ANALYSIS OF THE MECHANICS OF THE FRUIT PRESSURE TESTER

The Magness-Taylor pressure tester is used extensively as a firmness measuring device for convex-shaped fruits. Researchers have been interested to learn what is really being measured by this device when its probe is pressed against the fruit to the point of flesh rupture. This investigation analyzes the mechanics of deformation of Rome Beauty apples, with the skin sliced off, in two deformation stages namely prior to and after puncturing of the flesh. Theories of inelasticity and piercing problem are adopted in analyzing the theoretical stress distribution. With the stress distribution, the theoretical force-deformation relations were found which show the effect of probe size, shape, radius of curvature, and the mechanical properties of the fruit. Experimental tests included simple compression of cylindrical specimens of the flesh, punch shear tests of slices of the flesh, and pressure tests of whole pared fruits. With the 5/16-in. probe, agreement between experimental and theoretical results was within 1.5 % and 2.2 % for the first and the second deformation stage, respectively. Results showed that; (1) the Magness-Taylor pressure test involves a combination of shear and compression stresses; (2) an average conversion factor of 1.85 can be used to convert readings by a 5/16-in. rigid die to those by a 7/16-in. die; and (3) shearing strength of the flesh is the only mechanical property needed to predict the maximum force reading after puncturing of the flesh.