Development of food quality measurement methods based on near-infrared imaging spectroscopy: Applications to visualization of sugar content distribution in fresh fruits and fruit sorting

Abstract

A NIR spectral imaging apparatus was developed to obtain information on spectrally discriminated images and the location of material to be measured. The apparatus consisted mainly of a CCD camera, a liquid crystal tunable filter and a spectral illuminator, which consisted of a xenon lump as well as a grating spectrometer. The sample surface image could be captured at any wavelength from 400 nm to 1 100 nm. To investigate the performance of the apparatus, it was employed in the measurement of sugar content distribution at the surface of a fresh green melon cut in half and in the detection of foreign materials among blueberries. The absorbance at 676 nm was found to be highly correlated with the sugar content in fresh green melons. Next, the intensity of each pixel of the images was converted into sugar content. By assigning the sugar content to a linear color scale, the sugar distribution of the melon was visualized. The plant organs could be detected by the second derivative absorbance image at 680 nm, which is an absorption band of chlorophyll. The second derivative absorbances for blueberries and plant organs were determined in the image. The positions of pixels judged as plant organs in the detection image were in good agreement with the actual locations where plant organs had been placed. The apparatus demonstrated that plant organs contaminating the raw blueberry materials could be detected using the proposed methodologies.