Nondestructive firmness prediction of apple fruit using acoustic vibration response

In this research, firmness of Royal Gala cultivar apples during shelf life storage period was measured using acoustic vibration response method. Newly-picked apples were kept in two conditions of temperature and humidity (0 degrees C, 95 % and 20 degrees C, 40 % relative humidity). Ten apples from each group were tested every week during storage. Meanwhile, the volume, mass, density and the elasticity modulus of apples were calculated. Tests of acoustic vibration response firmness, puncture firmness and pressure were performed on each sample. The acoustic test was conducted as a nondestructive test with two sensors including a microphone to receive sound signals and a contact accelerometer to receive vibration signals. The puncture test was used as a reference evaluation. The dominant frequencies of sound (FS) and vibration (FV) were obtained and firmness indices (FIS, FIV) were calculated using the formula FI = f(2)m(2/3). Four new indices F-sum, F-dif, FIsum and FIdif were also calculated. Correlation coefficients of all those for indices were calculated for both groups with the puncture firmness (PF). The highest correlation in the first group belonged to FIS, Fly (98.57 %, 97.40 %) and in the second group belonged to FIsum, FIdif (96.78 %, 96.71 %). Results showed that these new indices were good indicators for determining of apple firmness. Furthermore, results revealed that acoustic vibration response is a promising technique as an inexpensive and fast way for apple texture measurement.