Firmness evaluation of watermelon flesh by using surface elastic waves

•Surface elastic waves were applied for the firmness evaluation of watermelon flesh.•The surface-wave velocity for two watermelon varieties provided shear elasticity.•The velocities correlated well with the sensory firmness evaluation.•The velocity showed a decrease of 10% after 10 days of storage.

We measured the velocity of surface elastic waves on watermelon flesh for firmness evaluation. The Rayleigh waves at frequencies ranging from 800 to 2400 Hz propagate on the flesh and are detected by a piezo-bimorph sensor in contact with the flesh. We determined the shear elasticity from the velocity for two types of Japanese watermelons, Matsuribayashi777 and Wasenissho, to be 1.18 and 0.74 MPa, respectively. These correlated well with a sensory firmness evaluation. The values of Young modulus obtained by the surface-wave measurements were nine times larger than those by the compression tests performed, which can be explained by the differences in measurement displacement and frequency. We also investigated the effect of storage on the surface-wave velocity for Matsuribayashi777. The velocity decreased by about 10% after ten days of storage. The present results suggest that this technique can be applied to estimating the elastic properties of various fruits.