Gelling Properties of Gellan Solutions Containing Monovalent and Divalent Cations

Gelling temperatures of gellan solutions with the addition of Na⁺ and K⁺ ranging from 15 to 450 mM or Ca⁺⁺ and Mg⁺⁺ from 2 to 40 mM were determined by dynamic rheological testing at four polymer concentrations between 0.4 and 2.0% (w/w). Gelling temperatures were much higher for gellan solutions containing divalent cations than for those containing the same amount of monovalent cations. Solutions containing K⁺ gelled at higher temperatures than those containing Na⁺. Effects of Ca⁺⁺ and Mg⁺⁺ on gelling temperatures were not significantly different. A general model was developed to predict the gelling temperature of gellan solutions as functions of cation and polymer concentrations.     

Polymer and Ion Concentration Effects on Gellan Gel Strength and Strain

Failure stresses and strains were measured in compressive, tensile and torsional modes on gellan gels at four polymer (0.6–1.8% w/v) and seven Ca⁺⁺ (1.5–60 mM) concentrations. Shear stresses at failure were equal in all three testing modes and proportional to gellan content. Low calcium gels increased linearly in strength with Ca⁺⁺ concentration until it reached a level of about 0.5 calcium ions per repeat tetrasaccharide unit of gellan gum polymer. Gel strength decreased linearly with Ca⁺⁺ at higher concentrations. Low calcium gels were extensible with failure strains decreasing as the logarithm of Ca⁺⁺; whereas high calcium gels were brittle and failed at a constant strain, the value of which was twice as high in compression and torsion as in tension.     

Effects of Phytic Acid, Divalent Cations, and Their Interactions on α-Amylase Activity

The effects of phytic acid and its interactions with divalent cations (Ca⁺⁺ and Mg⁺⁺) on α-amylase activity were investigated in model systems. Amylase activity was influenced by both preincubation time with phytate as well as phytate concentration. At 6-30 mM concentrations, Ca⁺⁺ and Mg⁺⁺ ions lowered the enzyme activity by 9-34% and 24-49%, respectively. When divalent cations were added simultaneously with phytate, a slight increase in enzyme activity with Ca⁺⁺ and lowered enzyme activity with Mg⁺⁺ were observed, as compared to when added independently. The enzyme activity was only moderately lowered when phytate was first preincubated with the divalent cations. Amylase inhibition by phytate was found to be of noncompetative type with an apparent inhibitor constant of 1.75 mM under the assay conditions.     

