Formation Of Weak Gels Research Articles

Propylene glycol alginate (PGA) gelled foams: A systematic study of surface activity and gelling properties as a function of degree of esterification

A series of propylene glycol alginates (PGA) with degrees of esterification (D.E.) in the range of 32–93% were investigated with respect to surface activity and gelling potential. The surface tension measurements revealed increasing surface activity with increasing contents of propylene glycol groups. The rheological characterization of the PGA series calcium gels uncovered that the gelling potential of the PGAs was surprisingly well conserved even at intermediate degrees of esterification, assumed to reflect heterogeneity in the esterification pattern. For the PGAs of highest degree of esterification the assumed electrostatic point associations combined with hydrogen bonding lead to the rapid formation of weak gels. The rheological properties of high D.E. PGA solutions could be easily modified through the addition of sodium chloride and urea. Self-supporting, heterogeneous and stable PGA and PGA/unmodified alginate gelled foams were prepared applying the internal setting method and characterized with respect to mechanical properties and density. Increasing foamability and corresponding decrease in densities of the gelled foams and their mechanical strength was observed with increasing D.E..

Effect of S. macrosiphon and L. perfoliatum seed gums on rheological characterization of bitter orange (Citrus aurantium L.) and pomegranate (Punica granatum L.) paste blends.

The steady shear flow properties of bitter orange and pomegranate pastes and blend of two pastes including 0.5% Salvia macrosiphon (SMG) and L. perfoliatum (LPG) seed gums, two traditional Iranian hydrocolloids, were determined. All treatments exhibited shear-thinning behaviour. LPG added sample showed stronger shear thinning behaviour than the other due to its high molecular weight and intermolecular through hydrogen bonds and polymer entanglement. Ostwald model was found the best model to describe steady shear flow behaviour among different time-independent rheological model applied. Treatments including 0.5% these two seed gums indicated a flow behaviour index less than 0.6 and consistency coefficient raised by increasing concentration from 60 °Bx to 76 °Bx in bitter orange paste (from 0.55 Pa s(n) to 32.58 Pa s(n)), pomegranate paste (from 0.55 Pa s(n) to 84.87 Pa s(n)) and mix of these two pastes (from 0.64 Pa s(n) to 56.9 Pa s(n)). Oscillatory shear data showed weak gel-like behaviour of bitter orange and pomegranate pastes treatments including seed gums with the elastic modulus predominating over the viscous one at lower frequency. However, after weak gel formation, G″ was higher than G' in the frequency range of 0.01 to 10 Hz. An Ostwald model was used to describe the changes of viscose modulus with frequency. The results indicate that the elastic properties of bitter orange/ pomegranate paste and bitter orange paste may be increased by the presence of LPG and SMG due to associations of ordered chain segments of these gums, resulting in a weak three-dimensional network.

Dual modification of native white sorghum (Sorghum bicolor) starch via acid hydrolysis and succinylation

The present study investigated the effect of succinylation, acid-thining and combination of both on functional, textural and storage characteristics of starch isolated from white sorghum (Sorghum bicolor) grains. Prior to succinylation, native sorghum starch (NS) was acid-thinned with 0.1 M, 0.5 M and 1.0 M HCl solutions, separately. Succinylation of both NS and acid-thinned starches reduced percent retrogradation in starches while peak, cold paste and setback viscosities increased significantly. The extent of changes in various textural attributes like gumminess, hardness and chewiness were significantly reduced on refrigerated storage of starch gels. Similarly, decline in paste clarity was less pronounced in dual modified and single modified succinylated starches on cold storage. The results suggested that Su-At0.1M and Su-At0.5M starches showed lower percent retrogradation than At0.1M and At0.5M starches. Also they produced less gummy and chewy textures on storage which imply their application in puddings, custards and dairy desserts where weak gel formation is desirable. Also it was observed that Su-At1.0M gels had high degree of clarity, low hot paste viscosity and lower retrogradation tendency than At1.0M starches, which makes former extremely important in preparation of frozen pie fillings where high degree of clarity and storage stability is required combined with low hot paste viscosity.

Fabrication of the weak gel based on hydrolyzed polyacrylamide crosslinked by inorganic-organic mixed crosslinker

A novel weak gel of hydrolyzed polyacrylamide (HPAM) was prepared using inorganic–organic mixed crosslinker. The inorganic crosslinker was aluminum citrate (AlCit), and the organic crosslinker was 1,3,4,6-tetrahydroxymethylglycoluril (TMGU). The viscosity measurements showed that the viscosity of HPAM solutions was effectively increased from 80 mPas to 2500 mPas in stable state when 130 mg L−1 AlCit and 600 mg L−1 TMGU were added, and the gelation time of the weak gel could also be well controlled by changing the concentration of AlCit. Scanning electron microscope showed the formation of crosslinking structure of HPAM crosslinked by AlCit and the interpenetrating polymer network of weak gel crosslinked by AlCit and TMGU mixed crosslinker. Viscoelastic measurements indicted the elastic nature of weak gel. Fourier transform infrared spectra indicted that the carboxyl group of HPAM worked with AlCit while the amide group of HPAM worked with TMGU in the formation of weak gel. Thermogravimetric analyses and viscosity measurement indicted that AlCit may play the major role in the formation of weak gel. The mechanism of the formation of the weak gel was also discussed. The weak gel fabricated with high viscosity may have potential use in oil recovery due to its high viscosity.

