Research noteNondestructive determination of watermelon flesh firmness by frequency response

•Quality of watermelon was nondestructively tested using laser vibrometery.•A stepwise method applied to the phase spectrum allowed firmness prediction.•Selected frequencies were contributed to make predictive model.•Laser vibrometery is a modern approach for accurately evaluating watermelon texture.

The flesh firmness is an important indicator to detect ripeness of watermelon (Citrullus lanatus). It can be used for sorting fruits. A nondestructive method based on frequency response function (FRF) was applied to determine firmness of watermelons of the variety Crimson Sweet. The fruit's response to vibration C. lanatus (Thunb.) var. lanatus was detected using Laser Doppler Vibrometry (LDV). Spectra were extracted from FRF throughout a wide frequency range. Firmness was measured at three points of the watermelon cross section by penetrometry. Stepwise multiple linear regression (SMLR) and partial least square regression (PLSR) were applied on extracted vibration spectra to construct prediction models of watermelon texture. The best performance was obtained with SMLR models applied to the phase spectrum. The determination coefficients (R2) of the calibration and cross validation models were 0.9999 and 0.9993 respectively. Evolution of viscoelastic properties of watermelon flesh during softening and ripening process was reflected by vibration response. The firmness of watermerlons could be predicted from their phase spectra.