Heat transfer processes are important for almost all aspects of food preparation and play a key role in determining food safety. Temperature difference between the source of heat and the receiver of heat is the driving force in heat transfer. Convection is the transfer of heat by the movement of groups of molecules in a fluid. The groups of molecules may be moved by either density changes or forced motion of the fluid. In a typical convective heat transfer a hot surface heats the surrounding fluid, which is then carried away by fluid movement. The convection heat transfer coefficient or h value is defined as the rate of heat that will be convected at the product surface–fluid interaction through a unit surface area of the material if a unit temperature gradient exists between the product surface and the surrounding fluid and is very important in modeling and design of frying systems for foods. Frying is a very turbulent process with random and dynamic movement of small bubble particles over the boundary layer of the product surface. Deep fat frying is a very fast method of food processing among conventional heat transfer methods. The frying by immersion can be divided into four stages: (1) initial heating (the temperature of the internal part is slowly increased to boiling point); (2) surfaces boiling (crust formation and higher oil turbulence); (3) falling rate (long period, vapor transfer at the surface decreases) and (4) bubble end point (dried product). These four states during frying can be generalized as nonboiling phases (stages 1 and 4) and boiling phases (stages 2 and 3). So, Convection heat transfer during immersion frying may be divided into two stages: 1) free convection during initial heating of the material and 2) forced convection during the boiling phase. In this study, the convective heat transfer coefficient investigated as a function of the water loss rate during frying process due to the effects of evaporation and boiling on this thermal parameter. Understanding of changes in heat transfer coefficient for thermal control of the frying process is very important to achieve optimum quality of product.
The potato stripes with specified size are fried at temperature of 145, 160 and 175 °C for 60, 120, 180 and 240 seconds using sunflower oil. The center and surface temperatures of potato stripes were recorded with two-second intervals using T type thermocouple and data logger. Moisture content of French fries was measured by drying them within an oven instrument. Mean moisture and oil content of potato stripes was measured. The h value was estimated and its changes studied during process using heat energy balance between the sample and oil by assuming that total heat transferred by convection from oil to potato is equal to the sum of energy spent on heating potato and energy spent on water evaporation. Moreover, changes in the fraction of total heat used for evaporation at different temperatures were expressed as an empirical model.
The results showed that the temperature at the center increased up to the evaporation temperature (“A” zone: about 60 seconds after process beginning). Then, it remained constant at this temperature for a while due to evaporation (“B” zone: about from 60 s till 160 s). This constant temperature period decreased as temperature increased. After this period, the center temperature approached to that of oil (“C” zone: increasing product surface temperature to oil temperature). These various stages of the process were separated. The moisture loss rate was high at the beginning of frying and oil uptake increased as the moisture content diminished. As oil temperature increases, the sample moisture content for the same frying period decreases since an increase in temperature results in a higher kinetic energy for water molecules leading to a more rapid moisture loss in form of vapor. The heat transfer coefficient is increased as the oil temperature increased and at the first time of process observed higher h value. It was due to more water loss rate and so the higher turbulence within the oil before crust formation and for elevated temperatures. The maximum heat transfer coefficient for the temperatures of 175, 160 and 145 °C is estimated 943.68, 847.81 and 682.64 W/m2°C, respectively. Estimated h value also shows a linear increase with water loss rate. Since most of the energy used is associated with the evaporation of water present in the potato, for lower temperatures, the fraction of total heat needed to complete the evaporation is higher and it also represents lower h values by reducing the oil temperature. Also, the variation in the fraction of total heat used for water evaporation showed exponential rise to maximum behavior.
The proposed model for studying evaporation heat fraction fitted experimental data properly, with standard error values range of 0.01-0.03. Evaporation heat fraction could have been affected by oil temperature changes, water loss rate and h value (linear depend with the water loss rate) during process.
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