Optical Properties of Fruits and Their Utilization in Automatic Sorting Facility [I]

Suitable Wave Length to Detect the Peel Color and Maturity of Citrus Unshiu

Abstract

The yield of Gitrus Unshiu was about 3.3 million tons in the year 1972, and there is a probability that it would continue to increase in the future. Then the need of many large packing houses and workers would become pressing in the production districts.
Grading of the citrus was done by manual sorting based on the external appearance and color.
Due to this manual labor within the grading line of citrus, it became quite impossible to increase the sorting rate of fruits together with sorting efficiency.
From this point of view, it becomes meaningful that an automatic color sorting facility to detect the maturity non-destructively should be planned and manufactured.
To establish the standards of design for this instrument, a spectrophoto analyzer attached with an integrating sphere was set up and the optical transmittance properties of citrus Unshiu were investigated. The results obtained were as follows:
1. The degree of the presence of chlorophyl introduced absorption bands within the range of wave lengths (557.5nm-700.0nm) with diminishing absorbance as the peel color varied from green to yellow or red. (Fig. 6)
2. There was a strong absorption band near the wave length of 670.0nm due to chlorophyl.
If the fruits were of the same dimensions, the degree of this absorption was proportional to the chlorophyl content in the peel.
The relationship between the chlorophyl content and optical density at the wave length of 671.5nm was
OD=0.543 CH+1.295 (Fig. 7)
3. Spectral absorbance curves of the fruits of difference sizes having the same degree of color, became parallel at the wavelength ranging 670nm to 700.0nm, By considering this, effect of size variation of the fruits could be neglected.
4. There was linear correlation between the chlorophyl content and ΔOD at a couple of wavelength, as shown in Table 2, the best one being that of (671.5nm-700.0nm.) or (681.5nm-700.0nm).
5. The effect of fruit's dimension on ΔOD could be neglected at a couple of wavelengths (681.5nm-700.0nm), (Tab. 3).
6. From the results mentioned above, it could be concluded that the color of peel which indicates the maturity of intact Citrus Unshiu can be precisely detected regardless of the size of samples by measuring ΔOD of the specimen at wavelength (681.5nm-700.0nm).