Non-contact bruise detection in apples by thermal imaging

Abstract

Thermal imaging is a non-destructive and non-contact infrared sensing technique. Such imaging creates a bit-map called a thermogram by detecting infrared radiation emitted from an object. Up to 100% of apple bruises were detected using thermal imaging during warming of the fruits by discriminating surface temperature between bruised and sound tissues. Apples were bruised by dropping them from 0.46 m onto a smooth concrete floor and then were held at 26 °C and 50% RH for 48 h. They were then thermally imaged using a ThermaCam™ PM390 (FLIR Systems, Inc., Portland, OR) during heating and cooling treatments. Thermal images of bruised tissue showed at least 1–2 °C difference from sound tissue within 30–180 s. The temperature differences between bruised and sound tissues were possibly due to the differences in thermal diffusivity. Under steady-state temperature, thermal imaging did not detect bruises, indicating that the temperature differences were not due to emissivity differences. The technique could provide a basis for automatic bruise sorting, and possibly a better understanding of bruised tissue.