Fracture properties of stored fresh and osmotically manipulated apple tissue

The purpose of this research was to use single-edge notch three-point bend and cutting tests to evaluate changes in the texture of Granny Smith apples under room temperature storage conditions and to relate these changes to changes in internal structure. Experiments were performed at 1 week intervals over a period of 4 weeks. Specimens were soaked in buffer solutions in order to attain maximum and constant turgor pressure. Confocal microscopy showed that internal air spaces increased and cell adhesion decreased with increased storage time. Fracture toughness (a parameter that quantifies the ease of propagating a crack due to stress concentration at its tip) and cutting energy (the energy required to cleave the bonds in the plane of cutting) did not follow the same patterns. Relative changes in fracture toughness were far larger ( ≈50%) than changes in cutting energy (≈30%) in fresh specimens. However, relative changes in cutting energy (≈30%) were far larger than in fracture toughness (≈5.5%) in soaked specimens. We conclude that in Granny Smith apples, cutting energy is a more sensitive measure of cell adhesion, whereas fracture toughness obtained from bend tests seems to be related more directly to cell turgor pressure. Received: 8 October 1999 / Revised version: 15 December 1999