THE ACOUSTIC IMPULSE-RESPONSE METHOD FOR MEASURING THE OVERALL FIRMNESS OF FRUIT

An acoustic impulse method has been used to measure the maturity of '' Golden Delicious'' apples throughout the growing season and during eig ht months cool storage at 1 degrees C and 96% r.h. Over the same perio d, penetrometer firmness was measured. Comparison of the results has e nabled correlations to be made between the elastic coefficient of appl e tissue and the mean firmness at a depth from 5 to 9mm below the skin . Under the experimental conditions used, the penetrometer measurement s provided better discrimination between immature fruit, while the aco ustic impulse response measurements were superior for ripe fruit. The possibility of applying the acoustic impulse response method to fruits with non-uniform internal structures was considered experimentally by studies on tomatoes. In this case, correlations were established betw een the coefficient of elasticity calculated from acoustic measurement s, colour measurements (lightness and hue), the force required to comp ress the whole fruit by 3% of its equatorial diameter, and the modulus of elasticity calculated from the whole fruit load-deformation curve. Acoustic response measurements gave a reliable indication of the chan ge in mechanical properties of fruit before, during and after harvest. In particular there were indications that the acoustic response may g ive additional information on fruit water status which is not detectab le by conventional firmness measurements. Given the non-destructive na ture of acoustic response measurements, the method appears to have con siderable promise as' a technique for the evaluation of the post-harve st condition of apples. (C) 1997 Silsoe Research Institute.