Non-destructive firmness assessment of apples using a non-contact laser excitation system based on a laser-induced plasma shock wave

Various indexes have been used to assess the ripeness of fruit, including peel color and firmness because added value is given to fruit when grade selection is determined objectively. In this paper, we realize a non-destructive firmness assessment for apples by means of a non-contact vibration test method. We investigate their natural frequencies and vibration mode shapes because these factors influence the ₀S₂ mode, which is related to firmness. A laser-induced plasma shock wave generated with a high-output Nd:YAG pulsed laser is applied to apples as an excitation force. Firmness is assessed with this non-contact and non-destructive method using the apple's vibration response spectra measured with a laser Doppler vibrometer. The effectiveness of this method is experimentally demonstrated through assessments of apples’ firmness, identification of the vibration mode shapes, and a follow-up survey on the flesh firmness of apples during storage.