Cation effects on sol-gel and gel-sol phase transitions of kappa-carrageenan-water system

Sol–gel and gel–sol phase transitions of κ-carrageenan in pure water and in KCl solution were studied using photon transmission technique. Photon transmission intensity, I_tr, was monitored against temperature to determine the sol–gel and gel–sol temperatures (T_sg and T_gs) and activation energies (ΔH_sg and ΔH_gs). It was observed that T_gs was notably higher than T_sg due to the hysteresis on the phase transition loops. T_gs and ΔH_gs values were also higher for gels containing KCl than for those without KCl. The increase in carrageenan content caused an increase in both critical temperatures and activation energies for the gels prepared in pure water and in KCl solution. Increases in the KCl/carrageenan ratio, raised both T_gs and T_sg. Similarly ΔH_sg was elevated by the increase in cation content of the gel. These results were interpreted as the formation of stronger gels in the presence of KCl in water.     

Effect of galactomannan addition on the thermal behaviour of κ-carrageenan gels

Gelation/melting cycles of κ-carrageenan/galactomannan (guar, tara and locust bean gums) binary systems have been studied by measuring dynamic rheological parameters. Two experimental conditions were used, (i) the total polysaccharide concentration was kept at 1% and the κ-carrageenan/galactomannan ratio fixed at 4:1 and (ii) the κ-carrageenan concentration was fixed at 0·75% and the galactomannan content varied from 0% to 1·2%. A thermal hysteresis was observed for all mixed systems and was found to depend on the galactomannan used. From a comparison of the gelation temperature (T_g) and melting temperature (T_m) to values obtained with κ-carrageenan alone, it was suggested that galactomannan interferes with gel structure by the formation of a secondary network provided that the M/G ratio is high enough.     

β-carrageenan: Isolation and characterization

β-Carrageenan, essentially devoid of ester sulfate, was isolated from the hot aqueous extracts of alkali-modified Eucheuma gelatinae, Eucheuma speciosa, and Endocladia muricatum by precipitating the more anionic moieties with a quaternary ammonium salt, isolating the fractions that did not precipitate, then treating these with an anion-exchange cellulose. The β-carrageenan was characterized by chemical analysis, optical rotation, and NMR. Gelling was found to be ion-independent, with T_g = 31–33°C and T_m = 63–70°C. Specific optical rotations of the isolated β-carrageenan samples were more positive than the κ-, λ-, and ι-carrageenans with which they were compared, while agarose, its stereoisomer, exhibited a negative specific rotation. Electrophoresis gels made from β-carrageenan were used to separate DNA fragments which exhibited faster migration than on an agarose gel of comparable concentration, indicating that β-carrageenan has a less restrictive pore structure.     

Thermal properties of corn gluten meal and its proteic components

Thermal properties of corn gluten meal (CGM) and of its extracted proteic components (zein and glutelin) at 0% moisture content, is studied by dynamic mechanical thermal analysis (DMTA) and modulated differential scanning calorimetry (MDSC). The glass transition temperature (T_g) on first heating, is measured at 176 and 174°C, respectively, for hot-air-dried and native CGM. For zein and glutelin isolated fractions, the measured T_g values are 164 and 209°C, respectively. The calculated T_g from using Matveev’s method (Matveev YI. Spec Publ R Soc Chem 1995;156;552) is in good agreement with experimental data for zein, a well defined protein. MDSC allows the measurement of change in heat capacity at T_g (ΔCp) with a single heating scan, avoiding sample alteration, and ΔCp values are 0.365 J/g per K for zein and 0.184 J/g per K for glutelin. The differences observed in T_g, relaxation temperatures, ΔCp and tan δ peak height are related to differences in the structure of the proteins, through the cross-linkages and hydrogen or van der Waals interactions. Experimental data from DMTA and MDSC, and the Couchman–Karasz thermodynamic approach indicate that CGM behaves as a miscible blend of its components, with high non-polar interactions between zein and glutelin proteins.     

