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.     

Mechanism of gel formation in κ-carrageenan

We present a general view of κ-carrageenan gelation and some new experimental results concerning their behavior in the presence of Na⁺, K⁺, or Rb⁺ counterions in semidilute concentration. The gelation mechanism is based on the aggregation of helical dimers.     

The conformational transition of κ-carrageenan using microcalorimetry

The interactions between inorganic cations and κ-carrageenan in aqueous solutions have been studied using microcalorimetry. The enthalpy of dilution and the enthalpy of mixing of κ-carrageenan in the coil conformation with an alkaline metal chloride have been investigated and are in agreement with theoretical predictions. The formation of helical dimer in a K⁺ salt form in the absence of a gel formation is confirmed.     

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.     

127I-NMR studies of anion binding to κ-carrageenan

A preliminary ¹²⁷I-NMR relaxation study of the binding of the I^? ion to κ-carrageenan is presented. Signal loss caused by motional correlation times out of extreme narrowing makes it difficult to analyze line widths. Longitudinal relaxation times are more suitable for a comprehensive relaxation study. From the observation of nonextreme motional narrowing, the lower limit of the residence time at the binding site is estimated to be on the order of 10 ns. © 1993 John Wiley & Sons, Inc.     

Small-angle x-ray scattering of κ-carrageenan based systems: Sols,gels, and blends with carob galactomannan

Small-angle x-ray scattering using a synchroton x-ray monochromatic radiation was carried out to investigate the structure of different polysaccharides in aqueous medium: carob galactomannan, κ-carrageenan in the sol and in the gel states, and κ-carrageenan-carob galactomannan mixed systems. Experiments performed on a 0.2% carob galactomannan solutions confirmed that this polysaccharide behaved as a neutral polymer in a good solvent. For K-carrageenan in the / state, either in the sodium form or in the cesium form, a maximum in the scattering curve was evidenced. Position and height of this maximum changed with K-carrageenan concentration in close agreement with what is expected for wormlike polyelectrolyte in semidilute solution. In the case of k-carrageenan in the gel state, in the cesium form, scattering curves also exhibited a maximum at an intermediate Q value. The position of this correlation peak did not change with concentration while its intensity increased. This effect was ascribed to a packing of rodlike structures by analogy with a suspension of colloidal elongated particles. This local structure could be viewed as bundles of parallel double helices. Addition of carobgalactomannan in κ-carrageenan gels induced dramatic structural changes. As the galactomannan concentration increased, the correlation peak tended to vanish. In contrast, no change in the cross-sectional radius of gyration was noticed. This phenomenon suggested a screening effect of the galactomannan, resulting in a loss of the correlation between the κ-carrageenan double helices. © 1995 John Wiley & Sons, Inc.     

Responsiveness of κ-carrageenan microgels to cationic surfactants and neutral salts

The objective of this study was to examine the responsiveness of chemically cross-linked κ-carrageenan microspheres to different types of neutral salt electrolytes as well as to surfactants of varying chain lengths. In the presence of increasing salt concentration microsphere size changed radically from D[4,3] values of 320 μm to approximately 160 μm. The level of salt concentration needed to bring about this change varied depending on electrolyte type. This common behaviour was attributed to the difference in free cationic counter-ions concentration between the inside and outside of the microsphere and can be explained due to the effect of the Donnan equilibrium. The rheological properties of these microgels in their swollen and collapsed states were also explored with results showing that the collapsed microspheres had a greater impact on the viscosity of the system probably as a result of some aggregation of the collapsed microgels at rest due to surface charge screening at these high salt concentrations. The effect of surfactant on microsphere size showed a dramatic drop in D[4,3] values from 320 μm to approximately 120 μm for BAC, DoTAB, MTAB and CTAB at specific critical concentrations. This critical aggregation concentration was found to increase linearly on a log–log scale with the critical micelle concentration of these surfactants in water, indicating that the alkyl chain length of the surfactants had an effect on the critical aggregation concentration.     

