Osmotic Dehydration of Apples Under Reduced Pressure Conditions
Abstract The aim of the study was to determine the effect of reduced pressure on the osmotic dehydration of apples. Tests were performed under vacuum of 8 kPa, 67 kPa, 80 kPa and under the atmospheric pressure (100 kPa). The samples were dehydrated in a sucrose solution with a concentration of 30°Bx, 50°Bx and 70°Bx. It has been shown that the effect of low pressure application depends significantly to the concentration of the osmotic solution. It has been found that the overall weight change significantly depend on the concentration of the solution, and after 3 hours of dehydration at a pressure of 80 kPa at solutions of 30°Bx, 50°Bx and 70°Bx total weight loss increased by 65%, 12% and 25% respectively, when compared to samples dehydrated at atmospheric pressure. From the studied variants of reduced pressure, the pressure of 80 kPa seems to be the optimal one, as evidenced by the lowest values of weight gain to water loss ratios for apples dehydrated in solutions of 50°Bx and 70°Bx.
Mass Transfer During Osmotic Dehydration of Fruits and Vegetables: Process Factors and Non-Thermal Methods
Osmotic dehydration of fruits and vegetables is a slow process due to resistance in mass transfer (MT). This resistance can be modified by varying the osmotic solution or process conditions, or by employing non-thermal treatments (NTT). Some NTT modify the tissue structure by formation of pores in the cell membrane (such as pulse electric fields) or microchannels (such as ultrasound and osmosonication), and others increase the contact surface area with the tissue through a decrease in atmospheric pressure (such as vacuum pulses) or an increase of the system pressure (such as high hydrostatic pressures). Changes in the rate of MT caused by modifications in experimental conditions can be described with mathematical models, such as Fick’s second law. The aims of this review are to analyze the different factors that modify MT rate during osmotic dehydration, examine the application of NTT to modify MT, and to study models that describe these processes.
Osmotic dehydration of pineapple as a pre-treatment for further drying
Osmotic dehydration can be used as a pre-treatment for tropical fruits with the final aim of obtaining high quality dried fruit products. This concept was applied to South African grown Cayenne type pineapple pieces. The effect of osmotic dehydration on mass fluxes (water loss, solids gain and weight reduction) was investigated. Pineapple cylinders of 2 cm in diameter and 1 cm thick were immersed in sucrose solutions of 45, 55 and 65 °Brix at 30, 40 and 50 °C for 20, 40, 60, 20, 180 and 240 min. Experiments were conducted at both atmospheric pressure and applying a 200 mbar vacuum pulse during the first 10 min. Water loss and solids gain increased with temperature and concentration. Applying a vacuum pulse facilitated water loss especially at the highest concentration and temperature. Furthermore, the yield was improved by applying a vacuum pulse, as mass loss was less in those cases. Temperature affected mostly the water loss while the concentration of the solution affected mostly the solids gain. Prototypes of high/low water loss and high/low solids gain combinations were selected for quality evaluation.