The use of a laser Doppler vibrometer to assess watermelon firmness
• The measured locations of watermelons did not affect the second resonant frequency f 2 . • The repeatability of f 2 measured by the LDV system was high. • The stiffness coefficient S 2 had better linear relationships with firmness variables. Firmness is an important factor in describing the quality of agricultural products and is correlated with the vibrational characteristics of the object. In this study, the vibration response of ‘Qilin’ watermelons at postharvest was measured with an experimental system based on a laser Doppler vibrometer (LDV) for firmness detection. The vibration excitation applied by an electrodynamic shaker was monitored simultaneously with an accelerometer. After the excitation and response signals were transformed to the same dimension and converted from time-domain into frequency-domain by fast Fourier transform (FFT) processing, the ratio of response to excitation was calculated to determine the second resonance frequency ( f 2 ). Subsequently, three widely used stiffness coefficients ( S 1 = f 2 2 m , S 2 = f 2 2 m 2 / 3 ρ 1 / 3 and S 3 = f 2 2 m 2 / 3 , where m is the sample mass and ρ is the sample density) were calculated. These coefficients were selected as vibration parameters in addition to f 2 . Moreover, a puncture test was conducted to obtain reliable firmness variables from force/deformation curves, including maximum force ( F max ) and mean force at a 3–10 mm distance ( F ave ). The effect of the measured locations of watermelons on f 2 was not significant, and a relatively stronger linear relationship was observed between S 2 and F max ( r = 0.410 with P < 0.01). However, no strong relations could be established between the vibration parameters and the firmness variables. This was most likely because of the firmness reference method, the watermelon variety or the small distributions of weight and density of the test samples. Further efforts are needed to identify the reasons for the weak relations.