Antilisterial and physical properties of biopolymer films containing lactic acid bacteria

Novel biopolymer films were developed and used to control Listeria innocua in an artificially contaminated synthetized medium. Two hydrocolloids, sodium caseinate (NaCas) and methylcellulose (MC), and two bacteriocin-producing lactic acid bacteria (LAB), Lactobacillus acidophilus and Lactobacillus reuteri, were tested. Bioactive cultures were added directly to the film forming solution and films were obtained by casting. In order to study the impact of the incorporation of bacterial cells into the biopolymer matrix, the water vapour permeability, optical and mechanical properties of the dry films were evaluated. Furthermore, the survival of LAB and the antimicrobial potential of bioactive films against L. innocua were studied. Results showed that the use of lactic acid bacteria altered the filmʹs physical properties. Films enriched with bacterial cells exhibit higher gloss and transparency whereas no significant modifications were observed in terms of tensile properties. These films were less-effective water vapour barriers, since a significant increase can be observed in the WVP values. As far as food safety is concerned, these films are an interesting, novel approach. In refrigeration conditions, these films permit a complete inhibition of L. innocua for a week. Viability of LAB was higher in sodium caseinate films, although bacteriocin production was greater in polysaccharide matrix. The best results were obtained for films made of methylcellulose, without differences between the two lactic acid bacteria tested.