Effects of High Pressure Treatments on Polymeric Films for Flexible Food Packaging

Effects of high pressure (HP) treatments on functional and morphological properties of commercial packaging films have been investigated, to assess their suitability for use in these processes. Experimental analyses have been performed on packaging films, used to make pouches containing four different types of food simulants, after HP pasteurization and sterilization treatments at 200, 500 and 700 MPa. The investigated polymeric flexible packaging structures were single layer as well as bilayer films. Effects of treatments on structural/morphological properties (i.e. crystallinity and density) and on thermodynamic and barrier properties were addressed. Quite all the investigated films were found to be suitable for HP pasteurization, which only determined a negligible effect on the glass transition temperatures and on density of the amorphous regions of some polymeric materials, without a significant effect on functional properties of the packaging. Only bilayer structures including metalized polyethyleneterephthalate were not able to withstand processing conditions because of an extensive delamination accompanied by loss of metallization layer. Conversely, in the case of HP sterilization, combined effects of HP and relatively high temperature caused significant modifications in some of the packaging materials, even those not including a metalized film. In particular, localized delamination occurred in the case of bilayer structures made by laminating polyethyleneterephtalate with linear low density polyethylene or with polypropylene. Films obtained by laminating cast polyamide and bioriented polyamide with polypropylene displayed no evidence of delamination. In the case of films not displaying delamination, only limited changes in gas and vapour barrier properties were detected.