The effect of ultrasound-assisted osmotic dehydration applied for different lengths of time on pineapple tissue structure was evaluated. Using distilled water as the liquid medium, ultrasound induced disruption of cells and formation of microscopic channels in the fruit structure but did not induce breakdown of the cells. Consequently, ultrasound application increased sugar loss and water diffusivity because of the formation of microscopic channels, which offered lower resistance to water and sugar diffusion. Osmotic dehydration induced gradual distortion of shape of the cells, disconnection between the cells and the formation of channels by breakdown of the tissue. The changes caused by the application of osmotic dehydration in the fruit tissue structure resulted in a higher water loss and higher sugar gain. During the air-drying process, the effective water diffusivity increased when an osmotic solution with low sugar content was used (35 degrees Brix). It decreased when an osmotic solution with high sugar content was used (70 degrees Brix) because the high sugar gain observed under this condition has saturated of the microscopic channels with sugar creating an extra mass transfer resistance for water and sugar diffusion through the channels. (c) 2008 Elsevier Ltd. All rights reserved.