Fabrication and characterization of antimicrobial biopolymer films containing essential oil-loaded microemulsions or nanoemulsions

New active packaging materials were prepared consisting of biopolymer films containing the antimicrobial self-microemulsifying (SME) essential oil formulations or nanoemulsion. All the biopolymer films were constructed using calcium alginate to form a gel network with propylene glycol as a plasticizer. A mixture of essential oil (thyme oil), organic acids (acetic or propionic acid), and non-ionic surfactant (Tween®80) was used to formulate the antimicrobial microemulsion-films. The microemulsion-films were transparent, had little color, and were mechanically as strong but more stretchable than the control films (calcium-alginate only). Microstructural analysis showed that the microemulsion-films had high internal porosity and surface roughness, which resulted in an elevated water vapor permeability, opacity, and wettability. No new molecular interactions were detected in the films after incorporation of the essential oil using FTIR spectral analysis. The thyme oil was completely released from the microemulsion-films while only 3% of thyme oil was released from the nanoemulsion film when the films were brought into contact with water. Dynamic light scattering showed that the mean diameter of the oil droplets released from the microemulsion-films formulated from acetic or propionic acid were 38 and 28 nm, respectively. An antimicrobial assay showed that the highest microbial inhibition zones were obtained for the microemulsion-films. When applied to ground meat, the antimicrobial efficiency of the microemulsion-films was much higher than that of the control films against coliforms, Staphylococcus aureus, yeast, mold, and lactic acid bacteria. Our results demonstrate the efficiency of self-microemulsifying thyme oil films for maintaining the microbial quality of ground meat.