Using Visible/Near-Infrared Reflectance Spectroscopy and Chemometrics for the Rapid Evaluation of Two Panamanian Watermelon (Citrullus lanatus) Varieties

In this study, visible/near-infrared (Vis/NIR) reflectance spectroscopy measurement in the 300-900 nm wavelength range, are used as external parameter to evaluate the internal parameters (Brix content and pH) of two local watermelon varieties, Quetzaly (N=8) and Mickylee (N=6), in a noninvasive manner. To assess this relationship three Chemometrics and machine learning methods were used: Principal Component Regression (PCR), Partial Least Squares (PLS) and Multilayer Perceptron Regression (MLPR). Spectral Data augmentation was used to generate more observations from the original spectra. Our results suggest Quetzaly has a higher Brix content and that all three methods can be used to predict it with confidence. However, PLS shows a higher correlation coefficient of 0.98 and standard errors of prediction of ≤0.1, closely followed by PCR with 0.96 and errors of ≤0.01 and ≤0.04, respectively. In the case of pH all methods produce good results, with a PLS achieving correlation coefficient of 0.91-0.94 with errors of prediction on the <0.05; and lower results for the other two methods. This study demonstrates the value of non-invasive Vis/NIR Reflectance spectroscopy as quality assessment in a watermelon classification system. More importantly, this system can be later implemented in local watermelon fruit packing centers in Panama. This can also be used in other export fruits or vegetables, helping local producers.

I. Introduction

Panama is positioned as one of the Central American countries with most watermelons exports. Profits over 5 million have been reported for the first quarter of 2018. This represents a net growth of 20% when compared to the first quarter of 2017, and have been increasing since. These exports usually end up in markets all over America, and Europe [1]. As is the case with many fruit exports, the export quality watermelons must comply with specific external and internal characteristics. For instance, externally watermelons are requested to be symmetrical, uniform and to have a waxy and shiny surface. Moreover, they must not have any type of surface defect. Also internally, they must comply with having a live red color, and having a specific pH (5.18-5.60, slightly acidic), which can be very informative about growing conditions, processing methods and overall maturity of the product [2]. Also, required is to ensure the total sugar content measured as Degrees Brix or basically the sugar content of an aqueous solution (also called Soluble solids content or SSC). By convention, 1°Bx being equivalent to 1 gram of sucrose in 100 grams of solution (or 1.0% sugar by mass), fruit extract. In the case of watermelon juice, it should provide a value of 10°Bx (or over) [3].