Although agricultural materials are generally nonhomogeneous, non-isotropic and non-elastic, it has been possible to define an elastic range of behaviour within which elastic parameters can be quantified. Mohsenin (1986) observed that under small strains, most agricultural materials exhibit extensive elasticity, to which Hertz’s theory of contact stress is applicable. However, owing to the complexity of the stress system and inability to determine the failure area with sufficient accuracy, the force–deformation response of an egg (Reece & Lot, 1976) and acocoapod (Faborode & Dinrifo, 1994) was expressed in terms of stiffness modulus rather than elastic modulus. The stiffness modulus is the ratio of the maximum load to the maximum deformation at the straight-line portion of the force–deformation curve. Toughness is defined as the work required to cause the material to rupture. This can be approximated by the area under the stress–strain or force–deformation curve up to the rupture point (Mohsenin, 1986).