Rheology and suspending properties of modified tapioca starches

Five modified tapioca starches, sodium carboxymethyl tapioca starch (SCMS), cross-linked sodium carboxymethyl tapioca starch (CSCMS), pregelatinized tapioca starch (PGTS), hydroxypropyl tapioca starch (HPTS) and cross-linked hydroxypropyl tapioca starch (CHPTS) were investigated for their properties as suspending agent comparing with commercial suspending agent such as Ultrasperse®2000 (UT), xanthan gum (XG) and tragacanth (TG). SCMS with DS of 0.11, 0.22 and 0.37 were prepared by Filbert’s method and CSCMS with % phosphate content of 0.17, 0.23 and 0.32 were obtained from Kerr and Cleveland’s method. Those materials were studied for their viscosity and rheological behavior. The pseudoplastic flow was observed in all suspending agents. The effect of ionic strength, pH and type of buffer (phosphate and citrate buffer) on the viscosity and rheological behavior of those materials was evaluated. CSCMS was excluded from the study because it was very sensitive to ionic strength, buffer and pH. SCMS, PGTS, HPTS, CHPTS, UT, XG and TG at three level concentrations were used as suspending agents in ibuprofen formulation. Their physical appearance, pH, viscosity, sedimentation volume and redispersibility were evaluated to find the proper concentration of those suspending agents after keeping in room temperature for 1 month. The ibuprofen preparations containing 3 % w/v SCMS, 4 % w/v PGTS, 6 % w/v HPTS, 3 and 4 % w/v CHPTS provided the preferable suspension comparing with 4 % w/v UT, 0.4 and 0.6 % w/v XG and 1 % and 1.5 % w/v TG. The evaluation parameters including physical appearance, sedimentation volume, pH, viscosity, rheological behavior, redispersibility and % drug content were investigated during storage at room temperature and heating-cooling condition. All preparations provided the acceptable chemical stability after standing 3 months at room temperature and 8 cycles of heating-cooling condition. SCMS, PGTS and HPTS were not suitable as suspending agent because they obviously decreased the viscosity after storage. Moreover, the difficult redispersibility of compact sediment was occurred in their preparations. CHPTS was a promising suspending agent in the preparation due to its good physical appearance, redispersibility, no caking, suitable rheological behavior and comparable to those of UT, XG and TG. CHPTS was a good modified starch candidate for the development as suspending agent in the pharmaceutical industry.