The influence of carrageenan on molecular mobility in low moisture amorphous sugars

The influence of less than 1% of κ-carrageenan on the mobility of glucose syrup was studied in the context of the glass–rubber transition using proton NMR relaxometry. Glass-transition temperatures, (T_g) were measured by differential scanning calorimetry (DSC) on glucose syrup samples containing 0 or 0.9% κ-carrageenan, between 0 and 1.4% KCl, and at water contents from 3.5 to 16% (wwb). Potassium chloride was added to vary the extent of gelation of the carrageenan in order to assess the effect of the biopolymer network on molecular mobility.

Contrary to the reported increase of the rheologically determined glass-transition temperature, in the presence of gelling agents, the addition of 0.9% κ-carrageenan to glucose syrup with and without KCl, had no effect on the DSC measured T_g. In addition, there was no effect on molecular mobility in the glassy region. The presence of carrageenan only significantly affected the mobile part of the NMR free induction decay at relatively high temperatures.     

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.     

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.     

Physical characteristics of calcium induced κ- carrageenan networks

The presence of an optimum counter-ion concentration in calcium-induced κ-carrageenan gels at low polymer concentrations of 5 and 10 g/l is observed. At approximately the stoicheometric molar ratio of 1 calcium per carrageenan sulphate, a gel with high elastic modulus, high optical clarity and fine network structure is observed. On further increase of counter-ion concentration beyond this optimum, elastic modulus decreases significantly associated with sharp increase in the gels turbidity together with a network characterised with coarse and large-pore mesh.

The quite complete characterisation of the various gel networks both mechanically by ways of oscillatory and static rheology and optically by turbidimetry and cryo-SEM shows that the extensive structural charge neutralisation of the polysaccharide by divalent calcium ions is responsible for a marked aggregation of the polymer strands reminiscent of precipitation. At lower counter-ion to polymer ratios, onset of gelation might prevent such phase separation.     

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.     

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.     

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.     

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.     

Mixed gels made from protein and κ-carrageenan

The rheological behavior of mixed gels made from soy or pea protein concentrates with the addition of κ-carrageenan was investigated using uniaxial compression and dynamic measurements. Pea protein concentrate (PPC) exhibited greater synergy with κ-carrageenan than soy protein concentrate (SPC) in relation to gel strength, gel stiffness and pH stability. A modified Takanayagi treatment of dynamic measurements indicated a shift in the continuous phase from protein to κ-carrageenan at concentrations of 4–8% κ-carrageenan in the total solids. This shift occurred at lower concentrations when PPC was used compared to SPC.     

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.     

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.     

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.     

Salt dependence and ion specificity of the coil–helix transition of furcellaran

The conformational transition and the cation-binding properties of aqueous furcellaran (a gel-forming, low-sulfated polysaccharide of the carrageenan family) in various salts and salt mixtures was studied by optical rotation and by ¹³³Cs-nmr. The results were compared with theoretical predictions based on the Poisson–Boltzmann cell model (PBCM). The conformational transition of furcellaran occurs in a single step, which implies a nonblocklike distribution of sulfate groups along the polymer chain. The chloride salts of sodium, lithium, and tetramethylammonium are equally potent in inducing helix formation of furcellaran, indicating that these ions act by nonspecific electrostatic interactions. In contrast, the potassium and cesium ions specifically promote helix formation and aggregation (gelation) of furcellaran. The divalent calcium and magnesium ions are nonspecific, but more potent than the nonspecific monovalent ions in inducing helices. Anions differ in their capacity to stabilize the furcellaran helix in the sequence Cl^? < NO < Br^? < SCN^? < I^?. The iodide and thiocyanate anions impede aggregation and gel formation. ¹³³Cs-nmr chemical shifts indicate specific binding of cesium ions to the furcellaran helix. Thus, with respect to its ion specificity and ion-binding properties, furcellaran, with 0.6 sulfate group per repeating disaccharide, resembles κ-carrageenan (1 sulfate/disaccharide) but differs from ι-carrageenan (2 sulfates/disaccharide). The conformational transition temperatures of furcellaran are, however, generally higher than those of κ-carrageenan under comparable conditions, and in mixtures of the two polysaccharides, separate transitions still occur, indicating that no mixed helices are formed. The observed ion sensitivity and cation-binding properties of furcellaran agree with predictions, by the PBCM, for a K-carrageenan with a reduced charge density.     

Conformation and association of κ-carrageenan in the presence of locust bean gum in mixed NaI/CsI solutions from rheology and cryo-TEM

Mixtures of locust bean gum (LBG) with κ-carrageenan (KC) in 0.1 M aqueous solutions of the mixed salts NaI/CsI were investigated by cryo-transmission electron microscopy (cryo-TEM) and dynamic viscoelastic measurements. Previous studies have shown that as the cesium content is increased in such mixed salt solutions, a transition occurs from molecularly dispersed helices to ‘superhelical rods’ of KC. We now found that LBG stabilises the superhelical rods, shifting the transition to a lower content of Cs for the mixtures than for KC alone. The formation of superhelical rods was evidenced both by cryo-TEM images and by an onset of thermal hysteresis in the coil–helix transition of KC. In the mixtures, the transition temperatures on cooling and heating were insensitive to the proportions of LBG and KC present at all cesium contents. Under conditions where no helix aggregation occurred (no hysteresis) the mixtures showed high tan δ values and low storage moduli. Under aggregated conditions, gels formed, and gels with added LBG had enhanced moduli compared to gels with KC alone. On the basis of these results we propose that LBG associates to the super-helical rods of KC.     

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.     

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.     

Effects of Coexistence of β-Casein on Gelation of κ-Carrageenan

The effects of caseins on the rheological properties of κ-carrageenan-calcium gel was investigated by measuring the gel breaking strength. The existence of β-casein in the system promoted the gelation of κ-carrageenan in the presence of calcium ion. Beta-casein increased the strength of calcium gels of κ-carrageenan with increasing NaCl concentration up to 80 mM and strengthened the κ-carrageenan-calcium gel at neutral pH. The values obtained from the slopes of the logarithmic plots of the gel strength versus concentration were 2.15 for κ-carrageenan gel and 2.27 for a β-casein-κcarrageenan mixture gel, suggesting that β-casein may participate in the gelation of κ-carrageenan through the mediation of calcium ions.     

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.     

Immobilized lipase Candida sp. 99-125 catalyzed methanolysis of glycerol trioleate: solvent effect

The immobilized lipase Candida sp. 99-125 catalyzed methanolysis of glycerol trioleate was studied in twelve different solvents in order to deduce the solvent effect through an attempt to correlate the highest yield with such solvent properties as hydrophobicity (log P), dielectric constant (epsilon), and Hildebrand solubility parameter (delta). The results showed that the conversion of glycerol trioleate and yield of oleic acid methyl ester were quite dependent on the solvent. The catalyst lipase in various solvents also needed different optimum amount of water to keep its maximum activity, and generally this lipase in more hydrophobic solvents required more water. The correlation between the highest yield and log P value was found to be reasonable except deviation of data points of certain solvents, while no obvious correlation existed between the other two parameters, dielectric constant (epsilon) and Hildebrand solubility parameter (delta), and the enzyme activity. The study revealed that more hydrophobic solvents such as n-hexane or cyclohexane were more suitable solvents for Candida sp. 99-125 catalyzed transesterification of glycerol trioleate to oleic acid methyl ester.