Hybrid mixture theory-based modeling of unsaturated transport in a deforming porous food matrix during frying

Abstract

Physics-based modeling of deep fat frying is daunting given the intricacies involved in the transport of different phases (liquid water, gas, and oil) in a continuously deforming unsaturated porous matrix. To simplify model development, previous models for frying either ignored volume changes or used empirical relations. The model developed in this study solved the hybrid mixture theory-based unsaturated transport equations and mechanistically accounted for the volume changes of the porous food (potato) matrix. Pore pressure, the effective pressure on pore walls, was used as the driving force governing the volume changes. A good agreement was found between the model predictions and experimental results. The % mean absolute error for moisture content, oil content, and temperature is 5.57%, 22.42%, and 13.35%, respectively. Evaporation and gas expansion during frying led to high pressures in the porous matrix with a peak gauge pore pressure of approximately 19.16 kPa at the center of the sample. The high pressure restricted the frying oil from penetrating beyond the surface layers. Oil uptake mainly occurred during the early stages of frying (t<50 s) when the pressure in the core was low, and towards the end of frying when the matrix was more susceptible to oil penetration because of decreasing pressure. The potato cylinder shrunk by 18.55% for a frying time of 300 s. The gauge pore pressure near the surface became negative, which led to the rapid contraction of the surface layers, and as a result, the porosity near the surface decreased. The average porosity was predicted to decrease by 5.06% after 300 s of frying. The evaporation zone expanded with frying time, and its peak progressively moved towards the core. The insights generated from the discussed mechanisms will guide the industry in optimizing frying techniques.