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Development of Encapsulation Systems from Zein and Metal-Organic Frameworks (MOFs) for Improved Functional Properties of Essential Oils
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Essential oils (EOs), which are derived from plants, have antifungal, insecticidal and antimicrobial activities, but they are slightly soluble in water and impart to the water their odor and taste, which limit their applications in food area. Zein, a prolamin from corn, is able to form nanoparticles by liquid-liquid dispersion process. These nanoparticles are well dispersed in water and stable, which can be further applied to encapsulate functional materials that are insoluble in water. We have developed zein nanoparticles to encapsulate thymol and carvacrol in order to improve their solubility. The DLS (dynamic light scattering) and SEM (scanning electron microscopy) proved that zein nanoparticles encapsulated with EO were formed. The particles size was between 200~300nm before lyophilizing. 65-75% EOs have been encapsulated in the nano-sized particles. DPPH assay results proved good antioxidant property of the product. For the Ferric-ion spectrophotometric assay, hydroxyl free radicals had been cleared by 60~90% in overall. In the antimicrobial experiment, the nanoparticles encapsulating EOs reduced 0.8-1.8 log units of E. coli after 48h incubation. Furthermore, we have applied Metal-Organic Frameworks (MOFs) to encapsulate thymol. Metal-Organic Frameworks (MOFs) or porous coordination polymers (PCPs) is a new class of hybrid materials, which are formed by the self-assembly of metal-connecting points and polydentate bridging ligands. MOFs in this study was synthetized by Zinc nitrate hexahydrate and 2-aminoterephthalic acid in N, N-dimethylformamide (DMF). Thymol was then loaded inside the MOFs at the loading rate of 3.95%. The structure of porous crystal MOFs was confirmed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Inhibition to E. coli O157:H7 was measured both in TSB medium and on TSA agar. An E. coli O157:H7 reduction of 4.4 log CFU/mL have been achieved at a thymol to broth ratio of 0.04g/100g. An inhibition area of 223.73 mm2 was observed after 12h incubation. With the two methods (zein nano-particles and MOFs), EOs can be encapsulated and well dispersed in water solution. The enhanced antioxidant activity and antimicrobial ability of the encapsulated EOs promise their further applications in food industries.