Blends of cross-linked high amylose starch/pectin loaded with diclofenac

Polymers blends represent an important approach to obtain materials with modulated properties to reach different and desired properties in designing drug delivery systems in order to fulfill therapeutic needs. The aim of this work was to evaluate the influence of drug loading and polymer ratio on the physicochemical properties of microparticles of cross-linked high amylose starch–pectin blends loaded with diclofenac for further application in controlled drug delivery systems. Thermal analysis and X-ray diffractograms evidenced the occurrence of drug–polymer interactions and the former pointed also to an increase in thermal stability due to drug loading. The rheological properties demonstrated that drug loading resulted in formation of weaker gels while the increase of pectin ratio contributes to origin stronger structures.

The effect of hydrogen peroxide on the growth of thermophilic lactic starter and acid gelation of UHT milk

The influence of H 2O 2 pre-treatment (10–250 mg L −1) of ultra-high temperature treated (UHT) milk devoid of lactoperoxidase activity on the growth of thermophilic starter in the process of yoghurt production was studied using isothermal batch microcalorimetry and dynamic rheological measurements. Typical dual-peak power-time curves of diauxic growth of starter bacteria in milk were registered. Even the lowest concentrations of H 2O 2 added into milk 1 h before inoculation hindered the growth of thermophilic bacteria, retarded the onset of milk gelation, and resulted in formation of weaker gels. UHT milk samples treated with H 2O 2 and subsequently with catalase showed certain bacteriostatic influence of H 2O 2 pre-treatment on the second exponential growth phase. However, hardly any difference in gelation and no changes in rheological characteristics of mature gels in milk free of residual H 2O 2 due to catalase treatment prior to inoculation in comparison with H 2O 2-free control samples were observed.

Weak-gel formation in dispersions of silica particles in a matrix of a non-ionic polysaccharide: Structure and rheological characterization

Oscillatory and steady-shear rheological methods as well as scanning electron microscopy were used to characterize silica (SiO 2)/galactomannan dispersions. Particle loading and surface charge are found to strongly influence the composite structure and dynamics. When the particles were added above a minimum concentration, a sol–gel transition was induced. The SiO 2/polysaccharide dispersions exhibited significant non-linearities, especially under ionic conditions that favor interparticle and particle–polysaccharide interactions, characterized by strong shear-thinning, diverging viscosities at low shear rates, apparent yield stress and high strain sensitivity. The mechanical spectra for the composite systems suggest that stress relaxation of these composites is effectively arrested by the presence of silica nanoparticles. Microstructural analysis revealed an ordering of the silica particles around the polymer chains. Polysaccharide chains act as templates for particle clustering leading to the formation of a particle-mediated transient biopolymer network, with particle clusters hindering polymer mobility and enhancing junction zones within the network.

Weak gel of chitin with ionic liquid, 1-allyl-3-methylimidazolium bromide

This paper reports the formation of weak gel of chitin with an ionic liquid, 1-allyl-3-methylimidazolium bromide (IL). When a mixture of 5% (w/w) chitin with IL was heated at 100 °C for 48 h, the clear liquid was obtained. The experimental process was observed by the CCD camera view and the SEM analysis. From a mixture of chitin with IL in the higher concentration (7%, w/w), a more viscous material, i.e., a gel-like material was obtained. The rheological evaluations showed that both 5% (w/w) and 7% (w/w) chitins with IL behaved as weak gels.

Effects of xanthan gum on physicochemical properties and stability of corn oil-in-water emulsions stabilized by polyoxyethylene (20) sorbitan monooleate

The influence of added xanthan gum on rheological and dispersion characteristics and stability of concentrated (50% w/w) corn oil-in-water emulsions, stabilized with 5% (percentage on oil amount) polyoxyethylene (20) sorbitan monooleate (Tween 80), have been investigated. Emulsion with no xanthan indicated coalescence and poor creaming stability. All emulsions, with and without xanthan, showed shear-thinning flow behavior. Addition of xanthan protected emulsions from coalescence during 15 days of storage. Increase in xanthan concentration led to decrease in droplet average radius and creaming index, and increase in elastic properties of emulsions. Decrease in the emulsions flow behavior indexes, which suggested the extent of non-Newtonian behavior of emulsions, was influenced by increase in xanthan concentration. Above 0.04% of xanthan concentration, G′ and G″ values indicated formation of weak gels. Gel structure existence arises from droplet network association, due to depletion flocculation. Standard deviation of emulsions droplet size mean diameter decreased while concentration of added xanthan increased.

