Relation between mechanical properties and structural changes during osmotic dehydration of pumpkin

Abstract

This work presents the changes in the mechanical properties of pumpkin (Cucurbita pepo L.) fruits when submitted to osmotic dehydration processes. Cylinders of the parenchymatic tissue were dehydrated with sucrose solutions, varying the concentration (30–60% w/w) and temperature (12–38 C) of the osmotic solution and process time (0–9 h). As an opposite process to dehydration, water soaking of some cylinders was also performed. Samples were submitted to uniaxial compression until rupture, and four parameters were analyzed: apparent modulus of elasticity, true stress at failure, Hencky strain at failure and failure work (toughness). Values of these mechanical properties for fresh material ranged from 0.96 to 2.53 MPa for apparent modulus of elasticity, 250–630 kPa for failure stress, 0.42– 0.71 for failure strain and 85–285 kJ/m 3 for toughness. Mechanical properties of osmodehydrated samples showed no dependence on concentration of the osmotic solution and process temperature, whereas they were found to be dependent on moisture content: apparent elastic modulus decreased and failure strain increased during dehydration; toughness and failure stress initially decreased with moisture content, and increased at advanced stages of the process. Water soaked samples showed a decrease in failure strain, failure stress and toughness, but the apparent elastic modulus increased. Simultaneous structural observation during compression showed that the material fails in the contact zones of its fibres. This fact and the observed structural profiles during dehydration could explain the changes in the failure properties (strain, stress, toughness) along the studied processes. Changes in the apparent modulus of elasticity were likely related with the changes in the turgor pressure of cells. 2007 Elsevier Ltd. All rights reserved.