Food hydrocolloids control the gelatinization and retrogradation behavior of starch. 2a. Functions of guar gums with different molecular weights on the gelatinization behavior of corn starch

Gelatinization behavior of starch in an aqueous system was studied in the presence or absence of various guar gum samples with different molecular weights in order to clarify the difference in functions of each guar to starch. Some native corn starches with different amylose/amylopectin ratios were used to clarify which fraction of starch the gums act on. Weight-average molecular weight Mw of the guar samples ranged from 4.7×105 to 34.6×105 g/mol, whereas z-average root-mean-square radius of gyration Rg ranged between 82 and 233 nm from static light-scattering technique. Relationship between Mw and Rg was expressed by the Flory exponent of 0.503. Also, intrinsic viscosity [η] of the guar samples in aqueous solution was in good agreement with Mw determined, and relationship between Mw and [η] was expressed by the Mark-Houwink-Sakurada exponent of 0.517. Gelatinization behavior of starch/guar system was investigated from its viscosity profiles. Guars with Mw values higher than 12.2×105 g/mol shifted the onset of viscosity increase for the system to lower temperatures and increased its peak viscosity upon heating at a relatively low starch concentration (e.g. 5 w/v%). The earlier onset of viscosity increase was independent of Mw of guar, while the increase in peak viscosity was dependent on its Mw. These guars shifted the onset of viscosity increase for the system upward, on the contrary, at a relatively high starch concentration (e.g. 15%). Interaction between guar and starch components, amylose and amylopectin, was hypothesized as one of the factors to govern the gelatinization behavior of starch.