Moisture adsorption isotherms of microwave-baked Madeira cake

Adsorption isotherms of microwave-baked Madeira cake, flour and sugar were determined using a standard static gravimetric method within the temperature range 5–60 °C and relative humidity range of 0.04–0.96. Microwave-baked Madeira cake and sugar samples exhibited Type III isotherm behaviour, whereas flour exhibited Type II characteristics, with the sorption capacity decreasing with increasing temperature for Madeira cake and flour. Conversely, the equilibrium moisture content of the sugar samples increased with increasing temperature at water activity values greater than 0.88. Experimental data obtained were fitted to several isotherm models and their goodness-of-fit evaluated in terms of mean relative error (MRE), standard error of estimate (SEE) and residual sum-of-squares (RSS). Non-linear least square regression software was used to evaluate the models parameters. The Ferro-Fontan model provided the best description of the experimental sorption behaviour of microwave-baked Madeira cake and flour. The net isosteric heat of sorption was determined from the equilibrium adsorption data, using the Clausius–Clapeyron equation. The heat of adsorption decreased with increase in moisture content and approached a constant value. It showed a power law relationship with moisture content for microwave-baked Madeira cake and exponential relationship for flour.