Flexible packaging structures for high pressure treatments

This paper reviews the possible effects determined on polymeric food packaging films by the simultaneous application of pressure (P) and temperature (T) in high pressure pasteurization and sterilization treatments of packaged foodstuffs. Both theoretical modelling and experimental issues are discussed. In fact reversible and irreversible effects that could occur in single film as well as in multilayer film structures have been reviewed and correlated to the variation of structural and morphological properties of the polymeric materials, under P/T conditions relevant for the specific application. The structural changes of the amorphous and crystalline domains of polymer films could possibly occur in response to pressure and/or temperature conditions, which, in turn, result in detectable effects on properties of packaging materials. Particular attention has been devoted to addressing issues like irreversible alteration of functional (i.e., barrier to gases and water vapor, scalping and migration phenomena) and mechanical properties of the packaging materials in a relationship with a modification of degree of crystallinity, aging of the amorphous phase, delamination of multilayer films, rubbery to glassy state transition promoted by pressure and plasticization induced by absorption of low molecular weight compounds. Industrial relevance: High pressure processing (HPP) is steadily gaining as a food preservation method that preserves natural sensory and nutritional attributes of food with a minimal quality loss. In fact, packaged foods processed by using this technique maintain most of their original texture and nutritional qualities, additionally exhibiting an extended shelf-life. Industrially, the selection of the proper polymeric materials as well as the packaging structure becomes extremely important when both high pressure and high temperature are employed for high pressure sterilization processes. In this context, an important issue that is relevant for industrial applications is to predict the possible effects of treatments on the structure and morphology of the materials which, in turn, can determine relevant effects on mechanical, functional (i.e., gas and vapor barrier properties) and aesthetic (e.g., see-through clarity) properties. In fact, modification of the crystallinity level, density change of amorphous regions and possible hydrolysis phenomena could hamper the use of the packaging material for the HPP applications. Moreover, multilayer flexible films can, in some cases, display delamination phenomena leading to unacceptable modifications of the aspect as well as of the integrity of the packaging structure. (C) 2012 Elsevier Ltd. All rights reserved.