Fruit Ripening Evaluation Using Acoustic Vibration Method in ‘Rainbow Red’ Kiwifruits

The degree of maturity of the kiwifruit ‘Rainbow Red’ (Actinidia chinensis) cannot be determined accurately because the progression of fruit ripening shows marked variation. Here, we evaluated the fruit characteristics by non-destructive testing using acoustic resonance measurements. The elasticity index determined from the second, third, and fourth resonant frequencies decreased with ripening. Strong correlations were found between acoustic resonance measurements and flesh firmness, soluble solid contents, and titratable acid levels. The ripening process from pre-harvesting to over-ripeness was divided into seven stages according to the elasticity index determined from acoustic resonance measurements. The optimal harvesting time was determined from the elasticity index from the second, third, and fourth resonant frequencies. The ripening patterns of the elasticity index were examined in identical fruit. The elasticity index was correlated with the flesh firmness measured using a fruit durometer, modeling the elasticity index and measurement value by single regression analysis. These findings suggested that acoustic resonance measurements were sufficiently accurate to determine the characteristics of the fruit, and that this is a useful alternative method to use for in the A. chinensis ‘Rainbow Red’.