Influence of chain length and polymer concentration on the gelation of (amidated) low-methoxyl pectin induced by calcium

The gelation of low-methoxyl pectin (LMP) induced by addition of Ca2+ was studied by measuring the storage modulus as a function of temperature during cooling. Samples with different molar masses were prepared by mechanical degradation. The effect of the molar mass and the pectin concentration on the gelation properties was investigated. The effect of partial amidation was studied by comparing LMP and partially amidated LMP with the same molar mass and degree of methylation. The results are compared to those from a model developed for Ca2+-induced pectin gelation, and good agreement is found except at low concentrations and low molar masses where the gels are weaker than predicted. At low concentrations intrachain bonding weakens the gel, while the presence of small pectin chains weakens the gel because it neutralizes binding sites on larger chains.     

Heat-induced gelation of bovine serum albumin/low-methoxyl pectin systems and the effect of calcium ions

Influence of low-methoxyl pectin (LM pectin) and calcium ions (3 mM) on mechanical behavior and microstructure of bovine serum albumin (BSA) gels (pH 6.8, in 0.1 M NaCl) was evaluated. Protein and LM pectin concentrations were fixed at 2, 4, and 8 wt % and 0.21, 0.43, and 0.85 wt %, respectively. Rheological measurements and confocal laser scanning microscopy coupled with texture image analysis by use of the co-occurrence method were performed. Heat treatment of BSA/LM pectin mixtures induced protein gelation and a phase separation process between the two biopolymers, which was kinetically trapped. Calcium ions induced pectin gelation and modified BSA gel properties. Depending on biopolymer concentrations, a balance between pectin and/or protein gel contribution on final gel strength exists. The microstructures of the mixed systems in the presence of calcium can be interpreted as interpenetrated structures. Texture image analysis allowed one to classify more precisely the different microstructures observed in relation with mechanical properties.     

Characterization of association and gelation of pectin in methanol-water mixtures

Turbidity, swelling, and rheological features of semidilute systems of pectin in methanol-water media of different composition have been investigated. By increasing the percentage of methanol in the mixture, the thermodynamic properties of the pectin/methanol/water system become poorer, as shown by increasing turbidity and decreasing swelling. Effects of oscillatory and steady shear flows on intermolecular associations and gelation of pectin in methanol/water mixtures are reported. The effects of methanol concentration on the growth and structure of shear-induced gels, stabilized through hydrogen bonds, are analyzed. Steady shear measurements on these systems reveal shear thickening at low shear rates and disruption of intermolecular associative junctions at high shear rates.     

Pectin-chitosan interactions and gel formation

The effect of chitosan concentration on the gelation of pectins differing in charge density and distribution was examined, through the determination of gel stiffness and the binding of chitosan to the gel network. Chitosan acts as a crosslinker of concentrated pectin solutions, with its effectiveness showing a dependency on charge on the pectin. The networks produced are clear even under conditions of charge neutralisation.     

Effects of calcium, pH, and blockiness on kinetic rheological behavior and microstructure of HM pectin gels

The kinetic behavior during gel formation and the microstructure of 0.75% high methoxyl (HM) pectin gels in 60% sucrose have been investigated by oscillatory measurements and transmission electron microscopy for three comparable citrus pectin samples differing in their degree of blockiness (DB). Ca2+ addition at pH 3.0 resulted in faster gel formation and a lower storage modulus after 3 h for gels of the blockwise pectin A. For gels of the randomly esterified pectin B, the Ca2+ addition resulted in faster gel formation and a higher storage modulus at pH 3.0. At pH 3.5, both pectins A and B were reinforced by the addition of Ca2+. In the absence of Ca2+, the shortest gelation time was obtained for the sample with the highest DB. Microstructural characterization of the gel network, 4 and 20 h after gel preparation, showed no visible changes on a nanometer scale. The microstructure of pectins A and B without Ca2+ was similar, whereas the presence of Ca2+ in pectin A resulted in an inhomogeneous structure.     

