Experimental verification of transport phenomena during frying of foods

Frying;Transport Phenomena

Sandhu, Jaspreet Singh

关键词: Frying;    Transport Phenomena;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/78396/SANDHU-DISSERTATION-2015.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Experiments were performed to measure moisture loss, fat uptake, heat transfer coefficient and mechanical properties of fried foods. The measured properties and variables were used in a Hybrid Mixture Theory based model solved using the finite element method to elucidate the phenomena affecting fat uptake. This provided further clarity on mechanisms involving fat uptake and helped develop avenues to reduce fat content of fried foods.Frying experiments were performed at two temperatures of 175°C and 190°C for 200 s 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 rapid moisture flash-off. As frying progressed, the temperature inside the samples increased whereas the gage pressure started decreasing and became negative. The onset of suction or 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 was 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 was expected to have caused rapid absorption of surface oil during both frying and cooling stages.The effect of frying parameters (temperature and time) on the properties of potato slices (surface pore characteristics, oil content, moisture loss and mechanical properties) was investigated. Scanning electron microscopy (SEM) was employed to develop surface topographic images and image pro plus software was used to determine the pore area of potato slices. The rheological behavior of potato slices was investigated using dynamic mechanical analyzer (DMA). Both frying temperature and frying time had a significant impact on the pore area, creep behavior, moisture loss and fat uptake of potato slices. The changes in surface porous structure (pore area) and creep compliance were dynamic and in turn affected the oil uptake and moisture loss rates. Average open pore area and percent open pore area increased to 1.15 μm2 and 14.04%, respectively, during middle frying stages. Higher frying temperature resulted in faster structural degradation, moisture loss and oil uptake during initial frying stages. Higher frying times tended to increase the percentage open pore area. The convective heat transfer coefficient was also measured experimentally using a controlled one-dimensional frying methodology. Hollow Teflon disc was used as a sample holder. Thermostable silicon glue was used to seal the sample in the Teflon disc. This insulated the edges of the potato disc from the frying oil thus restricting oil penetration from only the exposed top and bottom surfaces. This also rendered this set of frying experiments a one-dimensional frying process. The peak heat transfer coefficient values were determined to be 3617, 4517 and 7307 W/m2°C at frying temperatures of 150, 170 and 190 °C, respectively. The heat transfer coefficient reached its peak value towards the end of frying at all temperatures. Involvement of unsaturated transport and high temperatures during frying of foods makes it a challenging process to study via experiments and computer simulations. The hybrid mixture based unsaturated transport theory of Takhar (2014) was validated via controlled frying experiments. A hollow Teflon disc was used to insulate the edges of potato disc to ensure that frying was controlled, one-dimensional, and oil uptake and moisture loss happened only through top and bottom surfaces. The model was used to predict moisture and oil content, evaporation rates, temperature distribution, and pore and gas pressure profiles as a function of frying time and temperature.Percentage average absolute difference (AAD) between predicted and experimental values for moisture content was 3.89%, 5.7% and 5.5% and oil content was 14%, 31% and 20% at 150, 170 and 190 °C respectively. Simulations showed that oil penetrated to only 0.25mm into the potato disc. Removal of surface oil improved the prediction of experimental oil content. Maximum evaporation rate of 0.32 kg/m3s was observed near the surface of potato slice at 60 s frying time resulting in rapid moisture loss. Pore pressure remained negative beyond 60s frying time, which may act as a driving force for oil uptake.