Rheological properties of mixtures of protein-polysaccharide-dynamic viscoelasticity of blend gels of acylated gelatin, kappa-carrageenan, and agarose

Complex Young's modulus of blend gels of gelatin and kappa-carrageenan or agarose has been measured in order to clarify the protein-polysaccharide interaction in biological systems. The mixture of gelatin and kappa-carrageenan showed phase separation in the intermediate volume fraction of gelatin, and it formed a homogeneous gel when the volume fraction of gelatin is very large or very small. Since the dynamic Young's modulus for blend gels of kappa-carrageenan and gelatin was larger than the calculated one from a theory for dispersed systems, some structural reinforcing must occur. The mixture of agarose and gelatin showed the inverse tendency. It was concluded that the role of electrolytic groups was dominant in dilute gels, while molecular entanglement became more important in concentrated gels.     

Effects of Potassium Chloride and Sodium Chloride on the Thermal Properties of Gellan Gum Gels

The exothermic and endothermic peaks in cooling and heating curves of differential scanning calorimetry (DSC) for gellan gum gels without and with potassium chloride and sodium chloride were analyzed. The gelling and melting temperatures shifted to higher temperatures with increasing gellan and salt concentration in the concentration range of gellan from 0.3 to 2.0% (w/w). The exothermic and endothermic enthalpy increased with increasing gellan and salt concentrations. Cooling DSC curves showed one exothermic peak for samples with salts and at low gellan concentration. Heating DSC curves showed many peaks for all samples except 0.3% (w/w) gellan gum gels. The sol-gel transition of samples was examined numerically by using a zipper model approach. The introduction of cations increases the number of junction zones or zippers and decreases the rotational freedom of parallel links. This makes the structure of junction zones more heat resistant, and increases the elastic modulus of the gel.     

Gelation behavior of native and acetylated konjac glucomannan

Gelation kinetics of native and acetylated konjac glucomannan (KGM) samples in the presence of alkali (sodium carbonate) was studied by dynamic viscoelastic measurements. Molecular weight and other molecular parameters of KGM were determined by static light scattering and viscosity measurements. It was found that KGM molecules were degraded during acetylation treatment, but the molecular weights of acetylated samples were almost independent of the degree of acetylation (DA) and were about a half of that of a native sample. At a fixed alkaline concentration, increasing concentration of KGM or temperature shortened the gelation time, but increasing DA delayed it. The deacetylation reaction and subsequent aggregation process of acetylated samples needed longer time than that of native sample, and acetylated samples formed finally more elastic gels. It implied that the presence of acetyl groups exerts a strong influence on gelation behavior of KGM. It was suggested that the gelation rate of acetylated KGM and native KGM, which depends on the alkaline concentration and temperature, is an important factor that determines the elastic modulus of gels. This was supported by the experimental finding that the saturated elastic modulus tends to the same value when the ratio of alkali concentration to acetylated groups was kept constant. In slower gelation processes, junction zones are more homogeneously distributed and more numerous, leading to the more elastic gels.     

Synergistic Interaction between Kappa-Carrageenan and Locust-bean Gum in Aqueous Media

Although neither kappa-carrageenan nor locust-bean gum gelled alone, a mixed aqueous solution of the above gums gave a gel at the concentration of 0.6% total gums in a range of low temperatures. The solution also gelled even at the concentration of 0.4% total gums in the presence of 0.1% KC1. The maximum dynamic modulus was obtained with a series of the samples composed of kappa-carrageenan and locust-bean gum in the mixing ratios of 1:1 and 3:1 at the concentration of 0.6 and 0.8% total gums at 0°C. The dynamic modulus of a mixed solution of kappa-carrageenan and locust-bean gum was not influenced by pH between pH 7.0 to 11.5, but decreased in the acidic range.

We concluded that intermolecular interactions, at low temperature, between kappa-carrageenan and locust-bean gum may take place on the K⁺-bridge of the former and the backbone of the latter molecule at low concentrations, but at high concentration of the gums, self-association of kappa-carrageenan molecules might also occurred.     

