Dynamic rheology of agar gels: theory and experiments. Part II: gelation behavior of agar sols and fitting of a theoretical rheological model

A theoretical rheological model for agar gels, based on the bead spring model for linear flexible random coils and the model for crosslinked polymers, is successfully fitted to experimental gelation curves obtained over a wide range of cooling rates (0.5–20 °C min⁻¹) and agar concentration (1–3 wt%). The theoretical gelation temperature, T_gel^model increases with increasing agar concentration and decreasing cooling rate. The intrinsic net association rate increases significantly with increasing cooling rate. This increase is related to the higher probability of association of the non-associated agar molecules at higher cooling rates.     

Microstructure and rheological behavior of pure and mixed pectin gels

The microstructure and the rheological properties of pure HM (high methoxyl) and LM (low methoxyl) pectin gels and of mixed HM/LM pectin gels have been investigated. Gel formation of either the HM or LM pectin, or both, was initiated in the mixed gels by varying the sucrose and Ca(2+) content. The microstructure was characterized by transmission electron microscopy, light microscopy, and confocal laser scanning microscopy. HM and LM pectin gels showed aggregated networks with large pores around 500 nm and network strands of similar character. Small differences could be found, such as a more inhomogeneous LM pectin network with shorter and more branched strands of flexible appearance. LM pectin also formed a weak gel in 60% sucrose in the absence of calcium. A highly inhomogeneous mixed gel structure was formed in the presence of 60% sucrose and Ca(2+) ions, which showed large synergistic effects in rheological properties. Its formation was explained by the behavior of the corresponding pure gels. In the presence of 60% sucrose alone, a homogeneous, fine-stranded mixed network was formed, which showed weak synergistic effects. It is suggested that LM pectin interacts with HM pectin during gel formation, thereby hindering secondary aggregation leading to the aggregated networks observed for the pure gels.     

Effect of concentration and temperature on the rheological behavior of collagen solution

Dynamic viscoelastic properties of collagen solutions with concentrations of 0.5-1.5% (w/w) were characterized by means of oscillatory rheometry at temperatures ranging from 20 to 32.5 degrees C. All collagen solutions showed a shear-thinning flow behavior. The complex viscosity exhibited an exponential increase and the loss tangent decreased with the increase of collagen concentration (C(COL)) when the C(COL)> or =0.75%. Both storage modulus (G') and loss modulus (G') increased with the increase of frequency and concentration, but decreased with the increase of temperature and behaved without regularity at 32.5 degrees C. The relaxation times decreased with the increase of temperature for 1.0% collagen solution. According to a three-zone model, dynamic modulus of collagen solutions showed terminal-zone and plateau-zone behavior when C(COL) was no more than 1.25% or the stated temperature was no more than 30 degrees C. The concentrated solution (1.5%) behaved being entirely in plateau zone. An application of the time-temperature superposition (TTS) allowed the construction of master curve and an Arrhenius-type TTS principle was used to yield the activation energy of 161.4 kJ mol(-1).     

槐豆胶与黄原胶的协效性研究

对槐豆胶与黄原胶的协效性进行了研究,结果表明,槐豆胶和黄原胶有较高的协效性,其最佳配比（重量比）为2：8;当混合液浓度达到0.5%-0.6%时形成凝胶,因此槐豆胶可作为黄原胶的增稠剂和凝胶剂。     

Effect of calcium concentration on textural properties of high and low acyl mixed gellan gels

Textural properties of 1% low and high acyl gellan gels and their mixtures were studied using compression tests and the microcentrifuge-microfiltration based water holding capacity (WHC) method. Low acyl (1% LA), high acyl (1% HA) gels and mixtures of 1% 25/75 LA/HA, 50/50 LA/HA, 75/25 LA/HA gels with calcium concentrations ranging from 2 to 80 mM were studied. HA or mixed gels with a lower LA/HA ratio had a greater WHC and failure strain than that of LA or mixed gels with a higher LA/HA ratio. Gellan gels with a higher LA/HA ratio had a larger initial Young's modulus. Our study also indicates that a higher LA/HA ratio does not necessarily result in a gel with a larger failure stress, although LA gels are generally firmer than HA gels. Gel strength and WHC of HA and LA/HA mixtures may reflect both stabilization and destabilization effects of glycerate groups positioned at one of the glucose residues in each repeating tetrasaccharide unit of HA gels.     

