Acoustic Imaging of Defects in Flexible Food Packages

A study was conducted using a high-frequency acoustic imaging system: the scanning laser acoustic microscope (SLAM), operating at 100 MHz, to detect packaging defects to within the system's resolution limit of 20 μm. The purpose of the study was to assess the feasibility of high-frequency acoustic imaging to detect and classify channel defects that would have the potential for microbial contamination through visually undetected defects. The SLAM can characterize and image various materials and defects by exploiting the differences in acoustic (mechanical) transmission properties within different materials. Channel defects transverse to the heat-seal major axis were fabricated by sandwiching 10-, 16-, 25-, and 37-μm wire between two layers of either polyethylene or plastic retort-pouch laminate film which were then heat sealed. The wire was then pulled out, leaving a channel filled variously with saline solution, air, or both. The channel defects were then assessed using the SLAM and validated with confocal microscopy. The results indicate that the SLAM technology can readily detect channel defects as small as 10 μm, the smallest channel defects examined, which is one-half the imaging system stated resolution specification. This study has clearly demonstrated that acoustic microscopy can nondestructively image micrometer-scale channel defects in heat seals at and smaller than the SLAM's resolution limit.