In heat transfer, heat energy is transferred under the driving force provided by a temperature difference, and the rate of heat transfer is proportional to the potential (temperature) difference and to the properties of the transfer system characterized by the heat-transfer coefficient. In the same way, mass is transferred under the driving force provided by a partial pressure or concentration difference. The rate of mass transfer is proportional to the potential (pressure or concentration) difference and to the properties of the transfer system characterized by a mass-transfer coefficient.
Writing these symbolically, analogous to q = UA DT, we have
dw/dt = k'g A DY                                                     (7.3)
where w is the mass being transferred kg s-1, A is the area through which the transfer is taking place, k’g is the mass-transfer coefficient in this case in units kg m-2 s-1 , and Y is the humidity difference in kg kg-1.

Unfortunately the application of mass-transfer equation is not as straightforward as heat transfer, one reason being because the movement pattern of moisture changes as drying proceeds. Initially, the mass (moisture) is transferred from the surface of the material and later, to an increasing extent, from deeper within the food to the surface and thence to the air. So the first stage is to determine the relationships between the moist surface and the ambient air and then to consider the diffusion through the food. In studying the surface/air relationships, it is necessary to consider mass and heat transfer simultaneously. Air for drying is usually heated and it is also a major heat-transfer medium. Therefore it is necessary to look carefully into the relationships between air and the moisture it contains.

Copyright © 1983, R. L. Earle. :: Published by NZIFST (Inc.),from http://www.nzifst.org.nz