Rheopectic Behavior for Aqueous Solutions of Megamolecular Polysaccharide Sacran

Abstract

The rheopectic behavior of sacran aqueous solutions, a natural giant molecular polysaccharide with a molecular weight of 1.6 × 107 g/mol, was investigated. When a low shear was applied to 1.0 wt.% sacran solution, the shear viscosity increased from 7.2 to 34 Pa·s. The increment in the viscosity was enhanced as the shear rate decreased. The shear viscosity was independent of the time at a shear rate of 0.8 s−1; simultaneously, thixotropic behavior was observed at shear rates higher than 1.0 s−1. A crossover was observed at 0.15 wt.% for the concentration dependence of both the viscosity increase and zeta potential, which was the vicinity of the helix transition concentration or gelation concentration. It was clear that the molecular mechanism for the rheopexy was different at lower and higher regions of the crossover concentration.

The Use of Megamolecular Polysaccharide Sacran in Food and Biomedical Applications

“…Sacran is a sulfated natural polymer extracted from the extracellular jelly matrix of A. sacrum , a cyanobacterium in freshwater [ 26 ]. Sacran contains 11 types of monosaccharides (Glc, Gal, Pria, Xyl, Rha, Fuc, GalA, GlcA, and traces of Ara, GalN, and Mur), with a molecular weight of 1–2.2 × 10 7 g/mol and a chain length of more than 30 μm [ 27 , 28 ].…”

“…In solution, sacran is a polyelectrolyte, and because of its electric charge, the conformation of the sacran chain changes depending on its concentration. For instance, a helix transition concentration occurs when a chain of sacran changes from a random coil into a double helix at 0.09%, and gelation concentration occurs when sacran transitions from liquid to gel at 0.25% [ 28 , 29 ]. With increasing concentration, the sacran chain changes into a rigid rod form, showing liquid crystalline (LC) properties [ 30 , 31 ].…”

“…Sacran is a heteropolysaccharide composed of various sugar residues. From 11% of the monosaccharides, it contains a sulfate and carboxyl group [ 28 , 33 ]. Sacran is considered a safe biomaterial as A. sacrum is a cyanobacterium that has long been used by the people of Kyushu, Japan as a food to treat allergies and gastroenteritis.…”

“…A hydrogel film that has been generated by sacran at a 0.5% w / v concentration produced a film with a thickness of 0.05 mm and a 20 Wt/Wo swollen ratio, which is considered a large value [ 23 ]. Yusof et al [ 28 ] in their research also showed that the observed flow properties of sacran solution had a low rate of shear in which its shear viscosity increased with time [ 50 ], and it was constant in an increased viscosity of 6 Pa.s. for 900 s. The increased viscosity improved with the decrease in the rate of shear.…”

“…Sacran contains several sugar residues such as galactose, glucose, mannose, and galacturonic acid. In addition, a high content of alanine, galactosamine, and muramic acid is also found in sacran [ 28 , 33 ]. Sugars are very important ingredients for the human body, because sugar is one of the important components that regulate the body’s physiological functions.…”