analysis of co-gel moduli

The experimental moduli (G′ at 5 °C) reported in the preceding paper for gelatin–calcium pectinate co-gels (pH 3.9; 1.0 wt% pectin; stoichiometric Ca²⁺; 0–10 wt% gelatin) formed in the presence or absence of 1 M NaCl have been analysed using a single adjustable parameter, p, to characterise partition of solvent. The analysis of samples incorporating 1 M NaCl assumed complete segregation of calcium pectinate into dispersed particles in a continuous gelatin matrix, with p defined as the ratio of water/polymer in the gelatin phase divided by the corresponding ratio for the pectin phase. Relative phase volumes at each trial value of p were used to determine the polymer concentration in each phase, and the corresponding moduli were obtained from standard calibration curves. For solvent distributions where the calculated modulus of the continuous gelatin phase was higher than that of the dispersed calcium pectinate phase, co-gel moduli were derived using the Takayanagi isostrain model, and the isostress model was used for the converse situation. The p factors required to give perfect agreement with the moduli observed experimentally were tightly grouped around a single value (p=1.21) for all concentrations of gelatin studied, indicating that the assumption of complete segregation is reasonably valid. Calculated moduli for the gelatin phase were in good agreement with experimental values obtained by melting the gelatin network, centrifuging to sediment the dispersed calcium pectinate particles, and re-gelling the gelatin supernatant. The same p factor (1.21) was used to derive calculated moduli for co-gels formed in the absence of NaCl, where the mixed solutions remain monophasic, by application of the relationship proposed by Davies for bicontinuous composites. The modulus of the calcium pectinate gel, which is already present when the gelatin network forms, was calculated (i) on the assumption of dynamic cross-linking (i.e. using the concentration-dependence of G′ for calcium pectinate alone), and (ii) for permanent cross-linking (by application of deswelling theory). The experimental moduli moved from close agreement with the former model to close agreement with the latter as the gelatin concentration increased from 0 to 10 wt%, consistent with a progressive increase in the extent of rearrangement of the calcium pectinate network required to accommodate the compression introduced by gelation of gelatin.     

Characterization of gelation time and texture of gelatin and gelatin–polysaccharide mixed gels

The effects of cooling rate, holding temperature, pH and polysaccharide concentration on gelation characteristics of gelatin and gelatin–polysaccharide mixtures were investigated using a mechanical rheometer which monitored the evolution of G′ and G″. At low holding temperatures of 0 and 4 °C, elastic gelatin gels were formed whereas a higher holding temperature of 10 °C produced less elastic gels. At slow cooling rates of 1 and 2 °C/min, gelling was observed during the cooling phase in which the temperature was decreased from room temperature to the holding temperature. On the other hand, at higher cooling rates of 4 and 8 °C/min, no gelation was observed during the cooling phase. Good gelling behavior similar to that of commercial Strawberry Jell-O^® Gelatin Dessert was observed for mixtures of 1.5 and 15 g sucrose in 100 ml 0.01 M citrate buffer containing 0.0029–0.0066 g low-acyl gellan. Also, these mixed gels were stronger than Strawberry Jell-O^® Gelatin Desserts as evidenced by higher G′ and gel strength values. At a very low gellan content of 0.0029 g, increasing pH from 4.2 to 4.4 led to a decrease in the temperature at the onset of gelation, G′ at the end of cooling, holding and melting as well as an increase in gel strength. The gelation time was found to decrease to about 40 min for gelatin/sucrose dispersions in the presence of 0.0029 g gellan at pH 4.2 whereas the corresponding time at pH 4.4 was higher (79 min). In general, the gelation time of gelatin/sucrose dispersions decreased by a factor of 2 to 3 in the presence of low-acyl gellan. The addition of low-acyl gellan resulted in an increase in the gelation rate constant from 157.4 to 291 Pa. There was an optimum low-acyl gellan content for minimum gelation time, this optimum being pH dependent. Addition of guar gum also led to a decrease in gelation time to 73 min with a corresponding increase in the gelation rate constant to 211 Pa/min though these values were not sensitive to guar gum content in the range of 0.008–0.05 g. The melting temperature of gelatin/sucrose/gellan as well as gelatin/sucrose/guar mixtures did not differ significantly from that of pure gelatin or Strawberry Jell-O^® Gelatin Desserts. At pH 4.2, the melting rate constant was highest at a low-acyl gellan content of 0.0029 g whereas the rate constant was insensitive to low-acyl gellan content at pH 4.4. Addition of guar did not seem to affect the melting temperature or the melting rate constant.     

