Optical absorption and scattering properties of normal and defective pickling cucumbers for 700–1000 nm

Abstract

An effective optical inspection system for detecting defective pickling cucumbers is needed to help the pickle industry deliver consistent pickled products to the consumer. This research was intended to measure the optical absorption and scattering properties of normal and internally defective pickling cucumbers, using hyperspectral imaging-based spatially-resolved technique. Spatially-resolved hyperspectral scattering images were acquired from 50 freshly harvested ‘Journey’ pickling cucumbers in the summer of 2008 before they were subjected to rolling under mechanical load to induce internal damage. The damaged cucumbers were imaged 1 h and 1 day after the mechanical stress treatment. Spectra of the absorption and reduced scattering coefficients for pickling cucumbers were extracted from the spatially-resolved scattering profiles, using an inverse algorithm for a diffusion theory model, for the spectral range of 700–1000 nm. It was found that within 1 h after mechanical damage, changes in the absorption and reduced scattering coefficients for the cucumbers were minimal. One day after mechanical damage, the absorption coefficient for the cucumbers increased significantly (at 5–10% level) for the wavelengths of 700–920 nm, whereas the reduced scattering coefficient decreased significantly for the wavelengths of 700–1000 nm (at 10% level). Overall mechanical damage caused greater changes in absolute value to the scattering properties than to the absorption properties. This research suggests that effective defect detection can be achieved by enhancing scattering characteristics measurement in the optical evaluation of pickling cucumbers.