Microstructure and rheological properties of psyllium polysaccharide gel

The strong gelling property of psyllium polysaccharides is closely related to its health benefits and applications, such as use as a binding agent in the landscape industry. However, information about gelling properties of psyllium polysaccharides is very limited. To explore the gel properties of psyllium, the alkaline extracted gel fraction (AEG) of psyllium polysaccharides was studied in this investigation. The rheological and low-temperature electron microscopy methods including cryo-scanning electron microscopy (Cryo-SEM) and freeze-substitution for transmission electron microscopy (TEM) were used to investigate Ca2+ influence on the gel properties of AEG. AEG formed a weak gel with a fibrous appearance. AEG did not have a sharp melting point and exhibited no thermal hysteresis during heating and cooling procedure. The origin of this gelling behaviour was due to the fibrillar gel structure of psyllium polysaccharide. It was found that Ca2+ had a significant influence on the gel properties and microstructure. Elastic modulus G′ of gel increased as Ca2+ concentration increased. Critical strain S at first increased and then decreased as Ca2+ concentration increased. AEG gel became more resistant to temperature change on addition of Ca2+. Psyllium gel changed to aggregated gel with added Ca2+. The strands of gel appeared thicker and the density of the junction zone increased with increasing Ca2+ concentration as revealed by SEM and TEM. The changing bulk gelling properties of psyllium polysaccharide by adding Ca2+ was attributed to the change in gel structure.