COMPLEXES BETWEEN STARCH AND POLY(ETHYLENE-CO-ACRYLIC ACID) - VISCOSITY AND GEL RHEOLOGY OF JET-COOKED DISPERSIONS

Starch-poly(ethylene-co-acrylic acid) (EAA) complexes were prepared by jet-cooking cornstarch, waxy cornstarch, high amylose cornstarch (Amylomaize VII), wheat starch and potato starch in the presence of an aqueous ammonia solution of EAA. Much of the excess ammonia is volatilized during jet-cooking. and cooked dispersions had pH values of about 9, depending on the amount of EAA solution used relative to starch. Viscosities were continuously measured as dispersions were cooled from 90 to 22-degrees-C. Addition of as little as 0.5% EAA based on starch significantly increased the viscosity of a cooked starch dispersion, and viscosities increased with the amount of EAA added. The maximum viscosity and the shape of the viscosity versus time/temperature curve varied with the particular starch variety used. Hot starch-EAA dispersions were poured into molds to allow the formation of gels on cooling. Gel rigidity was measured by subjecting these gels to uniaxial compression in an Instron testing machine. The ability to form a gel depended on the starch variety used, but more importantly on the presence of EAA and the starch-EAA ratio. The most rigid gel tested was prepared from 5% Amylomaize VII (amylose content 70%) without EAA. Addition of 5% EAA based on starch softened the gel, indicating interference of EAA with amylose retrogradation during gelation. Contrary to these results, cornstarch (containing about 30% amylose) gave a firm gel at 5% solids when cooked with 5% EAA (based on starch), whereas gels too soft for testing were obtained with EAA levels less than 5%. With waxy cornstarch and potato starch, which do not form gels by themselves, EAA levels of 10% (based on starch) were required to obtain gels rigid enough to test. Addition of EAA may, therefore, either increase or decrease gel rigidity depending upon how readily a particular starch will form a gel by itself (through amylose retrogradation) in the absence of additives. Apparent cross-link densities were also calculated from compressional data.