Structure and rheology of the κ-carrageenan/locust bean gum gels

The structure and interaction of κ-carrageenan and locust bean gum (LBG) has been studied using rheology, cryo-SEM, conductivity and syneresis characterization. The rheological behaviour of the binary system has been characterized using both compression and shear measurements. Elimination of slip in the shear measurements yields G′ values of the order 10,000–30,000 Pa for a 1% κ-carrageenan gel in 0–0.2 M added KCl. These values are higher than previously reported. No synergistic peak was found with the addition of LBG as has been previously reported. The measured modulii for these gels yields a Poisson's ratio of 0.5. Compression rupture stress and strain were also monitored. The rupture measurements do show a synergistic peak indicating that the interaction does occur and is important at high strain amplitudes. The gel points as determined by conductivity for these systems show a decrease in temperature with increasing LBG concentration, which is consistent with rheological measurements. Syneresis results are reported for the range of κ-carrageenan/LBG ratios. The syneresis shown by the mixtures is the same as that shown by the same concentration of κ-carrageenan. Structures of the gels as determined by cryo-SEM are also reported. Characteristic length scales in these systems are of the order of tens of microns and show little change with LBG concentration. The reduction in the characteristic length scale with increasing LBG concentration is discussed in terms of the rheological behaviour.     

Influence of κ-carrageenan on the properties of a protein-stabilized emulsion

We report on the influence of κ-carrageenan (κ-CAR) on the surface activity of bovine serum albumin (BSA) and on the properties of BSA-stabilized oil-in-water emulsions. Surface tension data at low ionic strength indicate an electrostatic interaction at neutral pH which becomes much stronger at pH 6. The effect of the attractive BSA–κ-CAR interaction on the state of aggregation and creaming stability of protein-stabilized emulsions (20 vol% n-tetradecane, 1.7 wt% BSA, 5 mM) has been investigated at three pH values. At pH 6 the system behaviour is interpreted in terms of bridging flocculation leading to an emulsion droplet gel network over a certain limited polysaccharide concentration range. While the trend of behaviour is qualitatively similar to that reported recently for equivalent BSA+ι-carrageenan (ι-CAR) solutions and emulsions, the BSA–κ-CAR interaction is clearly weaker than the BSA–ι-CAR interaction under similar pH and ionic strength conditions. This means that a higher polysaccharide content is required to induce flocculation in systems containing κ-CAR, and also that the resulting emulsion gel network is weaker. The behaviour is consistent with the lower density of charged sulfate groups on the κ-CAR as compared with the ι-CAR.     

Differential scanning calorimetric studies of Philippines natural grade κ-carrageenan

Using differential scanning calorimetry, the gel-sol transition temperatures were compared for KCl-precipitated κ-carrageenan and Philippines natural grade (PNG) κ-carrageenan. The latter contains cellulose, which is associated with the carrageenan in seaweed. This leads to lower gel-sol transition temperatures. More heat absorption (-ΔH_m = 79 kJ/mol) is required for the formation of junction zones for the PNG κ-carrageenan than the KCl-precipitated κ-carrageenan (-ΔH_m = 40 kJ/mol). This behaviour can be attributed to the cellulose interspersing between κ-carrageenan aggregates in the PNG κ-carrageenan gels.     

Sweetening power of aspartame in hydrocolloids gels: Influence of texture

Equivalent sweetness of aspartame relative to two sucrose concentrations (10% and 20% w/w) were determined in water and in hydrocolloids gels. The influence of the texture of three hydrocolloids gelled systems—gellan gum, κ-carrageenan, and κ-carrageenan/locust bean gum (LBG)—at two gums concentrations (0.3% and 1.2% w/w) on the equivalent sweetness of aspartame were then studied. For the three gelled systems, the increase in hydrocolloid concentration produced a significant increase in the true rupture stress and in the deformability modulus values. For both κ-carrageenan and mixed gels the true rupture strain values increased when increasing hydrocolloid concentration while for gellan gels, decreased. For the same hydrocolloid concentrations the κ-carrageenan/LBG gels showed the largest strain at rupture and gellan gels the smallest (most brittle). For both soft (0.3% gum) and hard (1.2% gum) gellan gels and κ-carrageenan gels, the concentrations of aspartame needed to deliver a sweetness intensity equivalent to that of gels with 10% sucrose (0.079–0.087% w/w) were similar to those obtained for aqueous solutions (0.084% w/v). For hard κ-carrageenan/LBG gels the corresponding concentration of aspartame was slightly lower. For all gelled systems the concentrations of aspartame needed to deliver a sweetness intensity equivalent to that of gels with 20% sucrose were higher for soft gels than for hard gels.     

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.     

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.     

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 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.     

