Abstract The aim of the present study was to evaluate the effect of solar and microwave drying on raspberries (Rubus Idaeus) cv. Heritage. The efficiency of drying was evaluated in terms of weight reduction in function of the time. The quality properties evaluated were color, texture, antioxidant capacity and total polyphenols content. The results showed that solar drying and microwave drying allowed a good preservation of surface color in the final product. However, the solar drying achieved a desirable texture in dried raspberries compared to microwave drying. Neither solar drying nor microwave drying allowed a high retention of the antioxidant capacity. Nevertheless, both processes allow obtaining final stable fruits (low water activity) at any time of the year.

The mass and energy transfer during osmotic microwave drying (OD-MWD) process was studied theoretically by modeling and numerical simulation. With the aim to describe the transport phenomena that occurs during the combined dehydration process, the mass and energy microscopic balances were solved. An osmotic-diffusional model was used for osmotic dehydration (OD). On the other hand, the microwave drying (MWD) was modeled solving the mass and heat balances, using properties as function of temperature, moisture and soluble solids content. The obtained balances form highly coupled non-linear differential equations that were solved applying numerical methods. For osmotic dehydration, the mass balances formed coupled ordinary differential equations that were solved using the Fourth-order Runge Kutta method. In the case of microwave drying, the balances constituted partial differential equations, which were solved through Crank-Nicolson implicit finite differences method. The numerical methods were coded in Matlab 7.2 (Mathworks, Natick, MA). The developed mathematical model allows predict the temperature and moisture evolution through the combined dehydration process. Mathematical subject classification: Primary: 06B10; Secondary: 06D05.

A perda da água, o ganho do sacarose e a variação na concentração de outros açúcares naturais de fruta (glucose e fructose) foram estudados durante a desidratação osmótica de fatias de abacaxi, com 0.6 milímetro espessura, em solução de sacarose (60 % w/w) em três temperaturas (30, 40 e 50ºC). Como a temperatura foi elevada de 30 a 50ºC, as difusividades aparentes da água e da sacarose (Dw e Ds) aumentaram 3.8 e 2.8 vezes, respectivamente; conseqüentemente, o índice da eficiência da desidratação (Dw/Ds) aumentou com a temperatura. A perda de glucose e de fructose aumentou com temperatura. O índice do soluto é uma função linear do índice de umidade e esta relação nao é funcão da temperatura durante os primeiros 600 minutos da desidratação. As experiências da desidratação osmótica foram realizadas em 40°C com fatias de 10 milímetros de espessura para determinar os perfis da concentração da água e da sacarose em função da posição para valores diferentes de tempos da imersão.

Water loss, sucrose gain and the variation in concentration of other natural fruit sugars (glucose and fructose) were studied during osmotic dehydration of pineapple slices (0.6 mm thick) in sucrose solution (60 % w/w) at three temperatures (30, 40 and 50ºC). As temperature increased from 30 to 50ºC, the apparent moisture and sucrose diffusivities (Dw and Ds) increased 3.8 and 2.8 times, respectively; therefore, the dehydration efficiency index (Dw/Ds) increased with temperature. The loss of glucose and fructose increased with temperature too. It was found that the solute content was a linear function of the moisture content and this relation was independent of the temperature during the first 600 minutes of dehydration.