The effects of high-pressure processing (HPP) on the mechanical and physical characteristics of eight high-barrier multilayer films were investigated. These films were PET/SiO x /LDPE, PET/Al 2 O 3 /LDPE, PET/PVDC/nylon/HDPE/PP, PE/nylon/EVOH/PE, PE/nylon/PE, metallised PET/EVA/LLDPE, PP/nylon/PP and PET/PVDC/EVA. In addition, PP was evaluated as a monolayer film for comparison purposes. Pouches made from these films were filled with distilled water, sealed, then pressure processed at 600 and 800 MPa for 5, 10 and 20 min at a process temperature of 45 • C. Pouches kept at atmospheric pressure were used as controls. Prior to and after HPP, all films were tested for tensile strength, percentage elongation and modulus of elasticity (at 50 cm min −1 ) and imaged by scanning electron microscopy (SEM) and C-mode scanning acoustic microscopy (C-SAM). Results showed no significant changes in tensile strength, elongation and modulus of elasticity of all films after HPP. However, significant physical damage to metallised PET (MET-PET) was identified by SEM and C-SAM. Thus it could be concluded that MET-PET is not suitable for batch-type high-pressure-processed food packaging. It can also be concluded that the other materials investigated during this study are suitable for batch-type high-pressure-processed food packaging.

This study investigated the effectiveness of ultrasonic imaging for non-destructive assessment of seal strength in 335 ml polymeric trays. In the ®rst part of this study, the experimental procedures involved ®lling sample trays with beef enchilada, then sealing at temperatures of 170±260°C for 5 s at 3 bar. Ultrasonic measurements were conducted using an immersion technique in pulse/echo mode. The captured signal amplitudes of the re¯ections from the samples were digitized and analysed to construct high-resolution C-scan ultrasonic images of the seals. The acquired data were statistically analysed for each sample and used as a basis for comparison with the seal (peel) strength, measured as the energy at break. The energy at break was obtained from destructive peel strength analysis, performed using a tensile testing frame. In the second part of this study, ultrasonic imaging was used to predict the performance of the heat sealer used to fabricate the test trays by comparing the seal quality of each of the four quadrants of the samples trays. This study also investigated the minimum channel leak size that could be detected using this ultrasound system. The results of the peel strength analyses showed that a direct correlation exists between seal strength and sealing temperature (170± 227°C). Results of the ultrasonic imaging showed that the statistical scatter in the signal amplitude decreased as the sealing temperature increased. This reduction in the scatter of the signal amplitudes indicated a trend of increasing uniformity in the fusion of the two seal faces. Between 170°C and 193°C, sharp changes in the scatter were observed; however, little change was noted for sealing temperatures greater than 193°C, indicating that 204°C could be a minimum recommended sealing temperature. To con®rm this result, a drop test analysis was performed on trays ®lled with beef enchilada and sealed at temperatures of 170±260°C for 5 s at 3 bar. Results of this drop test con®rmed that 204°C was the minimum critical sealing temperature. Based on these results, ultrasonic imaging appeared to offer a promising non-destructive method for the prediction of seal quality in polymeric trays, sealing equipment monitoring and identi®cation of channel leaks 520 mm in diameter, and is comparable with the more established destructive peel strength analytical method.