# Automatic Brix Measurement for Watermelon Breeding
###### Abstract  
Sweetness or sugar content, represented by soluble solids contents (SSC), is a vital quality trait in watermelon breeding which can be assessed by the refractive index method. However, sampling watermelon juice out of the pulp is a process that is both labor-intensive and error-prone. In this study, we developed an automatic SSC measurement system for watermelon breeding to improve efficiency and decrease costs. First, we built an automatic cutting system to cut watermelons into precise halves, in which a laser rangefinder is used to measure the distance from the upper surface of the watermelon to itself, and thus, the diameter is estimated. The experiments showed a high correlation between the estimated diameters and the ground truths, with \(R^{2}~{}=~{}0.9608\) and \(RMSE=4.321\). Then, we built an automatic Brix measurement system to obtain the Brix data from a central point on the watermelon's section, where an image analysis procedure is applied to locate the testing point. This is then transformed to the camera coordination system, and a refractometer is driven by a 3-axis robotic arm to reach the testing point. Brix measurement experiments were conducted using three vertical gaps and four lateral gaps between the probe of the refractometer and the pulp. The result showed that the best parameters were a vertical gap of 4 mm and a lateral gap of 2 mm. The average accuracy reached 98.74%, which indicates that this study has the potential to support watermelon breeding research. waternalon; Brix; refractometer; cutting; image analysis; laser rangefinder +
Footnote †: journal: Medical Imaging  
Hunan Agricultural Equipment Research Institute, Changsha 410125, China 1  
between the transmission velocity and SSC of watermelons reached 0.81-0.95 for the different striking points of various growth statuses of watermelons. Due to the development of chemometrics, infrared spectroscopy, especially near-infrared spectroscopy, has been widely used in many nondestructive determination tasks, including determination of the SSC of watermelons. [PERSON] et al. [17] used a near-infrared transmission method to assess the SSC of honeybee melons, which resulted in a correlation coefficient of 0.85. The authors in [18] developed a method to nondestructively determine the SSC of watermelons by applying multiple regression with wavelengths of 902 nm and 872 nm, which resulted in a root mean square error of calibration of 0.87. [PERSON] et al. [19] developed a method to determine their SSC by combining near-infrared reflectance spectroscopy and dielectric properties of the watermelons, which resulted in a correlation coefficient of calibration of 0.84. The authors in [20] used a field type near-infrared spectroscopy instrument with a wavelength of 700-1050 nm to measure the sweetness of golden melons, which resulted in a correlation coefficient of prediction of 0.85%. Another technology that can be used in non-destructive SSC measurement of watermelons is hyperspectral imaging, which obtains the spectrum for each pixel in the image of an object [21]. The authors in [22] used the hyperspectral imaging method to non-destructively measure the SSC of grapes, achieving a correlation coefficient of 0.77. [PERSON] et al. [23] used features extracted from a hyperspectral reflectance imaging system of 400-1000 nm to non-destructively assess the SSC of strawberries, which resulted in a determination of prediction of 0.937. [PERSON] et al. [24] used a near-infrared hyperspectral imaging system of a wavelength of 874-1734 nm to estimate the SSC of harvested cherries, which resulted in a standard deviation of prediction error of 2.7. Due to the high efficiency and low-cost features of these techniques, non-destructive SSC measurement methods are applied extensively in the fruit industry. However, the low accuracy and robustness of these methods mean that they are not suitable to be used for the accuracy-sensitive breeding task, because it is difficult to compare various cultivars using an inaccurate tool.