Rheology and fracture of mixed ι- and κ-carrageenan gels: Two-step gelation

It is shown that under certain circumstances, on cooling mixed ι- and κ-carrageenan solutions, the two forms gel separately at different temperatures, with the ι form gelling first. This ‘two-step gelation’ was only observed when both sodium and potassium ions were present, with a sodium/potassium mole ratio of between 1 and 100. For such mixed gels, a κ fraction as low as 2·5% of the total carrageenan has significant effects on their rheology, both at low deformation and fracture. In these systems, the κ form, gelling in the presence of an existing ι gel, produces measurable rheological effects at much lower concentrations than if it were alone. This behaviour can be used as a sensitive test of the ‘rheological purity’ of samples of ι-carrageenan.     

Synthesis and properties of pullulan acetate. Thermal properties, biodegradability, and a semi-clear gel formation in organic solvents

Pullulan acetate (AcPL) with various degree of substitution (DS: 1.0–3.0) was synthesized by the reaction of pullulan with acetyl chloride in the presence of pyridine. The product was characterized by gel permeation chromatography (GPC), infra-red (IR) and ¹H NMR spectroscopy. The weight average molecular weights of the products did not decrease less than 190,000 (GPC) in the acetylation reaction. Thermogravimetric analysis (TGA) revealed that AcPL has a higher decomposition temperature (306–363 °C) than unmodified pullulan (295 °C). Differential scanning calorimetry analysis (DSC) revealed that all the AcPLs exhibit a clear T_g, which decreased with increasing DS value in the range of DS 1.0–2.4. The AcPL with DS 2.4 showed the lowest T_g (153 °C), and the AcPL with DS 3.0 had a slightly higher T_g (163 °C). Tensile modulus of AcPL films was comparable to that of a popular cellulose acetate film. The biodegration rate of AcPL decreased with increasing degree of acetylation. The AcPL with DS 3.0 was found to form a semi-clear gel in organic solvents such as dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), and 1,4-dioxane.     

Rheological and calorimetric study of the sol–gel transition of κ-carrageenan

Rheological and DSC techniques were used to study the effect of κ-carrageenan and KCl concentrations, 0–300 mM, on the sol–gel transition as well as on the linear viscoelasticity, at 25 °C, of the resulting gels. In heating and cooling DSC tests, the peak temperature was taken as the sol–gel transition point. In rheological tests, sol–gel transitions were determined from the variation of dynamic moduli with frequency and temperature, the independence of the phase angle on frequency and the evolution with temperature of dynamic moduli on cooling and heating at constant frequency and strain. Transition temperatures from DSC and rheology were in good agreement among them and with those previously reported. The three procedures yielded similar results, but the transition temperatures were more easily determined through the independence of the phase angle on frequency. Frequency sweeps showed gel behavior with stiffness increasing with polysaccharide and salt concentration. Below 100 mM KCl, G′ increased notably, whereas higher concentrations produced only marginal increases.     

Photon transmission study on swelling of kappa-carrageenan gels prepared in various concentrations

κ-Carrageenan gels prepared with various carrageenan concentrations in pure water were completely dried and then swelled in pure water. Photon transmission measurements were performed using a UV-Vis (UVV) spectrometer during the swelling of κ-carrageenan gels. Transmitted photon intensity, I_tr, increased exponentially as swelling time is increased for all gel samples. The behaviour of I_tr was interpreted by Monte-Carlo Simulation. The increase in I_tr was quantified by employing Li-Tanaka equation, from which time constants τ₁ and collective diffusion coefficients, D_o were determined for the gels in various carrageenan concentrations. Gravimetric and volumetric measurements were also carried out during swelling of gels. It is observed that gel with high carrageenan content possess more double helices and more lattice dislocations and swell slower than gels with low carrageenan content which may contain less double helices and less lattice imperfections. Increase in I_tr was interpreted by the homogeneous distribution of double helices in the carrageenan gel system.     

