Microstructural changes during osmotic dehydration of parenchymatic pumpkin tissue L. Mayor a , J. Pissarra b , A.M. Sereno a, * a REQUIMTE, Department of Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal b Department of Botany, Faculty of Sciences and Functional Plant Biology Unit – Institute for Molecular and Cell Biology, University of Porto, Rua do Campo Alegre, 823, 4150-180 Porto, Portugal Received 30 January 2007; received in revised form 17 June 2007; accepted 29 June 2007 Available online 25 August 2007 Abstract Pumpkin (Cucurbita pepo L.) fruit was selected to study the microstructural changes of vegetable tissue during osmotic dehydration. Cylinders (diameter: 1.5 cm; length 2.5 cm) of the parenchymatic tissue were dehydrated with 45% (w/w) sucrose solutions at 25 °C. The microstructure of fresh and processed samples was observed using light microscopy techniques. Fresh pumpkin cells showed average values of 0.015 mm 2 , 0.469 mm and 0.136 mm for cell area, cell perimeter and cell equivalent diameter, respectively; and 1.288, 0.831 and 0.871 for cellular elongation, roundness and compactness, respectively. The main modifications observed during osmotic dehydra- tion were shrinkage of cells, plasmolysis and folding of the cell walls. These changes led to the decrease of cellular area, equivalent diam- eter, roundness and compactness; elongation of cells increased whereas the perimeter was maintained essentially constant along the process. The observed changes were not homogeneously distributed in the material, and were dependent on cell location in the sample and on process time. Empirical quadratic functions were used to relate the average cellular shape and size parameters with the dehydra- tion parameters water loss, weight reduction and normalized moisture content. The equations showed a good fit of the experimental data, leading to correlation coefficients ranging 0.93–0.99 and average relative deviations ranging 0.7–2.8%. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Light microscopy; Microstructure; Pumpkin; Vegetable cells; Shape factors; Shrinkage 1. Introduction Osmotic dehydration is a common water removal pro- cess applied to food materials that consists of placing pieces of a biological tissue, such as a fruit or a vegetable, in a hypertonic solution. Since this solution has higher osmotic pressure and hence lower water activity, a driving force for water removal arises between solution and food, while the natural cell membrane acts as a semipermeable membrane. The diffusion of water is accompanied by the simultaneous counter diffusion of solutes from the osmotic solution into the tissue. Since the membrane responsible for osmotic transport is not perfectly selective, other solutes present in the cells can also be leached in the osmotic solu- tion (Giangiacomo, Torreggiani, & Abbo, 1987). The technique is often used as a pre-treatment of other processes (Tregunno & Goff, 1996), to produce intermedi- ate moisture foods with improved shelf life characteristics (Monsalve-Gonza ´lez, Barbosa-Ca ´novas, & Cavalieri, 1993), or as a pre-treatment to reduce the energy consump- tion and/or heat damage in other traditional dehydration processes (Hawkes & Flink, 1978; Kim & Toledo, 1987). When a fruit or vegetable is submitted to a dehydration process, associated heat and mass transfer gradients pro- duce changes in the chemical, physical and structural char- acteristics of the plant tissue, such as changes in volume/ porosity (Lozano, Rotstein, & Urbicain, 1983; Mayor & 0260-8774/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2007.06.038 * Corresponding author. Fax: +351 22 508 1449. E-mail address: sereno@fe.up.pt (A.M. Sereno). www.elsevier.com/locate/jfoodeng Available online at www.sciencedirect.com Journal of Food Engineering 85 (2008) 326–339