Drying kinetics and rehydration characteristics of microwave-vacuum and convective hot-air dried mushrooms

Microwave-vacuum dehydration characteristics of button mushroom (Agaricus bisporus) were evaluated in a commercially available microwave oven (0-600 W) modified to a drying system by incorporating a vacuum chamber in the cavity. The effect of drying parameters, namely microwave power, system pressure and product thickness on the drying kinetics and rehydration characteristics were investigated. The drying system was operated in the microwave power range of 115-285 W, pressure range of 6.5-23.5 kPa having mushroom slices of 6-14 mm thickness. Convective air drying at different air temperatures (50, 60 and 70 degrees C) was performed to compare the drying rate and rehydration properties of microwave-vacuum drying with conventional method. Microwave-vacuum drying resulted in 70-90% decrease in the drying time and the dried products had better rehydration characteristics as compared to convective air drying. The rate constants of the exponential and Page's model for thin layer drying were established by regression analysis of the experimental data which were found to be affected mainly by the microwave power level followed by sample thickness while system pressure had a little effect on the drying rate. Rehydration ratio was significantly affected by the system pressure. Empirical models are also developed for estimating the drying rate constant and rehydration ratio as a function of the microwave-vacuum drying process parameters. (c) 2005 Elsevier Ltd. All rights reserved.