Magnetic and Temperature‐Sensitive Release Gels from Supramolecular Polymers

AbstractSupramolecular gels consisting of trivalent polyisobutylene and bivalent poly(ethylene oxide) are generated. Strong hydrogen bonding interactions, affixed to the end‐group moieties of the respective polymers (binding constant Kassn = 105 M–1), serve as molecular glue, leading to the formation of weak gels. Two different gels were prepared: one, with a short telechelic poly(ethylene glycol) (PEG) segment (gel A), and one with a longer PEG segment (number‐average molecular weight Mn = 2000 g mol–1) (gel B). Both gels show a significant increase in viscosity upon mixing of the two polymeric components, with a lag time of several minutes, indicative of nucleation mechanisms as the formation principle. However, only gel A displays classical gel‐like behavior, with a loss modulus G′ larger than the storage modulus G″ after formation. Both gels display microphase‐separated behavior with a spacing between 4–5 nm as probed via small‐angle X‐ray scattering (SAXS) and transmission electron microscopy (TEM) measurements. The incorporation of magnetic nanoparticles (Fe2O3; radius r = 3.5 nm) is successfully achieved, generating new magnetic gels with strongly thermoresponsive properties, displaying a strong temperature‐dependent release profile of included dye molecules. Magnetic measurements indicate a superparamagnetic behavior of the incorporated nanoparticles, prospecting the application as magneto‐sensitive delivery gels for pharmaceutical purposes.

Rheological properties of organoclay suspensions in epoxy network precursors

This paper deals with the evolution of the state of dispersion of organically modified montmorillonites in epoxy or amine precursors. The epoxy prepolymer is a diglycidyl ether of bisphenol A (DGEBA) and the curing agent is an aliphatic diamine with a polyoxypropylene backbone (Jeffamine D2000). The clay dispersion is evaluated at the platelet scale (nanoscopic scale) from X-ray spectrometry [wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS)] and at the aggregates scale (microscopic scale) from rheological analysis. The organoclays used form gels in the monomers above the percolation threshold if no shear is applied and present a mechanical gel/sol transition when shear stress increases. Gel strength and viscosity at high shear rates are linked to the nanometric state of dispersion and reveal the existence of two different organizations depending on organoclay/monomer interactions: (i) When the clay shows good interactions with the monomer, a significant swelling of the clay galleries by the monomer is obtained. These swollen particles lead to formation of weak gels which after shearing give high relative viscosity fluids. (ii) When the clay develops poor interactions with the monomer, the clay tends to reduce its exchange surface with the monomer and leads to a strongly connected gel. Shear breaks down this physical network leading to a very low relative viscosity fluid composed of nonswollen particles keeping a high aspect ratio.

PH Induced Aggregation and Weak Gel Formation of Whey Protein Polymers

ABSTRACT:Whey protein polymers were formed by heating (80 °C) a 4% (w/v) whey protein (WP) isolate dispersion at pH 8.0 for 15, 25, 35, 45, or 53 min. Dispersions were adjusted to pH 6.0, 6.5, 7.0, 7.5, or 8.0 after heating and the rheological properties were determined. Viscosity increased with increased heating time and decreased pH. At pH 7.0 and 7.5, high‐viscosity dispersions with pseudoplastic and thixotropic flow behavior were formed, while weak gels were formed at pH 6.0 and 6.5. The storage (elastic) and loss (viscous) moduli of pH‐induced gels increased when temperature was increased from 7 °C to 25 °C, suggesting that hydrophobic forces are responsible for gelation. Key Words: weak‐gels, whey proteins, polymers, gelation, functionality

Salt, Cryoprotectants and Preheating Temperature Effects on Surimi‐like Material from Beef or Pork

ABSTRACTGel forming ability, measured by cooked gel hardness, and water‐holding capacity of surimi‐like pork were enhanced by addition of NaCl at 1.5 or 3% although 4.5% or 6% NaCl did not further increase hardness. Cooked gel strength was unaffected by freezing of beef or pork surimi‐like materials for 48 hr. Addition of cryoprotectants (3% or 6% sorbitol, 3% glycerol, or 3% sucrose) before freezing had no effect on gel forming ability. Gel hardness was not increased by preheating prior to cooking. Preheating surimi‐like pork at 50°C resulted in formation of weaker gels which corresponded to myosin proteolysis. Myosin heavy chain (mw 200 kD) decreased, and new peptide fragments of mw about 138 kD, 128 kD, 120 kD, 86 kD, 77 kD, and 64 kD appeared. Proteolysis was salt‐dependent and occurred at pH 6.8–7.5.

