Nondestructive detection of split pit in peaches using an acoustic vibration method

Split-pit in peach fruit is a problematic disorder. Split-pit fruit cannot be detected based on external appearance, and contamination of fruit by split-pit reduces its reliability in the marketplace. Here, we demonstrate that split-pit fruit can be identified by a nondestructive acoustic vibration method and a unique approach based on the ratio of the third (f₃) to the second (f₂) resonant frequency. The response-resonant frequency spectra showed that the peaks of f₂ frequencies in split-pit fruit were shifted to much lower values than those in normal fruit, whereas those of f₃ frequencies showed only small shifts. The calculated f₃/f₂ ratios in most normal fruit were in the range of 1.35–1.4, whereas those in split-pit fruit were 1.45–2.0. Analysis of more than 300 fruit samples revealed that by setting the f₃/f₂ cut-off value at >1.45, 95% of split-pit fruit in the fruit samples were detected, whereas only 1.5% of normal fruit were missorted as split-pit fruit. A model for simulating the vibration properties of peach fruit was developed by using the finite element method. The simulated vibration patterns showed that f₃/f₂ values were increased by the insertion of split pit, indicating that, at least partially, the observed high f₃/f₂ values in split-pit fruit directly reflected split-pit occurrence. These results clearly demonstrate that the use of f₃/f₂ ratios obtained using an acoustic vibration method can effectively detect fruit with split-pit. The possibility of installing acoustic vibration devices in peach sorting lines and the application of portable devices to unpicked fruit on the tree are discussed.