Evaluation of fruit tissue texture and internal disorders by laser Doppler detection

Ripening-dependent changes in fruit texture and the development of physiological disorders affecting texture were sensed remotely by laser Doppler detection based on changes in surface vibrations. Intact fruit of kiwifruit (Actinidia deliciosa (A. Chev.) Liang et Ferguson, 'Hayward'), peach (Prunus persica Batsch., 'Akatsuki'), and Japanese pear (Pyrus pyrifolia Nakai, 'Cyouju'), at different maturity stages, were placed on a vibration generator stage and the sample was subjected to sine wave excitation through a frequency series from 5 to 2000 Hz. Vibration transmitted through the fruit to the upper surface was measured by a laser Doppler vibrometer. Phase shift caused by the transmission through the fruit was determined based on the comparison of the difference between input and output vibration signal. To reconcile changes in the phase shift with other methods used for texture analysis, a probe with a load sensor was inserted into the sliced fruit and displacement force was measured. The correlation coefficient (r) between displacement force and phase shift at either 1200 or 1600 Hz was 0.81 to 0.92 (P < 0.01). The laser Doppler technique was also capable of detecting citrus fruit (Citrus tamurana Hort. ex Tanaka) afflicted with internal defects. These data suggest that laser Doppler detection of fruit vibration signals has the potential for use as a versatile remote sensing tool for evaluation of fruit tissue firmness and maturation. It also offers a possible means of evaluating otherwise concealed internal defects that render fruit unacceptable. 1999 Published by Elsevier Science B.V. All rights reserved.