A Low-Cost Handheld Photoacoustic (PA) Probe for Rapid and Non-Destructive Detection of Watermelon Ripeness

For improving the quality and profitability of watermelon production, rapid and non-destructive detection of watermelon ripeness is important. Conventional acoustic and optical testing methods are based on indirect measurements, which require complex modeling and suffer from low accuracy. This article reports the initial investigation of using photoacoustics (PAs), a hybrid modality of light and sound, for non-destructive detection of watermelon rind thickness and inner flesh redness, the two visual hallmarks of watermelon ripeness. A new low-cost handheld PA probe is designed and fabricated to enable rapid and reliable PA measurements in air by simply touching the watermelon surface. The data acquisition capability of the PA probe is tested on personal watermelon samples with different rind thicknesses and embedded red-ink solutions to simulate different levels of flesh redness. The rind thickness and flesh redness are estimated based on the time-of-flight (ToF) and correlation with the peak-to-peak amplitude of the acquired PA signal, respectively. The captured PA signals are also used to differentiate the rind thickness and flesh redness with machine learning. The results show that the new PA probe could provide a promising solution for rapid and non-destructive detection of the ripeness of watermelons, especially those in the field before harvesting.