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

摘要

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 M_w of the guar samples ranged from 4.7 x 10~5 to 34.6 x 10~5 g/mol, whereas z-average root-mean-square radius of gyration R_g ranged between 82 and 233 nm from static light-scattering technique. Relationship between M_w and R_g 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 M_w determined, and relationship between M_w 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 M_w values higher than 12.2 X 10~5 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 M_w of guar, while the increase in peak viscosity was dependent on its M_w. 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 ypothesized as one of the factors to govern the gelatinization behavior of starch.