Nondestructive measurement of kiwifruit ripeness using a laser Doppler vibrometer

A laser Doppler vibrometer was applied to evaluate the viscoelastic propert ies of kiwifruit [Actinidia deliciosa (A. Chev.) Liang et Ferguson, cv Hayw ard] at various stages of ripeness. A stiffness coefficient (S) was defined as f(n=2)(2) m(2/3), where f(n=2) was the frequency of the second resonanc e peak and m was the fruit mass. A loss coefficient (eta) was defined as (f (2)-f(1))/f(n=2), where frequencies f(1) and f(2) were determined at 3 dB b elow the second resonance peak (f(2) > f(1)). Fruit firmness of the samples that had been used for the laser Doppler measurement Mns determined by mea suring the force required to insert a conical probe 5 mm into the cut surfa ce of a fruit slice. There was a highly significant relationship between th e stiffness coefficient and the firmness of the kiwifruit core (r(2) = 0.96 7). The loss coefficient correlated well with soluble solids content. The l ass and sriffness coefficients changed during ripening at 20 degrees C; the stiffness coefficient decreased, while the loss coefficient increased. The re was a characteristic inverse relation between the loss and stiffness coe fficients. When the stiffness coefficient decreased to 1 x 10(7) in the ear ly stages of ripening, the loss coefficient increased only from 0.1 to 0.2. When the stiffness coefficient decreased further from 1 x 10(7) to 0.1 x 1 0(7) in the late stages of ripening, the loss coefficient increased drastic ally from 0.2 to 0.8. The results indicate that early stages of fruit softe ning are reflected by the stiffness coefficient and late stages are reflect ed by the loss coefficient. Therefore, the two coefficients clearly disting uish between ripe and unripe kiwifruit.