Characterization and improvement of rheological properties of sodium caseinate glycated with galactose, lactose and dextran

The aim of this work was to investigate the effect of non-enzymatic glycosylation with galactose, lactose, and 10 kDa dextran on the rheological properties of sodium caseinate. To promote the formation of covalent complexes, the reaction was done in solid state (a_w = 0.67), pH 7.0 (0.1 M sodium phosphate buffer), and temperature set at 50 and 60 °C. The progress of Maillard reaction was indirectly traced by measuring the formation of the Amadori compound, through furosine (2-furoylmethyl-lysine) analysis, and brown polymers, and the resulting glycoconjugates were characterized by LC/ESI-MS and SEC. Results showed a higher reactivity of galactose than lactose and dextran to form the glycoconjugates, due to its smaller molecular weight. Glycation with galactose and lactose increased the viscosity of caseinate and also altered its flow characteristics from Newtonian to shear-thinning. Oscillatory testing showed a higher elastic modulus (G′) in glycoconjugates when compared to non-glycated caseinate, especially with galactose, where a gel-like behaviour was observed after long incubation times. Glycation with dextran did not produce substantial improvements in the rheological properties of caseinate, probably due to the limited extent of the reaction. Our results show that by controlling the rate and extent of the Maillard reaction is a technologically feasible operation to improve the viscosity and gelling properties of sodium caseinate-based ingredients.     

Effect of hydrocolloid type and concentration on flow behaviour and sensory properties of milk beverages model systems

Formulation of low-calorie flavoured milk beverages involves the use of hydrocolloids to obtain an acceptable mouthfeel. Sodium alginate and κ-carrageenan are the most commonly used. In this paper, the rheological behaviour of model solutions containing either of these two hydrocolloids, with or without the addition of sucrose, in water or milk, has been studied. All solutions fitted well to the Ostwald de Waele model. The analyses of variance showed that in κ-carrageenan solutions, the effect of the medium–hydrocolloid interaction on both flow index (n) and apparent viscosity at 1 s⁻¹ (η₁) was significant. Milk-based solutions were more pseudoplastic and more viscous than the aqueous solutions, due to the well known κ-carrageenan–casein molecular interaction. In alginate solutions, the medium–hydrocolloid interaction was also significant but the differences in both n and η₁ values were of less entity. Sensory viscosity differences of more viscous chocolate milk model systems were well explained by apparent viscosity values at low shear rate (10 s⁻¹), while for less viscous samples apparent viscosity at higher shear rate (300 s⁻¹) values were appropriate. At similar viscosity, κ-carrageenan systems showed better flavour-releasing properties than alginate.     

Structural and mechanical characterization of κ/ι-hybrid carrageenan gels in potassium salt using Fourier Transform rheology

The mechanical and structural properties of κ/ι-hybrid carrageenan gels obtained at various concentrations in the presence of 0.1 m KCl were studied with Fourier Transform rheology (FTR) and cryoSEM imaging. FTR data show that gels formed at concentration below 1.25 wt% exhibit a strain hardening behavior. The strain hardening is characterized by a quadratic increase of the scaled third harmonic with the strain and a third harmonic phase angle of zero degree. Both features are weakly depending on the concentration and conform to predictions from a strain hardening model devised for fractal colloidal gels. However, the phase angle of the third harmonic reveals that κ/ι-hybrid carrageenan gels obtained at higher concentrations show shear thinning behavior. Colloidal gel models used to extract structural information from the concentration scaling of gel equilibrium shear modulus G₀ and the strain dependence of FTR parameters suggest that κ/ι-hybrid carrageenan gels are built from aggregating rod-like strands (with fractal dimension x = 1.13) which essentially stretch under increasing strain. The mechanically relevant structural parameters fairly match the gel fractal dimension (d = 1.66) obtained from the cryoSEM analysis.     

Effect of high-methoxy pectin on properties of casein-stabilized emulsions

We report on the effect of high-methoxy pectin on the stability and rheological properties of fine sunflower oil-in-water emulsions prepared with α_s1-casein, β-casein or sodium caseinate. The aqueous phase was buffered at pH 7.0 or 5.5 and the ionic strength was adjusted with sodium chloride in the range 0.01–0.2 M. Average emulsion droplet sizes were found to be slightly larger at the lower pH and/or with pectin present during emulsification. Analysis of the serum phase after centrifugation indicated that some pectin becomes incorporated into the interfacial layer at pH 5.5 but not at pH 7.0. This was also supported by electrophoretic mobility measurements on protein-coated emulsion droplets and surface shear viscometry of adsorbed layers at the planar oil–water interface. A low pectin concentration (0.1 wt%) was found to give rapid serum separation of moderately dilute emulsions (11 vol% oil, 0.6 wt% protein) and highly pseudoplastic rheological behaviour of concentrated emulsions (40 vol% oil, 2 wt% protein). We attribute this to reversible depletion flocculation of protein-coated droplets by non-adsorbed pectin. At ionic strength below 0.1 M, the initial average droplet sizes, the creaming behaviour, and the rheology were found to be similar for emulsions made with either of the individual caseins (α_s1 and β) or with sodium caseinate. At higher ionic strength, however, whereas emulsions containing β-casein or sodium caseinate were stable, the corresponding α_s1-casein emulsions exhibited irreversible salt-induced flocculation which was not inhibited by the presence of the pectin.     