Behavior of κ-carrageenan in glycerol and sorbitol solutions

The solubility of κ-carrageenan in low water-content solvents is important in food applications where complete solubilization is required for proper development of structure and rheology. The effect of glycerol and sorbitol on the gelation and conformational helix transition of κ-carrageenan was studied using rheology and optical rotation. Glycerol/water solutions from 0–100 wt% glycerol and sorbitol solutions from 0–100% saturation were studied over the temperature range 0–90°C. The results were analyzed in terms of solvent solubility parameters, water chemical potential, and solvent dielectric constant. Effective cohesive energy density parameters could not be inferred for the carrageenan, but the gelation temperature could be correlated with solvent dielectric constant. Hydrogen bonding interactions control the carrageenan helix formation. The cohesive energy density as a measure of solvent quality accounts for hydrogen bonding but not Coulombic interactions, and the Coulombic interactions scale on dielectric constant. This indicates the dominant role of electrostatics on the gelation process.     

Synthesis and NMR structural analysis of O-succinyl derivative of low-molecular-weight κ-carrageenan

Low-molecular-weight (LMW) κ-carrageenan was achieved through mild hydrochloric acid hydrolysis of κ-carrageenan. The acylation of LMW κ-carrageenan was performed by use of tetrabutylammonium (TBA) salt of the anionic polysaccharide fragments, succinic anhydride, 4-dimethylaminopyridine and tributylamine under homogeneous conditions in N,N-dimethylformamide at 80 °C. Investigation of FT-IR spectrum of the succinylated LMW κ-carrageenan showed that a monoester derivative with succinyl group was formed when LMW κ-carrageenan reacted with succinic anhydride. The ¹H and ¹³C NMR spectroscopy has been used to characterize the fine structure of O-succinyl derivative of the LMW κ-carrageenan. The ¹³C and ¹H NMR chemical shifts of disaccharide unit of O-succinyl LMW κ-carrageenan have been fully assigned using 2D NMR spectroscopic techniques.     

Physio-chemical properties of polymyxan 88A–water system

The system exopolysaccharide polymyxan 88A–water was studied at several temperatures. The temperature dependence of viscosity at cooling and heating was obtained in order to estimate the phase separation temperature (T_s) and the gelation temperature (T_g). The experimental values of T_s and T_g were used to plot the phase diagram of the system under study at polymer concentrations below 1.5 wt%. Viscous flow in the system was examined by the cylinder–cylinder rotation method. It has been found that: (i) at shear rates within 1–100 s⁻¹ the dependence of viscosity on shear rate can be fairly expressed by the power low; (ii) the activation enthalpy of viscous flow practically does not depend on shear rate; and (iii) the activation entropy of viscous flow is negative, most likely due to an orienting action of mechanical field.     

O-maleoyl derivative of low-molecular-weight κ-carrageenan: Synthesis and characterization

Low-molecular-weight (LMW) κ-carrageenan was achieved through mild hydrochloric acid hydrolysis of κ-carrageenan. The acylation of low-molecular-weight κ-carrageenan was performed by use of tetrabutylammonium (TBA) salt of the anionic polysaccharide fragments, maleic anhydride, 4-dimethylaminopyridine and tributylamine under homogeneous conditions in N,N-dimethylformamide at 60 °C. Analysis of FT-IR spectrum of O-maleoyl κ-carrageenan showed that a monoester derivative with maleoyl group was formed when LMW κ-carrageenan reacted with maleic anhydride. Investigation of ¹H NMR spectroscopy revealed that the maleoylation took place regioselectively at C-2 of the κ-carrageenan 3-linked unit.     

Xanthan–galactomannan interactions as related to xanthan conformations

The influence of xanthan conformation on the physicochemical behaviour of their mixtures with galactomannan from Schizolobium parahybae mannose:galactose ratio (M/G=3), was studied by viscoelastic measurements, differential scanning calorimetry (DSC) and chiroptical (circular dichroism) methods. The results suggested a more effective interaction of the galactomannan with disordered xanthan segments, which are more abundant in low salt concentrations but are still present in lower proportion at temperatures lower than the temperature of xanthan conformational transition (T_m). The dependence of ellipticity with temperature in a circular dichroism (CD) spectra suggested an ordering of the xanthan chains induced by galactomannan at the temperature of gel formation (T_g≈25°C), under conditions where xanthan alone exhibits a disordered conformation. The lower T_g value found (≈25°C) compared with that (60°C) usually described in the literature is certainly related to the M/G ratio and the galactosyl unit distribution along the mannan main chain.     

