The Acoustic Impulse Response Method for Measuring the Overall Firmness of Fruit

Abstract

An acoustic impulse method has been used to measure the maturity of ‘‘Golden Delicious’’ apples throughout the growing season and during eight months cool storage at 1°C and 96% r.h. Over the same period, penetrometer firmness was measured. Comparison of the results has enabled correlations to be made between the elastic coefficient of apple tissue and the mean firmness at a depth from 5 to 9mm below the skin. Under the experimental conditions used, the penetrometer measurements provided better discrimination between immature fruit, while the acoustic 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 between the coefficient of elasticity calculated from acoustic measurements, colour measurements (lightness and hue), the force required to compress 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 change in mechanical properties of fruit before, during and after harvest. In particular there were indications that the acoustic response may give additional information on fruit water status which is not detectable by conventional firmness measurements. Given the non-destructive nature of acoustic response measurements, the method appears to have considerable promise as a technique for the evaluation of the post-harvest condition of apples.