Bioengineering, Food, and Natural Products zyxw Moisture Transport in Shrinking Gels during Saturated Drying zy Srinivas Achanta zyxwv Process Systems Technology, The Procter zyxwvu & Gamble Company, Cincinnati, OH 45232 Martin R. Okos Dept. of Agricultural and Biological Engineering, Purdue University, West Lafayette, IN 47907 John H. Cushman Center for Applied Mathematics, Mathematical Sciences Building, Purdue University, West Lafayette, IN 47907 David P. Kessler School of Chemical Engineering, Purdue University, West Lafayette, IN 47907 zyxw The 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 glassy state - during drying. The proposed theory is applied to the drying of a model cylindrical starch -gluten gel system. The predicted dying characteristicsdepend on the Deborah number, a ratio of the characteristicrelax- ation time to the characteristicdifision time. At low Deborah numbers, drying is Fick- ian. At intermediate and high Deborah numbers, however, drying is non-Fickian, lead- ing 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 itspredictions with experimental data from dying of starch - gluten gels at 22.5 and zyxwvuts 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. introduction Drying of high-moisture biopolymers such as processed foods, gels, fruits, and vegetables occurs predominantly in the saturated flow regime, where shrinkage almost entirely com- pensates for the loss of water. As shown by Jomaa and Puig- gali (1991) for cellulosic gels, Lartigue et al. (1989) for wood, Ketelaars et al. (1992) for clays, Karathanos et al. (1993) for celery, and Crapiste et al. (1985) and Suarez and Viollaz (1991) for potato slabs, drying of high-moisture food biologi- cal materials results in a bulk-density change that is linearly related to the change in product moisture content. Hence, Correspondence concerning this article should be addressed to zyxwvuts M. R. Okos accounting for shrinkage in a mechanistic or mathematical model of the drying of biological materials is important. Sev- eral models exist in the literature that account for matrix shrinkage during drying, either analytically (Viollaz, 1985) or numerically by using finite difference (Crapiste et al., 1985) or finite-element formulation (Ketelaars et al., 1992). These and other related works provide a good understanding of vol- ume/density changes during drying and serve as better mod- els for drying of biological materials than those models that do not account for shrinkage. However, they are still incom- plete because they generally are empirical in nature and ne- glect the nonequilibrium characteristics of shrinkage. August 1997 Vol. 43, No. 8 AIChE Journal 2112