Synergistic gel formation of xyloglucan/gellan mixtures as sudied by rheology, DSC, and circular dichroism

The gelation behavior of mixtures of tamarind seed xyloglucan (TSX) and sodium form gellan (Na-G) was investigated. The storage and loss shear moduli, G' and G', of the mixtures showed that a thermoreversible gel was obtained although each polysaccharide alone did not form a gel at experimental conditions. The viscoelastic behavior of the mixtures showed a gel formation of TSX and Na-G induced by synergistic interaction. This synergistic interaction was also revealed by differential scanning calorimetry (DSC) and circular dichroism. Although TSX alone did not show any peak in DSC curves, mixtures with only a small amount of Na-G, which by itself did not show any peak, showed a single peak. With increasing Na-G content, another peak began to appear at the same temperature at which a peak of Na-G alone appeared. Thermally induced changes in circular dichroism of the mixtures were different from those expected from the individual behavior of TSX and Na-G.     

SAXS and ATR-FTIR studies on EBT-TSX mixtures in their sol-gel phases

Our previous study demonstrated that mixtures of tamarind seed xyloglucan (TSX) with appropriate concentrations of eriochrome black T (EBT) produced a gel that could be of benefit for medical use. Here, the sol-gel systems of various fresh and aged mixtures were further investigated using rheological measurements. The nanostructural changes of EBT-TSX sol-gel phases were analyzed using SAXS. The interactions between EBT and TSX in the sol and gel states were examined using ATR-FTIR. SAXS data analysis demonstrated that the mixture containing lower concentration of EBT formed rod-like structures and that with higher concentrations of EBT produced flat particles. The sizes of the TSX structures from the aged mixtures in the gel stage were larger than those from the same mixtures in the sol state. ATR-FTIR spectral changes revealed that the azo and sulfonic acid groups of EBT interacted with the TSX, and the characteristic spectrum of the sulfonic acid group of EBT could discriminate between the sol and gel state of the EBT-TSX systems. The interactions between EBT and TSX may cause conformational changes to TSX and facilitate the sol-gel transition or formation of a gel.     

Viscoelastic and Thermal Properties of Collagen–Xanthan Gum and Collagen–Maltodextrin Suspensions During Heating and Cooling

The purpose of this work was to investigate the viscoelastic properties of aqueous suspensions of crude collagen powder extracted from bovine hides and nonsubmitted to the hydrolysis reaction that leads to gelatin. The studied variables included the collagen concentration and the addition of xanthan gum or maltodextrin at varied concentrations during heating/cooling of the mixtures. Differential scanning calorimetry thermograms showed that the addition of polysaccharides decreased the endothermic peak areas observed at the denaturation temperature of collagen. The rheological properties of the pure collagen suspensions were highly dependent on concentration: 4% and 6% collagen suspensions presented a great increase in the storage modulus after heating/cooling, whereas for concentrations of 8% and 10% G′ decreased during heating and did not recover its original value after heating/cooling. The frequency sweeps showed that the thermal treatment was responsible by the strengthening of the interactions that formed the polymer network. Addition of 0.1% xanthan gum to collagen suspensions increased the gel strength, especially after heating/cooling of the system, whereas increasing gum concentration to 0.3% resulted in a weaker gel, which could indicate thermodynamic incompatibility between the biopolymers. Mixtures of collagen and maltodextrin resulted in more fluid structures than those obtained with pure collagen at the same collagen concentration and the range of temperatures in which these mixtures behaved as a gel decreased with increasing concentrations of both collagen and maltodextrin, suggesting incompatibilities between the biopolymers.     

The reinforcement of gellan gel network by the immersion into salt solution

The effect of immersion into salt solutions on rheological properties of gellan gels was investigated. The storage Young's modulus of gellan gels increased with time during the immersion into salt solutions. The increase of the storage Young's modulus can not be explained solely by change in the concentration of gellan. The ellipticity at 202 nm decreased by the immersion, suggesting the formation and aggregation of gellan helices. It was considered that during immersion cations penetrated into gellan gels to induce the formation and aggregation of gellan helices in gels, resulting in reinforcement of the gel network.     

A rheological study of the order-disorder conformational transition of xanthan gum

The rheological properties of a moderately concentrated solution of xanthan gum in both the ordered and the disordered state have been studied. Oscillatory shear, steady shear flow, and extensional flow experiments have been performed at different temperatures, covering the order-disorder transition determined by differential scanning calorimetry (DSC). The principle of time/temperature superposition was applied to the xanthan solutions for the different types of flow. Although a master curve covering six decades of frequency could be obtained for the storage modulus over the entire investigated temperature range, less agreement was found for the other modulus. This indicates that the order-disorder transition reflects changes on the molecular scale and slight modification of the physical network structure. To the authors' knowledge, this is the first time that this transition has been observed using these different rheological techniques.     

