Relationship between density and stiffness of apple flesh

The parenchyma of a number of varieties of apple was tested in torsion. It is necessary to use small samples (cylinders 10 mm long × 5 mm diameter) orientated to take account of the radial air spaces. Over a range of nine varieties of apple, the torsional stiffness (0.5 to 7 MPa) is largely dependent on the average density (0.6 to 0.9 g cc −1 ) both within and between varieties. At a given density, the outer parenchyma is stiffer than the inner. As the apple matures, density and stiffness reduce and their mutual dependence is also less. A model is presented which can convert density (or volume fraction) of packed cells into the degree of mutual cellular contact. The calculated degree of contact (never greater than 0.4 of the wall area of a cell) correlates well with stiffness, though the nature of the correlation varies with the variety of apple and its maturity. The anisotropy and density/stiffness relationship must be considered in any mechanical (and presumably, therefore, organoleptic) testing of apples.