Vibrational and Impact Response Properties of Agricultural Products for Non-destructive Evaluation of Internal Quality (I)

Relation between Resonant Frequency and Texture of Watermelon

Abstract

The vibrational resonant frequency of watermelon was investigated in order to evaluate the internal quality nondestructively. The results obtained are supposed to be applicable for the determination of the maturity date for picking in the field and for the automatic mechanization of grading for watermelon in the packing line.
1. Sugar content, specific gravity and compressive strength of flesh had a high correlation with the accumulated temperature of watemelon in the field, and proved to be useful factors for the quality determination.
2. The resonant frequency of sound watermelon was not changed by the direction of vibration in both type A and B (Fig. 1). Therefore, the internal quality was evaluated by the one-direction vibration.
3. The resonant frequency appeared in the lower frequency ranging from 150Hz to 450Hz, and the change of resonance pattern was not affected by the type of vibration (type A and B).
4. The amplitude of resonance for watermelon with cavity decreased remarkably as compared with that of sound one. The decreasing amount of the amplitude was proportional to the volume of cavity in the watermelon. The pattern of resonance changed with the direction of vibration applied to the watermelon.
5. The resonant frequency of watermelon transferred to the lower side due to the increase of the weight. Supposing the watermelon as 2-media elastic sphere model, the relation among resonant frequency (f), weight (m) and shear modulus of flesh (μ₁) was obtained as follows;
f²·m^2/3_??_μ₁
This indicated that f²·m^2/3 was not affected by the weight of watermelon, and agreed with experimental results.
6. The index of f²·m^2/3 had a high correlation with compressive strength of flesh, a correlation with specific gravity, but no correlation with sugar content of the flesh.