Characterization of Organogel Prepared from Rice Bran Oil with Cinnamic Acid

Cinnamic acid (CA) was added to rice bran oil (RBO) at concentrations range from 2 to 12% (w/w) to prepare functional fat-like organogel. The oil binding capacity and gel formation time of the obtained organogels were determined. The results showed that the oil binding capacity was increased and the gel formation time was significantly (p < 0.05) decreased with the increase in concentration of CA up to 10% (w/w). Based on these results, organogels prepared from RBO with 6, 8, and 10% CA (w/w) were chosen and subjected to further characterization analyses. The microscopy analysis showed different gel network structures and crystalline behavior for the organogels prepared with varying concentrations of CA. The rheological and thermal properties of the organogel were improved with the increase in CA concentration up to 10% (w/w). The X-ray diffractometer and Fourier Transform Infrared spectroscopy analyses indicated that the gel network formed based on crystalline of CA and physical entanglements among the organogel components. In addition, the organogel of RBO with 10% CA (w/w) showed a uniform and homogenized structure during storage at 5 °C for 28 days compared to the organogels prepared with 6 and 8%. Therefore, it can be suggested that cinnamic acid is a good organogelator for preparing functional fat-like organogel from rice bran oil.     

Rheology of concentrated isotropic and anisotropic xanthan solutions: 3. Temperature dependence

The oscillatory rheology of one rodlike and one semiflexible xanthan sample has been investigated as a function of temperature in the range of xanthan concentrations where the polymer forms a lyotropic liquid crystalline phase in aqueous NaCl solutions. Readily observed changes in the rheological observables at temperatures corresponding to phase boundaries permit construction of the biphasic chimney region of the temperature-composition phase diagram. The chimney region leans toward larger values of the polymer concentration with increasing temperature, presumably as a consequence of a reduction in the effective axial ratio of the helical polymer with increasing temperature. The results permit construction of plots of the rheological observables as a function of polymer concentration at temperatures T in the range 20 相似文献   

Structure and properties of a bacterial polysaccharide from a Klebsiella strain (ATCC 12657)

The chemical structure and the rheological behavior of the Klebsiella polysaccharide ATCC 12657 was studied and compared with data described in the literature and obtained for similar polysaccharides. The acetylated polysaccharide presents in solution a normal viscoelastic behavior with no evidence of an ordered conformation whatever the experimental conditions are. The deacetylated form can induce the formation of physical gels, in the presence of salt excess or ethanol. Microcalorimetry, optical rotation, and rheology experiments demonstrate that a thermally reversible and highly cooperative conformational transition occurs at the same temperature than a sol-gel transition. The melting of the gel and the conformational transition temperatures are dependent on the nature of cations and ionic concentration, whereas the gel strength is only influenced by polymer concentration.     

Rheology and gelation of water-insoluble dextran from Leuconostoc mesenteroides NRRL B-523

Dynamic oscillatory testing has been used to study the rheology of water-insoluble dextran. The rheological properties (storage and loss moduli) of dextran gel were measured and dextran was found to be neither a strong gel nor a weak gel, but an entanglement network at a concentration of 250 mg/ml. The extent of gelation, illustrated by the gel elastic modulus G′, is found to decrease with increasing concentration of calcium ions. This was confirmed by shift of crossover frequencies towards higher values on the dynamic spectra and lower yield stress τ values obtained from stress ramp experiments. Finally, a comparison between gelation of dextran and alginate (a similar biopolymer) was made for clear understanding of effect of calcium ions on the dextran gelation.     

