Rheological Behavior Of Gellan Gum

Gellan gum is a kind of novel microbial polysaccharide with a high commercial value. Gellan gum have played a significant role in the field of food industry and have been used as texture modifiers, water adsorption agents and thicking agent because of their low dosage, resistant to acid and heat. In order to understand the gelation kinetics thoroughly and obtain the better properties, it is important for us to explore the rheological behavior of gellan gum. In this thesis, the rheological properties and gelation behavior were studied. Especially, modeling of stress relaxation of high acyl gellan gum was emphasized.Studies on the steady and dynamic rheological behavior of high acyl gellan gum were carried out. Parameters studied include the effects of concentration, temperature, pH and ion concentration. The steady-shear viscosity measurements indicated that high-acyl gellan gum aqueous solution posses a typical pseudoplastic fluid. The viscosity increased with increase in concentration. At a specified shear rate (100s-1), the effect of temperature on viscosity can be described by Arrhenius function within the temperature range from20to50℃, and the power law model was found to be more suitable than the exponential model in expressing the relationship between concentration and viscosity. Dynamic rheological measurements of the system suggested that high-acyl gellan gum aqueous solution were more elastic than viscous and G’ increased as the gum concentration increased. Cox-Merz rule was not applicable to high-acyl gellan gum aqueous solution. Adding cations in a proper range could make the gellan solution gelation, while adding excessive cation would decrease the viscosity for the competition between the cations.The stress relaxation behavior of high acyl gellan gum was investigated by using uniaxial compression. Results showed that the stress relaxation behavior of high acyl gellan gum gels fitted well with seven elements Maxwell model and modified Tang model. Meanwhile, cation species and cation concentrations had a pronounced effect on the stress relaxation behavior of high-acyl gellan gels. The values of residual stress (σe) correlated well with gel hardness (R2=0.868～0.976) indicating that σe could represents the gel strength. It appeared that addition of divalent cations at low levels results in an increase in gel strength compared to gels with monovalent cations. The tendency of initial decay rate (C1) varied with ion concentration was similar to that of The effect of testing parameters on stress relaxation behavior of high acyl gellan gel was investigated. Crosshead speed had little influence on relaxation test, while the compression strain affect the relaxation behavior significantly. Higher strains resulted in higher initial decay rate as well as the equilibrium stress. Equilibrium stresses and compression strains showed a positive correlation (R=0.986～0.987). It could be concluded from this thesis that the mechanism of stress relaxation of high acyl gellan gel was due to the shifting of the crosslinks, rather than the release of the hydraulic pressure.Influences of concentrations, temperature, pH and cations(Ca2+, Na+) on the rheological and texture properties of low acyl gellan gum were investigated by rheological and compression tests. Results reveals that gellan gum solutions under steady shear appear a shear-thinning fluid (n=0.12～0.34) and Power law model can describe the shear rate dependence of apparent viscosity. Moreover, flow behavior index (n) increase with increase of temperature, and consistency index (K) decrease with increase of temperature. At a specified shear rate, the effect of temperature on apparent viscosity can be described by Arrhenius function within the temperature range from10to40℃. On the other hand, increase of shear rate results in a decrease in flow activation energies (Ea), however Ea increase with the increase of the concentration of gellan gum solution. pH effect the viscosity of gellan gum solutions. It is found through the dynamic measurements that the dynamic storage modulus (G) is greater than the dynamic loss modulus (G") in the frequency (ω) range from0.1to100rad/s. G’and G" increase with ω increase while complex viscosity ((?)η(?)) decrease. Magnitudes of G’and G" increased with increase of concentration. It is suggested that the Cox-Merz rule is invalid for such system due to the formation of weak gel in gellan gum solutions. Maximal force represented gel strength. The fracture force increased with increase of gellan gum concentrations. At optimum cation levels, gellan gels with Ca2+were stronger than with Na+. Above the optimum cation levels, the strength of the gels formed with gellan polymer decreased with increasing cation concentrations. The calcium concentrations in low acyl gellan gum gels significantly affect water holding capacity.