Functions of iota-carrageenan on the gelatinization and retrogradation behaviors of corn starch in the presence or absence of various salts

Gelatinization and retrogradation behaviors of the aqueous starch/iota-carrageenan composite system (0.5 w/v% starch and 0.2% carrageenan) were investigated. Purified iota-carrageenan in sodium form was mixed with corn starch in the presence or absence of various salts. Functions of iota-carrageenan on starch depended on salts added. During gelatinization, the addition of iota-carrageenan lowered the incipient swelling temperature of the composite system with decreased peak viscosity in the absence of added salts and in the presence of monovalent cations, while the temperature increased in the presence of divalent cations. It also increased the rate constant for short-term (<24 h) retrogradation of the composite system with depressed gel-like characters in the presence or absence of salts. The effect of iota-carrageenan to increase the rate constant was the greatest in the absence of added salts and was greater in the presence of monovalent cations than divalent cations. The effect of the polysaccharide to depress gel-like characters was the greatest in the presence of LiCl and NaCl. The molecular mechanism was deduced from microscopy to explain these rheological behaviors mainly from the structural similarity between iota-carrageenan and amylose. Thermodynamic incompatibility between these two substances may explain the early onset of viscosity increase during gelatinization (i.e., upon heating) and the acceleration of subsequent short-term retrogradation due to increased effective concentration of starch or amylose, whereas physical interactions (both molecular complexes and steric hindrance) between the substances may explain the retardation of structural ordering during retrogradation (i.e., upon cooling) due to inhibited amylose aggregation.