Gelation of gellan – A review

Gellan is an anionic extracellular bacterial polysaccharide discovered in 1978. Acyl groups present in the native polymer are removed by alkaline hydrolysis in normal commercial production, giving the charged tetrasaccharide repeating sequence: → 3)-β-d-Glcp-(1 → 4)-β-d-GlcpA-(1 → 4)-β-d-Glcp-(1 → 4)-α-l-Rhap-(1 →. Deacylated gellan converts on cooling from disordered coils to 3-fold double helices. The coil–helix transition temperature (T_m) is raised by salt in the way expected from polyelectrolyte theory: equivalent molar concentrations of different monovalent cations (Group I and Me₄N⁺) cause the same increase in T_m; there is also no selectivity between different divalent (Group II) cations, but divalent cations cause greater elevation of T_m than monovalent. Cations present as counterions to the charged groups of the polymer have the same effect as those introduced by addition of salt. Increasing polymer concentration raises T_m because of the consequent increase in concentration of the counterions, but the concentration of polymer chains themselves does not affect T_m. Gelation occurs by aggregation of double helices. Aggregation stabilises the helices to temperatures higher than those at which they form on cooling, giving thermal hysteresis between gelation and melting. Melting of aggregated and non-aggregated helices can be seen as separate thermal and rheological processes. Reduction in pH promotes aggregation and gelation by decreasing the negative charge on the polymer and thus decreasing electrostatic repulsion between the helices. Group I cations decrease repulsion by binding to the helices in specific coordination sites around the carboxylate groups of the polymer. Strength of binding increases with increasing ionic size (Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺); the extent of aggregation and effectiveness in promoting gel formation increase in the same order. Me₄N⁺ cations, which cannot form coordination complexes, act solely by non-specific screening of electrostatic repulsion, and give gels only at very high concentration (above ∼0.6 M). At low concentrations of monovalent cations, ordered gellan behaves like a normal polymer solution; as salt concentration is increased there is then a region where fluid “weak gels” are formed, before the cation concentration becomes sufficient to give true, self-supporting gels. Aggregation and consequent gelation with Group II cations occurs by direct site-binding of the divalent ions between gellan double helices. High concentrations of salt or acid cause excessive aggregation, with consequent reduction in gel strength. Maximum strength with divalent cations comes at about stoichiometric equivalence to the gellan carboxylate groups. Much higher concentrations of monovalent cations are required to attain maximum gel strength. The content of divalent cations in commercial gellan is normally sufficient to give cohesive gels at polymer concentrations down to ∼0.15 wt %. Gellan gels are very brittle, and have excellent flavour release. The networks are dynamic: gellan gels release polymer chains when immersed in water and show substantial recovery from mechanical disruption or expulsion of water by slow compression. High concentrations of sugar (∼70 wt % and above) inhibit aggregation and give sparingly-crosslinked networks which vitrify on cooling. Gellan forms coupled networks with konjac glucomannan and tamarind xyloglucan, phase-separated networks with kappa carrageenan and calcium alginate, interpenetrating networks with agarose and gelling maltodextrin, and complex coacervates with gelatin under acidic conditions. Native gellan carries acetyl and l-glyceryl groups at, respectively, O(6) and O(2) of the 3-linked glucose residue in the tetrasaccharide repeat unit. The presence of these substituents does not change the overall double helix structure, but has profound effects on gelation. l-Glyceryl groups stabilise the double helix by forming additional hydrogen bonds within and between the two strands, giving higher gelation temperatures, but abolish the binding site for metal ions by changing the orientation of the adjacent glucuronate residue and its carboxyl group. The consequent loss of cation-mediated aggregation reduces gel strength and brittleness, and eliminates thermal hysteresis. Aggregation is further inhibited by acetyl groups located on the periphery of the double helix. Gellan with a high content of residual acyl groups is available commercially as “high acyl gellan”. Mixtures of high acyl and deacylated gellan form interpenetrating networks, with no double helices incorporating strands of both types. Gellan has numerous existing and potential practical applications in food, cosmetics, toiletries, pharmaceuticals and microbiology.     

Sol-gel transition temperatures of high acyl gellan with monovalent and divalent cations from rheological measurements

The sol–gel transition temperatures of 0.1–1.0% high acyl gellan (HAG) with 0–200 mM NaCl or KCl and 0–20 mM CaCl₂ or MgCl₂ were determined using rheological measurements. Transition temperatures for monovalent cations, Na⁺ and K⁺, in the range of 50–80 °C were not significantly different (p > 0.5). Absence of thermal hysteresis was the salient feature. However, thermal hysteresis (∼4.4 °C) was observed for 0.1% HAG without added salt, but disappeared on increasing HAG and counterion concentrations. Few concentrations of HAG and added monovalent and divalent cations showed thermal hysteresis not higher than 2.5 °C. Transition temperatures for divalent cations were similar to those for monovalent cations although for considerably lower concentrations of Ca²⁺ or Mg²⁺. Increasing concentrations of monovalent and divalent counterions give rise to higher transition temperatures but not to higher storage moduli. This was interpreted as a lack of cross-link formation in the three-dimensional network structure of the gels. A single sol–gel transition diagram for monovalent cations is proposed, in which different zones associated with the presence of ordered and disordered conformations serve to identify the conditions in which HAG can exist in aqueous media.     

