Sorting of peeled mandarin segments with orange core based on characteristic fluorescent signal

Abstract: The peeled mandarin segment with orange core (central column residue) is one kind of defective mandarin segment during the production of canned mandarin. So it is necessary to remove the defective mandarin segments according to the requirement of quality before canning. Due to the differences of physical and chemical properties between orange core and orange gizzard, the fluorescence spectra of orange core and orange gizzard are different theoretically. For the purpose of offering direct and accurate signals to the machine to identify the peeled mandarin segments with orange cores, in this study, the peeled mandarin segments were taken as the samples, and the 3D (three-dimensional) fluorescence spectra of orange core and orange gizzard were determined by Cary Eclipse fluorescent spectrophotometer (excitation wavelength in the range of 250-700 nm, 5 nm interval; emission wavelength in the range of 300-800 nm, 1 nm interval; discharge voltage of 600 V, scanning speed of 1 200 nm/min). The differences of fluorescence characteristics of orange core and orange gizzard were found according to the 3D fluorescence spectra after removing Rayleigh and Raman scatter. In order to verify the differences further, the fluorescence spectra of orange core and orange gizzard of 50 peeled mandarin segments were determined by Cary Eclipse fluorescent spectrophotometer (at an excitation wavelength of 370 nm, an emission wavelength in the range of 400-700 nm with the interval of 1 nm, a discharge voltage of 600 V, a scanning speed of 1 200 nm/min). The differences of fluorescence intensities at 440 nm for orange core and orange gizzard were evaluated by means of drawing the box-plots of the fluorescence intensity at 440 nm. The accuracy rate of the discrimination of orange core and orange gizzard was analyzed, and the average value between fluorescence intensity of orange core at the lower quartile and that of orange gizzard at the upper quartile at 440 nm was used as classification criterion. The camera obscura was taken as a platform, and 2 sets of ultraviolet light sources with center wavelength of (375±2 nm) were selected as imaging light sources. The fluorescence images of the mandarin segments with and without orange core were taken using a monochrome fluorescence image acquisition system, which was set up by putting a band-pass filter (440±5 nm) in the front of Image soft G445 camera objective. The grayscale images were obtained by extracting B color component from RGB images, and then the binary images were generated by applying the threshold values, with the IN Vision Assistant software. The results showed that there were significant differences between orange core and orange gizzard of peeled mandarin segments at an excitation wavelength of 370-390 nm and an emission wavelength of 440 nm. According to the classification criterion the discrimination accuracy of orange core and orange gizzard was 85%. Meanwhile, using the fluorescence image acquisition system, the monochrome images of the mandarin segments with and without orange core were obtained respectively. There was significant difference between them, and thus the mandarin segments with orange core and the mandarin segments without orange core could be distinguished effectively. Overall, the study indicates that the fluorescence signal at an excitation wavelength of 370-390 nm and an emission wavelength of 440 nm can be the exclusive light signal of the orange core for on-line, nondestructive sorting, which is help to eliminate the peeled mandarin segments with orange core.