Structural and functional evaluation of modified native rice starch for tablet formulations

The Gorakhnath-509 variety of Nepalese-Indian rice, characterized by its high amylose content, was investigated as a source of a modified tablet binder through chemical modification of its starch using malonic acid in an aqueous medium without the use of organic solvents. Esterification of the starch was supported by the appearance of a new carbonyl absorption at 1720 cm⁻¹ in the infrared spectrum, while a reduction in the intensities of the characteristic A-type X-ray diffraction peaks was qualitatively consistent with alteration of the native semicrystalline structure. The degree of substitution, determined by titrimetry, was 0.081 ± 0.009 (n = 3). Qualitative swelling of the modified starch under aqueous conditions indicated altered hydration behaviour. Tablet formulations containing 1%, 3%, and 5% w/w modified starch were compared with corresponding formulations containing native starch at the same binder concentrations. At the 1% binder level, modified starch produced tablets with higher hardness (F2: 16.5 ± 0.46 N) and lower friability (0.15%) than the corresponding native starch formulation (F1: 15.66 ± 0.57 N; 0.37%). At the 3% and 5% binder levels, however, modified starch formulations exhibited lower hardness than their corresponding native starch formulations, indicating that the effect of modification on tablet hardness was concentration-dependent. Friability remained below the commonly applied pharmacopoeial acceptance criterion of 1.0% across all formulations. In vitro dissolution studies showed higher cumulative diclofenac sodium release from the modified starch formulations than from their corresponding native starch formulations, with F4 achieving approximately 96% release compared with 77% for F3. The Korsmeyer–Peppas model provided the best fit among the models evaluated (R² = 0.9897–0.9997), with release exponent values > 1.0 consistent with Super Case-II transport according to the model classification. Overall, malonic-acid-modified Gorakhnath-509 rice starch demonstrated concentration-dependent effects on tablet mechanical properties and sustained diclofenac sodium release, supporting its potential as a rice-derived pharmaceutical excipient for further investigation.