MECHANICAL DAMAGE-INDUCED BY CONTROLLED FREEZING IN APPLE AND POTATO

Freezing was used to induce controlled damage to apple and potato pare nchyma. As the tissue freezes ice crystals form extra- or intracellula rly, pushing the cells apart or rupturing cell walls producing large v oids within the tissue. Mechanical tests (wedge penetration, tensile a nd compression) on specimens frozen to a certain temperature at a cont rolled rate and thawed show that the changes in the mechanical behavio ur of the material are directly related to the degree of cell damage. As the tissue freezes, the fastest rate of tissue damage occurs betwee n -3C and -10C. In this range there is a marked decrease in stiffness and an increase in the deformation required to cause failure. In apple s, slow freezing leads to the intercellular spaces expanding, increasi ng their aspect ratio and hence, the anisotropy of the tissue. The inc reasing number of voids increases crack-stopping and ductility; more e nergy is dissipated in flow elevating the fracture toughness. Toughnes s then decreases with further damage. This effect can be mimicked with brittle paper: fracture toughness of tracing paper initially increase s with increasing number of holes punched randomly in it.