Linear viscoelatic retardation phenomena in selected polysaccharides systems
Advancing the chemical engineering fundamentals
Rheology (T2-4)
Keywords: viscoelasticity, Burger model, retardation, polysaccharides
Polysaccharides are main constituent of human’s food. They are used also as food texturing agents and to grant food with proper resistance. Polysaccharides, due to their biodegrability and renewability found wide applications in pharmaceutical, ceramic, and textile industries, and also in biotechnology. Discussed compounds are characterized by very complicated structure, and exhibit many interesting, from practical point of view, rheological phenomena. Complex hydrodynamic behavior of polysaccharides and their solutions allows to apply them as pilot testing materials in emerging technologies. It seems to be reasonable to undertake researches dealing with viscoelastic phenomena of water based solutions of polysaccharides. It gives unequivocal opportunity to characterize investigated material in linear area, and also it is start point to model phenomena connected with non-linear viscoelasticity and thixotropy.
The aim of present study was to determine the viscoelastic properties of the model mixtures of the selected pasted polysaccharides. The initial material was corn starch (CS), mixed with guar gum (GG).
Starch is plant storage material, which is composed of two alpha glucans: essentially linear amylose, and branched amylopectin. The linear amylose (chain length 500 to 6000 glucose units) has a small degree of branching but it is predominantly regarded as a single chain. The chain of amylopectin contains only up to 30 glucose units. Regular corn starch (CS) produced by National Starch was used in this research. The production of guar gum bases on its extraction from the leguminous shrub (Cyamopsis tetragonoloba). By means of chemical compound guar is a galactomannan with (1-4)-linked beta-D-mannopyranose backbone with branch points from their 6-positions linked to alfa-D-galactose. Guar gum (GG) was supplied by Regis (Poland).
Rheological measurements were performed using rheometer RS-150 (Haake, Germany) equipped with coaxial cylinder system. Temperature was controlled by ultratermostat F9 (Haake, Germany) with 0.1˚C accuracy. The principle of rheological measurements was to search for linear viscoelastic area by step function, and finally to create creeping curves. This investigation relied on conducting few deformation measurements in time function for different stress tau0 values. Next, the function of susceptibility for creeping was calculated according to following formula: J(t) = gamma(t)/tau0 (gamma - strain, t - time) and if obtained curves matched each other that indicated on linear viscoelasticity of the system. Obtained in such way curves, also known as creeping susceptibility curves, were used for further analysis. The measurements were executed in the following manner: starch paste or hydrocolloid solution (95˚C, prepared in water bath (95˚C ±1˚C) and mixed for 30 min., was placed in measuring cylinder previously heated to 95˚C Next sensor was cooled down to desired temperature during one hour, and was kept at this temperature for the next 30 minutes. Next measurement was performed.
One of the relationships describing retardation phenomenon is continuous Burger model, which was used here. It contains two unknown parameters in Maxwell's element and unknown continuous function L(lambda), so called retardation spectrum, in Kelvin-Voigt's element (lambda - retardation time).
To estimate model parameters, regularization method of Tikhonov was used. The most often used idea in determining of estimators of mathematical model parameters are minimization of square sum of difference between observed value and calculated value based on model. There are many modifications of this method; one of them is called regularization method. It is applied, when analysed problem belongs to group of so called “ill posed”. Such problems are very sensitive to data fluctuation. Even small such event may lead to results diametrically opposed to original data. Such disturbances have no influence on solution of well-posed problems. Basing on regularization method set of numerical procedures was created, allowing to estimate retardation spectra L(lamda) of viscoelastic materials. Moreover there was possibility to calculate error of estimation.
It is concluded, that starch pastes and mixtures of gelatinised starch and hydrocolloid diversified viscoelastic properties exhibited. Standard Burger rheological model could be applied to describe viscoelastic properties of pastes and their mixtures with hydrocolloids. Data from experiments with step function for starch–hydrocolloid systems continuous viscoelastic models could be fitted.
See the full pdf manuscript of the abstract.
Presented Thursday 20, 16:00 to 16:20, in session Rheology (T2-4).