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.     

Physical and stability characteristics of Burkholderia cepacia lipase encapsulated in κ-carrageenan

In this work, a novel approach for lipase immobilization was exploited. Lipase from Burkholderia cepacia was encapsulated into κ-carrageenan by co-extrusion method to form a liquid core capsule. The diameter of the encapsulated lipase was found to be in the range of 1.3–1.8 mm with an average membrane thickness of 200 μm and 5% coefficient of variance. The encapsulation efficiency was 42.6% and 97% moisture content respectively. The encapsulated lipase was stable between pH 6 and 9 and temperature until 50 °C. The encapsulated lipase was stable until disintegration of the carrier when stored at 27 °C and retained 72.3% of its original activity after 6 cycles of hydrolysis of p-NPP. The encapsulated lipase was stable in various organic solvents including methanol, ethanol, iso-propanol, n-hexane and n-heptane. Kinetic parameters K_m and V_max were found to be 0.22 mM and 0.06 μmol/min for free lipase and 0.25 mM and 0.05 μmol/min for encapsulated lipase respectively.     

Anion effects on equilibria and kinetics of the disorder–order transition of κ-carrageenan

The effect of a number of tetramethylammonium salts on the equilibria and kinetics of the disorder to order transition in the polysaccharide κ-carrageenan have been investigated. Data from the temperature dependence of optical rotation show that anion stabilization of the ordered form follows the lyotropic series I^? > Br^? > NO > Cl^? > F^?. Stopped-flow polarimetry was used to study the kinetics of conformational ordering following a rapid increase in salt concentration. The transition to the new equilibrium position was shown to be biphasic for all of the tetramethylammonium salts studied. The rate equation for the fast phase and the temperature dependence of the observed forward rate constant accord with a cooperative dimerization process. Activation parameters for helix nucleation, ΔH^* and ΔS^*, vary with both salt concentration and (at constant ionic strength) the anion type, increasing through the lyotropic series from I^? to F^?. The slow phase shows second-order kinetics, and is interpreted as further stabilization of the ordered form either through limited aggregation or annealing. The rate constant for the slow phase also follows the lyotropic series. Thus we have shown that both the growth and nucleation processes are anion dependent.     

The influence of locust bean gum and dextran on the gelation of κ-carrageenan

The interaction of κ-carrageenan with locust bean gum and dextran has been studied by rheology, differential scanning calorimetry (DSC), and electron spin resonance spectroscopy (ESR). Rheological measurements show that the carrageenan gel characteristics are greatly enhanced in the presence of locust bean gum but not in the presence of dextran. Carrageenan/locust bean gum mixtures show two peaks in the dsc cooling curves. The higher temperature peak corresponds to the temperature of gelation and its intensity increases at the expense of the lower temperature peak as the proportion of locust bean gum in the mixture increases. Furthermore, the DSC heating curves show enhanced broadening when locust bean gum is present, indicating increased aggregation. These results are taken as evidence of carrageenan/locust bean gum association. The gelation process has also been followed by ESR using spin-labeled carrageenan. On cooling carrageenan solutions, an immobile component appears in the ESR spectra signifying a loss of segmental mobility consistent with chain stiffening due to the coil → helix conformational transition and helix aggregation. For carrageenan/locust bean gum mixtures, carrageenan ordering occurs at temperatures corresponding to the higher temperature DSC setting peak and the temperature of gelation. Similar studies using spin-labeled locust bean gum show that its mobility remains virtually unaffected during the gelation process. It is evident, therefore, that carrageenan and locust bean gum interact only weakly. It is proposed that at low carrageenan concentrations the gel network consists of carrageenan helices cross-linked by locust bean gum chains. At high carrageenan concentrations the network is enhanced by the additional self-aggregation of the “excess” carrageenan molecules. For carrageenan/dextran mixtures, only one peak is observed in the dsc cooling curves. The onset of gelation shifts to higher temperatures only at very high (20%) dextran concentrations and this is attributed to volume exclusion effects. Furthermore, there is no enhanced broadening of the peaks in the DSC heating curves as for the carrageenan/locust bean gum systems. It is therefore concluded that carrageenan/dextran association does not occur. The difference in behavior between locust bean gum and dextran is attributed to the greater flexibility of the dextran chains. © 1996 John Wiley & Sons, Inc.     

