Rheological properties of native and hydrolyzed starches in dependence on composition and concentration

The structure formation of starch polysaccharides in aqueous solutions is determined by the ratio of amylose to amylopectin and the molecular properties of these components. Our research is focused on establishing defined correlations between composition, molecular structure in diluted solutions and rheological properties of concentrated aqueous starch polysaccharide solutions. Diluted solutions were investigated by size exclusion chromatography with multi angle laser light scattering detector. Measurements of concentrated aqueous solutions were carried out by a Bohlin cs-rheometer with programmed stress using a cone-plate geometry of 40 mm diameter and a cone angle of 4 degrees. Gels were characterized by oscillatory measurements taking into account the frequency dependence of the storage and loss moduli and the influence of a stress sweep on the moduli. The concentration dependence was investigated with starches of potato, wheat, maize and wrinkled pea. Starches with quite similar amylose content as from potato, wheat and maize, show different behavior in rheological properties. Further differences in structure formation were obtained by enzymatic hydrolysis of potato and wheat starch with bacterial alpha-amylase. The hydrolyzing conditions were chosen such that the degradation led to molecular weights between 5*10(5) and 10(7) g/mol. Detailed information about molecular composition was obtained by fractionation of degraded starches. The amylopectin was found to be degraded more strongly than the contained amylose. In comparison to native starch polysaccharide fractions the amylopectin hinders the gelation process in dependence on its molecular weight distribution and the length of the outer chains.