Solvent-induced lysozyme gels: effects of system composition and temperature on structural and dynamic characteristics

The gelation process of lysozyme in water/tetramethylurea in the presence of salt was investigated as a function of temperature and system composition by rheology, infrared spectroscopy, and microcalorimetry. Times and temperatures of gelation were determined from the variation of the storage (G') and loss (G') moduli. It was found that gelation times follow exponential decays with both protein and tetramethylurea (TMU) concentrations and with temperature. The activation energy for the overall process shows a linear dependence on TMU mass fraction. A strongly increased beta-sheet content and reduced alpha-helix occur with the increase of TMU concentration in the binary solvent. Also, a linear decrease of lysozyme denaturation temperature and enthalpy on TMU concentration is found for the TMU mass fraction up to 0.5, above which no denaturation signal can be detected.     

A crude sword bean (Canavalia gladiata) extract is gelated by cooling

White sword bean (Canavalia gladiata) seeds have the potential to be utilized in the manufacturing of processed foods owing to their high protein and carbohydrate content. Our previous reports explored the use of the sword bean as a source of food materials by preparing extracts in distilled water. In the present study, we found that one such extract can be gelated by cooling. The gelling substances were extracted by boiling and simultaneously stirring a suspension containing ground beans. Few proteins were present in the gelated extract. We also examined the conditions under which gelation occurred and the gel melting temperature. The extract gelated at temperatures below 10 °C, and the resulting gel melted at those above 65 °C. This is the first report that gelling substances can be extracted from sword beans in large quantities. We expect that this gelling agent can be used for the production of processed foods.     

Target concentration dependence of DNA melting temperature on oligonucleotide microarrays

The design of microarrays is currently based on studies focusing on DNA hybridization reaction in bulk solution. However, the presence of a surface to which the probe strand is attached can make the solution‐based approximations invalid, resulting in sub‐optimum hybridization conditions. To determine the effect of surfaces on DNA duplex formation, the authors studied the dependence of DNA melting temperature (T_m) on target concentration. An automated system was developed to capture the melting profiles of a 25‐mer perfect‐match probe–target pair initially hybridized at 23°C. Target concentrations ranged from 0.0165 to 15 nM with different probe amounts (0.03–0.82 pmol on a surface area of 10¹⁸ Ų), a constant probe density (5 × 10¹² molecules/cm²) and spacer length (15 dT). The authors found that T_m for duplexes anchored to a surface is lower than in‐solution, and this difference increases with increasing target concentration. In a representative set, a target concentration increase from 0.5 to 15 nM with 0.82 pmol of probe on the surface resulted in a T_m decrease of 6°C when compared with a 4°C increase in solution. At very low target concentrations, a multi‐melting process was observed in low temperature domains of the curves. This was attributed to the presence of truncated or mismatch probes. © 2012 American Institute of Chemical Engineers Biotechnol. Prog., 2012     

Kinetics and modeling of temperature effects on batch xanthan gum fermentation

Batch fermentation kinetics of xanthan gum production from glucose by Xanthomonas campestris at temperatures between 22 degrees C and 35 degrees C were studied to evaluate temperature effects on cell growth and xanthan formation. These batch xanthan fermentations were modeled by the logistic equation for cell growth, the Luedeking-Piret equation for xanthan production, and a modified Luedeking-Piret equation for glucose consumption. Temperature dependence of the parameters in this model was evaluated. Growth-associated rate constants increased to a maximum at approximately 30 degrees C and then decreased to zero at approximately 35 degrees C. This temperature effect can be modeled using a square-root model. On the contrary, non-growth-associated rate constants increased with increasing temperature, following the Arrhenius relationship, in the entire temperature range studied. The model developed in this work fits the experimental data very well and can be used in a simulation study. However, due to the empirical nature of the model, the parameter values need to be reevaluated if the model is to be applied to different growth conditions.     