Conformational change and aggregation of κ-carrageenan studied by flow field-flow fractionation and multiangle light scattering

The relatively novel combination of flow field-flow fractionation (FFF) and multiangle light scattering (MALS) was employed to study a nondegraded κ-carrageenan in different 0.1M salt solutions. The applicability of the technique was tested, and the effects of salt type and salt composition on the molar mass and radius of gyration were studied. A conformational ordering was induced at room temperature by switching the solvent from 0.1M NaCl (coil form) to 0.1M NaI (helix form). An approximate doubling of the average molar mass and an increase in radius of gyration was then observed, in agreement with results obtained previously using size exclusion chromatography–MALS. This increase in size was attributed to conformational ordering and to the formation of double helices. Severe aggregation was observed above 40% CsI in the 0.1M mixed salt solution of CsI and NaI. This was ascribed to the association of helices into large aggregates. For these large associates, having molar masses of several millions, a reversal of the elution order in flow FFF was detected. © 1998 John Wiley & Sons, Inc. Biopoly 45: 85–96 1998     

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.     

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.     

Effect of locust bean gum and konjac glucomannan oh the conformation and rheology of agarose and κ-carrageenan

Mixing with locust bean gum (LBG) induces obvious gel-like character in very dilute solutions of K⁺ κ-carrageenan (< 0.01% w/w in 100 mM KCl). At higher concentration (0.085%), addition of LBG (0.036%) gives a shoulder on the high-temperature side of the DSC (differential scanning calorimetry) exotherm associated with the carrageenan disorder-order transition, with an accompanying increase in gelation temperature and enhancement in gel strength (storage modulus, G′). On substitution of LBG by konjac glucomannan (KM) the shoulder in DSC cornverts to a discernable peak. Van't Hoff analysis of optical rotation data indicates that the high-temperature thermal processes could arise from association of LBG or KM chains to the carrageenan double helix as it forms, with the main transition at lower temperature corresponding to ordering of surplus carrageenan. With K -carrageenan in the nongelling tetra-methylaminonium salt form, addition of LBG causes no delectable change in DSC; rheological enhancement at high concentration (1% w/w) is limited to development of a very tenuous network, and in dilute solution a decrease in viscosity is observed. Agarose shows only a very slight increase in the disorder-order transition temperature on addition of KM, and it shows no detectable change with LBG. These observations are interpreted as showing that efficient binding of mannan or glucomannan chains requires some aggregation of the algal polysaccharide helices, but that extensive aggregation restricts synergistic interaction by competition with heterotypic association. © 1995 John Wiley & Sons, Inc.     

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     

NMR investigations of the 4-ethyl guaicol self-diffusion in iota (ι)-carrageenan gels

Self-diffusion coefficient of an aroma molecule (4-ethyl guaicol) was measured using the pulsed field gradient spin echo NMR (PGSE-NMR) method in order to investigate the influence of a macromolecular matrix on its diffusion and release processes. Iota (ι)-carrageenan was used for its ability to form thermoreversible gels in aqueous salt solutions. Variations of the ι-carrageenan and the salt concentrations permitted various gels with different thermal and rheological properties to be obtained. These latter were modified by an isotope effect obtained by preparing gels in D₂O. The NMR self-diffusion measurements realised for water and the aroma molecules indicated neither chemical interactions with ι-carrageenan, nor obstruction effects from the polysaccharide chains. In ι-carrageenan gels, the diffusional phenomenon was highly dependent on the heterogeneous gel structure and controlled by hydrodynamic interactions due to frictional drag between each molecule of the system and water microviscosity changes.     

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.