Non-destructive determination of fruit maturity by a resonant vibration technique

The resonant frequency of fruit obtained by vibrating the fruit with a range of frequencies (100-3000 Hz) of sinusoidal wave is closely related with the firmness of fruit, which is generally related to its maturity. An apple vibrated by a vibrator showed several resonant peaks. The lowest resonant peak (f₁) was found to reflect the local information of the area attached by the vibrator. The second lowest resonant peak involves the vibration of the whole fruit. The vibration mode at 2^nd resonant frequency was physically defined as ₀S₂, one of the spheroidal modes (_nS_m). An elastic modulus (E) of the fruit can be calculated with the 2^nd resonant frequency (f₂) as, E=α×f₂²×ρ^(1/3)×m^(2/3) (α is a shape constant; ρ, fruit density; m, mass of fruit). We defined Elastic Index (EI) as EI= f₂²×m^(2/3), since α is constant, and ρ is nearly unit. Correlation coefficients between kiwifruit firmness measured by a conventional penetrating method and EI were over 0.9 in core, locule interface and outer pericarp. ΕΙ was correlated with the fruit maturity by sensory evaluation. In melon, correlation coefficient between EI and sensory score was 0.96, while that between soluble solid (%) and the sensory score was only 0.26. EI of immature melon predicts days waiting for full maturity and relishing period of melon fruit. Abnormal low f₂ distinguished core rot fruit of pear with impaired volume more than 5% of total fruit volume from those less than 5%.