Résumé
Mathematical modeling of the frying process requires precise knowledge of the thermal conductivity of different regions in fried product. While thermophysical properties of fresh and fried foods are well documented in the literature, thermophysical properties of the crust and core regions are not available. This study investigated potato thermal conductivity during deep-fat frying, specifically in the crust and core regions. Changes in characteristics of the crust and core in relation to frying time are discussed. A fried product has a porous structure, especially in the crust region. An improved Lees apparatus, in which the sample is enclosed between plates, was successfully used to directly determine the effective thermal conductivity of porous samples. The potato was cut into 0.05 x 0.05 x 0.006 m(3) slabs and fried for 110 min at 170 degrees C. Top and bottom crusts were separated from the core region after each frying time and their thermal conductivities were individually measured. Results showed that the effective thermal conductivity (k) of the core increases with frying time, reaching a maximum value of 0.6 W/(m K) at 3 min of frying and then decreases to 0.4 W/(m K) at the end of frying. This behavior can be related to physico-chemical changes such as starch gelatinization and moisture loss in the core region. Top and bottom crust thermal conductivities decrease with frying time, due to their specific moisture losses and porous structure formations.