Gelation of high methoxy pectin in the presence of pectin methylesterases and calcium

High methoxy pectin was submitted to various amounts of a fungal pectin methylesterase (PME) from Aspergillus aculeatus and of a plant PME from orange in the presence of calcium. The systems were characterized by rheological means during the gelation process. By the way of in situ demethoxylation with low amount of orange PME, it was possible to gel pectin from the beginning of the reaction although its high degree of methylation around 70. To understand this unusual properties, the behaviour of the two enzymes was investigated in pectic gels and in solution through the analysis of content and distribution of the remaining methyl esters. In the gel, the degree of methylation decreased slowly with orange PME and rapidly with Aspergillus PME. The degree of methylation and degree of blockiness after treatment with each PME in solution or in gels were slightly different. Possible explanations for this are evolving visco-elastic properties, including gel formation or influence of calcium on the enzyme–substrate complex.     

Rheological and structural properties of aqueous alginate during gelation via the Ugi multicomponent condensation reaction

Turbidity, structure, and rheological features during gelation via the Ugi multicomponent condensation reaction of semidilute solutions of alginate have been investigated at different polymer and cross-linker concentrations and reaction temperatures. The gelation time of the system decreased with increasing polymer and cross-linker concentrations, and a temperature rise resulted in a faster gelation. At the gel point, a power law frequency dependence of the dynamic storage modulus (G' proportional, variant omega(n)(')) and loss modulus (G' ' proportional, variant omega(n)(' ')) was observed for all gelling systems with n' = n' ' = n. By varying the cross-linker density at a fixed polymer concentration (2.2 wt %), the power law exponent is consistent with that predicted (0.7) from the percolation model. The value of n decreases with increasing polymer concentration, whereas higher temperatures give rise to higher values of n. The elastic properties of the gels continue to grow over a long time in the postgel region, and at later stages in the gelation process, a solidlike response is observed. The turbidity of the gelling system increases as the gel evolves, and this effect is more pronounced at higher cross-linker concentration. The small-angle neutron scattering results reveal large-scale inhomogeneities of the gels, and this effect is enhanced as the cross-linker density increases. The structural, turbidity, and rheological features were found to change over an extended time after the formation of the incipient gel. It was demonstrated that temperature, polymer, and cross-linker concentrations could be utilized to tune the physical properties of the Ugi gels such as structure, transparency, and viscoelasticity.     

Agarose-acrylamide gradient gel electrophoresis of proteins

A new agarose-acrylamide gradient slab gel electrophoresis system is described. The preparation of this new gel has been facilitated by the use of agarose with a relatively low gelation temperature. Fractionation of marker proteins and crosslinked proteins from a subcellular cytoskeletal preparation on agarose-acrylamide gradient gels is compared to that found using other acrylamide gel electrophoresis systems.     

Structural characterisation of thermoreversible anionic polysaccharide gels by their elastoviscous properties

Previously reported results obtained for the elastoviscous properties of some thermoreversible gels formed from anionic polysaccharides (high methoxyl pectin, furcellaran and κ-carrageenan) and also gelatin and maltodextrin are discussed and some conclusions about the structure of the gels are presented.The rate at which the relaxation processes take place in the gel is independent of the polymer concentration suggesting that the gels are structurally inhomogeneous.If the helical conformation of the individual macromolecule is stable the standard enthalpy change on crosslink breakdown is less than 45 kJ mol^?1. A relatively small decrease in standard enthalpy is sufficient for network stability because of the low standard entropy loss on gelation which is typical of semi-rigid chain polymers. If, however, the helical conformation is unstable the gelation process is cooperative and the standard enthalpy change on crosslink breakdown exceeds 200 kJ mol^?1.     

