Dynamic Viscoelastic Properties of Potato Starch Solutions in 1N Potassium Hydroxide

Abstract

The dynamic modulus and dynamic viscosity of a potato starch solution in 1N KOH were measured in concentrations from 2.0 to 7.0g/100ml by using a forced torsional vibration rheometer of various frequencies at temperature 25°C. The similar measurement was applied to the starch solutions irradiated by gamma ray. The master curves of the reduced dynamic modulus and reduced dynamic viscosity were derived in fairly good single lines, according to Ferry's time-concentration reduction principle.
The concentration dependence of the dynamic modulus G'∝C^m, the dynamic viscosity η_d∝C^n', and of the zero angular frequency viscosity η/η₀∝Cⁿ were respectively determined. The authors' m values suggest that the network structure in the starch solutions is formed by some trifunctional crosslinkages. The n value generally indicates the degree of intermolecular interaction. The authors' n values lie between that of the gelatin solution and that of the collagen solution.
The relaxation spectra of the starch solutions were obtained in curves of nearly linear shapes, and those of the irradiated starch solutions shifted to shorter relaxation time with steeper inclinations in accordance with increasing total dose. It was suggested from the range of logτ observed that the relaxation spectra were in the flow region of the whole relaxation spectra, and that they were attributable to the relaxation mechanism caused by the macro-brownian movements of the starch molecules. It is to be noted that the relaxation spectra derived from the time-concentration reduction principle agree well with those derived from the time-temperature reduction principle which were reported for the solutions in formamide in the previous paper. This proves that the starch solutions in formamide and 1N KOH are thermorheologically simple, and that they can be treated as linear viscoelastic fluids. Moreover, the similarity in the relaxation spectra for both kinds of solvent may be due to the good solubility of the starch in these solvents.