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The process of quasi-isothermal dehydration of thin films of pure water and aqueous sugar solutions is investigated with a dual experimental and theoretical approach. A nanoporous paper disk with a homogeneous internal structure was used as a substrate. This experimental set-up makes it possible to gather thermodynamic data under well-defined conditions, develop a numerical model, and extract needed information about the dehydration process, in particular the water activity. It is found that the temperature evolution of the pure water film is not strictly isothermal during the drying process, possibly due to the influence of water diffusion through the cellulose web of the substrate. The role of sugar is clearly detectable and its influence on the dehydration process can be identified. At the end of the drying process, trehalose molecules slow down the diffusion of water molecules through the substrate in a more pronounced way than do the glucose molecules. [ABSTRACT FROM AUTHOR]

The isothermal dehydration process was investigated by calorimetric experiments on water samples, pointing out a substantial correspondence with gravimetric results, even though the heat flow measurement is characterized by a higher resolution on approaching the end of the evaporation. The role of the geometry of the sample on water evaporation rate is analyzed with reference to two model geometries, resembling the spherical and the planar surface. Specific attention was given to the planar surface for the description of the water evaporation from thin film. The development of a theoretical model, which provides information regarding the water activity, was supported by investigating the effect of the several parameters involved in the isothermal dehydration, such as sample mass and temperature. The data collected revealed some limits in the valuable use of the theoretical model when the experimental conditions are outside the range of quasi-equilibrium thermodynamic condition, e.g., at temperature higher that 50 °C. Attention has also been given to control the initial thermal unbalance in order to improve the measurements. [ABSTRACT FROM AUTHOR]