Molecular structure of potassium kappa carrageenate gels: a rheological and pulsed electric birefringence study

Measurements have been made of the shear modulus as a function of biopolymer concentration for pure potassium kappa carrageenate gels. The results have been discussed on the basis of rubber elasticity theory to investigate the free polymer linkages between junction zones within the gels. Pulsed electric birefringence studies have been made on segmented kappa carrageenan as a model for the junction zones of the gel.     

Influence of thermal history on the structural and mechanical properties of agarose gels

Using a multitechnique approach, two temperature domains have been identified in agarose gelation. Below 35 degrees C, fast gelation results in strong, homogeneous and weakly turbid networks. The correlation length, evaluated from the wavelength dependence of the turbidity, is close to values of pore size reported in the literature. Above 35 degrees C, gelation is much slower and is associated with the formation of large-scale heterogeneities that can be monitored by a marked change in the wavelength dependence of turbidity and visualised by transmission electron microscopy. Curing agarose gels at temperatures above 35 degrees C, and then cooling them to 20 degrees C, produces much weaker gels than those formed directly at 20 degrees C. Dramatic reductions in the elastic modulus and failure strain and stress are found in this case as a result of demixing during cure. An interpretation, based on the kinetic competition between osmotic forces (in favor of phase separation) and elastic forces (that prevent it) is proposed.     

Rheological study into the ageing process of high methoxyl pectin/sucrose aqueous gels

The ageing process of high methoxyl pectin (HMP)/sucrose gels was followed at different ageing temperatures by small amplitude oscillatory experiments. Dynamic mechanical measurements allowed the characterisation of the point at which the system undergoes the sol/gel transition. The HMP/sucrose system is extremely sensitive to temperature variation during ageing, especially in the lower temperature range. The viscoelastic behaviour through the gel point changes with the ageing temperature, probably due to variations in mobility of the pectin chains, and consequently, in the lifetime of junction zones. Weaker pectin networks are formed under thermal conditions unfavourable to the development of hydrophobic interactions. Gel time and elastic modulus have a complex dependence on temperature, which could be attributed to the different thermal behaviour of the intermolecular interactions that stabilise the nonpermanent cross links of these physical networks.     

Interpenetrating network formation in agarose--kappa-carrageenan gel composites

Thermal, mechanical, turbidity, and microscope evidence is provided which strongly suggests molecular interpenetrating network (IPN) formation by mixtures of the seaweed polysaccharides agarose and kappa-carrageenan. Over a range of ionic strength, and potassium content, there is no evidence for synergistic coupling of the networks, and simple phase separation (demixing) can definitely be ruled out. At low ionic strength, where the agarose gels first, differential scanning calorimetry evidence shows some influence of the carrageenan on the agarose ordering enthalpy, particularly at higher polymer concentrations. As the potassium level is increased, however, and the order of gelling is reversed, this effect disappears. Cure behavior for the systems at high ionic strength can be described as a simple summation of the pure component contributions. At low ionic strength, on the other hand, the modulus behavior is more complex, suggesting either a modification, in the mixture, of the kappa-carrageenan gelling parameters or a more complex modulus additivity rule.     

Steady-state and pulsed NMR studies of gelation in aqueous agarose

Steady-state and pulsed NMR techniques have been used to investigate molecular motion in sols and gels of agarose. In passing through the sol–gel transition, the molecular mobility of water molecules is reduced only by a small amount, whereas motion of the polymer chains is greatly attenuated. The results are discused in terms of the network theory of gelation, with references to the role of water in the process and the nature of the “junction zones” between polymer chains. T₂ and line-width measurements are dominated by exchange broadening. The effects of exchange rate and differences in relaxation time between the exchanging sites are discussed. The temperature hysteresis behavior of agarose gels has been investigated and the effects of “ageing” correlated with changes in nuclear relaxation times. The synergistic increase in gel strength obtained on adding locust bean gum (LBG) to agarose has been investigated. The results indicate that LBG does not form double-helix junctions and may decrease rates of gelation by steric effects. At high agarose concentration, the LBG remains mainly in solution in interstitial water, but at low agarose concentration, it is suggested that the LBG can link gel aggregates together into a self-supporting structure, producing a synergistic increase in gel strength. Comparisons have been made between the nature of the agarose–LBG interaction and agarose–cellulose interactions in biological systems.     

