Microencapsulation of Forsythia suspensa Essential Oil Based on Complex Coacervation

Abstract

Plant essential oils possess valuable antibacterial and antioxidant properties, but their high volatility and chemical instability severely limit their direct use in functional materials. In this study, Forsythia suspensa essential oil was successfully encapsulated within a gelatin wall material by complex coacervation, with sodium hexa-metaphosphate as a coacervation promoter and transglutaminase as a green crosslinking agent. The influence of the core-to-wall ratio, coacervation temperature, pH and stirring speed on the encapsulation efficiency was first evaluated through single-factor experiments, after which a three-factor, three-level Box–Behnken design was employed to optimize the dominant variables. A reduced hierarchical quadratic model was statistically significant (p = 0.0084; R² = 0.7724; adjusted R² = 0.6359), with the response governed primarily by significant quadratic curvature in all three variables; the individual linear effects were not statistically significant and were therefore not ranked. Under the optimised conditions of a core-to-wall ratio of 1:1.1, a temperature of 39 °C and a pH of 4.69, a predicted encapsulation efficiency of 97.52% was obtained. The resulting microcapsules displayed a particle size distribution of 0.25–10 µm (D90 = 4.76 µm) and a zeta potential of −32.25 mV, indicating appreciable electrostatic repulsion. SEM and AFM analyses revealed predominantly rounded micron-scale particles with smooth continuous surfaces (mean diameter 2.38 µm; RMS roughness 1.245 nm). Microcapsules displayed clear sustained-release behaviour and were successfully deposited onto cotton and linen textiles by a pad–dry–cure process.