Effect of oxidised starch on calcium pectinate gels

Small-deformation oscillatory measurements have been used to study the effect of low-viscosity oxidised starch on the rheology of calcium pectinate gels formed by controlled cooling from the sol state at high temperature. Large reductions in modulus (G′; 0.5% strain; 10°C) were observed at starch concentrations below the minimum critical concentration for gelation of oxidised starch alone under the same conditions (32 wt%). This behaviour is tentatively ascribed to thermodynamic incompatibility between the two polymers causing incipient precipitation of calcium pectinate within the gel network.     

Maturation effects in fish gelatin and HPMC composite gels

This study assessed the effectiveness of using hydroxylpropylmethylcellulose (HPMC) to enhance mechanical strength and thermal stability in fish skin gelatin (FG). The significant increase in absorbance (A₄₀₀) observed after HPMC had been added to FG and then matured indicated successful formation of a composite gel. Increased gel strength and storage modulus (G′) indicated the enhanced gelation ability of the matured composite gel, while increased melting temperature (T_m) and enthalpy (ΔH) indicated its improved thermal stability. Maturation-related rheological property improvements were more noticeable at 4 °C than 10 °C, but no apparent differences in T_m improvement were observed between 4 °C and 10 °C maturation. Nevertheless, the composite gel exhibited reversible cold and thermal gelation properties.     

Role of cosolutes in gelation of high-methoxy pectin. Part 1. Comparison of sugars and polyols

Mixtures of high-methoxy pectin (DE 70; 1.0 wt%; pH 3.0) with ethan-1,2-diol, glycerol, xylitol, sorbitol, glucose, fructose or sucrose at concentrations of 50, 55, 60 and 65 wt% were prepared at 95 °C and changes in storage modulus (G′) and loss modulus (G″) during cooling to 5 °C, heating to 90 °C and re-cooling to 5 °C (at 1 °C/min) were measured at 1 rad s⁻¹ and 0.5% strain. In all cases, the onset temperature for gelation during cooling and the moduli recorded at 5 °C increased with increasing concentration of cosolute. Both values, however, were substantially lower for the liquid cosolutes (ethan-1,2-diol and glycerol) than for mixtures incorporating the same concentrations of the solid cosolutes. The difference is attributed to inhibition of pectin–pectin interactions by pectin–cosolute interactions, which in turn are inhibited by cosolute–cosolute interactions, these being weaker for the liquid cosolutes than for the solids (as indicated by lower melting points). On heating, there was an initial reduction in modulus, with the same temperature-course as the increase on cooling; for the solid cosolutes, this was followed by an increase attributable to hydrophobic association of methyl ester substituents. No such increase was seen with the liquid cosolutes, but differential scanning calorimetry studies showed two (reversible) thermal transitions in all cases, one over the temperature-range of the initial gelation process on cooling and the other coincident with the increase in modulus on heating in the presence of the solid cosolutes. The absence of any detectable increase in modulus on heating with the liquid cosolutes is attributed to accumulation of cosolute around the polymer chains (i.e. pectin–cosolute interactions) promoting hydrophobic association between methyl ester groups on the same chain, or within small clusters of chains, with, therefore, no contribution to network structure. At high concentrations of the solid cosolutes, the increase in modulus on heating was followed by a decrease at higher temperature; this is attributed to excessive aggregation, and was reflected in lower moduli on subsequent re-cooling to 5 °C, in contrast to the enhancement in gel strength after heating and cooling that was observed at lower concentrations of the same cosolutes.     

On solid-like rheological behaviors of globular protein solutions

Dynamic viscoelastic and steady flow properties of β-lactoglobulin, bovine serum albumin, ovalbumin, and α-lactalbumin aqueous solutions were investigated at 20°C. When a sinusoidal strain in the linear viscoelastic region was applied, the solutions of the globular proteins except for α-lactalbumin showed typical solid-like rheological behavior: the storage modulus G′ was always larger than the loss modulus G″ in the entire frequency range examined (0.1–100 rad/s). Under a steady shear flow, strong shear thinning behavior was observed with increasing shear rate from 0.001 to 800 s⁻¹, for the globular proteins except for α-lactalbumin. The values of the steady shear viscosity η were lower than those of the dynamic shear viscosity η* at a comparable time scale of observation, violating the Cox–Merz rule, and thus suggesting that a solid-like structure in a globular protein solution was susceptible to a steady shear strain. During isothermal gelation of the protein colloids at 70°C, no crossover between G′ and G″ was observed so that the gelation point was judged by an abrupt increase in the modulus or a sudden decrease in tanδ.     