Bound Cations in Cucumber Pickle Mesocarp Tissue as Affected by Brining and CaCl2

The effects of brining, fermentation, storage, time of CaCl₂ addition and concentration of NaCl and CaCl₂ on Ca⁺⁺, Mg⁺⁺, Na⁺, and K⁺ bound to middle lamella-cell wall material of cucumber pickle mesocarp tissue were examined. Changes in the amount of each bound cation occurred rapidly during the first 2 days of brining; levels then remained relatively stable during fermentation and storage if concentrations of NaCl or CaCl₂ in brines were not altered. NaCl reduced levels of bound Ca⁺⁺, Mg⁺⁺, and K⁺, and the presence of CaCl₂ increased the amount of bound Ca⁺⁺ by displacing the other cations. Delayed addition of CaCl₂ to brines enhanced the content of bound Ca⁺⁺, indicating that levels of bound Ca⁺⁺ may not be related to maintenance of firmness.     

Effects of Cation Properties on Sol-gel Transition and Gel Properties of κ-carrageenan

The phase transition temperatures, rheological properties and gel‐network characteristics for gelation of κ‐carrageenan‐salt (NaCl, KCl and CaCl₂) solutions and their aged gels were investigated. The effectiveness of increasing gelling and gel‐melting temperatures at the salt concentrations examined followed the sequence of K⁺ > Ca2⁺ > Na⁺. This sequence was also true for the gel strength and the melting enthalpy (DH) of the most crosslinked junction zones of aged gels at low salt concentrations. Nonetheless, a different order (Ca⁺⁺ > K⁺ and Na⁺) was found for increasing storage modulus and gelation rate during early‐stage gelation, thermal hysteresis and the DH of aged gels in some salt‐carrageenan systems.     

Gelling Temperatures of Gellan Solutions as Affected by Citrate Buffers

Effects of citrate buffers at pH 3.5 and 5.0 on gelling temperatures of gellan solutions with 0.4–1.8% gellan and 1.5–60 mM Ca²⁺were studied. Partial dissociation of the carboxyl groups in gellan polymer in pH 3.5 solutions resulted in weakened gels. The pH 3.5 buffer exhibited weak chelating ability for Ca²⁺. The gelling temperature of gellan solutions at pH 3.5 was quantitatively related to polymer and cation concentrations using a similar model to that for gellan water solutions. The pH 5.0 buffer exhibited strong chelating ability. Gelling temperatures at pH 5.0 were generally lower than those at pH 3.5, except at low calcium concentrations.     

Gel—sol transition in gellan gum solutions. II. DSC studies on the effects of salts

The thermal properties of sodium form gellan gum solutions with and without sodium chloride, potassium chloride, calcium chloride and magnesium chloride were studied by differential scanning calorimetry (DSC). The DSC cooling or heating curves for 1% gellan gum solutions without salt showed a single exothermic or endothermic peak at ~30°C. DSC cooling curves showed a single exothermic peak, with the setting temperature (Ts) shifting to progressively higher temperatures with increasing concentration of the added NaCl or KCl. At low concentrations of NaCl or KCl, DSC heating curves showed a single endothermic peak; however with more addition of salt the endothermic peak gradually developed a bimodal character and eventually split into more than two distinct peaks. The onset of detectable splitting occurred at a high salt concentration which coincided with that at which elastic gels are formed at even a higher temperature as was observed by viscoelastic measurements. With a sufficient addition of monovalent cations the endothermic curve became again a single peak shifting to higher temperatures. In the presence of divalent cations, although Ts shifted to higher temperatures with increasing concentration of added CaCl₂ or MgCl₂, the melting temperature (Tm) in heating DSC curves shifted to higher temperatures (up to a certain temperature) and then shifted to lower temperatures with increasing concentration of salt. With increasing concentration of CaCl₂ or MgCl₂, the exothermic and endothermic enthalpies estimated for a main peak increased up to a certain salt concentration and then decreased; however many other peaks were observed at higher temperatures. The endothermic peaks for gels with excessive divalent cations were too broad to be resolved from the baseline; in contrast the exothermic peaks were much sharper and readily recognized. In comparing thermal properties with rheological properties, gellan gum solutions with excessive divalent cations form firm gels on cooling to below the setting temperature, and then it was difficult to remelt them. This was quite different from the behaviour of thermoreversible gels formed in the presence of monovalent cations. It seems that the mechanism of gel formation in gellan gum with divalent cations is markedly different from that with monovalent cations.     

