Measuring the mechanical properties of apple tissue using modal analysis

Along with the grade of intactness, fruit firmness is an important factor for the quality criteria on which the consumers' acceptance for applies is based. Two methods for measuring the fruit firmness (a destructive one and a nondestructive one) are compared. The nondestructive method uses modal analysis to define the lower vibration modes. This enables the use of the 3-media elastic sphere model developed by Cooke and Rand to calculate the dynamic elastic moduli of the whole apples. The destructive method determines the dynamic elastic moduli of cylindrical samples of apple tissue. This method is based on the vibration characteristics of a mass-spring-damper system. This system consists of a cylindrical apple sample and a mass. Measuring the direct transfer function of this one-degree-of-freedom system results in values of its system parameters, from which the dynamic elastic moduli of the specimen can be calculated. The results of the two methods applied on Golden Delicious apples were compared, and the suitability of each model was evaluated.