Influence of spent brewer’s yeast β-glucan on gelatinization and retrogradation of rice starch

The effects of β-glucan (BG) prepared from spent brewer’s yeast on gelatinization and retrogradation of rice starch (RS) were investigated as functions of mixing ratio and of storage time. Results of rapid visco-analysis (RVA) indicated that addition of BG increased the peak, breakdown, setback, and final viscosities, but decreased the pasting temperatures of the rice starch/β-glucan (RS/BG) mixtures. Differential scanning calorimetry (DSC) data demonstrated an increase in onset (T_o), peak (T_p), and conclusion (T_c) temperatures and a decrease in gelatinization enthalpy (ΔH₁) with increasing BG concentration. Storage of the mixed gels at 4 °C resulted in a decrease in T_o, T_p, T_c, and melting enthalpy (ΔH₂). The retrogradation ratio (ΔH₂/ΔH₁) and the phase transition temperature range (T_c − T_o) of the mixed gels increased with storage time, but this effect was reduced by the addition of BG. BG addition also slowed the syneresis of the mixed gels. Results of dynamic viscoelasticity measurement indicated that the addition of BG promoted RS retrogradation at the beginning and then retarded it during longer storage times. The added BG also retarded the development of gel hardness during refrigerated storage of the RS/BG mixed gels.     

Studies on κ-carrageenan/locust bean gum mixtures in the presence of sodium chloride and sodium iodide

The solution properties of κ-carrageenan and κ-carrageenan/locust bean gum mixtures have been studied by small deformation oscillation measurements and differential scanning calorimetry (DSC) in the presence of sodium chloride and sodium iodide. Both salts induced the κ-carrageenan to undergo a coil-helix conformational change as noted by an increase in the storage and loss moduli (G′, G′) and by an exothermic peak in the DSC cooling curves. The enthalpy ΔH_c-h and temperature of the conformational transition T_c-h were higher in Nal compared to NaCl and T_c-h increased with increasing the concentration of both electrolytes. Gelation was not observed for carrageenan or carrageenan/locust bean gum mixtures in the presence of up to 200 mM Nal. Although carrageenan alone did not gel in the presence of 100 mM NaCl, a weak gel was obtained for a mixture containing 0.9%/0.1% carrageenan/locust bean gum. Furthermore, the mixture showed hysteresis in both the rheological and DSC cooling and heating curves. A strong gel was produced for carrageenan alone in the presence of 200 mM NaCl and the gel strength increased on adding a small proportion of locust bean gum (0.9%/0.1%). © 1997 John Wiley & Sons, Inc. Biopoly 41: 657–671, 1997     

A solid state NMR study of locust bean gum galactomannan and Konjac glucomannan gels

This paper describes a ¹³C solid state NMR study of hydrated powders and gels of locust bean gum galactomannan-LBG and Konjac glucomannan-KGM. Changes in relative spectral intensities, cross-polarization dynamics (T_CH, T_1ρH) and relaxation times (T_1C, T_1H, T_2H) show that hydration (0–90%) of LBG powders increases the 10⁸ Hz frequency molecular motions, probably reflecting the enhanced motion of non-aggregating segments and chain ends. Slower motions (10⁴–10⁵ Hz) are enhanced only slightly at 90% hydration. LBG gel shows higher spatial distinction between aggregated and non-aggregated segments than the hydrated powder and relaxation times indicate higher mobility for galactose-ramified segments, compared to linear mannose segments. While the dynamics of KGM hydration is similar to that of LBG, i.e. mainly affecting fast 10⁸ Hz motions, the gel is significantly more rigid. Both spectra and relaxation times show that glucose residues in KGM gel are particularly hindered, probably due to their preferential involvement in chain aggregation.     

