Laser ultrasonic monitoring of the elastic properties of an apple

Fruit firmness estimates usually require contacting or destructive methods. However, we propose to estimate firmness from an all optical, non-contacting method that generates and detects elastic waves in an apple. The compressional and shear wave velocities are governed by the fruit's elastic constants and density, which can be used to estimate firmness, non-destructively. High energy pulses of light are used to excite elastic waves via thermo-elastic expansion. Elastic waves are detected using a laser Doppler vibrometer. By exciting the apple surface at many locations we accurately estimate the compressional, shear and Rayleigh wave velocities of an apple. Repeated realizations of this experiment over 15 days to observe changes in apple elastic properties over time. Softening of the apple decreases the resonant mode frequencies and elastic constants, as well as increases wave attenuation. We find that attenuation is the most sensitive parameter to the aging of the apple. We believe that our novel method of apple interrogation could be automated, and extended to other fruits