Application of various polysaccharide gums to improve gelation and rheological properties of hydroxypropyl starch hydrocolloids

This work aims to develop starch-based low-cost vegetarian soft capsules used for medicine and health products through compounding with different polysaccharide gums. In this work, the phase diagrams, gelation behaviors, rheological properties, and microstructures of hydroxypropyl starch (HPS) blending with various polysaccharide gums, including ι and κ-carrageen (LC and KC), curdlan (CL) and agar (AG) have been investigated. The materials in both solution and hydrogel were systematically characterized in the review of practical processing and application based on the requirements of fabricating soft capsules. The phase diagram was firstly drawn based on critical gel concentration at 30–60 °C; the rheological behaviors of various blending systems were investigated under both oscillatory and dynamical conditions; and the gel microstructures were studied by SEM. The phase diagrams showed that HPS-AG has the lowest critical gel concentration while HPS-CL has the highest one since AG itself has shown the lowest critical gel concentration. The order of both critical gel concentration and viscosity of the HPS-based suspensions is HPS-CL > HPS-KC > HPS-LC > HPS-AG. In contrast, the gel strengths of HPS-based hydrogels are almost opposite to the order of viscosity HPS-AG > HPS-KC > HPS-LC > HPS-CL. Some of the blending systems (HPS-KC and HPS-CL) showed higher viscosity then the summary of the individual material, indicating some polymeric chain intergradation. All the compounding systems showed the porous structure of typical hydrogel, and cell size generally decreased after adding various gums. The hydroxyl bound and network between HPS and gums (carrageen and curdlan) results in higher gel strength of the blending system than that of any single substance. The relationships among the phase diagrams, microstructures, and rheological properties were compared and discussed. The results provide a guideline to design and develop capsules using multi-gums systems based on exploring their individual behaviors.