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.     

Gelation behaviour of aqueous solutions of different types of carrageenan investigated by low-intensity-ultrasound measurements and comparison to rheological measurements

The gelation of different carrageenans in aqueous solutions was investigated by ultrasound at 7.8 MHz and by oscillating rheological measurements. The gelation of κ-, κ/ι-hybrid-carrageenan as well as a mixture of κ- and ι-carrageenans causes an increase in ultrasonic attenuation and a decrease in ultrasonic velocity. However, the gelation of ι-carrageenan, which could be detected by the oscillating rheological measurement, did not cause detectable changes in the ultrasonic measurement. Different gelation behaviours of a κ/ι-hybrid-carrageenan and a mixture of κ- and ι-carrageenans were observed by both techniques. This illustrates that the analytical techniques studied, which are based on different principles, can differentiate the gelation behaviour of different carrageenan systems. This might have important implications on the elucidation of gel formation mechanisms which, for complex systems, cannot be explained using only one single analytical technique.     

Interfacial composition and stability of emulsions made with mixtures of commercial sodium caseinate and whey protein concentrate

The interfacial composition and the stability of oil-in-water emulsion droplets (30% soya oil, pH 7.0) made with mixtures of sodium caseinate and whey protein concentrate (WPC) (1:1 by protein weight) at various total protein concentrations were examined. The average volume-surface diameter (d₃₂) and the total surface protein concentration of emulsion droplets were similar to those of emulsions made with both sodium caseinate alone and WPC alone. Whey proteins were adsorbed in preference to caseins at low protein concentrations (<3%), whereas caseins were adsorbed in preference to whey proteins at high protein concentrations. The creaming stability of the emulsions decreased markedly as the total protein concentration of the system was increased above 2% (sodium caseinate >1%). This was attributed to depletion flocculation caused by the sodium caseinate in these emulsions. Whey proteins did not retard this instability in the emulsions made with mixtures of sodium caseinate and WPC.     

Rheological Behavior of High-Concentration Sodium Caseinate Dispersions

ABSTRACT: Apparent viscosity and frequency sweep (G′, G″) data for sodium caseinate dispersions with concentrations of approximately 18% to 40% w/w were obtained at 20 °C; colloidal glass behavior was exhibited by dispersions with concentration ≥23% w/w. The high concentrations were obtained by mixing frozen powdered buffer with sodium caseinate in boiling liquid nitrogen, and allowing the mixtures to thaw and hydrate at 4 °C. The low-temperature G′−G″ crossover seen in temperature scans between 60 and 5 °C was thought to indicate gelation. Temperature scans from 5 to 90 °C revealed gradual decrease in G′ followed by plateau values. In contrast, G″ decreased gradually and did not reach plateau values. Increase in hydrophobicity of the sodium caseinate or a decrease in the effective volume fraction of its aggregates may have contributed to these phenomena. The gelation and end of softening temperatures of the dispersions increased with the concentration of sodium caseinate. From an Eldridge–Ferry plot, the enthalpy of softening was estimated to be 29.6 kJ mol⁻¹. Practical Application: The results of this study should be useful for creating new products with high concentrations of sodium caseinate.     

Gelling property of soy protein–gum mixtures

Gelling properties of soy protein–gum mixtures were determined by small deformation oscillation measurement and the experimental data were analyzed with blending laws of polymers. Gel strength of soy protein–carrageenan mixture was found to follow either upper or lower bounds depending on the concentration of the constituents, suggesting the occurring of phase shift. G′ of soy protein–xanthan mixed gel always followed the upper bound, indicating that soy protein was the continuous phase regardless variations of the gum concentration. Combination of soy protein with propylene glycol alginate (PGA) produced a mixed gel with high gel strength and stayed above the upper bound at all gum concentrations examined. Covalent bonds between PGA and soy protein was suggested to contribute to the rigidity. Storage modulus of the mixture of soy protein–locust bean gum (LBG) was below the lower bound at low gum concentrations due to the limited demixing process of LBG. G′ values of the mixture of soy protein and LBG–xanthan followed the lower bound but approached upper bound on reducing protein concentration, suggesting that the presence of soy protein might inhibit LBG–xanthan mixture from forming continuous networks.     

Viscoelastic properties of xanthan gum hydrogels annealed in the sol state

Viscoelastic properties of xanthan gum aqueous solutions and hydrogels were investigated using a cone-plate type rheometer. The change of viscoelasticity during annealing the solution and cooling to gelation temperature was examined as functions of annealing time, temperature and frequencies. In the annealing process, the storage modulus G′ increases with increasing annealing time. In the subsequent cooling process, G′ of the annealed solution increased, whereas the G′ of non-annealed solution remained almost constant. G′ of hydrogels increased with the increase of annealing temperature and concentration. Based on the experimental results obtained, the structural change of the solution in the annealing process and the structure of gels were investigated.     

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.     

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.