Thermally induced protein gelation: gelation and rheological characterization of highly concentrated ovalbumin and soybean protein gels

In order to optimize the use of proteins as functional ingredients in foods, one needs more insight into the effects of environmental conditions (pH, ionic strength, and temperature) on the functional properties of protein. This paper summarizes the results of an extensive study on heat-induced gelation of ovalbumin (egg-white protein) and soybean protein in the concentration range from 10 to 35 g/100 g. It was the aim of the study to relate the rheological properties of thermally induced protein gels to the microstructure of the gel and the physicochemical properties of the constituent protein. The gelling behavior of the protein was quantified with rheological techniques, and the physical properties of the gels were determined, at small and large deformations. From the swelling/dissolving behavior of the gels in various media, the nature of the crosslinks was determined qualitatively. The microstructure of the gels was determined with electron microscopy. Nmr-spectroscopy was applied in order to elucidate changes in conformation during heating. It was found that the formation of a continuous covalently crosslinked network is not a prerequisite for thermally-induced protein gelation. The properties of a gel strongly depend on the pH at which the gel is formed. When heat-set at high pH(pH～10), a homogeneous, strong, and almost transparent gel is formed, consisting of flexible crosslinked protein gels. Heat-setting at low pH (pH 5) leads to the formation of a heterogeneous and weak gel, which easily exudes water. This gel consists of crosslinked aggregated protein. The ionic strength of the solvent in which the protein is dissolved and heat-set has a much lower effect on gel properties.     

Unique gelation behavior of cellulose in NaOH/urea aqueous solution

A transparent cellulose solution was prepared by mixing 7 wt % NaOH with 12 wt % urea aqueous solution which was precooled to below -10 degrees C and which was able to rapidly dissolve cellulose at ambient temperature. The rheological properties and behavior of the gel-formed cellulose solution were investigated by using dynamic viscoelastic measurement. The effects of temperature, time, cellulose molecular weight, and concentrations on both the shear storage modulus (G') and the loss modulus (G") were analyzed. The cellulose solution having a viscosity-average molecular weight (M(eta)) of 11.4 x 10(4) had its sol-gel transition temperature decreased from 60.3 to 30.5 degrees C with an increase of its concentration from 3 to 5 wt %. The gelation temperature of a 4 wt % cellulose solution dropped from 59.4 to 30.5 degrees C as the M(eta) value was increased from 4.5 x 10(4) to 11.4 x 10(4). Interestingly, at either higher temperature (above 30 degrees C), or lower temperature (below -3 degrees C), or for longer gelation time, gels could form in the cellulose solutions. However, the cellulose solution remains a liquid state for a long time at the temperature range from 0 to 5 degrees C. For the first time, we revealed an irreversible gelation in the cellulose solution system. The gel having been formed did not dissolve even when cooled to the temperature of -10 degrees C, at which it was dissolved previously. Therefore, this indicates that either heating or cooling treatment could not break such stable gels. A high apparent activation energy (E(a)) of the cellulose solution below 0 degrees C was obtained and was used to explain the gel formation under the cooling process.     

Enzymatically cross-linked tilapia gelatin hydrogels: physical, chemical, and hybrid networks

This Article investigates different types of networks formed from tilapia fish gelatin (10% w/w) in the presence and absence of the enzymatic cross-linker microbial transglutaminase. The influence of the temperature protocol and cross-linker concentration (0-55 U mTGase/g gelatin) was examined in physical, chemical, and hybrid gels, where physical gels arise from the formation of triple helices that act as junction points when the gels are cooled below the gelation point. A combination of rheology and optical rotation was used to study the evolution of the storage modulus (G') over time and the number of triple helices formed for each type of gel. We attempted to separate the final storage modulus of the gels into its chemical and physical contributions to examine the existence or otherwise of synergism between the two types of networks. Our experiments show that the gel characteristics vary widely with the thermal protocol. The final storage modulus in chemical gels increased with enzyme concentration, possibly due to the preferential formation of closed loops at low cross-linker amount. In chemical-physical gels, where the physical network (helices) was formed consecutively to the covalent one, we found that below a critical enzyme concentration the more extensive the chemical network is (as measured by G'), the weaker the final gel is. The storage modulus attributed to the physical network decreased exponentially as a function of G' from the chemical network, but both networks were found to be purely additive. Helices were not thermally stabilized. The simultaneous formation of physical and chemical networks (physical-co-chemical) resulted in G' values higher than the individual networks formed under the same conditions. Two regimes were distinguished: at low enzyme concentration (10-20 U mTGase/g gelatin), the networks were formed in series, but the storage modulus from the chemical network was higher in the presence of helices (compared to pure chemical gels); at higher enzyme concentration (30-40 U mTGase/g gelatin), strong synergistic effects were found as a large part of the covalent network became ineffective upon melting of the helices.     