A study of some factors influencing amylose gelation

Amylose fractions were prepared by aqueous leaching from pea, maize and patato starch granules. The fractions were characterised by iodine binding, β-amylolysis and viscometry. Amylose starts to form a gel rather than a precipitate on cooling aqueous solutions to room temperature at concentrations above the coil overlap concentration C¹. Amylose gels are almost purely elastic, with negligible viscous flow at room temperature. The rigidity modulus is strongly dependent on concentration, c, in that above 1·5% w/w the modulus increases as a function of c⁷. The modulus of a matured gel falls only slightly with increasing temperature; at temperatures below 100°C the gel could not be melted. The non-equilibrium nature of the system is shown by the dependence of rigidity on thermal history. The shear modulus is also dependent on amylose type; higher molecular weight amylose fractions produced less rigid gels at a given concentration.     

Rheology of kappa-carrageenan in mixtures of sodium and cesium iodide: two types of gels

Recent studies on dilute solutions (Borgström et al. (1996), Int. J. Biol. Macromol. 18, 223) have shown that kappa-carrageenan helices associate into superhelical rigid rods in mixed 0.1 M aqueous solutions of NaI and CsI above a critical mole fraction (x_Cs = 0.4) of Cs. This work concerns the temperature-dependent rheology of more concentrated systems in mixed and pure solutions of the same salts. Gels with low moduli were even found in NaI alone, although this salt is known to impede the gelation of kappa-carrageenan, but only above 0.9% (w/w) of carrageenan. These gels were reminiscent of iota-carrageenan gels in two respects: the (low) magnitude of the shear storage modulus (G′), and the absence of hysteresis in the sol-gel transition. On the other hand, both the threshold concentration for gelation and the ratio between the loss and storage moduli were substantially higher for the kappa-carrageenan gels in NaI. In mixed solutions of CsI and NaI, two types of kappa-carrageenan gels could be distinguished, depending on the cesium content. The transition occurred at x_Cs = 0.4, as in the previous studies on dilute solutions. Below x_Cs = 0.4, the gels were similar to those in NaI alone. Above x_Cs = 0.4, the gels were similar to ‘conventional’ kappa-carrageenan gels, formed in salts such as KC1: a pronounced thermal hysteresis appeared in the sol-gel transition, the gels showed tendencies for syneresis, and G′ increased dramatically with increasing cesium content.     

Linear viscoelastic model of a maturing gelatin solution

The linear viscoelastic model proposed in this work considers the viscoelastic nature of maturing gelatin solutions through a relaxation modulus that depends on temperature and maturation. This modulus is defined in the conceptual contexts of the classical rubber elasticity theory and the rheometric gel theory. An analysis of the relationship between the equilibrium elastic modulus and the percolation variable around the gel point is also included yielding a percolation exponent close to 1.7 as expected from previous theoretical predictions. Additionally, a simple kinetic model is proposed to follow the microstructural changes obtained as a consequence of the generation of junction zones, the number of which vary with time during the dynamic rheometric tests used in this work. Thus, the storage and loss moduli are measured at different temperatures and frequencies, during the period of gelatin maturation. The theoretical aspects of the rheological model are presented emphasizing the quantitative changes of rheological parameters with the maturation.     

Dynamic mechanical properties of agarose gels modeled by a fractional derivative model

The complex modulus (E*) and elastic modulus (E') of agarose gels (2% to 4%) are measured with a dynamic mechanical analyzer in frequency sweep shear sandwich mode between 0.1 and 20 Hz. The data showed that E* and E' increase with frequency according to a power law which can be described by a fractional derivative model to characterize the dynamic viscoelasticity of the gel. The functions between the model parameters including storage modulus coefficient (H) and the power law exponent (beta) and the agarose concentration are established. A molecular basis for the application of the fractional derivative model to gel polymers is also discussed. Such an approach can be useful in tissue culture studies employing dynamic pressurization or for validation of magnetic resonance elastography.     

