Experimental measurement of physical pressure in foods during frying

Two important factors affecting the oil uptake in foods during deep fat frying are water content and pressure development. In the past frying studies, the physical pressure has not been measured experimentally but was calculated using computer models, which has resulted in disagreements about its magnitude. The present study tries to explain the complex mass transfer mechanisms taking place during deep fat frying with respect to real time pressure variations inside potato discs and chicken nuggets. Frying experiments were performed at two temperatures of 175 °C and 190 °C for 200 and 240 s for potato discs and chicken nuggets, respectively. The gage pressure increased rapidly above the atmospheric pressure immediately after the samples were introduced into the hot oil. The rise in pressure was greater in potato discs with greater initial moisture content. This was expected due to sudden moisture flash-off. As frying progressed, the temperature inside the samples increased whereas the gage pressure started decreasing and became negative. The onset of negative pressure was observed during initial stages of frying for chicken nuggets, but in the middle of frying for potato discs. The negative pressure values before the product is taken outside the fryer may cause increased oil uptake during frying itself. During the post frying cooling, the pressure further decreased and reached negative values. The negative pressure is expected to have caused rapid absorption of surface oil during both frying and cooling stages.