EFECT OF pH BUFFERS ON MECHANICAL PROPERTIES OF GELLAN GELS

The strength and deformability of calcium cross-linked gellan gels as affected by pH 3.5 and 5.0 citrate and acetate buffers were measured by large compressive deformation test until failure. The trend of dependence of gel strength on polymer and calcium concentrations was similar to gels formed in distilled water without pH adjustment. A critical calcium concentration was observed for each gellan concentration. Gels formed at the critical calcium concentration exhibited the maximum strength. The chelating effect of pH 5.0 citrate buffer greatly increased the critical calcium concentration. The failure strain, representing the deformability, of gellan gels formed in buffers behaved differently from gels formed in distilled water. In the pH 3.5 buffer systems, gellan gels were brittle regardless of gellan and calcium concentrations. In the pH 5.0 buffer systems, gellan gels were brittle at high calcium concentrations and ductile at calcium concentrations less than 24 mM in citric buffer and less than 6 mM in acetate buffer.     

高酰基结冷胶/酪蛋白酸钠复合凝胶粘弹行为的研究

本文研究了酪蛋白酸钠浓度、结冷胶浓度、离子和测试条件对高酰基结冷胶/酪蛋白酸钠复合凝胶粘弹性的影响。结果表明:高酰基结冷胶/酪蛋白酸钠共混体系为典型的切力变稀流体,表观粘度随着酪蛋白酸钠浓度的升高而降低,而随着阳离子浓度的增大出现先增大后减小的变化趋势。压缩速度对复合凝胶硬度几乎无影响,而内聚性和弹性则随着压缩速度的增加而增大。内聚性随着压缩应变的增大出现先增大后减小的变化趋势。复合凝胶的硬度和弹性随着酪蛋白酸钠浓度的增加而下降,但复合凝胶的内聚性对酪蛋白酸钠浓度不敏感。高酰基结冷胶浓度越高,复合凝胶的硬度和弹性越大。相对于一价离子而言,二价离子形成的凝胶更强且用量更少。钾离子的添加对复合凝胶保水性影响较弱,而钙离子的添加则可以提高复合凝胶的保水性。     

17—THE INTERACTION OF NEGATIVELY CHARGED POLYURETHANE-LATEX PARTICLES WITH WOOL FIBRES

An investigation is reported of the adsorption of an anionic polyurethane latex, Impranil DLN, onto wool in the presence of inorganic salts. The divalent metal cations Pb⁺⁺, Ba⁺⁺, and Mg⁺⁺ promoted adsorption of the polymer particles at concentrations at which homocoagulation of the particles was absent or at least slow, but the trivalent cations Fe³⁺ and Al³⁺ were ineffective under similar conditions. The rate of adsorption increased with decreasing pH. The effect of surfactants on both adsorption and desorption of polymer particles was also studied. Adsorption of Impranil DLN in the presence of electrolyte was easily suppressed by non-ionic and anionic surfactants. Conversely, desorption of polymer particles from wool could be brought about by non-ionic surfactants, but only at concentrations considerably higher than those needed to suppress adsorption.     

A Rapid Method for the Evaluation of Emulsion Stability of Non-Dairy Creamers

A rapid method based on the Lowry protein assay was developed for the determination of the emulsion stability of nondairy creamers in a model system. The method used the detergent and crystallization properties of the anionic surfactant sodium dodecyl sulfate to solubilize the casein present in the creamers. Increasing concentrations of the divalent cations Ca⁺⁺ and Mg⁺⁺ resulted in an increasing degree of emulsion breakdown in a commercial nondairy creamer. Significant differences in the emulsion stability of five commercial nondairy creamers were also demonstrated with this method.     

