Firmness evaluation of watermelon and melon using velocity dispersion of surface-acoustic-wave

Abstract

The surface-acoustic-wave (SAW) velocity was measured in the frequency range of 400–7000 Hz in watermelon and melon to evaluate the firmness nondestructively. We showed that the positive velocity dispersion (velocity increases with increasing frequency) observed was caused by the fruit structure, which consists of a hard pericarp and underlying soft flesh. In watermelon, the low-frequency limit of the velocity dispersion curve observed for the pericarp predicted the SAW velocity in watermelon flesh, which was measured to be independent of frequency. In melon, the positive velocity dispersions observed for the pericarp as well as the flesh suggested a distribution of elasticity in the depth direction. Ripening for fourteen days caused a decrease in the SAW velocity by 34%–57% depending on the frequency. The present results demonstrate that the SAW velocity dispersion is a good measure of the firmness and ripening of fruits.