The impact of microwave‐assisted thermal sterilization on the morphology, free volume, and gas barrier properties of multilayer polymeric films

Abstract

Microwave-assisted thermal sterilization (MATS) is an advanced thermal process that utilizes microwave (MW) energy for in-package food sterilization. Benefits include much shorter processing times than conventional retort sterilization. This research explores how MATS affects the performance of high-barrier multilayer polymeric films compared with conventional retort sterilization. The gas barrier, morphological, and free volume packaging properties of these films may influence the shelf-life of shelf-stable foods. In this study, we applied X-ray diffraction (XRD) and positron annihilation lifetime spectroscopy in order to investigate film morphology and free volume characteristics, respectively. Results show that the conventional retort process affected gas barrier properties more than MATS processing did which could be explained by the morphological and free volume changes in the polymeric films. XRD revealed improved crystalline morphology of MW-treated films in terms of overall crystallinity as compared with retort sterilization. On the other hand, higher free volume increase in MW-treated films could be explained by the different heating mechanisms involved in MATS and retort sterilization. Overall, the oxygen transmission rate for both films remained below 2 cc/m 2 -day after MATS and retort sterilization required for packaging applications for shelf-stable foods. This work provides the basis for understanding the gas-barrier changes of multilayer polymeric films after MATS application using Materials Science techniques.

Development of an Oxygen Sensitive Model Gel System to Detect Defects in Metal Oxide Coated Multilayer Polymeric Films

“…The structural changes in polymers also contribute to reduction in gas barrier properties after thermal treatment. However, morphological changes have less influence on the barrier properties of metal oxide coated films than EVOH‐based multilayer structures (Bhunia et al., ; Dhawan et al., ; Zhang et al., ). Nevertheless, several other factors that may have contributed to changes in film barrier properties include moisture absorption by hygroscopic nylon in the multilayer structure, an increase in free volume and the formation of smaller crystals with increasingly distorted crystal lattices (Bhunia et al., ; Zhang et al., ).…”

“…The change in overall enthalpies of crystallinities in the films may be attributed to changes in chain structures of the polymers caused by the annealing effect induced by sterilization conditions under high moisture environment. In the case of PET and nylon, thermal processing can bring about changes to the molecular structure, resulting in enhanced crystallinity after thermal treatment (Bhunia et al., ; Dhawan et al., ; Zhang et al., ). The PET crystallinity decreased immediately after thermal processing or remained unchanged, while that of nylon increased (Dhawan et al., ; Zhang et al., ).…”

“…In the case of PET and nylon, thermal processing can bring about changes to the molecular structure, resulting in enhanced crystallinity after thermal treatment (Bhunia et al., ; Dhawan et al., ; Zhang et al., ). The PET crystallinity decreased immediately after thermal processing or remained unchanged, while that of nylon increased (Dhawan et al., ; Zhang et al., ). This may have resulted in nylon losing its mechanical strength upon thermal processing, leading to a more brittle structure overall.…”

Linking morphology changes to barrier properties of polymeric packaging for microwave‐assisted thermal sterilized food

“…It is vital to understand the morphological changes during processing and storage of polymer‐based food packages in order to advance sterilization technologies based on microwaves, high‐pressure, and irradiation . This is particularly true for pre‐packaged foods that require an extended shelf life.…”

“…Reduction of oxygen and water vapor barrier properties is a major concern, since this can lead to faster oxidation and textural changes that shorten shelf life . Several factors can lead to a loss of barrier properties, including a rougher surface barrier coating and an increased free volume caused by a less regular polymer chain alignment . These effects vary according to the type of polymer, barrier coating, and film processing methods .…”