Application of vibration response for the nondestructive ripeness evaluation of watermelons.

The assessment of watermelon ripeness on the basis of its apparent properties, such as size or skin colour, is very difficult as traditional methods have various problems and limitations. These include lack of uniformity, concentration of excitation energy within narrow bands and need for physical contact between a fruit and the measuring device. In this study a new method making use of Laser Doppler Vibrometry technology (LDV) has been applied to evaluate the ripeness of watermelons, without many of those limitations. At first a watermelon is excited by a shaker as vibration generating device within a range of frequencies. At the same time, the vibrating response of the upper side of the fruit is measured by LDV. The device emits a laser beam on a spot above the sample. The beam reflected from that point is received by the LDV and finally the vibration response of the sample is measured and the signal is sent to the computer. Using a fast fourier transform algorithm and the ratio of input to response signals, the frequency response of the fruit sample was processed and the desired results extracted. After the nondestructive tests were completed, the watermelon ripeness was evaluated by means of a destructive method. The sugar and firmness of the samples were measured as ripeness indices. In this study four modal properties were derived and used for developing predictive models to relate the vibration response results with the watermelon sugar and firmness. Interaction models by including the fruit mass were found more accurate than other common multiple models. The nonlinear regression model has five variables as sample mass, first and second resonant frequency and also damping ratios of them. For this model the coefficients of determination and the mean square error for the estimation of the fruit sugar were 0.9 and 0.79 respectively. For the estimation of the fruit firmness, they were 0.89 and 0.035. This study demonstrated feasibility of information which is derived from vibration response curves for predicting fruit ripeness. The vibration response of watermelon using the LDV method is measured without direct contact of the device with watermelon; it is accurate and timely, which could result in significant advantage for the commercial-scale classifying of watermelons based on consumer demands.