ANISOTROPY IN THE FRACTURE PROPERTIES OF APPLE FLESH AS INVESTIGATED BY CRACK-OPENING TESTS

The texture of apple flesh is important in assessing the eating qualities of the fruit. Texture is in turn related to the structure of the parenchyma. Crack-opening tests (wedge penetration tests and notch tensile tests) have shown the tissue to have marked anisotropy in its fracture properties. These differences can be detected by sensitive mechanical instruments and also in the mouth. The flesh of the apple is split much more easily along the fruit's radius than, for example, in a direction parallel to the fruit's tangent. This was shown with the fracture tests as well as discriminated by a taste panel. In tangential orientation the wedge, or teeth, have to penetrate to a greater distance exerting a greater force to initiate a free-running crack, and fracture toughness is about 50% greater than in radial orientation. The mechanical behaviour of apple parenchyma is directly related to its structural composition. The radially elongated intercellular spaces ease the passage of radially travelling cracks, i.e. along the direction of the spaces, and act as crack stoppers and crack deflectors to tangentially travelling cracks, i.e. at right angles to the spaces. This increases the energy requirement for crack propagation for tangential cracks hence increasing the fracture toughness in that orientation.