COMPRESSIVE STIFFNESS AND FRACTURE PROPERTIES OF APPLE AND POTATO PARENCHYMA

The compressive mechanical properties of fruit and vegetable parenchym a are related to the morphology of the material such as size, shape an d orientation of cells and intercellular spaces, and cellular adhesion . Uni-axial compression tests on geometrical specimens of apple and po tato flesh have shown that apple parenchyma is mechanically very aniso tropic whereas potato shows no such behaviour. Apple flesh is highly o rientated in the cortex into radially elongated cells and intercellula r spaces. If the flesh is compressed along the rows of cells (radial) it generally fractures by a collapse of single layer of cells at right angles to the force. If the flesh is compressed at right angles to th e rows of cells (tangential) it fails in shear. It is stiffer radially but tougher tangentially requiring greater deformation to fail. There is no orientation of such morphological structures in potato and henc e the compressive properties are not dependent on the direction. It al ways fails in shear. Mature late season apples tend to be stiffer and tougher than early apples as they have smaller cells and thicker cell walls. Early season apples become mealy quickly and cracking occurs by cell separation due to the early breakdown of pectins in the weak int ercellular lamellae. This makes them mechanically much weaker than lat e season apples where cell adhesion is strong and cracking occurs by c ell rupture, a process requiring greater force and energy.