Gelation by phase separation in a whey protein system: in-situ kinetics of aggregation

The aggregation and gelation properties of beta-lactoglobulin (BLG), a globular protein from milk, was studied in aqueous ethanol solutions at room temperature. The phase state diagrams as a function of pH and ethanol concentration showed that a gel structure appeared after a period ranging from 1 min to 1 week, depending on the physico-chemical conditions. The in-situ kinetics of aggregation were followed by several methods in order to obtain a better understanding of the building of aggregates by the addition of ethanol. It was shown that the aggregation kinetics highly depended upon the pH, the process being fastest at pH 7. Viscoelasticity and infrared measurements indicated that alcohol-induced gelation would proceed via a two-step mechanism: small aggregates loosely connected between them were first built up; a real network took place in a second step. The coarse and irregular structures formed in aqueous ethanol gels revealed by confocal laser scanning microscopy could be analysed in terms of a phase separation. This observation was supported by a syneresis phenomenon visible in the final gel state. BLG in water-ethanol solution would undergo either an inhibition of the demixing by gelation or a binary phase separation accompanied by an irreversible gelation transition.     

Aqueous gel formation of a synthetic peptide derived from the beta-sheet domain of platelet factor-4

We observed gelation of a 23-residue peptide derived from the beta-sheet domain of platelet factor-4 (PF4(24)(-)(46)). The gels were primarily heterogeneous mixtures of 50-200 microm spherical aggregates in a less-dense gel matrix. Infrared and circular dichroism spectroscopies showed gelation involving the conversion of PF4(24)(-)(46) from random coil to beta-sheet. We used aggregation-induced NMR resonance broadening to show that temperature, pH, and ionic strength influenced PF4(24)(-)(46) gelation rates. Under identical solution conditions, gel formation took days at T /= 50 degrees C. Gelation was most rapid at pH values near the pK(a) of the central His35 residue. Increases in solution ionic strength reduced the critical gelation concentration of PF4(24)(-)(46). Our results suggest that PF4(24)(-)(46) gels by a process combining aspects of both heat-set and beta-fibril gelation mechanisms.     

Microstructure and kinetic rheological behavior of amidated and nonamidated LM pectin gels

The microstructure, kinetics of gelation, and rheological properties have been investigated for gels of nonamidated pectin (C30), amidated pectin (G), and saponified pectin (sG) at different pH values, both with and without sucrose. The low-methoxyl (LM) pectin gels were characterized in the presence of Ca(2+) by oscillatory measurements and transmission electron microscopy (TEM). The appearance of the gel microstructure varied with the pH, the gel structure being sparse and aggregated at pH 3 but dense and somewhat entangled at pH 7. During gel formation of pectins G and C30 at pH 3 there was a rapid increase in G' initially followed by a small increase with time. At pH 7 G' increased very rapidly at first but then remained constant. The presence of sucrose influenced neither the kinetic behavior nor the microstructure of the gels but strongly increased the storage modulus. Pectins G and C30 showed large variations in G' at pH values 3, 4, 5, and 7 in the presence of sucrose, and the maximum in G' in the samples occurred at different pH values. Due to its high Ca(2+) sensitivity, pectin sG had a storage modulus that was about 50 times higher than that of its mother pectin G at pH 7.     

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.     

Rheological study of genipin cross-linked chitosan hydrogels

This paper reports the rheological behavior of chitosan solutions that have been cross-linked with different amounts of genipin, at body temperature and physiological pH. The effect of the cross-linker loading on the rheological properties of hydrogels has been evaluated. The oscillatory time sweep method was used to monitor the dynamic viscoelastic parameters during in situ (i.e., in the rheometer) gelation experiments, enabling the determination of the gelation time. The stress and frequency sweeps were employed to measure G' of the cured hydrogels. It was found that the solutions of chitosan cross-linked with genipin, under physiological conditions, could form relatively strong elastic gels when compared to those of pure chitosan. Moreover, the gelation time obtained from the crossover of G' and G' was in excellent agreement with the value obtained from the Winter-Chambon criterion. A significant reduction on this parameter was achieved even at low genipin concentrations. This behavior suggests that these formulations are able to be produced in situ and thus constitute promising matrices for cells and bioactive molecule encapsulations.     