Whey protein concentrate/λ-carrageenan systems: Effect of processing parameters on the dynamics of gelation and gel properties

The thermal characteristics, dynamics of gelation and gel properties of commercial whey protein concentrate (WPC), WPC/λ-carrageenan (λ-C) mixtures (M) and WPC/λ-C spray-dried mixtures (DM) have been characterized. In a second stage, the effect of the gelling variables (T, pH, total solid content) on gelation and textural properties of DM was evaluated through a Doehlert uniform shell design.The presence of λ-C either in mixtures (M) or in DM promoted the WPC gelation at lower concentration (8%). M showed higher rates of formation and better gel properties (higher hardness, adhesiveness, springiness and cohesiveness) than DM.Nevertheless, when the effects of pH (6.0–7.0), heating temperature (75–90 °C) and total solid content (12–20 wt%) on gelation dynamics and gel properties of DM were studied, gels with a wide range of rheological and textural properties were obtained. While pH did not affect the gelation dynamics, it had some effect on rheological and textural properties. Total solid content and heating temperature were the most important variables for the dynamics of gelation (gelation rate (1/t_gel), gelation temperature (T_gel), rate constant of gel structure development (k_G), elastic modulus after cooling (G′_c) and textural parameters (hardness, springiness and cohesiveness).     

Effect of food matrix and heat treatment on the rheological properties of salmon-based baby food

Shelf stable baby foods from Alaskan salmon were developed from red and pink salmon with and without bones. The effect of salmon type, presence of bones and thermal treatment (121 °C for 55 min) on the dynamic (viscoelastic) and flow models were evaluated. Rheological behaviors of all samples were also tested over a temperature range of 25–55 °C. All samples had a higher viscoelastic behavior with consistently higher storage modulus (G′) than loss modulus (G″) over the entire frequency range used (0.5–100 rads/s at 25 °C). Thermal treatment had a significant effect (p < 0.0001) on viscoelastic behavior of baby foods when the exponential model (G′ or G″ = A(ω)^b) was used. A values were higher for the processed food for G′ and G″, and b values. The Casson model was found to be the best for the shear rate – shear stress for all types of tested samples. Retorted samples exhibited lower yield stress than their unretorted counterparts. Retorted samples were susceptible to temperature change more than unretorted samples as shown by energy of activation values (E_a) when the effect of temperature (25–55 °C) was studied with an Arrhenius type model. Red salmon without bone had higher E_a values among all samples and with pink salmon with bone recording a lowest among all of them.     

Rheological behaviour of uncross-linked and cross-linked gelatinised waxy maize starch with pectin gels

Rheological characterisation of uncross-linked (UPS) and cross-linked (CPS) waxy maize starch with pectin was conducted to determine the influence of pectin on the properties of the starch. The viscoelastic behaviour of 5% (w/v) gel systems containing UPS and CPS polysaccharides at 25 °C was evaluated by small angle deformation oscillation rheometry. Viscoelasticity measurements of the cross-linked polysaccharides indicated that the elastic component increased after cross-linking. Among all gels studied, the properties of the CPS mixtures (ratios 2:3 and 3:2) showed quite high storage (G′) and loss (G″) moduli (compared with gels of other ratios), indicating that gels of these two particular ratios had the greatest degree of elasticity and were very well structured. The results suggest that cross-linking between starch and pectin molecules can give rise to novel rheological properties.     

Physicochemical properties of pectins from okra (Abelmoschus esculentus (L.) Moench)