Fracture Analysis of Alginate Gels

The fracture properties of alginate gels were investigated using torsion and compression. The gel fracture stress correlated with Ca²⁺ and alginate concentration, whereas the fracture strain was insensitive to composition. Considering the relationship of fracture stress with gel network crosslink density and the energy to break covalent and noncovalent bonds, the fracture of alginate gels is hypothesized to result from the disruption of junction zones. Consequently, the fracture stress was the stress required to overcome electrostatic forces that formed junction zones. The fracture stress‐strain relationship for alginate gels can be described by the Blatz, Sharda, adn Tschoegl (BST) equation, suggesting that for a given gel, the fracture strain can be predicted based on fracture stress, small‐strain shear modulus, and a fitted parameter describing nonlinearity of the gel. In addition, the fracture properties were affected by deformation rate. The influence of deformation rate on fracture was ascribed to structural changes among the alginate junction zones.     

Gelling Temperature of Gellan Solutions Containing Calcium Ions

Gelling temperatures were measured using a dynamic rheological testing method, a spectrophotometric method and direct visual examination combined with use of a thermocouple. Results from the dynamic testing method were most consistent. Gelling temperatures of gellan solutions increased from 30 to 72°C, as polymer concentrations increased from 0.4 to 2.0% w/v and Ca⁺⁺ concentrations increased from 2 to 40 mM. A mathematical model was developed based on the van't Hoff equation to relate gelling temperatures to composition and polymer molecular weight. The heat of cross-linking in Ca-gellan gels was estimated to be 428.6 kJ/mole, about 1.4 to 2.1 times greater than for gelatin gels.     

Viscoelastic Properties of Xanthan Gels Interacting with Cations

Viscoelastic properties of gels were greatly affected by xanthan gum concentration and types of cations. The storage moduli (G'), measured at 1.0 Hz, were 8.3, 10, and 2700 Pa for xanthan gels at 0.5% polymer concentration and 37, 42, and 13000 Pa for xanthan gels at 1.0% polymer concentration in the presence of Na⁺, Ca²⁺, or Fe³⁺, respectively. The elastic recovery was 27.9, 61.6, and 66.3% for 0.5% xanthan gels, and 38.5, 22.5, and 69.1% for 1.0% xanthan gels, in the presence of Na⁺, Ca²⁺, or Fe³⁺, respectively. The generalized Kelvin model simulated creep compliance and ferric ion was an excellent crosslinker for a rigid, firm gel.     

Factors that determine the fracture properties and microstructure of globular protein gels

Protein gel matrices are responsible for the texture of many foods. Therefore an understanding of the chemical reactions and physical processes associated with fracture properties of gels provides a fundamental understanding of select mechanical properties associated with texture. Globular proteins form thermally induced gels that are classified as fine-stranded, mixed or particulate, based on the protein network appearance. The fundamental properties of true shear stress and true shear strain at fracture, used to describe the physical properties of gels, depend on the gel network. Type and amount of mineral salt in whey protein and β-lactoglobulin protein dispersions determines the type of thermally induced gel matrix that forms, and thus its fracture properties. A fine-stranded matrix is formed when protein suspensions contain monovalent cation (Li⁺, K⁺, Rb⁺, Cs⁺) chlorides, sodium sulfate or sodium phosphate at ionic strengths ≤0.1 mol/dm³. This matrix has a well-defined network structure, and varies in stress and strain at fracture at different salt concentrations. At ionic strengths >0.1 mol/dm³ the matrix becomes mixed. This network appears as a combination of fine strands and spherical aggregates, and has high stress values and minimum strain values at fracture. Higher concentrations of monovalent cation salts cause the formation of particulate gels, which are high in stress and strain at fracture. The salt concentration required to change microstructure depends on the salt's position in the Hofmeister series. The formation of a particulate matrix also occurs when protein suspensions contain low concentrations (10–20 mol/dm³) of divalent cation (Ca²⁺, Mg²⁺, Ba²⁺) chloride salts or di-cationic 1,6-hexanediamine at pH 7.0. The divalent cation effect on β-lactoglobulin gelation is associated with minor changes in tertiary structure involving amide—amide interproton connectivities (determined by ¹H NMR) at 40–45°C, increasing hydrophobicity and intermolecular aggregation. The type of matrix formed appears to be related to the dispersed or aggregated state of proteins prior to denaturation. Mixed and particulate matrices result from conditions which favor aggregation at temperatures (25–45°C) which are much lower than the denaturation temperature (~65°C). Therefore, general (e.g. Hofmeister series) and protein-specific factors can affect the dispersibility of proteins and thereby determine the microstructure and fracture properties of globular protein gels.     

