INFLUENCE OF STARCH ORIGIN ON RHEOLOGICAL PROPERTIES OF CONCENTRATED AQUEOUS SOLUTIONS

The rheological properties of corn and potato starch concentrated aqueous solutions were investigated at 25 degrees C. The starches were previously dispersed in water and the solutions were obtained by heating of dispersions at 115-120 degrees C for 20 min. The solutions of potato starch were transparent, while the corn starch solutions were opalescent. The results of dynamic mechanical measurements showed that the values of viscosity eta, storage modulus G', and loss modulus G '', of the corn starch solutions increased with the storage time. This phenomenon was not observed for the potato starch solutions. It was assumed that the increase of eta, G' and G '' is the result of starch solutions retrogradation. The potato starch solutions retrogradation did not occur probably because of the phosphates presence. The viscosity of 2 mass% corn starch solution was less than the viscosity of 2 mass% potato starch solution. By increasing the concentration of corn starch solution the gel with elastic behavior was formed. The corn starch solutions formed gels as early as at 4 mass% concentration, while potato starch solutions achieved the gel state at the concentration of 5 mass%. The value of exponent m (G' and G '' proportional to omega(m)) during the transition of potato starch solutions to gel is 0.414, which gives the fractal dimensions for corn starch of 2.10. The obtained value of fractal dimension corresponds to slow aggregation. The corn starch solutions with the starch concentrations higher than 4 mass% formed weak gels. For these solutions, the values of modulus in the rubber plateau were determined. It was found that the modulus in rubber plateau increased with the concentration by the exponent of 4.36. Such high exponent value was obtained in the case when the tridimensional network is formed, i.e., when supermolecular structures like associates or crystal domains are formed.