Date Published: February 2, 2017
Publisher: Public Library of Science
Author(s): Anna Ptaszek, Paweł Ptaszek, Marek Dziubiński, N. Mirosław Grzesik, Marta Liszka-Skoczylas, Xiao-Dong Wang.
This research study analysed the rheological properties of potato amylose and potato amylopectin in binary solutions of the following water and dimethyl sulfoxide concentrations: 90% DMSO (1), 80% DMSO (2) and 50% DMSO (3), with preparation methodology involving the dissolution at the temperature of 98°C. The studies of dynamic light scattering on the biopolymer coils and the determination of main relaxation times of the solutions were carried out. For the amylose solutions, the fast relaxation phenomena are predominant. The results of the quality tests of the hysteresis loop showed, that the amylose solutions in the solvents (1) and (2) are rheologically stable and shear-thickened. The amylose solutions in solvents (3) reveal oscillatory alterations of viscosity in the time. Amylopectin solutions are characterized by 80% share of slow relaxation phenomena, very low diffusion coefficients and hydrodynamic radii in the range of 2000 nm. The amylopectin solutions are rheologically unstable.
The structure-forming properties of starch are a result of its composition, and the interactions between its polysaccharide chains and the solvent’s molecules. There are only few known systems in which starch can dissolve , i.e., in water, dimethyl sulfoxide (DMSO) and N,N-dimethyloacetamide supplemented with LiCl. Dissolution of starch requires a total destruction of the starch granule, which is composed of layers of crystalline structure, as well as amorphous layers. As a result, a solution containing both linear and branched chains is obtained. The chains’ conformation, as well as their behaviour, are distinct and depend on the type of the solvent. Amylose (AM), which in aqueous solutions adopts a helix conformation, undergoes a process of gelation immediately after it is released from a granule; the gelation process begins upon crossing the c* concentration (overlap concentration) [2, 3]. Amylopectin (AP) chains adopt a coil conformation in water; the coils form a network due to the entanglement of the side branches. The values of average radii of gyration and weight average molecular mass obtained in the experiments with static light scattering (SLS)  and with the help of chromatography studies, are comparable  and indicate, that amylopectin assumes a spherical or globular conformation in DMSO, whereas amylose adopts a conformation of an elastic chain.
The research materials comprised solutions of starch (1% and 5% w/w) in a binary solvent, formed of water and DMSO at various ratios . Pure potato amylose (Sigma, Germany) and pure potato amylopectin (Eliante Avebe, the Netherlands) were used in this research.
The dissolution of potato amylopectin, or amylose, in the mixture of DMSO and H2O in the ambient temperature turned out to be ineffective (Method I). The values of the second osmotic virial coefficient, determined for the solutions of the analyzed starches in pure DMSO and in water at 30°C are low ; this explains difficulties in dissolution of starch in lower temperature conditions. Only when the temperature increased to 100°C, the solutions became clear (Method II).
The rheological properties of AP and AM solutions in binary solvents are quite different from each other. In the case of AP solutions, certain characteristics—defined as time-dependent—can be observed. The AM solutions behave as shear-thickened, while AP solutions produce shear-thinned systems. Moreover, amylose solutions are characterised by decidedly lower apparent viscosity, compared to the corresponding amylopectin solutions.
The results of the quality tests of the hysteresis loops revealed, that AM solutions in solvents formed of a mixture of H2O and DMSO with lower water concentration, are rheologically-stable fluids and exhibit a shear-thickening phenomenon. The AM solutions in a solvent composed of H2O and DMSO at the ratio of 500/500, display complex rheological behaviours manifested by oscillatory changes of viscosity in the function of shear time (at a stable shear rate). Due to a small radius of gyration and low expansion coefficient, a deformation of the AM coil during shear is possible, which enables the accumulation of energy. The AP solutions are rheologically unstable fluids. The results demonstrate, that instability of the starch solutions stems from the phenomena of energy accumulation. The capability of chains to accumulate energy results from the conformation they adopt in the analyzed solutions.