Diffusion coefficients as mass transfer properties and water ad/desorption (drying)

Abstract

Mass transfer is important in many food processes such as drying, extraction and cooking. In many cases such processes are governed by diffusion. Here, we first discuss on diffusion coefficients at dilute solutions by explaining the definition of the diffusion coefficient. Then, the diffusion coefficients in gel are discussed. The self-diffusion coefficient and the mutual diffusion coefficient are also explained. Thirdly, the diffusion coefficients at concentrated and supersaturated solutions are reviewed. As a good illustrative example, a sucrose-water system is chosen. The diffusion coefficient decreases with decreasing water concentration sharply, especially below 0.3 kg-water/kg-total, where the solution becomes supersaturated. The activation energy for diffusion increases from 20 kJ/mol in this supersaturated solution region. The desorption isotherm and the isosteric heat of adsorption data also show that the water in this region is strongly bound to the solute. Desorption behavior of water from gelled sugar solutions can be grouped into two categories. Gelled sugar solutions in one category such as agar-gels were not affected by the gel properties and the diffusion in sugar solutions is dominant. On the contrary the desorption of water from gelled sugar solutions such as gelatin was governed by the surface properties of dense gel layers near the surface. The importance of the effect of the surface concentration and the concentration distribution on the desorption curve is discussed. The drying behavior of sugar solutions based on the water concentration dependent diffusion coefficients is explained with emphasis on the flavor retention due to the selective diffusion.