Effect of freeze-drying and oven-drying on the physicochemical and morphological properties of marjoram essential oil beads produced by ionic gelation

Abstract

This study explored the impact of different drying methods – freeze-drying (FD) and oven-drying (OD) – on the physicochemical properties of marjoram essential oil (MEO) beads produced by ionic gelation. Microencapsulation was performed by extrusion–dripping of emulsions containing 2 g/100 g MEO, 1.25 g/100 mL sodium alginate and 1.25 g/100 mL whey protein isolate into a 0.175 mol L⁻¹ CaCl₂ gelling bath. The beads were dried either by freeze-drying (FD) for 24 h or by hot-air oven drying (OD) at 60 °C for 24 h. After drying, the beads were characterized in terms of encapsulation efficiency, chemical composition by GC-MS and GC-FID, moisture content, and thermal degradation by thermogravimetric analysis (TGA). The beads' morphology was evaluated by scanning electron microscopy (SEM). FD beads retained a higher MEO content (51.25%) compared to OD beads (33.75%) (p < 0.05). Morphological analysis indicated a brittle structure and larger pores in FD beads, whereas OD beads exhibited a denser and less porous surface. Moreover, FD beads showed higher thermal stability than OD beads. The study concludes that the choice of drying technique plays a crucial role in the beads' physicochemical and morphological properties. Overall, the results demonstrate that the drying method plays a crucial role in the physicochemical and morphological properties of the beads, with freeze-drying being more effective in preserving essential oil content and thermal stability.