BRUISING AND ENERGY DISSIPATION IN APPLES

A strong correlation was obtained between bruise volume and energy absorbed for both impact and slow compression of Granny Smith apples. Apple tissue was more easily bruised by slow compression than by impact. For the same amount of energy, bruise volumes were approximately 40% higher under slow compression. There was no correlation between the stiffness (force/deflection) of apple flesh and bruise volume. It is unlikely that the theories of failure based on the theory of elasticity will prove useful in predicting the extent of bruising. The application of elastic theory to bruise initiation was not studied in this paper. A simple model was formulated to explain the behaviour of apples under compression. Apple tissue was likened to an orderly arrangement of liquid-filled, spherical cells bounded by viscoelastic membranes with air-filled interstitials. It is postulated that on initial compression the cells are deformed into ellipsoids under a stress distribution similar to that found in an elastic sphere. Further compression would then result in cell wall fracture, i.e., cell bursting in regions of high shearing stress. Distortion and bursting of cells explains the energy dissipative mechanism. The work showed that energy absorbed during compression is a good predictor of bruising and is, thus, a useful parameter in evaluating handling and packing systems.