Effect of physicochemical parameters on the formation of chitosan-based gels

The possibility for forming physical gels based on Pchelozan (bee chitosan with a molecular weight of 230 kDa and an acetylation degree of 26-65%) has been demonstrated. Conditions for obtaining the gels (1% solution in 1% glycolic acid, 25 degrees C, pH 5.5-7.5) were selected. The effects of (1) the concentration of the original solution, (2) the degree of acetylation of Pchelozan, and (3) the value of pH on the process of gel formation, were studied. The gels obtained may be classified with reversible toxotropic systems. These gels are stable for a long time when stored within a temperature range of 18-55 degrees C. The gel with a degree of acetylation of 34% was characterized by irreversible syneresis.     

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.     

Thermodynamic incompatibility and complex formation in pectin/caseinate mixtures

The instability of polysaccharide/protein mixtures occurs because of either thermodynamic incompatibility or complexation. We studied which instability mechanism dominated given the external conditions. Therefore the effect of temperature, pH, and biopolymer concentration on the phase separation of pectin/caseinate mixtures was investigated. At pH > 6, thermodynamic incompatibility with spinodal decomposition was observed in pectin/caseinate mixtures resulting in the formation of water-in-water emulsions in intermediate stages of the phase separation process. The demixing rate of these emulsions and appearance of two macroscopic phases (lower phase enriched with caseinate and upper phase with pectin) was retarded when the pectin concentration increased or when the storage temperature decreased due to a higher viscosity of the mixtures at those conditions. As the pH of the mixture was lowered below 6, pectin accumulated in the caseinate-rich phase. Complexation of pectin and caseinate led to the formation of microparticles (approximately 3 microm), whose shape depends on the biopolymer concentration ratio and rate of acidification. These pectin/caseinate particles do not coalesce and are insensitive to salt addition.     

Temperature—composition phase diagram and gel properties of the gelatin—starch—water system

The gelatin-starch-water system has been studied at different temperatures, at a total biopolymer concentration of 5.0 wt%. The weight ratios (W) of gelatin/ starch used were 9:1, 8:2.4. 2:8, 1:9, with pH values between 5.82 (at W = 9:1) and 6.50 (at W = 1:9). The systems were characterized rheologically and by turbidity measurements to construct a phase diagram in the temperature (T) and composition (W) variables. The T-W quadrant consists of three regions: a singlephase solutions region (A) and regions of complete and incomplete phase separation (B and C, respectively). The system in region C is a gel. Region B, lying between A and C, corresponds to two co-existing liquid phases. The transition from A to C (obtained by cooling the system at constant W) involves crossing region B. The properties of the resulting gels depend on the rate of this intersection. Gels formed on rapid cooling have an even distribution of turbidity, whereas slow cooling gives two gel layers of different turbidity. The gelation temperature and gel strength of the mixed systems are dominated by the gelatin component, with no indication of network formation by starch.     

Gelation of konjac glucomannan crosslinked by organic borate

A series of thermoreversible konjac glucomannan gels crosslinked by organic borate were prepared. The gel network was formed through the crosslinking reaction between borate ions dissociated from organic borate and the cis-diol hydroxyl groups on the mannose units of polysaccharide chains. The rheological properties of the complex gels were studied by dynamic viscoelastic measurement. The gelation kinetics of the gels were studied and the critical gelation points of the gels were exactly determined by the Winter–Chambon criterion. The effects of temperature and composite ratio on the shear storage modulus (G^′), the loss modulus (G^″), and the sol-gel transition points were investigated. The critical gel-sol temperatures of the complex gels were successfully elucidated by Winter–Chambon criterion. The effect of crosslinking density on the critical gelation temperature and the elasticity of the gels were discussed.     

Chitosan gels: 1. Study of reaction variables

A study has been made of the gelation behaviour of chitosan solutions on reaction with acyl anhydrides. The effect on the time to onset of gelation of varying the nature and concentration of the acyl anhydride, the chitosan concentration, the temperature, and the nature of the cosolvent has been examined. The results indicate that gelation occurs because of decreased solubility of the polymer molecules due to the increased extent of $\text{N-}\text{acylation}$, and that variations of the above parameters influence the rate of gelation through influencing the rate of $\text{N-}\text{acylation}$. The syneresis of the gels produced has been briefly examined.