Non-destructive Quality Evaluation Of Watermelon And Melon Based On Its Acoustic Properties And Dynamic Finite Element Analysis

The watermelon is grown in 90 countries on five continents, and is a favorite fruit of people. The yield of watermelon in the world exceeds 50 billion pounds per year. Consumers of watermelons always expect a high quality of them. Quality of watermelon or melon consists of different aspects such as size, shape and appearance, especially the internal characteristics ripeness. Several indicators determine the ripeness of watermelon. Among them, firmness is commonly considered to be an indicator of the state of maturity of a watermelon. In order to decide the suitable harvesting date and subsequently to fix the marketing date, to monitor shelf-life quality and to evaluate firmness evolution of the fruit, the measurement technique should be non-destructive. While all the available techniques give results, which are reasonably objective and reproducible, were destructive. The vibration characteristics of fruits and vegetables are governed by their elastic modulus (firmness), mass, and geometry. Therefore, analysis of the vibration responses of a fruit was a potential approach to predict the firmness of watermelon or melon non-destructively.The objective of this research is to optimize the size and material of the impactor, determine the optimum location of the excitation, choose suitable sensors, set up the finite element models of watermelon, correlate the firmness of watermelon and melon to the vibration characteristics, and at last develop a nondestructive method for evaluating the firmness of watermelon and melon.In this study, the watermelon or melon was excited by a ball and the vibration was detected by an accelerometer. The Instron device was used to measure the static elastic modulus of the inner, middle and outer portions of melon flesh. The finite element (FE) technique was used to determine the optimum excitation location of the excitation to chosen measurement sensor and to analyze the mode shape fruits. The main contents and results of this study were listed as follows:1. Giling watermelons were selected as the samples of our study. The optimum location of the force excitation and the suitable response measurement sensor for measuringthe resonant frequency were determined. The middle part of the fruit surface was suggested to be used as the location of the force excitation while estimating the fruit firmness by the resonant frequency of the first-type mode. To measure the torsional mode, the directions of the exciting force and sensory axis of the response measurements sensor should be concentric and perpendicular to the tangent of the fruit surface at the location of force excitation or response measurement sensor. Moreover, to measure the vibration signal of the mode, excitation and sensors should be placed on the watermelon surface far away from the nodal lines, and in accordance with vibration directions. The selection of location and direction for the force excitation and the response measurement sensor are most important for detecting a mode.2. The acoustic impulse generating method, type of material and size dimension of impactors did affect the resonant frequency of watermelons. Compared to the other two types of balls made from rubber and steel, the higher significant resonant frequency mean was recorded when the wooden impactor ball was used. Additional, compared to the other four wooden balls with the diameters of 15 mm, 20 mm, 35 mm and 45 mm, the higher significant resonant frequency mean was found when the wooden ball with a diameter of 30 mm was used. Therefore, the wooden ball with a diameter of 30 mm was selected as the ball to generate the excitation in our experiment.3. The finite element models (FEM) of watermelon were set up using ANSYS version 7.0 (SAS IP. Inc), with its homogenous, linear and elastic properties. Three kinds of mode shapes of watermelon were found by the method of FE simulation, which included (a) first-type spherical mode, (b) second-type spherical mode, and (c) torsional mode. The comparison between the experimental results and the finite element estimated results confirm that the model is reasonably agreement with real watermelon. The lowest bending mode is the first-type spherical vibration mode for watermelon, and which related to fruit firmness.4. The high correlation between the resonant frequency and Young's modulus was found (r2 =0.90), the linear regression equation was Y = 59.79069 + 37.25189 * X.