2019 ASHS Annual Conference

We previously reported that the technique for measuring watermelon texture by using elastic surface waves (ASHS annual meeting 2009). Although previous study provided a useful method for measuring elasticity of watermelon fruit flesh, the method was not able to be used for the screening of a whole fruit, because this was a destructive method. This study aims to develop a non-destructive method to evaluate the elasticity of the fruit flesh.

Surface waves were excited with an oscillator composed of a shaker, a function generator and a power amplifier. In this study we newly employed a laser Doppler vibrometer for detecting surface elastic waves. Signals were recorded with 2 mm increment of propagation distance by moving the sliding stage supporting a laser mirror. The time shifts of the detected peaks were plotted as a function of the propagation distance between the oscillator and the detecting laser, from which the surface wave velocities were obtained. Small-sized watermelons (Citrullus lanatus (Thunb.) Matsum. et Nakai) ‘Hitorijime-BonBon’ with diameter of approximately 16 cm were obtained from a farmer (Ibaraki, Japan).

We obtained velocity dispersion curves for whole fruits in the frequency range of 400–7000 Hz. The velocity in the low frequency limit corresponded to the value for the fruits, the peel of which were removed. This result suggested that we can obtain the velocity for flesh from non-destructive measurement of whole fruit. Also, we made measurements after we kept watermelon fruits in the refrigerator for several days, the velocity was reduced from the value of the fresh fruits, indicating that the firmness was reduced by maturation.

Even we still have to improve the system, we demonstrated the possibility for the non-destructive measurement of the degree of the firmness of watermelon fruit in this study.