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.
Effects of Vacuum and Normal Pressure Impregnation on Water Loss and Solid Gain of Apple (Malus pumila Mill)
The rates of water loss (WL) and solid gain (SG) by osmotic dehydration (OD) are important criteria for evaluating the dehydration efficiency and commercial values of a certain OD technique. For comparing the effect of OD with and without vacuum treatment on the WL and SG of fruits, round apple slices were impregnated in sucrose solution at different concentrations (30–60°Brix) and different temperatures (10–55C) with vacuum (20 kPa, VI) or without vacuum (NVI, atmospheric pressure). The values of WL were significantly (P  ≤  0.05) lower and those of SG higher by VI than by NVI, irrespective of concentration, temperature, time of impregnation or water content of the sample. This study suggests that VI is unfeasible for the OD of apple slices if the pretreatment is for the processing of dried products with the lowest sugar content. 
 
 
 
Practical Applications 
 
Osmotic dehydration (OD) has been extensively studied, and vacuum impregnation (VI) is usually combined with OD to enhance product quality. According to a report, VI increases water loss (WL), but does not increase solid gain (SG) compared with normal OD. While our results suggested vacuum or negative pressure is unfavorable to increasing the WL rate, but favorable to increasing the SG rate of OD, thereby conflicting with former reports to some extent. This study puts forward some practical references for further research efforts on VI and for commercial applications of OD.
Osmotic Dehydration Kinetics of Pumpkin Fruits Using Ternary Solutions of Sodium Chloride and Sucrose
The objective of this work was to obtain experimental data and modeling of osmotic dehydration kinetics of pumpkin fruits (Cucurbita pepo L.) with aqueous NaCl/sucrose solutions. For this purpose, effective diffusion coefficients for water, sucrose, and NaCl were calculated by means of a simple model based on Fick's second law. Water loss achieved 80%, sucrose 13%, and NaCl 6% of the initial sample weight. Effective diffusion coefficients ranged from 0.58–1.40 × 10−9 m2/s, 0.75–1.23 × 10−9 m2/s, and 2.60–4.11 × 10−9 m2/s for water, sucrose, and NaCl, respectively. The proposed model gave a good correlation of the experimental data. The quality of the operation was evaluated by analysis of the values of WL/SG ratio.