Mixtures of pregelatinised maize starch and κ-carrageenan: Compatibility, rheology and gelation

Compatibility, flow and visco-elastic properties of a pregelatinised maize starch mixed with κ-carrageenan were investigated. After cooking of the pregelatinised starch, some undissolved granules remained in solution. Aqueous mixtures of κ-carrageenan and starch were studied at 60 °C and 20 °C by combining rheological measurements and microscopic observations under conditions allowing gelation of carrageenan and non-gelation of starch. The viscometric study of mixed dilute solutions of amylose from pregelatinised starch and carrageenan showed that the components are slightly incompatible. Mixture viscosity and elastic modulus were studied at 60 °C in details as a function of mixture composition for a total polymer concentration of 3%; both were found to be significantly higher than the corresponding theoretical additive values. This finding was interpreted by starch granules excluded volume effect. At 20 °C, no noticeable increase of mixture elastic modulus was found as compared with the additive value. The absence of the synergistic effect is supposed to be due to the formation of highly inhomogeneous gels with agglomerates of undissolved granules.     

The determination of the kinetics of polysaccharide thermal degradation using high temperature viscosity measurements

Information about the thermal degradation of the polysaccharides sodium alginate, carrageenan and carboxymethyl cellulose has been obtained from the time dependence of the viscosity at high temperatures measured using a slit viscometer. The viscosity is related to the molecular weight using previously-published relations between the zero shear specific viscosity and the coil overlap parameter in conjunction with the appropriate Mark-Houwink equation. It is found that alginate is much less stable than carboxymethyl cellulose and carrageenan. Activation energies for depolymerisation obtained from Arrhenius plots in the presence of oxygen ranged from 50 kJ/mol for alginate to 105 kJ/mol for κ-carrageenan.     

Definition of the rheological glass transition temperature in association with the concept of iso-free-volume

Small deformation dynamic oscillation was used to develop an index of physical significance for the rationalisation of the mechanical properties of high co-solute/biopolymer systems during vitrification. The index is based on the combined framework of Williams–Landel–Ferry equation with the free volume theory and is called the ‘rheological glass transition temperature, T_g’ thus differentiating it from the empirical calorimetric T_g used in thermal analysis. The rheological T_g is located at the conjunction of two distinct molecular processes, namely: free-volume effects in the glass transition region and the predictions of the reaction-rate theory in the glassy state. The method of reduced variables was used to shift the mechanical spectra of shear moduli to composite curves. The temperature dependence of shift factors for all materials was identical provided that they were normalised at suitably different reference temperatures, which reflect iso-free-volume states. The treatment makes free volume the overriding parameter governing the mechanical relaxation times during vitrification of high co-solute/biopolymer systems regardless of physicochemical characteristics. We believe that potential applications resulting from this fundamental work are numerous for the food and pharmaceutical industries.     

Rheology and microstructure of Ca- and Na-K-carrageenan and locust bean gum gels

The viscoelastic and microstructural influences of 0.1-0.6% locust bean gum on 0.5 or 1.0% κ-carrageenan gels, in different ionic environments, have been studied using small deformation oscillatory measurements and transmission electron microscopy (TEM). The results from the Theological measurements showed synergistic effects in the storage modulus, G', as locust bean gum, of two different mannose to galactose ratios (3 and 5), was mixed with ion-exchanged Na- and Ca-κ-carrageenan, in 0.25 M NaCl and 0.030 M CaC1₂, respectively. The increase in G' was dependent on the mannose to galactose ratio, polymer concentrations, and ionic environment.

At the supermolecular level, the microstructure of dilute samples has been visualised using low angle rotary metal shadowing for TEM. In the presence of sodium and calcium ions, the self-association of κ-carrageenan helices is moderate to low. Locust bean gum did not influence the supermolecular structure of κ-carrageenan to any large extent. The microstructure of the gels at the network level was studied using plastic embedding and thin sectioning for TEM. In both sodium and calcium ionic environments, the mixed gels showed a more homogeneous and connective network structure.