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Strawberry is a non-climacteric fruit with a limited harvesting period. Because of high susceptibility to mechanical damage, strawberry has a small postharvest life. In this research, an experiment was designed to study the mechanical damage phenomena in strawberry during the harvest and postharvest operations together with some physical properties of strawberry. Influences of some other factors such as variety, fruit position in the box as well as box position on the truck were also investigated. Results indicated that the variety, operation stage, fruit position in the box, and box position on the truck, had significant effects on the extent of the fruits' mechanical damage. Maximum damage index was related to picking stage. The variety Gaviota showed more susceptibility to mechanical damage than Selva. The maximum damage occurred at the bottom rows in the boxes. Furthermore, it was observed that the higher the position of a box on the truck, the more the susceptibility of fruits would be to mechanical damage.

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Impact forces during the contact are the main sources of damage in Peach fruit. In order to reduce this damage, it is necessary to model the impact forces during handling and transporting. An experimental procedure was conducted to determine the parameters of a viscoelastic normal contact force model. Peaches were subjected to dynamic loading by means of a pendulum at the levels of impact. In order to estimate the values of the parameters (spring, "k" and damping, "c") of the Kuwabara and Kono viscoelastic contact force models for normal impacts the nonlinear least squares technique to be calculated. Two prediction models were constructed for the parameters "k" and "c" using multiple linear regression analyses. In these models, impact velocity and two fruit properties (the effective radius of curvature and acoustic stiffness) were used as independent variables. Significant effect of impact velocity, the effective radius of curvature and acoustic stiffness and some interactions on the parameters "k" and "c" were obtained at 5% probability level with the 6 Corresponding 483 coefficient of determination of 0.99 for models. It was concluded that lowering the impact velocity and increasing the effective radius of curvature and acoustic stiffness will enhance the parameters "k" and "c" of the peach fruit, but there was no significant effect of the acoustic stiffness on the damping parameter. PRACTICAL APPLICATIONSA very promising approach for the simulation of fruit impact damage during transport and handling is the contact force discrete element method (DEM). In order to do so, models for the forces acting between particles (like fruits) in contact need to be specified. Forces acting between the two particles are decomposed into normal and tangential components. In this paper the focus is set on normal contacts. The presented research determines the parameters of normal contact forces models suited for DEM simulation of viscoelastic materials (peach fruit). 484E. AHMADI ET AL.The most frequently employed models in discrete element simulations are the linear models. In order to include energy dissipation, their normal force always consists of two parts, one for modeling the elastic repulsion, the other DYNAMIC MODELING OF PEACH FRUIT 485 DYNAMIC MODELING OF PEACH FRUIT 487

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