Shear-Induced Aggregation and Ordering of Amylopectin Dispersion Affected by Hydrocolloids

Fang Fang, Purdue University

Abstract

Amylopectin is a naturally highly-branched biomacromolecule. Amylopectin molecules with re-ordered structure have high viscosity and viscoelastic properties that may have potentially good nutritional functions such as providing long-lasting energy to the human body and delaying stomach empty among other properties. However, the amylopectin molecular re-ordering process under static conditions normally takes much longer time than amylose to achieve a desired and relatively stable structure. A simple but novel approach consisting in applying a constant shear rate of 20 s-1 at a temperature of 5°C for 24 hours was proposed in this study to accelerate the re-ordering process of gelatinized waxy amylopectin dispersions. Two different shear-induced mechanisms were studied: the first, which included shear-induced aggregation of selected waxy amylopectin dispersions occurring within a short period of time and the second that was due to shear-induced molecular re-association after a 24-hour period of shearing with a constant shear rate of 20 s-1 at a temperature of 5°C. Waxy potato and waxy corn amylopectin dispersions displayed shear-thickening behavior in a moderate shear rate range (approximately 15 to 25 s-1 ), a phenomenon that was not observed in waxy rice amylopectin dispersion. The rheology of the former dispersions was shear rate, time and temperature dependent. At low temperatures, the shear-thickening behavior of potato amylopectin was more noticeable than that observed in the waxy corn amylopectin dispersion. After shearing with a shear rate of 20 s-1 for 10 min, no aggregation was formed in waxy rice amylopectin, whereas small aggregates and large aggregates were observed in waxy corn and waxy potato amylopectin dispersions, respectively, indicating that shear-induced aggregation may be the reason for the shear-thickening behavior of these dispersions. The presence of neutral and anionic hydrocolloids significantly influenced the shear-thickening behavior and viscoelasticity of waxy potato amylopectin dispersions. Hydrocolloids with flexible chains were hypothesized to associate with long-chain branches of amylopectin, which was promoted by shear forces. The electrostatic interactions between flexible chains of anionic hydrocolloids (e.g. pectin and sodium alginate) and chain segments of potato amylopectin binding phosphate groups were likely to increase the molecular movement of chains and decrease the viscosity and elasticity of these waxy amylopectin dispersions. However, the enhanced mobility favored the formation of shear-induced aggregation upon the application of shear forces. The interactions between neutral hydrocolloids (e.g. guar gum and konjac glucomannan) and waxy potato amylopectin dispersions were hypothesized to be mainly through hydrogen-bonding interaction, chain-chain associations and likely entanglements showing an increased viscosity with a concomitant change in their viscoelastic properties characterized by a lower elasticity. The presence of anionic hydrocolloids with rigid chain conformation (e.g. agar and xanthan gum) in waxy potato amylopectin dispersions lead to products with strong shear-thinning behavior and a higher elasticity. The interaction between potato amylopectin and highly-branched hydrocolloids (e.g. gum arabic) was mainly due to hydrogen-bonding. The presence of gum arabic in waxy potato amylopectin dispersions enhanced the shear-thickening behavior and decreased their elasticity. These apparently contradictory results were explained noting that although an increased mobility of the chains by the addition of the hydrocolloid implies less elastic samples, the action of shear forces on more mobile chains may increase the likelihood of interaction between them. The shear-thickening behavior did not change upon storage of waxy corn amylopectin dispersions up to 7 days, but disappeared in waxy potato amylopectin dispersions after 7-day storage at 4°C and under static conditions. During long-time storage, waxy potato amylopectin dispersions were more ordered than other dispersions prepared with waxy corn and waxy rice amylopectin. The formation of a more ordered structure in waxy potato amylopectin dispersions was related to the characteristics of their chains, i.e. longer-chain branches and a lower degree of branches than the other amylopectin used in this study. The formation of ordered structures in waxy potato amylopectin dispersions was a function of pH, which showed that electrostatic interactions caused by the negatively charged phosphate groups in waxy potato amylopectin was also a contributor factor to the structure forming mechanism. The re-association of potato amylopectin was significantly accelerated by applying a constant shear rate of 20 s-1 for 24 hours, a phenomenon that was not observed in waxy corn amylopectin and waxy rice amylopectin dispersions. It was hypothesized that the mechanism of this phenomenon was the result of the stretching of long-chain branches of the waxy potato amylopectin molecule by a moderate shearing action to a conformation that was prone to form double-helical structures. (Abstract shortened by ProQuest.)

Degree

Ph.D.

Advisors

Hamaker, Purdue University.

Subject Area

Food Science

COinS