Moisture transport in shrinking gels during saturated drying

Abstract

AbstractThe transport of moisture in shrinking food gels during drying is studied based on a novel thermomechanical theory accounting for a structural transition in the material—from the rubbery to the galssy state—during drying. The proposed theory is applied to the drying of a model cylindrical starch–gluten gel system. The predicted drying characteristics depend on the Deborah number, a ratio of the characteristic relaxation time to the characteristic diffusion time. At low Deborah numbers, drying is Fickian. At intermediate and high Deborah numbers, however, 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 Biot number but also a function of the “Shell” number, a ratio of the Deborah and Biot numbers. The model is verified by comparing its predictions with experimental data from drying of starch–gluten gels at 22.5 and 40°C. The model predictions agree with experimental data and capture the observed sigmoidal shape of the experimental drying curves in the saturated flow regime. The predicted moisture profiles show shell formation and growth during drying, compatible with the experimental moisture profiles from the literature.