Encapsulation of Nigella sativa Essential Oil by Complex Coacervation Using Gelatin and Gum Arabic

This study aimed to encapsulate Nigella sativa essential oil (NSEO) using gelatin (Ge) and gum arabic (GA) via complex coacervation to produce NSEO microcapsules (NSEO-MCs), optimize encapsulation conditions, characterize the microcapsules, and evaluate antimicrobial activity. Response surface methodology (RSM) based on a Box-Behnken design was used to optimize pH, wall material concentration, and oil concentration. Optimal conditions were pH 4.09, wall material concentration 1.52%, and oil concentration 0.64%, yielding a predicted encapsulation efficiency (EE) of 87.38%. Freeze-dried microcapsules exhibited moisture content (MC) of 4.65%, hygroscopicity of 13.08%, water solubility (WS) of 19.44%, and moderate flowability, with a Carr's index (CI) of 17.78% and Hausner ratio (HR) of 1.22. X-ray diffraction (XRD) revealed an amorphous character in NSEO-MCs, with increased diffraction intensity at 2θ ≈ 20° attributed to the compact Ge-GA coacervate matrix. Fourier transform infrared spectroscopy (FTIR) confirmed successful encapsulation through attenuation of NSEO-characteristic C─H stretching bands and Amide I profile changes, indicating noncovalent interactions between Ge and GA. Thermogravimetric analysis (TGA) demonstrated enhanced thermal stability, with the derivative thermogravimetry (DTG) peak shifting to higher temperatures and broadening relative to free NSEO, confirming the protective role of the wall matrix. Antimicrobial evaluation by agar well diffusion showed dose-dependent inhibitory activity of both NSEO-MCs and free NSEO against Escherichia coli and Staphylococcus aureus. No significant difference was observed between NSEO-MCs and free NSEO at any tested concentration, confirming that encapsulation preserved antimicrobial efficacy. These results indicate that Ge-GA complex coacervation effectively protects NSEO while retaining its bioactive properties.