Effect of cationic size on gelation temperature and properties of gelatin hydrogels

Effect of Na+, K+ and Ca2+ on gel transition temperature (Tg) of gelatin hydrogels (5%, w/v) has been studied by oscillatory rheology in the salt concentration range I = 0.01-0.1 M, which showed increase in Tg with salt concentration with the trend for Tg showing Ca2+ > K+ > Na+. The dynamic light scattering (DLS) measurements in the sol state (T>Tg) showed two distinct relaxation modes whereas only a gel mode was observed in the gel state in all the samples which contained significant amount of heterodyne contribution. Low frequency (1.5 rad/s) isochronal storage modulus data revealed the formation of strong gel in presence of CaCl2 compared to that of NaCl and KCl situations. The slow mode relaxation and heterodyne parameter obtained from DLS data indicate the presence of larger clusters in Ca2+ gels.     

Tuning the morphology of cellulose acetate gels by manipulating the mechanism of phase separation

The effect of dihydric alcohol (nonsolvent) addition on the rheological and microstructural behavior of cellulose acetate (CA) in a ternary CA, N,N-dimethylacetamide (DMA), nonsolvent system was investigated. Increasing the dihydric alcohol concentration led to enhanced steady shear viscosity and dynamic viscoelastic properties that were dependent on CA concentration. Changing the dihydric alcohol from 1,2-ethanediol to 1,4-butanediol and 1,6-hexanediol increased the moduli and decreased the concentration of nonsolvent at which the sol-gel transition occurred. At 10 wt % CA concentration the modulus and gel morphology of the 1,2-ethanediol and 1,4-butanediol systems were quite similar and distinctly different from that of 1,6-hexanediol. In the former, the gel morphologies were more heterogeneous, evident of more extensive coarsening, and likely obtained via nucleation and growth and spinoidal decomposition of off-critical mixtures. The latter exhibited more uniform dense network morphology, indicative of a spinoidal decomposition of near-critical mixtures. The gels were fractal in nature and exhibited different fractal dimensions in-line with the observed differences in microstructure; D ～ 1.87 ± 0.02 (1,2-ethanediol and 1,4-butanediol) and D ～ 1.97 ± 0.02 (1,6-hexanediol). However, at 15 wt % CA content, the gels exhibited more similar viscoelastic behavior and gel microstructures; D ～ 1.97 ± 0.02 for all three dihydric alcohol systems.     

The effect of the 540-kilodalton actin cross-linking protein, actin-binding protein, on the mechanical properties of F-actin

This study describes the effect of actin-binding protein derived from rabbit lung macrophages on the mechanical properties of F-actin. The dynamic storage modulus, G'(omega), and loss modulus, G"(omega) of F-actin, at concentrations from 1 to 4 mg/ml, in the absence or presence of actin-binding protein at molar ratios to actin of 1:1000 to 1:125, were measured at frequencies ranging from 3 X 10(-3) to 0.5 Hz. Actin-binding protein increased the dynamic moduli of F-actin, but this increase was much greater as either the actin-binding protein/actin ratio or the total protein concentration increased. Moreover, there was a convergence of the values of G' and G" at high frequencies for F-actin which became more prominent upon the addition of actin-binding protein. The value of the modulus obtained by an extrapolation of these data to actin concentrations similar to that found in the cell cortex was close to values which have been obtained by direct measurements. The addition of actin-binding protein to an F-actin solution enabled it to reach an equilibrium strain following the application of a stress, in contrast to pure F-actin. These data allow a more rigorous definition of the "sol" to "gel" transition and suggest that the cross-linking of actin filaments by actin-binding protein leads to the formation of a network structure whose underlying mechanism of mechanical behavior is short range intrafilament bending in contrast to the classical rubber network.     

Viscoelastic behavior of cellulose acetate in a mixed solvent system

The effect of increasing water composition on the rheological and microstructural behavior of a ternary cellulose acetate (CA)/N,N-dimethylacetamide (DMA)/water system is examined. Addition of water to the CA/DMA system results in enhanced steady shear viscosity and dynamic viscoelastic properties and ultimately to phase-separated gel formation. The changes in dynamic rheological behavior of the system during gelation correlate well with the combined solubility parameter (delta) and, in particular, the Hansen hydrogen-bonding solubility parameter index (delta(h)) of the solvent system, suggesting hydrogen-bonding interactions may be the major route initiating the sol-gel process. For all gels studied, the elastic modulus and the critical stress to yield shifts to higher values with increasing CA concentration and/or water content. In addition, the elastic modulus exhibits a power-law behavior with water content, with the same power-law exponent observed for gels containing different CA concentrations. Addition of water leads to formation of a denser gel network, as evidenced from direct visualization of the gel microstructure through confocal microscopy.     