Weak gels of fat crystals in oils at low temperatures and their fractal nature

AbstractThe formation of fat crystal gels in soybean oil has been studied by sedimentation in a low concentration region at 10–25°C. At 10°C, weak gels were formed with 1% crystals, and no gels formed at concentrations of 2–5%. At temperatures of 15–25°C, no gels were formed at concentrations of 1–5%, and samples sedimented. Stronger gels of fat crystals were formed with ∼10% fat crystals at all temperatures examined. Formation of weak gels is a consequence of the fractal nature of fat crystal aggregates and sediments. At low temperature, the interaction is weak. The fractal dimension is then high, and the floc size is large for low crystal concentrations. These large flocs form a three‐dimensional network that act as a weak gel and withstand gravitational force. When the temperature is increased, the fat crystal interaction becomes stronger, fractal dimension decreases, and floc size decreases. Smaller flocs have a higher density, pack more easily, and sediment. Similar effects are observed when the concentration of fat crystals is increased at low temperature due to a decrease in floc size.

Viscoelastic properties of solutions of ovine submaxillary mucin

The linear viscoelastic and rheological properties of high molecular weight ovine submaxillary mucin (OSM) solution have been investigated in terms of the Newtonian steady-flow viscosity [eta(gamma)], the complex oscillatory viscosity [eta*(omega)], and the storage and loss shear moduli [G'(omega) and G"(omega)]. It was observed that tau(gamma), eta*(omega), and G'(omega) are always higher when OSM is dissolved in 0.1M NaCl than when at the same concentration in 6M GdnHCl. This is consistent with previous observations that submaxillary mucins self-associate in 0.1M NaCl to form large aggregates, which are disrupted in 6M GdnHCl. As the OSM concentration increases, the appearance of a plateau shear modulus indicates the formation of a gel network in both solvents. The results suggest gelation involves specific intermolecular interactions, perhaps due to hydrophobic forces between interdigitated oligosaccharide side chains. The viscoelastic behavior of OSM solution at high concentration is thus similar to that reported in the literature for porcine gastric mucin (PGM). However, the OSM gels are mechanically weaker, having moduli that are an order of magnitude lower than those for PGM gels of comparable concentration. The oligosaccharide side chains of OSM consist of only 1-2 sugar units compared to 10-15 for PGM, but it appears that this is sufficient to allow for intermolecular interaction and the formation of weak gels.

Gel-Forming Characteristics of Milk Proteins, 2. Roles of Sulfhydryl Groups and Disulfide Bonds

The gel-forming characteristics of milk proteins were investigated by employing skim milk either nonpreheated or preheated at 80°C, and coagulating them at 70 or 80°C with glucono-delta-lactone. The solubility of each gel in the phosphate buffer (pH 7.0) containing urea and 2- mercaptoethanol was examined. The disulfide bonds played a more important role in the gel coagulated at 80°C from skim milk preheated at 80°C than in the gel coagulated at 70°C from nonpreheated skim milk.The effect of the reduction treatment with 2-mercaptoethanol was more pronounced on preheated skim milk than on nonpreheated skim milk. Sulfhydryl groups and disulfide bonds, which were buried in the molecules of whey proteins in their native state, were rendered accessible following heat treatment at 80°C.The gel prepared from skim milk pretreated with oxidizing agent, hydrogen peroxide (i.e., skim milk has no accessible sulfhydryl groups as a result), or the gel prepared from skim milk pretreated with reducing agent, 2-mercaptoethanol, (i.e., skim milk has few disulfide bonds as a result), displayed weak gel formation. But the gel prepared from the mixture of these skim milks with appropriate ratio displayed higher gel firmness. These findings suggest that intermolecular disulfide bonds are formed by the exchange reaction between sulfhydryl groups and disulfide bonds during gel formation.

Modern grouts and their uses : Dodd, M Tunn Tunnlg, V14, N10, Nov 1982, P20–21

The article discusses the uses of liquid grout materials in tunnelling applications. The grout is forced into soil or rock cavities and sets to render the treated ground impervious and increase its strength prior to excavation. The main differences between the types of grout are detailed under the following sections - particulate grouts and solution grouts. Particulate grouts cover cements and clays--the former bind soil with a cementitious mass forming a strong cohesive mass, while clay grouts do not impart strength but depend for their action on the formation of weak gels with a thixotropic action. Solution grouts depend on the reaction of two or more chemicals in solution to form a gel and are able to penetrate into soils of low permeability. This class of grout is described as being divided into two further classes - colloidal, silicate-based materials and organic polymerising solutions. The first are used for the same economic reasons as particulate grouts while the second, known as resin grouts, have found application in situations where there are permeability problems. The range of resin grouts available are detailed and also their use in particular cases. (TRRL)