Antimicrobial activity and physical properties of protein films added with cell-free supernatant of Lactobacillus rhamnosus

Antimicrobial and physical properties of whey protein isolate (WPI) or calcium caseinate (CC) films added with cell-free supernatant of Lactobacillus rhamnosus NRRL B-442 were evaluated. Antimicrobial activity was probed against Listeria monocytogenes Scott A, Staphylococcus aureus ATCC 29413, Escherichia colt ATCC 25922, or Salmonella enterica. serovar Typhimurium ATCC 14028. Cell-free supernatant was obtained by centrifugation and filtration from cultures of L. rhamnosus in MRS broth, freeze-dried, and rehydrated to add 6, 12, or 18 mg/ml to protein film solution. Films' inhibition zones were determined by agar disk diffusion assay against tested microorganisms. Films' thickness, puncture strength, color, moisture content, solubility, and water vapor permeability (WVP) were evaluated. Noticeable antimicrobial activity (about 3 mm) was observed against E. coli, L. monocytogenes, S. aureus, or S. Typhimurium when 18 mg/ml of cell-free supernatant were added. Gradual and significant (p < 0.05) increments were obtained in thickness, color, and solubility when different increasing concentrations of supernatant were added. Reductions in WVP and puncture strength were observed when adding supernatant. Antimicrobial films were brown colored and with variable moisture contents. WPI films were significantly (p < 0.05) better than CC films for most evaluated properties. Protein films added with cell-free supernatant are an interesting alternative to develop natural antimicrobial films that can have a potential food application as wrappings or coatings. (C) 2015 Elsevier Ltd. All rights reserved.