Preparation And Rheological Properties Of Low Acyl Gellan Gum Fluid Gel

Fluid gel is a suspension of gelled particles dispersed in a non-gelled continuous medium.They are formed by applying shear to a biopolymer during its ordering gelation where it would otherwise form a large-scale gel network.Fluid gels behave like solid at rest,but flow above a critical value of applied stress.Their rheological properties may be finely controlled to meet requirements for desired applications.In this paper,low acyl gellan gum fluid gels?LAG?were prepared.The effects of gellan concentration,cooling rate,shear rate,cation and glucono-?-lactone?GDL?on the formation and rheological properties of fluid gel were investigated.Low acyl gellan fluid gels were prepared via heat-induced gelation of low acyl gellan solution under controlled temperature and shear.The effects of polymer concentration,shear rate,cooling rate,ion species,ion concentration and LAG/GDL weight ratio on the formation and microstructure of LAG fluid gel were carried out.The results indicated that the low acyl gellan concentration had a pronounced effect on the formation of low acyl gellan fluid gels.The transition temperature,fluid gel particles size and final viscosity of fluid gels increased with the increasing of low acyl gellan concentration.When the concentration is increased further to 1.0%w/v the particles were much larger and irregular in shape.The viscosity of the fluid gels during formation increased with increasing cooling rates and viscosity decreased when shear rate was increased.The micrographs indicate that a smaller particle size can be obtained by decreasing cooling rate and increasing the shear rate when forming the fluid gels.For low acyl gellan,there existed a critical cooling rate.The cooling rate is almost no influence on the transition temperature of low acyl gellan fluid gels at or above the critical value.The higher the ion concentration,the higher the transition temperature and viscosity of fluid gel increased.Moreover,the fluid gel particle sizes increased with the increasing of ion concentration.Viscosity of fluid gels with Ca²⁺were larger than that with Na⁺.The lower the LAG/GDL weight ratio,the higher the transition temperature and the viscosity of fluid gel,the looser the microstructure.The rheological properties of LAG fluid gel were investigated.The results showed that the low acyl gellan concentration and temperature had a significant effect on the steady rheological behavior of low acyl gellan fluid gels.With the increasing of low acyl gellan concentration,the flow type of LAG fluid gel changed from the Newtonian fluid to shear thinning fluid.The higher the cooling rate and the lower shear rate,the higher the viscosity of LAG fluid gel.With the increasing of low acyl gellan concentration,the storage modulus G?and loss modulus G?increased.The fluid gel behaves like a liquid in the low content range,while it behaves like solid in the high content range.The higher the cooling rate and shear rate,the lower the dynamic modulus of fluid gel.During the cooling,the LAG fluid gel undergo the sol-gel transition.The results indicated that the gelation temperature increased with the increasing of low acyl gellan concentration.But the shear rate and cooling rate had almost no effect on the gelation temperature.The process method of fluid gel affects the thixotropy remarkably.The thixotropy of LAG fluid gels enlarged with the increase in low acyl gellan concentration and decrease in shear rate.With the increasing of temperature,the area of thixotropic loop increased firstly and then decreased.The transient results showed that the higher the low acyl gellan concentration,the harder the structure reconstruction.Moreover,the effects of ionic crosslinking and acidification on the rheological properties of LAG fluid gel were investigated.The results revealed that fluid gel is typical non-newton fluid.With the increase of ion concentration,the viscosity,dynamic modulus and transition temperature of fluid gel increased.The thixotropy enhanced with the increasing of Ca²⁺concentration.LAG/GDL weight ratio had almost no effect on the viscosity of fluid gel.Moreover,the fluid gel with different LAG/GDL weight ratio behaves like the solid.