Solid State Characterization of Two Tropical Starches Modified by Pregelatinization and Acetylation: Potential as Excipients in Pharmaceutical Formulations

Abstract

Aims: To pre-gelatinize, acetylate and characterize Dioscorea dumetorum (bitter yam) and Dioscorea oppositifolia (Chinese yam) starches using solid-state parameters such as differential scanning calorimetry (DSC), X-ray powder diffraction (PXRD), Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance spectroscope (1H-NMR) and scanning electron microscopy (SEM). The pH, swelling, density, flow and rheological properties of the starches were also determined. Commercial native corn and pre-gelatinized potato starches were used as standards. Methodology: Pregelatinization and acetylation were conducted by thermal and chemical modification respectively. Acetylated starches of bitter and Chinese yams with degrees of substitution of 2.56 and 2.70 respectively were obtained. The presence of acetyl groups was confirmed by FT-IR, 1H NMR spectroscopy, SEM, DSC and PXRD. Results: The DSC endotherms revealed that pre-gelatinized bitter and Chinese starches had higher gelatinization temperatures than the native forms (111.56 to 123.52°C and 101.14 to 108.88°C respectively), whereas the acetylated starches had lower gelatinization temperatures (84.69°C and 69.85°C respectively). The modified starches showed lower enthalpies (ΔH values 154.0 to 91.91 J/g and 263.57 to 120.70 J/g for the respective pre-gelatinized starches; 8.09 J/g and 7.02 J/g for the respective acetylated starches). Acetylation was confirmed by FT-IR and NMR spectroscopy. The PXRD spectra showed characteristic semi-crystalline structures of the native starches (2θ between 13°C and 23°C) were disrupted by acetylation and pregelatinization, forming amorphous structures. Viscosity of 5% slurry of the starches decreased with increasing shear rate; flow curves were typical of non-Newtonian shear thinning fluids. Bitter and Chinese yam starches had lower bulk density (0.33±0.02 and 0.38±0.01 gcm-3 respectively) and better compressibility (40±2.00 and 27.17±2.18% respectively) relative to commercial native corn, suggesting their potential effectiveness as binders in tablets. Pre-gelatinized yam starches had higher (p<0.01) swelling than their native forms and standards, suggesting their potential as disintegrants. Acetylation altered the nature of the starches from hydrophilic to hydrophobic and reduced swellability, suggesting their suitability for controlled release application. Conclusion: Bitter and Chinese yam starches in their native, pre-gelatinized and acetylated forms could find applications as cheaper alternative excipients in various pharmaceutical formulations.