Okra pectin obtained by hot buffer extraction (HBSS) consists of an unusual pectic rhamnogalacturonan I structure in which acetyl groups and alpha galactose residues are substituted on rhamnose residues within the backbone. The okra Chelating agent Soluble Solids (CHSS) pectin consists of slightly different structures since relatively more homogalacturonan is present within the macromolecule and the rhamnogalacturonan I segments carry slightly longer side chains. The rheological properties of both okra pectins were examined under various conditions in order to understand the unusual slimy behaviour of okra pectins. The viscosity of the okra HBSS pectin was 5–8 times higher than the viscosity of the okra CHSS pectin. The okra HBSS pectin showed an elastic behaviour (G′ > G″) over a wide range of frequencies (10⁻¹–10 Hz), at a strain of 10%, while okra CHSS and saponified okra HBSS/CHSS pectin showed predominantly viscous responses (G′ < G″) over the same frequency range. The results suggest that the structural variation within the okra pectins greatly affect their rheological behaviour and it is suggested that acetylation of the pectin plays an important role through hydrophobic associations. Dynamic light scattering was used to study the association behaviour of both okra pectins at low concentration (0.001–0.1% w/w). Results showed that the saponified okra pectins did not exhibit a tendency to aggregate in the concentration range studied, whereas both non saponified samples showed a substantial degree of association. These results suggest that the unusual slimy behaviour of the non saponified samples may be related to the tendency of these pectins to associate, driven by hydrophobic interactions.     

Effect of inulin on the rheological properties of silken tofu coagulated with glucono-δ-lactone

This study investigated the rheological properties of inulin-containing silken tofu coagulated with glucono-δ-lactone (GDL) upon heating. Inulin (Raftiline^® HP-gel) was added to a soy protein isolate-enriched cooked soymilk at 0%, 1%, 2%, 3% and 4% (w/v) levels along with 0.4% (w/v) GDL to prepare acid-induced silken tofu. Gelation was induced by heating the soymilk mixture from 20 to 90 °C at a constant rate (1 °C/min) or isothermally at 90 °C for 30 min. The gelling properties were measured with dynamic small-deformation mechanical analysis and static large-deformation compression tests. The rheological changes in soymilk during gelation were dependent upon both the pH decline (hydrolysis of GDL) and the specific temperature of heating. Control samples heated to 50 °C, with the pH lowered to 5.95, started to gel, showing a rapid increase in storage (G′) and loss (G″) moduli afterwards. The addition of 2% inulin lowered the on-set gelling temperature by 2.8 °C and improved (P < 0.05) both rheological parameters of the tofu gel as well as hardness and rupture force (textural profile analysis) of the formed silken tofu. The results indicated that inulin enhances the viscoelastic properties of GDL-coagulated silken tofu, and the textural effect of inulin is an added benefit to its current application mainly as a prebiotical ingredient in food.     

Solution viscosity and structural modification of pumpkin biopectin

Solutions of ‘biopectin’ obtained from pumpkin pulp by digestion with the multi-enzyme culture supernatant from Bacillus polymyxa (strain 88A) were prepared in 0.10 M NaCl and characterised by rotational viscosity measurements at 20 °C. The resulting double-logarithmic plot of specific viscosity versus the product of concentration (c) and intrinsic viscosity ([η], from combined Huggins and Kraemer extrapolation to c=0) showed a sharp increase in slope at c[η]≈1 in comparison with the normal value of c[η]≈4 for disordered coils, suggesting a branched structure, possibly arising from Ca²⁺-mediated association of constituent chains.Pumpkin biopectin is non-gelling, in marked contrast to the pectin obtained from the same source by conventional extraction with acid, although the yield is more than doubled. Attempts to induce gelation (with 70 wt% solids at low pH or with stoichiometric Ca²⁺ at neutral pH) by removing the high content of divalent cations naturally present in the biopectin and by chemical deacetylation under acidic conditions (pH 1.2; 4 days; 40 °C) proved unsuccessful. Further research using enzymic deacetylation is suggested.     

Phase transitions of pigskin gelatin

Edible films are flexible thin materials based on biopolymers. It is therefore necessary to know the physicochemical properties of those macromolecules in order to obtain films with desirable characteristics. Dried gelatin is a partially crystalline polymer that exhibits glass and helix–coil transitions. The knowledge of phase properties is important for the choice of the type and concentration of the plasticizer to be utilized to obtain a flexible and easy-to-handle film. The objective of this work is to determine the phase transitions of pigskin gelatin as a function of moisture content in the hygroscopic domain. Pure gelatin was conditioned over different saturated salt solutions at 25°C to allows samples with various moisture content. After equilibrium was established, the samples and an empty pan, as reference, were heated twice between −100 and 250°C at the rate of 5°C/min, in a DSC (TA 2010) after quench-cooling with liquid nitrogen. Gelatin was placed in a pan with a perforated cover, and maintained at 105°C for 24 h in the DSC cell before thermal analyses, to obtain completely dried samples. The glass transition temperature of these samples was found to be 220.2°C. The DSC traces obtained in the first scan, except those conditioned at 11% relative humidity, showed a glass transition followed by two endothermic peaks due to two sol–gel transitions in the gelatin. The plasticizing effect of moisture on Tg was observed in all the samples conditioned by absorption and in the second scan with the samples conditioned by desorption. This behavior was well represented by the Gordon and Taylor model, with κ=5.26 and R²=0.96. Also, a plasticizing effect of moisture over the sol–gel transitions was observed. The Flory-Huggins model was applied to experimental data with: χ₁=1.94 and R²=0.999, for the first peak Tm, and χ₁=1.90 and R²=0.989, for the second peak.     