Morphology and gelation of thermosensitive xyloglucan hydrogels

Galactose modified xyloglucan is a thermally reversible hydrogel that is increasingly used in the biomedical field due to the ease of altering the gelation time and temperature by modifying the galactose removal ratio. However there is little information concerning the morphology and rheological properties of the hydrogel under physiological conditions. Differential scanning microcalorimetry (DSmicroC) showed the thermal gelation process to occur over a broad temperature range (5-50 degrees C). The rheological properties of the hydrogels were investigated as a function of concentration, temperature and ionic strength. The final elastic moduli of the hydrogels increased with increases in concentration. Isothermal rheology suggests that the gelation occurred in two distinct stages, which was influenced by the solution media. Scanning electron microscopy (SEM) was used to characterize the morphology of the xyloglucan which were thermally gelled at 37 degrees C. The resultant morphology was strongly dependent on the concentration of the hydrogel. Strong hydrogels were only obtained at 3 wt.% at 37 degrees C, and the morphology characterized by an open 3-dimensional network, comprised of thin membranes. It is proposed that the first stage of the isothermal gelation is the formation and growth of the thin membranes, followed by the formation of a three dimensional network.     

Biosynthesis of polyhydroxyalkanoate copolyester containing cyclohexyl groups by Pseudomonas oleovorans

Production of polyhydroxyalkanoates (PHAs) substituted with cyclohexyl groups by Pseudomonas oleovorans grown with 4-cyclohexylbutyric acid (4-CHB) and its mixtures with nonanoic acid (NA) was investigated. Addition of NA to medium gave rise to an increase in the total concentration of 3-hydroxy-4-cyclohexylbutyrate repeating unit in the PHAs, indicating that the bioconversion rate of 4-CHB to polyester was significantly improved by the cometabolic effect. Increasing the proportion of NA from 1.0 to 7.5 mM at a concentration of 10 mM total carbon substrate also accelerated the uptake speed of 4-CHB by the organism and resulted in an increase of the ratio of 3-hydroxynonanoate to 3-hydroxyheptanoate from 1.28 to 2.05. Differential scanning calorimetric analysis of the PHAs bearing the corresponding functional groups showed one melting transition and one glass transition temperature varying according to the composition. These results indicated that random copolyesters were obtained from the carbon substrates used in this study.     

Enhanced hydration of dipalmitoylphosphatidylcholine multibilayer by vinblastine sulphate

Vinblastine sulphate, an antimitotic and anti-inflammatory agent, modifies the thermal behaviour of the model membranes: the dipalmitoylphosphatidylcholine DPPC bilayers. The mixed DPPC and vinblastine sulphate multibilayers in the range of DPPC mole fraction 0.4 to 1 display clearly the gel-liquid crystal (chain melting) transition on the thermograms obtained with a differential scanning microcalorimeter. The molar enthalpy of this transition is slightly depressed by vinblastine sulphate (less than 10%). The temperature-composition phase diagram corresponds to a total insolubility of vinblastine sulphate inside the frozen (gel) bilayers and to a solubility of 0.2 (mole fraction) of vinblastine sulphate inside the fluid (liquid crystalline) bilayers. The dissolved vinblastine sulphate depresses the cooperativity number of the frozen ? fluid transition of the bilayers very strongly (4- to 5-times). Up to its solubility concentration, vinblastine sulphate increases the amount of the structural water of the bilayers and modifies the thermal behaviour of this water. The ‘expelled’ vinblastine sulphate molecules are retained by the polar groups of DPPC molecules and screen their electrostatic interactions with the structural water molecules. Below 0°C, the amount of the structural water, which forms the aqueous separation between two bilayers, is enhanced by vinblastine sulphate. However, the drug reduces (screens) the bilayers interaction with the structural water molecules.     

Controlled gelation of xanthan by trivalent chronic ions

Addition of trivalent chromic ions to xanthan solutions gives rise to gel formation. The dynamic shear storage and loss moduli (0.01 – 10 rad/s) of xanthan solutions with polymer concentrations ranging from 1 to 7 mg/ml and Cr³⁺ concentrations ranging from 0 to 50 m have been studied. It is found that the rate of gel formation is strongly dependent on the Cr³⁺ concentration, but to a much smaller extent on the xanthan concentration. The gelation time is less than 1 h for 50 m Cr³⁺ and about 40 h for 2 m Cr³⁺. It is found that the minimum Cr³⁺ concentration needed to give gelation of 1–7 mg/ml xanthan is 1–2 m .