Evaluation of the firmness measurement of fruit by using a non-contact ultrasonic technique

In this study, we investigated the feasibility of using a novel non-contact ultrasonic technique to measure fruit firmness. The intensity of the reflected ultrasonic signal that bounces back from the fruit surface is dependent on the firmness of the fruit surface. However, differences in fruit size cause the reflected intensity to change with the distance separating the non-contact ultrasonic transducer and fruit surface. Therefore, it is necessary to eliminate the effect of fruit size on signal intensity to accurately predict fruit firmness when using a reflected ultrasonic signal. In this study, we developed and evaluated distance correction factors, which may reduce the size effects of fruit on the reflected ultrasonic signal. The distance correction factors were proposed using a combination of ultrasonic parameters obtained from the first and second reflected ultrasonic signals in the time and frequency domain. Fruit firmness parameters, such as bioyield strength and apparent elastic modulus, were measured and correlated with the distance corrected non-contact ultrasonic parameters. Multiple linear regression models to predict fruit firmness using ultrasonic parameters were assessed. The results demonstrated the potential utility of the distance corrected non-contact ultrasonic technique for the non-destructive measurement of fruit firmness.