Gelation behaviour of gelatin and alginate mixtures

Mixtures of alginate and gelatin were studied by rheology as a function of different parameters, such as temperature, biopolymer concentrations, calcium concentration and ionic strength. In particular conditions, the formation of a mixed gel of alginate and gelatin is obtained. A slow release of calcium ions leads first to an irreversible alginate gel and cooling results in a reversible gelatin gel. Depending on experimental conditions, non-linear behaviours upon gelation of alginate occur and a collapse of alginate gel is directly observable by rheology. These trends are favoured between 35 and 45 °C, by a high total biopolymer concentration or a high calcium concentration and ionic strength. Different mechanisms could be responsible for this collapse, such as a competition between alginate gelation and phase separation in the biopolymer mixture or an over-association of alginate chains at high Ca²⁺ concentration, favoured by the presence of gelatin.     

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.     

Gelling properties of kefiran, a food-grade polysaccharide obtained from kefir grain

The physicochemical and gelling properties of kefiran, a water-soluble glucogalactan with probed health-promoting properties, were investigated. Gel permeation chromatograms revealed a single distribution of molecular weight corresponding to 10⁷ Da. Intrinsic viscosity of kefiran determined using Huggins extrapolations was 6.0 dl/g and using Kramer approximations was 5.95 dl/g.Kefiran has a Newtonian behaviour in diluted solutions, which becomes pseudoplastic at higher concentrations. Rheological behaviour of the solution before and after freeze drying was evaluated by small deformation oscillatory rheological measurements. The mechanical spectrum of solution corresponded to an entangled network behaviour. After freeze–thaw treatment of the solution, a rheological behaviour transition from a liquid-like system to a gel was observed. The storage modulus (G′) in cryogels was 35 times higher than the value obtained for the solution. Rheological characteristics of the cryogel were influenced by kefiran concentration. As the polymer concentration increased, higher number of interactions was evident for the increment in both moduli (G′ and G″). The behaviour of kefiran cryogels about 37 °C determines its ability to melt at mouth temperature. These results suggest that kefiran cryogels could be an interesting alternative for its application in food formulations.     

Factors that determine the fracture properties and microstructure of globular protein gels

Protein gel matrices are responsible for the texture of many foods. Therefore an understanding of the chemical reactions and physical processes associated with fracture properties of gels provides a fundamental understanding of select mechanical properties associated with texture. Globular proteins form thermally induced gels that are classified as fine-stranded, mixed or particulate, based on the protein network appearance. The fundamental properties of true shear stress and true shear strain at fracture, used to describe the physical properties of gels, depend on the gel network. Type and amount of mineral salt in whey protein and β-lactoglobulin protein dispersions determines the type of thermally induced gel matrix that forms, and thus its fracture properties. A fine-stranded matrix is formed when protein suspensions contain monovalent cation (Li⁺, K⁺, Rb⁺, Cs⁺) chlorides, sodium sulfate or sodium phosphate at ionic strengths ≤0.1 mol/dm³. This matrix has a well-defined network structure, and varies in stress and strain at fracture at different salt concentrations. At ionic strengths >0.1 mol/dm³ the matrix becomes mixed. This network appears as a combination of fine strands and spherical aggregates, and has high stress values and minimum strain values at fracture. Higher concentrations of monovalent cation salts cause the formation of particulate gels, which are high in stress and strain at fracture. The salt concentration required to change microstructure depends on the salt's position in the Hofmeister series. The formation of a particulate matrix also occurs when protein suspensions contain low concentrations (10–20 mol/dm³) of divalent cation (Ca²⁺, Mg²⁺, Ba²⁺) chloride salts or di-cationic 1,6-hexanediamine at pH 7.0. The divalent cation effect on β-lactoglobulin gelation is associated with minor changes in tertiary structure involving amide—amide interproton connectivities (determined by ¹H NMR) at 40–45°C, increasing hydrophobicity and intermolecular aggregation. The type of matrix formed appears to be related to the dispersed or aggregated state of proteins prior to denaturation. Mixed and particulate matrices result from conditions which favor aggregation at temperatures (25–45°C) which are much lower than the denaturation temperature (~65°C). Therefore, general (e.g. Hofmeister series) and protein-specific factors can affect the dispersibility of proteins and thereby determine the microstructure and fracture properties of globular protein gels.     

