MECHANICAL DAMAGE INDUCED BY CONTROLLED FREEZING IN APPLE AND POTATO

摘要

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