Effects of non-ionic polysaccharides on the gelatinization and retrogradation behavior of wheat starch

Gelatinization and retrogradation behavior of wheat starch in an aqueous system was studied by rheological and thermal techniques in the presence or absence of non-ionic polysaccharides, including guar gum, tara gum, locust bean gum, and konjac glucomannan. Macromolecular characteristics of each polysaccharide, including weight-average molecular weight Mw and radius of gyration Rg, were determined by static light-scattering, resulting in (1.0-3.2)×106 g/mol for Mw and 104-217 nm for Rg, respectively. During gelatinization, addition of each polysaccharide (0.5-1% w/v) increased peak viscosity for the starch system (13%): 163-231 unit larger than the control at 0.5%, whereas 230-437 unit larger at 1%. Among the galactomannans tested, the order of this effect (locust>tara>guar) was contrary to that of the molecular size (guar>tara>locust). During short-term retrogradation, addition of each polysaccharide (0.5%) increased dynamic mechanical loss tangent (tanδ) for the starch system (5%) after storage at 4 °C for 24 h: (16.5-26.9)×10−2 unit larger than the control. Among the galactomannans tested, the larger the molecular size, the greater the effect to increase tanδ, and this effect of polysaccharide was not explained simply by the difference in the amount of amylose leached during gelatinization. During long-term retrogradation, addition of each polysaccharide (0.5%) decreased the rate constant expressing the relationship between storage time (for 14 days at 4 °C) and creep compliance for the starch system (15%): (0.9-1.5)×10−2 unit smaller than the control. Among the galactomannans tested, the larger the molecular size, the greater the effect to decrease the rate constant. Functions of polysaccharide to starch were hypothesized considering structural compatibility and molecular interactions between polysaccharide and starch components; amylose and amylopectin.