Active biodegradable sodium caseinate films manufactured by blown-film extrusion: Effect of thermo-mechanical processing parameters and formulation on lysozyme stability

Sodium caseinate-based edible/biodegradable films containing lysozyme were prepared directly by extrusion processes to obtain antimicrobial films. Thermoplastic pellets containing I% (w/w) lysozyme were blown by an industrial-like blown-film-extruder to obtain thermoplastic thin films. Lysozyme stability depended mainly on processing temperature and glycerol content. A correct choice of these parameters allowed obtaining pellets and films with 40.8 +/- 0.5% and 26.4 +/- 0.6% lysozyme residual activity, respectively. Whatever the storage temperature (-20 degrees C or room temperature), lysozyme activity of pellets remained constant for at least 5 weeks of storage. Antimicrobial activity against Micrococcus luteus was assayed in Tryptone Soya Broth (TSB): no bacterial growth was observed for 25 h at 37 degrees C in the presence of 0.05 g.L-1 pellets, while a 2.4 log increase of bacterial population was observed in the presence of control pellets without lysozyme. Traditional properties of films such as their water sorption and mechanical properties were also investigated and proved that addition of lysozyme at a 1% concentration did not affect these properties as well as their ultimate aerobic biodegradability. The application potential of such edible antimicrobial films is discussed in relation with these properties. (C) 2014 Elsevier B.V. All rights reserved.