Pressure development and volume changes during frying and post-frying of potatoes

Pressure evolution and material deformation during frying and post-frying of foods affect transport processes and food's textural attributes. Pressure and volume measurements have received limited attention in the frying literature. In this study, the changes in moisture content, oil uptake, temperature, pressure, and volume were measured as a function of frying time for potato cylinders. The volume change immediately after frying and after the cylinders had cooled was measured using a modified liquid displacement method. Fried cylinders continued to shrink post-frying. During frying, pressure and temperature showed similar increasing trends followed by plateauing of the temperature and a drop in pressure in the core. The measured gauge pressure remained positive throughout frying (t=300s) and up to 100s after frying. The measured gauge pressure dropped to negative values for the remaining cooling period. The temperature decreased during cooling, which is expected to have led to water vapor collapse and a drop in gas pressure. A high capillary pressure is expected to have developed in the food matrix due to water loss, causing the effective gauge pressure to become negative. The negative gauge pressure is expected to drive oil uptake and shrinkage of the food matrix post-frying.