Fabrication of Zein Nanoparticle-Biopolymer Complexes to Deliver Essential Oils in Aqueous Dispersions

Zein, alcohol-soluble corn proteins, easily precipitate as nanoparticles after dispersing a stock zein solution with a polar solvent into water. The process can be used to nanoencapsulate essential oils (EOs). However, the stability of hydrophobic zein nanoparticles is a challenge for their application in aqueous food systems. The goal of this dissertation reseach was to stabilize zein nanoparticles in aqueous dispersions by forming complexes with water-soluble biopolymers. The stabilization of zein nanoparticles was first studied using sodium caseinate by dispersing hot 50% aqueous ethanol solution with both polymers in water. The nanoparticles contained κ-casein and zein, and the spray-dried nanoparticles were easily re-dispersed in water with good stability at pH 7.0 during storage. Before advancing to encapsulate EOCs, the distribution of free EO components (EOCs) in milk was studied in relation to their antimicrobial activity. It was found that, when the concentration of EOCs in the milk serum was above the minimum bactericidal concentration (MBCs) for Listeria monocytogenes Scott A (established in tryptic soy broth), complete inhibition was observed in milk. Conversely, when zein/casein complexes were used to co-encapsulate eugenol and thymol, the bactericidal effect against Escherichia coli O157:H7 and bacteriostatic effect against L. monocytogenes were observed at concentrations of EOCs in the milk serum well-below their corresponding MBCs. This suggested a maintenance of, and possible synergistic activity, of EOCs after nanoencapsulation. The spray-dried nanocapsules with co-encapsulated thymol and eugenol were easily hydrated in water as stable dispersions with particles smaller than 200 nm. Because zein/casein nanocomplexes were stable only at around neutral pH, gum arabic (GA) was also studied as another biopolymer to form complexes with zein nanoparticles. Zein was pre-dissolved in propylene glycol, with peppermint oil as an EO model, and dispersed in water to form nanoparticles. The subsequent addition of GA formed nanocomplexes smaller than 200 nm and stable at pH 3.0-8.0. A gradual release of peppermint oil from freeze-dried samples was observed at pH 2.0-8.0, with a faster release at lower pHs. Therefore, water-soluble biopolymers such as caseins and GA can stabilize zein nanoparticles to enable the delivery of bioactive compounds in aqueous food dispersions.