Elastic Moduli of Collagen Gels Can Be Predicted from Two-Dimensional Confocal Microscopy

We quantitatively compare data obtained from imaging two-dimensional slices of three-dimensional unlabeled and fluorescently labeled collagen gels with confocal reflectance microscopy (CRM) and/or confocal fluorescence microscopy (CFM). Different network structures are obtained by assembling the gels over a range of concentrations at various temperatures. Comparison between CRM and CFM shows that the techniques are not equally sensitive to details of network structure, with CFM displaying higher fidelity in imaging fibers parallel to the optical axis. Comparison of CRM of plain and labeled collagen gels shows that labeling itself induces changes in gel structure, chiefly through inhibition of fibril bundling. Despite these differences, image analyses carried out on two-dimensional CFM and CRM slices of collagen gels reveal identical trends in structural parameters as a function of collagen concentration and gelation temperature. Fibril diameter approximated from either CRM or CFM is in good accord with that determined via electron microscopy. Two-dimensional CRM images are used to show that semiflexible polymer theory can relate network structural properties to elastic modulus successfully. For networks containing bundled fibrils, it is shown that average structural diameter, rather than fibril diameter, is the length scale that sets the magnitude of the gel elastic modulus.     

Transient electric birefringence of agarose gels. II. Reversing electric fields and comparison with other polymer gels

The transient electric birefringence of low electroendosmosis (LE) agarose gels oriented by pulsed unidirectional electric fields was described in detail in Part I [J. Stellwagen and N. C. Stellwagen (1994), Biopolymers, Vol. 34, p. 187]. Here, the birefringence of LE agarose gels in rapidly reversing electric fields, similar in amplitude and duration to those used for field inversion gel electrophoresis, is reported. Symmetric reversing electric fields cause the sign of the birefringence of LE agarose gels, and hence the direction of orientation of the agarose fibers, to Oscillate in phase with the applied electric field. Because of long-lasting memory effects, the alternating sign of the birefringence appears to be due to metastable changes in gel structure induced by the electric field. If the reversing field pulses are equal in amplitude but different in duration, the orientation behavior depends critically on the applied voltage. If E < 7 V/cm, the amplitude of the birefringence gradually decreases with increasing pulse number and becomes unmeasurably small. However, if E > 7 V/cm, the amplitude of the birefringence increase more than 10-fold after ～ 20 pulses have been applied to the gel, suggesting that a cooperative change in gel structure has occurred. Because there is no concomitant change birefringence must be due to an increase in the number of agarose fibers and /or fiber bundles orienting in the lectric field, which in turn indicates a cooperatice breakdown of the noncovalent “junction zones” that corss-link the fibers in to the fgel matrix. The sign of the birefringence of LE agarose gels is always positive after extensive junction zone breakdown, indicating that the agarose fibers and fiber bundles preferentially orient parallel to the lectric field when they are freed from the constraints of the gel matrix. Three other gel-forming polymers, high electroendosmosis (HEEO) agarose (a more highly changed agarose), β-carrageenan (a stereoisomer of agarose), and polyacrylamide (a chemically corss-linked polymer) were alos studied in unidirectional and rapidly reversing electric fields. The birefringence of HEEO agarose backbone chain. The β-carrageenan gels exhibit variable orientation behavior in reversing electric fields, suggesting that its internal gel structure is not as tightly interconnected as that of agaroise gels. Both HEEO agarose and β-carrageenan gels exhibit a large increase in the amplitude of the birefringence with increasing pulse number when asymmetric reversing pulses > 7 V/cm are applied to the gels, suggesting that junction zone breakdown in a common feature of polysaccharide gels. Chemically cross-linked polyacrylamide gels exhibit very small birefringence signals, indicating that very little orientation occurs in pulsed lectric fields. The sign of the birefringence is independent of the polarity of the lectric field, as expected from the Kerr law, and normal orientation behavior is observed in reversing electric fields. Hence, the anomalous change in sign of the birefringence observed for agarose gels in reversing electric fields must be due to the metastable junction zones in the agarose gel matrix, which allow gel fiber rearrangements to occur. © 1994 John Wiley & Sons, Inc.     