Determination of melting temperature and temperature melting range for DNA with multi-peak differential melting curves

Many factors that change the temperature position and interval of the DNA helix–coil transition often also alter the shape of multi-peak differential melting curves (DMCs). For DNAs with a multi-peak DMC, there is no agreement on the most useful definition for the melting temperature, T_m, and temperature melting width, ΔT, of the entire DNA transition. Changes in T_m and ΔT can reflect unstable variation of the shape of the DMC as well as alterations in DNA thermal stability and heterogeneity. Here, experiments and computer modeling for DNA multi-peak DMCs varying under different factors allowed testing of several methods of defining T_m and ΔT. Indeed, some of the methods give unreasonable “jagged” T_m and ΔT dependences on varying relative concentration of DNA chemical modifications (r_b), [Na⁺], and GC content. At the same time, T_m determined as the helix–coil transition average temperature, and ΔT, which is proportional to the average absolute temperature deviation from this temperature, are suitable to characterize multi-peak DMCs. They give smoothly varying theoretical and experimental dependences of T_m and ΔT on r_b, [Na⁺], and GC content. For multi-peak DMCs, T_m value determined in this way is the closest to the thermodynamic melting temperature (the helix–coil transition enthalpy/entropy ratio).     

Molecular structure of potassium kappa carrageenate gels: a rheological and pulsed electric birefringence study

Measurements have been made of the shear modulus as a function of biopolymer concentration for pure potassium kappa carrageenate gels. The results have been discussed on the basis of rubber elasticity theory to investigate the free polymer linkages between junction zones within the gels. Pulsed electric birefringence studies have been made on segmented kappa carrageenan as a model for the junction zones of the gel.     

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.     

Dynamic rheology of agar gels: theory and experiments. Part I. Development of a rheological model

A theoretical rheological model for agar gels is proposed, based on the bead and spring model for linear flexible random coils and the model for crosslinked polymers. The model introduces the concept of a temperature dependence of the monomeric friction coefficient, ζ₀, of the agar molecule. The model has a random coil-like behavior at high temperatures (close to 373 K), and contributions from a three-dimensional network at low temperatures (close to 273 K). A proposed temperature dependence of the net association rate allows the calculation of the fraction of associated molecules as a function of time and temperature. The proposed model predicts the gelation behavior of agar gels utilizing time–temperature data (cooling curves).     

Effect of carbon dioxide on the rheological behavior of submerged cultures of Chlorella minutissima in stirred tank reactors

The objective of this study was to quantify the effect of algal biomass concentration on the rheology of the algal culture broth. Batch cultivations of Chlorella minutissima were carried out with air and carbon dioxide in a stirred tank bioreactor with a working volume of 1.8 L. The apparent viscosity of the culture broth was significantly affected by the cell mass concentrations in the bioreactor. Culture broth containing 50 g/L cell mass from air fed was found to exhibit an apparent viscosity of 1.52 mPa.s. The apparent viscosity of the carbon‐dioxide‐fed cultivations was found to increase by 20% at a shear rate of 100 s^?1. The flow behavior of the system was adequately described by the Herschel–Bulkley model with a small yield stress.     

Effects of amylopectin structure and molecular weight on microstructural and rheological properties of mixed beta-lactoglobulin gels

Nongelling amylopectin fractions from potato and barley have been used to form mixed beta-lactoglobulin gels. The amylopectin fractions were produced by varying the time of alpha-amylase hydrolysis followed by sequential ethanol precipitation. The molecular weights, radius of gyration, chain length distribution, and viscosity of the fractions were established. The mixed gels were analyzed rheologically with dynamic mechanical analysis in shear and microstructurally with light microscopy, transmission electron microscopy, and nuclear magnetic resonance spectroscopy. The result of the gel studies clearly showed that small differences in the molecular weight of amylopectins have a significant influence on the kinetics of protein aggregation and thereby on the gel microstructure and the rheological behavior of the gel. Both an increase in the molecular weight and a higher concentration of amylopectins resulted in a more open protein network structure, with thicker strands of larger and more close-packed beta-lactoglobulin clusters, which showed a larger storage modulus. The transmission electron micrographs revealed that degraded amylopectins were enclosed inside the protein clusters in the mixed gels, whereas nondegraded amylopectin was only found outside the protein clusters. The volume-weighted mean value of the molecular weight of the amylopectins was found to vary between 3.2 x 10(4) and 5.0 x 10(7) Da and the ratio of gyration between 14 and 61 nm. The maximum in chain length distribution was generally somewhat distributed toward longer chain lengths for potato compared to barley, but the differences in chain length distribution were minor compared to those seen in the molecular weight and ratio of gyration between the fractions.     

