PH—Postharvest Technology: Modelling the Load Distribution of Stacked Apples and other Spherical Objects

When fruit are stacked in regular or random arrays, the probability of damage (e.g. bruising) depends on the depth of the stack of fruit, and the distribution of pressure amongst the fruit in the lower layers. Studies have shown that the load is not uniformly distributed between all fruit in the lowest layer, but that there is a wide range of pressure on each individual fruit. To date no suitable explanation for this phenomenon has been offered. In this paper, a laboratory simulation using steel spheres was used to confirm the variability found in field experiments on apples. A simple computer model is proposed which generates pressure distributions that vary in a similar fashion to those found in practice. The model is based on the assumption of a random allocation of pressure to each object in a load bearing position. The model is based on a regular arrangement of spheres, but the nature of the model, and the choice of load-sharing factor, enables the model to be effective for random distributions. The model predicts that the load on individual apples or spheres can be many times greater than the average load which would be expected if the pressure were distributed uniformly between all potential load-bearing spheres or apples on the bottom layer of a stack.