MOISTURE TRANSPORT IN SHRINKING GELS DURING SATURATED DRYING

The transport of moisture in shrinking food gels during drying is stud ied based on a novel thermomechanical theory accounting for a structur al transition in the material-from the rubbery to the glassy state - d uring drying. The proposed theory is applied to the drying of a model cylindrical starch-gluten gel system. The predicted drying characteris tics depend on the Deborah number, a ratio of the characteristic relax ation time to the characteristic diffusion time. At low Deborah number s, drying is Fickian. At intermediate and high Deborah numbers, howeve r drying is non-Fickian, leading to an apparent mass-transfer shutdown , which is a result of surface dryout and skin/shell formation. Based on a time-dependent surface boundary condition the model proposes that surface drying is not only a function of the Blot number bur also a f unction of the ''Shell'' number, a ratio of the Deborah and Blot numbe rs. The model is verified by comparing its predictions with experiment al data from drying of starch - gluten gels at 22.5 and 40 degrees C. The model predictions agree with experimental data and capture the obs erved sigmoidal shape of the experimental drying curves in the saturat ed flow regime. The predicted moisture profiles show shell formation a nd growth during drying compatible with the experimental moisture prof iles from the literature.