Assessment of the mechanical properties of Tarocco orange fruit under parallel plate compression

Abstract

In this work the mechanical compressive properties of orange fruits of the Tarocco variety were assessed using a Universal Testing Machine, equipped with upper and lower plates made of transparent graduated Plexiglas® and three cameras positioned along the axis lines x, y and z. By submitting each fruit to different engineering strains ( ε z ) along the z-axis in the range of 3–21% of the initial fruit height, the momentary contact area of each fruit under testing was precisely determined by approximating its outline to a polygon and reconstructing it via the Elliptic Fourier Analysis, or an ellipse inscribed in a bounding box. Both estimates yielded no statistically significant difference and varied linearly with ε z . Their prediction using the ASABE Standard Method (2008), or assuming no variation in the volume of the fruit undergoing compression, resulted be under- or over-estimated by 45% or 19%, respectively. Use of image analysis allowed the apparent Poisson’s ratio and modulus of deformability ( E O) of the whole orange fruit to be estimated as equal to 0.16 ± 0.09 and 353 ± 3 kPa, respectively. By assimilating the orange fruit to a deformable membrane ( flavedo) filled with an internal incompressible juice, it was possible to estimate the compressive stress acting on the equatorial horizontal cross-section of the epicarp only and thus estimate the apparent modulus of elasticity of the orange peel (=375 ± 33 kPa), and a burst pressure of 533 ± 103 kPa at a rupture strain ε z R of 0.30 ± 0.03 for rind thicknesses ranging from 0.8 to 4.0 mm.