The texture of apple flesh is important in assessing its eating qualities. The texture in turn is related to the structure of the parenchyma. The parenchyma cells of the fruit are arranged in radial quasi-columnar form with radial spaces in between. This anisotropy has a marked effect on the fracture properties such that it is much easier to drive a crack between the columns (radially) than to drive it across them (tangentially). The fracture tests used were simple crack-opening tests under tension or using a wedge. This difference was also detected by a taste panel. The radial spaces ease the passage of cracks travelling along them, and act as crack stoppers for cracks travelling at right angles to them. They also allow the cells to deform more in one orientation more giving the structure ductility and making the apple tougher in that orientation. It is possible to increase this effect by controlled damage such as slow freezing which causes the intercellular spaces to expand increasing the crack-stopping mechanisms and increasing the ductility, therefore increasing the fracture toughness. Toughness first increases, then decreases with increasing damage. This effect can be mimicked with brittle paper: fracture toughness of tracing paper initially increases if holes are punched randomly in it.
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