Firmness Measuring System of Fruits and Vegetables with a Light-Contact Vibrating Probe

Abstract

A system consisting of a probe, a vibrator and displacement sensors, and a servo analyzer was developed to evaluate firmness of fruits and vegetables nondestructively. The probe whose top has spherical surface was set with initial loading force of 5 gf to a sample. The sample was vibrated with multi-sine wave which has the frequency ranged from 20 to 200 Hz. Both the displacement (input) and force (output) signals were measured at the same probe and transferred to the servo analyzer. A transfer function was calculated by the servo analyzed. With theoretical consideration, proposed that anti-resonant frequency, f_u, could be used as an index of firmness. The relationship between f_n and dynamic modulus, G', was examined in model specimens made of silicone rubber, which gave a logarithmic relationship with a correlation coefficient of 0.930. As an application to fruits and vegetables, tomatoes and strawberries in several storage conditions were measured by this system. In addition, conventional penetration test was also carried out to each sample. The initial force-displacement rate, w/d, which is equivalent to elastic modulus, was determined by the penetration test. There was the logarithmic relationship between f_n and w/d, which is the same as the model specimens' we observed, and the correlation coefficient was 0.899. These results indicate that this system is available for the easy and intact firmness evaluation for fruits and vegetables.