THERMAL-DIFFUSIVITY OF GRANULAR AND POROUS FOODS AT LOW MOISTURE-CONTENT

Thermal diffusivity is an important transport property of foods, which is useful in computations, simulation and control of several unsteady -state food processing operations. Limited data on thermal diffusivity are available, especially, at low moistures, which are of interest in drying and storage processes. Experimental values of the thermal diff usivity (alpha) in the low moisture range were obtained directly for f reeze-dried coffee, potato and meat, using the transient method of Dic kerson (1965), and indirectly for granular starch, flour, rice, and ra isins, using thermal conductivities, obtained by the heated probe meth od. A nonlinear relationship was found between (alpha) and moisture co ntent in granular and! porous foods in the low moisture region. An eva luation of published data indicates a strong effect of physical struct ure (especially porosity) on the thermal diffusivity. A similar effect has been observed on the moisture diffusivity. These anomalies may be related to the state of water (adsorbed or capillary) in the porous s tructure of the low-moisture foods. (C) 1997 Elsevier Science Limited.