Pectin–sucrose–Ca2+ interactions: effects on rheological properties

The effects of varying concentrations of pectin (4.5–6.5%, w/w), sucrose (40–60%, w/w) and calcium (20–60 mg/g pectin) on the viscoelastic properties of pectin dispersions at pH 3.0 were investigated. Pectin samples used were extracted from pomelo fruit peels (Citrus grandis) grown in Malaysia. The dynamic rheological parameters (G′, G″, δ and η*) of pectin–sucrose–calcium dispersion were determined at 1.5% strain from 90–20°C at a cooling rate of 3°C min⁻¹. Plots of G′ and G″ against frequency (rad s⁻¹) showed G″>G′ throughout the frequency range with no occurrence of crossover for most of the pectin dispersions. In addition both storage (G′) and loss (G″) moduli of the dispersions increase on cooling. Increasing pectin, sucrose and calcium concentrations increased G′ and G″ with pectin having the greatest effect. Interactions amongst the three factors were also studied. At lower pectin concentrations, addition of Ca²⁺ increased G′ at all temperatures. This effect was also observed at higher pectin concentrations at 20°C but not at 90°C. The opposite effect was observed with the addition of sucrose, i.e. addition of sucrose at a higher pectin concentration increased G′ whereas at a lower pectin concentration no effect was observed. Interaction between calcium and sucrose gave rise to an increase in G′ when Ca²⁺ was added at high sucrose concentrations, but a decrease in G′ was evident at low sucrose concentrations. Dispersions of pectin alone or in combination with sucrose exhibited a more liquid-like behaviour with G″>G′. However, in the presence of Ca²⁺, mechanical spectra of G′>G″ were obtained.     

Effects of drying on the gel strength and cation mobility of furcellaran

Thermal stability, by means of air drying a furcellaran powder, and its impact on gel strength and cation mobility were studied. Halogen heating in the temperature range 90–115°C for 15 min resulted in loss on drying (L_D, %). These results can be described by polynom L_D=−9.583+2.989τ−0.249τ²+0.00729τ³+0.1034t (R²=0.9976), indicating a gradual decomposition of carbohydrates. Air-drying induced a decrease in gel strength and the partial removal of potassium, calcium and sodium ions from the matrix. Air drying above 115°C yielded a remarkable destruction of polysaccharides with a total collapse in gelling power.     

Shear-induced structuring of particulate whey protein gels

The effects of steady shear on particulate whey protein isolate (WPI) gels, at pH 5.4, have been investigated by light microscopy (LM) and dynamic oscillatory measurements. The steady shear was performed on suspensions at constant rates between 0.5 and 126/s. The gel point under static conditions (T_g) was around 78 °C and the shearing was performed during heating from 20 to 76 or to 82 °C. The gel point was postponed by the shear up to 82 °C. Steady shear up to 76 °C, at rates less than 6/s, resulted in a weaker storage modulus (G′), less frequency dependence and a higher stress at fracture compared to the unsheared gel. Steady shear up to 82 °C, at rates below 6/s, resulted in the formation of two different types of network structure. One structure was similar in appearance to the unsheared network, showing pores in the range of 50 μm. The other structure was dense, composed of smaller particles than the unsheared network and with pores in the range of 10 μm. The gels composed of two structures showed a lower G′ and stress at fracture compared to the unsheared gel. A shear rate above 24/s up to 76 °C resulted in irregular networks, which were composed of two different types of structures. One was loose and open, similar in appearance to the unsheared network structure. The other structure was dense and compact, and was present as individual aggregates. These gels also showed a weaker G′ than the unsheared gel. A shearing up to 82 °C at rates above 24/s resulted in a coarse, inhomogeneous network structure. The gels showed a weak G′, indicating aggregate break-up during the steady shear.