Oxygen transmission of multilayer EVOH films after microwave sterilization

Classical industrial retort sterilization processes expose food packages to high temperature, moisture, and pressure conditions. Migration of water into hydrophilic polymers such as ethylene vinyl alcohol (EVOH) sharply reduces their oxygen barrier ability. This research studied the effect of short time microwave sterilization on oxygen transmission rates (OTR) of two multilayer films containing EVOH and compared with that of conventional retorting. Film A had a laminated structure of EVOH sandwiched between oriented polyethylene terephthalate (PET) and cast polypropylene (PP). Film B consisted of PET laminated to a 7-layer co-extruded structure of PP/tie/Nylon 6/EVOH/Nylon 6/tie/PP. The films were used as lidstocks for trays containing mashed potato and processed by microwave or retort sterilization to achieve lethality of F0 = 3 min or F0 = 6 min. For both films the classical retort treatments resulted in higher OTR than the microwave treatments. In all cases, the oxygen barrier property of film A was better than that of film B. Storage of the food packages for 2 months at room temperature helped with recovery of more than 50% of the oxygen barrier lost by the films. The oxygen barrier slowly deteriorated beyond 2 months in storage. Over the 12 months storage, the OTR for both films after F0 = 3 min microwave process remained below 2 cc/m2 day, a value comparable to commercially available polyvinylidene chloride (PVDC) laminated films currently used in the USA as lid film for shelf-stable products.