Detecting volumetric edible rate of thick-skinned citrus using X-ray three-dimensional reconstruction

Citrus rich in nutrients is one of the most favorite fruits in recent years. But the thick-skinned citrus is often accompanied by the fruit hollow, skin thick floating, leading to the low actual volumetric edible rate. Furthermore, traditional nondestructive testing cannot accurately and rapidly detect the volumetric edible rate. In this study, the line array X-ray image acquisition and three-dimensional reconstruction were developed to detect the volumetric edible rate of thick-skinned citrus fruits, including the fruit rotation and lifting, data acquisition, radiation protection and motion control. The fruit rotating device consisted of a rotating table and a gear motor; the fruit lifting device consisted of a lifting platform and a stepping motor. The data acquisition device included an X-ray emission source, a line array X-ray detector, and upper computer software. The radiation protection device was a lead plate with a thickness of 2mm, which was used to prevent the radiation generated by X-rays from leaking to the external environment. The motion control part consisted of a programmable logic controller and upper computer software. Taking the Ugli fruit as the detection object, the information entropy of the X-ray projection map was evaluated to optimize the detection parameters. The optimal tube voltage and current of X-ray source were obtained to be 67 kV and 0.92 mA, respectively. The integration time of the line array detector was 1 ms. Dark-field correction and bright-field compensation were used to remove the uneven pixel distribution and the noisy background in the initial state of the X-ray detector before the experiment. A series of 180 X-ray projections were collected in the circumferential direction at intervals of 2.0° rotation. The samples were rotated by 120° with the mid-axis of the fruit as the center of rotation. A series of citrus X-ray projection maps were captured at three angles of 0°, 120° and 240°, respectively. The X-ray projection maps were converted to sinograms using the Radon transform. After that, the sinograms under the three angles were reconstructed as the slice maps using the FBP (Filtered Back Projection). Image segmentation was carried out on the slice maps using image filtering, enhancement, and thresholding segmentation, in order to form the background, pericarp, pulp, and cavity region. Moreover, the regional area ratio was defined using the ratio of citrus pulp region to fruit region, whereas, the slice map edible rate was defined using the regional area ratio. The physical parameters were measured, such as transversal diameter, vertical diameter, mass, volume, density and fruit shape index of citrus fruits. The true volumetric edible rate of citrus was calculated using the specific gravity of citrus pulp to the volume of the whole fruit. There was the better correlation between the physical parameters and volumetric edible rate of citrus. The results showed that there was the higher correlation between fruit density and slice map edible rate and citrus volumetric edible rate, with the highest correlation 0.93 for slice map edible rate. Finally, the slice map edible rate was selected as the input feature of the model. The linear regression model was used to quantify the volumetric edible rate of thick-skinned citrus, with the values of R_\mathrmp^2 (coefficient of determination of prediction), RMSEP (root mean square error of prediction), and RPD (residual predictive deviation) of 0.86, 4.81%, and 2.71, respectively. In conclusion, it is feasible to quantitatively analyze the volumetric edible rate of thick-skinned citrus using X-ray three-dimensional reconstruction. The developed approach can also be applied in the nondestructive testing of the quality of agricultural products. Therefore, the nondestructive testing techniques can be expected to evaluate the internal tissue lesions and external quality of agricultural products.