Changes in starch structure during manufacturing of starch microspheres for use in parenteral drug formulations: Effects of temperature treatment

Starch microspheres were produced by emulsification of a starch dispersion in an aqueous polyethylene glycol (PEG) solution. Crystalline/ordered structure was formed within these starch droplets during incubation at 6 °C for 25 h followed by incubation at 37 °C for 28 h. After incubation at 37 °C the crystalline structure in the samples was of type B. The crystallization process of microspheres was compared with crystallization in a model system. The crystalline structure of the microspheres melted at temperatures almost 20 °C lower than in the model system incubated under the same conditions, as determined by differential scanning calorimetry. It was thus concluded that the crystallization process within microspheres was different than that of bulk starch and the ability of the starch molecules to reorganize themselves within the dispersed starch phase of an aqueous two-phase system at the higher incubation temperature was limited. It was also observed that the presence of PEG or carbonate buffer protected the molecular order formed by the starch molecules during incubation from breakdown during freeze-drying.