Stability and interfacial viscoelasticity of oil-water nanoemulsions stabilized by soy lecithin and Tween 20 for the encapsulation of bioactive carvacrol

Abstract

The rheology of oil-in-water (O/W) droplet interfaces stabilized by food-grade emulsifiers (soy lecithin or Tween 20) under controlled aqueous conditions was investigated to elucidate its contribution in the kinetic stabilization of nanoemulsion-based delivery systems containing carvacrol, a naturally-derived antimicrobial compound. Dilational rheology of surfactant-laden O/W interfaces was measured using axisymmetric drop shape analysis. The kinetic stability of corresponding nanoemulsions (containing mixtures of carvacrol and medium-chain triglyceride (MCT) oil dispersed in water (pH 7)) was characterized using dynamic light scattering. Zwitterionic lecithin molecules adsorbed to the O/W interface for 24h formed a notably viscoelastic layer, compared to nonionic Tween 20 molecules. The kinetic stability within the first 24h for each nanoemulsion was strongly dependent upon encapsulated carvacrol concentration, with higher carvacrol concentrations leading to lower kinetic stability. Lecithin-stabilized nanoemulsions (pH 7) were highly stable, yielding monodispersed droplet size distributions and high resistance to increases in droplet size over 30days. Contrarily, corresponding Tween 20-stabilized nanoemulsions showed large increases in the droplet size and developed a bimodal droplet size distribution over time. The initial size of oil droplets stabilized by lecithin was slightly dependent on pH, yielding smaller droplets at pH 7 and larger droplets at pH 3; however, the extended kinetic stability was not greatly impacted by pH modulation. Determining a positive association between interfacial viscoelasticity and nanoemulsion stability may potentially be very useful for food manufacturers seeking to optimize the encapsulation and delivery of lipophilic antimicrobial molecules using food-grade emulsifiers.