微波诱变提高黄原胶的主要理化性能

用微波诱变黄原胶产生菌－野油采黄单孢菌,得到产胶量,胶粘度,抗盐性,pH稳定性及温度稳定性都高于出发株的P402－21突变株,P402－21的产胶浓度为3．484％,1％胶粘度为1673cp,能耐受1％－11％,NaCl,在pH4-14范围内稳定,25－100摄氏度胶粘度没有明显的改变。     

Texture properties of high and low acyl mixed gellan gels

The strength, deformability, and firmness of high acyl (H) and low acyl (L) mixed gellan gels were studied by compression tests. The gels were prepared with total polymer concentrations of 0.5, 1.0, and 1.5% at H/L weight ratios of 25/75, 50/50, 75/25, and calcium concentrations 2–80 mM. The mixed gels were much more deformable, with failure normal strains ranging from 0.6 to 1.5, but had similar strength compared to low acyl gellan gels. Both H/L ratio and total polymer concentration affected the textural properties, but H/L ratio was a more important factor. Maximum synergistic interaction was observed at H/L=50/50. The mixed gels exhibited excellent texture properties compared to other common food gels.     

Chain conformation and rheological behavior of an extracellular heteropolysaccharide Erwinia gum in aqueous solution

The chain conformation of a heteropolysaccharide Erwinia gum (EG) consisting of Glc, Gal, Fuc, and GlcA in aqueous solution was investigated by using viscometry and static and dynamic light scattering. The Huggins constants k′ ranging from 0.31 to 0.35, and the larger second virial coefficient A₂ of the order of 10⁻⁴ and even 10⁻³ mol g⁻² cm³ for EG fractions having different molecular weights in 0.03 M NaCl aqueous solution at 25 °C, suggested that 0.03 M NaCl aqueous solution is a good solvent for EG polysaccharide. Smidsrød’s ‘B-value’ characterizing chain stiffness was estimated to be 0.028-0.045 for EG fractions indicating that the backbone of EG polysaccharide is semi-stiff having similar stiffness to the semi-stiff Alginate and CMC. The hydrodynamic factor ρ (1.69-1.89), Flory-Fox factor Φ, and the product of ρΦ/N_A (0.16-0.22) also confirmed the semi-stiffness of EG polysaccharide chains. Compared with general flexible polymers, the first remarkable shear-thinning and then Newtonian flowing behaviors in steady shear tests for EG polysaccharides were ascribed to the alignment of extended semi-stiff chains on shearing. The dynamic oscillatory shear experiments indicated that addition of certain amount of NaCl effectively prohibited its gelation in pure water even at high concentration and low temperature for long time, suggesting that 0.03 M NaCl aqueous solution of EG has good stability and ability of antigelation, and thus is a promising additive in food field.     

Correlation of Aspergillus niger broth rheological properties with biomass concentration and the shape of mycelial aggregates

Aspergillus niger was grown in a 7-L chemostat at biomass levels of 7 to 9 gL(-1); dilution rates of 0.03, 0.05, 0.075, and 0.009 h(-1); and dissolved oxygen tensions of 7%, 12%, and 40% of air saturation. Broth rheological measurements were made on-line, while off-line image analysis was used to measure mycelial morphology, including characterization of mycelial aggregates (clumps). Under all conditions, more than 87% of the hyphase were in clumps, the shape of which determined the rheological characteristics of the broth. In particular, the power law consistency index could be correlated with the biomass concentration and the roughness factor of the clumps, which describes their hairiness. A decrease in specific growth rate decreased roughness, possibly due to changes in the amount of clump breakup. However, decreases of roughness with increasing dissolved oxygen tension might rather imply some effect on hyphal-hyphal interactions within the clumps. (c) 1993 John Wiley & Sons, Inc.     

The influence of headgroup structure and fatty acyl chain saturation of phospholipids on monolayer behavior: a comparative rheological study

This paper compares six phospholipidic monolayers at the water/chloroform interface by performing dilational rheological measurements with a drop tensiometer apparatus. The chosen lipids differ both in their headgroup structure and fatty acyl chain saturation or symmetry. The study concentrated on monolayers formed with DPPC, DPPE, DOPC, DOPE, POPC and POPE. Using a generalized Maxwell rheological model, transposed at the interface, the intimate intermolecular interactions between amphiphilic molecules are studied on and off the monolayer plane. The equilibrium and nonequilibrium phenomena are analyzed and, respectively, correlated with monolayer cohesion and with monolayer/sub-surface interactions. The purpose of this work is to gain further insights into the influences (as slight as they are) of the weak changes in phospholipid structure and on the behavior of the monolayers. The results, widely described, provide further details on nuances existing between very similar molecules, and likewise, on the synergies created between the different effects.