Apple cubes were osmotically dehydrated at 25, 40 and 60C, using sucrose or sorbitol, and the mass ratio of sample to solution of 1:4 and 1:10, at atmospheric pressure or vacuum pressure of 150 mbar. Six mathematical models were tested to describe the mass transfer kinetics of water loss (WL) and sugar gain (SG). Crank's, Azuara's, Peleg's, Page's and Weibull's models could fit well the experimental data, but the Penetration model resulted in a poor fit. The mass ratio of sample to solution did not have an influence on the mass transfer kinetics at the atmospheric pressure. The increase of temperature and the use of sorbitol as the osmotic agent resulted in an increase of the osmotic process rate at both pressures used. Therefore, sorbitol is a good alternative to sucrose. The vacuum presented a tendency to increase the initial rate of WL. PRACTICAL APPLICATIONSThis work confirms the potential to use sorbitol as an alternative to sucrose as the osmotic agent in OD. Besides presenting low calories, and being less sweet and less cariogenic than sucrose, sorbitol is a prebiotic. It induced an increased process rate, in comparison with sucrose. This is advantageous for a faster OD process. However, the benefit in the process time reduction may not justify the material costs. The mass ratio of sample to solution of 1:4 was identified as an alternative to 1:10 in the OD at the atmospheric pressure, as lower quantities of osmotic solution and, therefore, solute are required to carry out the OD process to the same level of dehydration. This work shows also that simple models, such as Peleg's and Page's, can be used to predict the mass transfer kinetics of WL and SG during OD processes at different conditions.

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The aim of the present work was to study the effect of the osmotic dehydration (OD) pre-treatment on the mass transfer kinetics and water activity (a) of apple cubes during hot air drying. The adequacy of different mathematical models to describe the moisture content of the product during this process was also evaluated. Apple cubes were osmotically dehydrated with sucrose or sorbitol solutions at 60 °C, and then dried by air at 25-80 °C. Overall, the OD and rise of the air temperature resulted in an increased water loss rate and a reduction of the a. The osmotic agent used in the OD was not relevant to the air drying kinetics, but the pre-treatment with sorbitol solutions produced dried samples with lower a. Newton's, Page's, modified Page's, Henderson and Pabis', Two-term, Two-term exponential, Logarithmic, Midilli et al.'s models could describe the moisture content well during the air drying process.

Effects of the osmotic pre-treatment (OD) with sucrose or sorbitol solutions and different drying methods on the rehydration kinetics of apple cubes were studied. Samples were dried by hot air (HAD) at 70 ºC, by microwave (MWD) at 500 W, and by freeze-drying (FD). The rehydration of dried samples was performed in water at 80 ºC and at a mass ratio of sample to water of 1:100. The fi rst order kinetic model was found to provide the best fi t of the rehydration data. The pre-treatment and the drying method did not affect the rehydration kinetics, but the rehydration capacity after 12 min of rehydration was higher in the control samples than in the osmotically dehydrated ones, for all three drying methods. The rehydration rate and the rehydration capacity of the apple cubes dried by HAD, MWD, or FD were not affected by the solute used in the pre-treatment, sucrose and sorbitol.