Abstract—It is difficult to judge ripeness by outward characteristics such as size or external color. In this paper a non- destructive method was studied to determine watermelon (Crimson Sweet) quality. Responses of samples to excitation vibrations were detected using laser Doppler vibrometry (LDV) technology. Phase shift between input and output vibrations were extracted overall frequency range. First and second were derived using frequency response spectrums. After nondestructive tests, watermelons were sensory evaluated. So the samples were graded in a range of ripeness based on overall acceptability (total desired traits consumers). Regression models were developed to predict quality using obtained results and sample mass. The determination coefficients of the calibration and cross validation models were 0.89 and 0.71 respectively. This study demonstrated feasibility of information which is derived vibration response curves for predicting fruit quality. The vibration response of watermelon using the LDV method is measured without direct contact; it is accurate and timely, which could result in significant advantage for classifying watermelons based on consumer opinions. Keywords—Laser Doppler vibrometry, Phase shift, Overall acceptability, Regression model ,Resonance frequency, Watermelon I. INTRODUCTION ATERMELON is a popular fruit and it has different properties and applications. According to FAO statistics published in 2008, Iran has been ranked third among watermelon producing countries. Nondestructive quality determination of watermelons has been a challenge for its customers since it has different structure from the other fruits. The subjective methods are usually based on appearance or sound caused by slap. Both are not reliable because these methods are prone to human factor errors. Researchers have studied different objective methods to evaluate watermelon quality: acoustic and dynamic technology [1]-[3]-[7]-[15]- [22], electrical and magnetic technology [8]-[14], X-ray and computed tomography [21], and near infrared (NIR) spectroscopy [5]-[6]-[16]. In this paper Laser Doppler Vibrometry (LDV) technology is used to detect overall quality of watermelon. In recent years using LDV has been studied by researchers as a new 1-Faculty of Agricultural Engineering and Technology, University of Tehran, Karaj, Iran, Abaszadeh@ut.ac.ir 2-Faculty of Agricultural Engineering and Technology, University of Tehran, Karaj, Iran, arajabi@ut.ac.ir 3- Faculty of Agricultural Sciences and Engineering, University of Tehran, Karaj, Iran, delshad@ut.ac.ir 4- School of Mechanical Engineering, University of Tehran, Tehran, Iran, mmahjoob@ut.ac.ir 5- Faculty of Agricultural Engineering and Technology, University of Tehran, Karaj, Iran, hjahmadi@ut.ac.ir nondestructive technique to test the quality of some fruits. Muramatsu did comparison between the use of accelerometer and LDV to measure the firmness of some varieties of apple, pear, kiwi and citrus. Their results of measurements carried out using the LDV expressed more accurate than the accelerometer results [10]. In addition, Muramatsu evaluated the texture and ripeness of some varieties of kiwi, peach and pears. They excited samples at different stages of ripeness, by the sine wave with frequencies from 5 to 2000 Hz and the vibration responses at top point of the fruit were measured by LDV. Then the phase shift between input and output signals was compared with the data obtained from the method of force-displacement. A significant relationship between these two methods obtained in 1200 and 1600 Hz excitation frequencies. The ability of the LDV technique for detection of internal defects of some citrus varieties was approved [11]. Muramatsu also used the method to conduct some tests and determine fruit texture changes during the ripeness. This technique was used for persimmon, apple and kiwi. In the range of 1,200 to 1,600 Hz, phase shift as a function of the ripeness significantly changed. They also found resonance frequency for all fruit under test was a function of the ripeness [12]. Terasaki used LDV to assess properties of kiwifruit at different stages of ripeness. They considered two factors S = f n = 2 2 m 2/3 and η = (f 2 -f 1 ) /f n = 2 where f n = 2 : second peak resonance frequency, m: mass of fruit and f2 and f1 are frequencies determined at 3 dB below peak resonance. The relationship between S as elasticity index and firmness of kiwifruit was significantly high. η also showed a good correlation with soluble solids content [20]. The potential of measuring the vibration response with a laser vibrometer was explored in plums by Bengtsson. Phase shifts at selected frequencies were highly correlated to postharvest storage time, plum weight, plum length and plum width [2]. Murayama conducted research on ripeness by the LDV in which pears harvested at different times and in different periods of storage were tested. Results showed that correlation coefficients between firmness and elasticity index were significantly high [13]. Taniwaki also conducted a separate investigation to review the trend of change in elasticity index figures from the melon, persimmon and pear after harvest period. The second resonance frequency of sample was obtained using LDV. The samples were evaluated by panelists’ senses considering features such as appearance, sweetness, firmness and etc. (each separately). The overall acceptability was sensory evaluated by subjective scoring. High correlation between the elasticity index and the mentioned properties were observed [17]-18]-[19]. The main objective of present study is establishing a relation among R.Abbaszadeh 1 , A.Rajabipour 2 M.Delshad 3 , M.J.Mahjub 4 , H.Ahmadi 5 Prediction of Watermelon Consumer Acceptability based on Vibration Response Spectrum w World Academy of Science, Engineering and Technology International Journal of Agricultural and Biosystems Engineering Vol:5, No:6, 2011 365 International Scholarly and Scientific Research & Innovation 5(6) 2011 scholar.waset.org/1307-6892/2289 International Science Index, Agricultural and Biosystems Engineering Vol:5, No:6, 2011 waset.org/Publication/2289