Compression resistance,sweetener's diffusion and sweetness of hydrocolloids gels

The effects of the type and concentration of two hydrocolloids—κ-carrageenan and gellan gum—and of the type and concentration of two sweeteners—sucrose and aspartame—on the gel resistance to compression, on the sweetener diffusion and on the intensity of the gel sweetness and the relationships between the gel physical properties and their perceived sweetness were studied. The gels true rupture stress increased with hydrocolloid concentration, this increase being higher for gellan gels. Gellan gels showed lower true rupture strain values, which in contrast with carrageenan gels, decreased on increasing hydrocolloid concentration. The addition of sucrose produced a bigger increase in gel strength at the higher hydrocolloid concentration. The main effect detected on the sweeteners’ diffusion constant was the higher value observed in low concentration (3 g L⁻¹) κ-carrageenan gels. Gellan gels were perceived as sweeter than κ-carrageenan gels. The decrease in sweetness due to an increase in hydrocolloid concentration was greater in gellan than in carrageenan gels. Variations in sweetener concentration, true rupture strain, and deformability modulus values explained 93% of the variability in sweetness for gels with sucrose and 94% for gels with aspartame.     

EFFECT OF SOME MONOVALENT AND DIVALENT METAL IONS ON THE FLOCCULATION OF BREWERS YEAST STRAINS*

In agreement with previous reports, it has been found that both Mg⁺⁺ and Mn⁺⁺ ions can imitate Ca⁺⁺ as inducers of flocculation, though the intensity of the flocculation is considerably reduced. This reduction is not dependent upon the ionic concentration and a 10-fold increase in Mg⁺⁺ or Mn⁺⁺ from the normal concentration of 10 mg ion/litre fails to increase the flocculation intensity. Low concentrations (1–10 mg ion/litre) of Na⁺ or K⁺ induce flocculation in those strains displaying intense flocculation with Ca⁺⁺, but high concentrations of either Na⁺ or K⁺ (50–100 mg ion/litre) antagonize floc formation. It is suggested that divalent ions act by bridging cells through negative charges on the cell surface, whereas monovalent ions induce flocculation via a “counter ion” effect where the repellent forces of the negative charges on the cell surface are neutralized, thus allowing some floc formation due to hydrogen bonding or other types of non-ionic bonding between cells. The antagonism of high concentrations of monovalent ions towards flocculation may be due to the fact that all available cell surface charges are neutralized, resulting in insulation of the cells and thus preventing cell-to-cell hydrogen bonding.     

Effect of Various Gelling Cations on the Physical Properties of “Wet” Alginate Films

In this study, the physical properties of “wet” alginate films gelled with various divalent cations (Ba²⁺, Ca²⁺, Mg²⁺, Sr²⁺, and Zn²⁺) were explored. Additionally, the effect of adding NaCl to the alginate film‐forming solution prior to gelling was evaluated. Aside from Mg²⁺, all of the divalent cations were able to produce workable “wet” alginate films. Films gelled with BaCl₂ (without added NaCl) had the highest (P < 0.05) tensile strength and Young's modulus while films gelled with CaCl₂ (alone) had the highest puncture strength. The Zn‐alginate and Sr‐alginate films had the highest elongation at break values. Adding NaCl to the alginate film‐forming solution increased the viscosity of the solution. Films with added NaCl were less transparent and had lower tensile strength, elongation, and puncture strength than films formed without NaCl in the film‐forming solution. ATR‐FTIR results showed a slight shift in the asymmetric COO^? vibrational peak of the alginate when the “wet” alginate films were gelled with Zn²⁺.