Rheological properties and mechanical stability of new gel-entrapment systems applied in bioreactors

The mechanical stability of gels applied for entrapment and retention of biocatalysts in bioreactors is of crucial importance for successful scale-up applications. Gel abrasion in agitated reactors will depend on liquid shear, bubble shear, and wall shear, as well as collisions between the gel particles. As a simplified standardized model system, abrasion of gel beads was studied in 1-m-high bubble columns with controlled aeration, and quantified by measuring the loss of gel material into solution. Gel beads were also taken out to measure stress-strain response during controlled compression. More general rheological properties of different gels were studied by applying a variety of regimes of controlled compression of standardized gel cylinders: Gel strength was measured by recording the fracture properties and the Young's modulus. Viscoelastic properties were revealed by recording creep during compression as well as recovery after compression. Oscillation tests up to 1000 cyclic compressions were applied to compare the fatigue of different gels. Results obtained for Ca-alginate gels, gels of chemically modified polyvinyl alcohol with stilbazolium groups (PVA-SbQ) as well as mixed gels of Ca-alginate and PVA-SbQ are compared with previously published data for kappa-carrageenan, agar, and polyethylene glycol (PEG) gels. It is concluded that material fatigue rather than mechanical properties such as stiffness or fracture stress should be considered when selecting a suitable gel material on the basis of abrasion resistance. The very soft and superelastic PVA-SbQ gel showed no significant fatigue in mechanical tests and no abrasion was detected in the standardized model system used. Ca-alginate gels, however, showed severe irreversible changes due to fatigue at oscillating loads and creep at constant load. Due to their similarities with kappa-carrageenan gels in mechanical tests, it is likely that Ca-alginate would also be sensitive to abrasion. Mixed gels of Ca-alginate and PVA-SbQ represent a complex system with intermediate properties, showing significant fatigue and creep, but elastic properties from the PVA-SbQ gel make it less sensitive than the pure Ca-alginate gel.     

In vivo measurement of flavour release from mixed phase gels

Flavour release was investigated from pure gelatin, pure agarose and mixed gelatin-agarose gels, all containing 25% sucrose and flavoured with p-cymene, ethyl butyrate, pyrazine and ethanol. Gels were characterised by optical microscopy, and rheological techniques to determine phase separation, elastic modulus and melting temperature. Volatile release was measured by monitoring the four volatiles in the expired air from one individual eating the gels, using Atmospheric Pressure Chemical Ionisation-Mass Spectrometry. The release pattern of p-cymene was not affected by gel type. The release of ethanol, ethyl butyrate and pyrazine was affected to different extents by the matrix suggesting that both the properties of the volatile and the matrix determine volatile release in vivo.     

Viscous solutions, networks and the glass transition in high sugar galactomannan and kappa-carrageenan mixtures

Small-deformation rotational oscillation was used to examine the effect of small additions of galactomannan and kappa-carrageenan on the vitrification of glucose syrup at a total level of solids of 83%. The method of reduced variables allowed construction of composite curves covering the glass transition and glassy state (from 10(5) to 10(9.5) Pa) over a wide frequency range (up to 15 orders of magnitude). The combined WLF/free volume framework was employed to determine the rheological glass transition temperature (T(g)), fractional free volume and thermal expansion coefficient of the samples. It was found that the WLF-predicted glass transition temperature matched the cross over of experimental modulus traces in the passage from the glass transition (GG') to the glassy state (GG"). This coincides with the mechanistic transformation from free volume effects to the Arrhenius-type phenomena, thus ascribing physical significance to the rheological T(g). The T(g) value of 83% glucose syrup at a scan rate of 2 degrees C min(-1) was -25.3 degrees C. Replacing, for example, 1% glucose syrup with guar gum shifted the T(g) of the mixture to -19.7 degrees C. Network formation via the K(+)-supported junction zones of the kappa-carrageenan chains further increased the T(g) to about -1 degrees C. It appears that the low rates of relaxation processes and diffusion mobility in the presence of a polysaccharide network accelerate the collapse of the free volume thus inducing vitrification of the high sugar/polysaccharide mixture at high temperatures.     

Temperature dependence of the elastic modulus of alginate gels

The temperature dependence of the elastic modulus for alginate gels was studied using two different gel systems: covalently crosslinked Na-alginate gels and in-situ prepared Ca-alginate gels. The modulus of physically crosslinked gels showed a complex behaviour. The temperature coefficient of the modulus of covalently crosslink gels changed from positive for the lowest crosslinked gels to negative for the highest crosslinked gels. This suggests a change from rubberlike to enthalpy-driven elasticity with an increasing degree of crosslinking for these gel networks.