Microwave Frying Compared with Conventional Frying via Numerical Simulation

Transcription

1 DOI 1.17/s x ORIGINAL PAPER Microwave Frying Compared with Conventional Frying via Numerical Simulation Ilkay Sensoy & Serpil Sahin & Gulum Sumnu Received: 25 October 211 / Accepted: 6 February 212 # Springer Science+Business Media, LLC 212 Abstract Microwave heating can be combined with other means of heating to yield a unique heating profile. In the study, microwave frying, a combination of convective and microwave heating, was compared with conventional frying. Frying experiments were performed by inserting a single food sample (chicken breast meat) in the hot oil at 18±1 C for both frying methods. Center temperature of the sample and the oil temperature were recorded during both frying methods. Simulations were performed to predict heat transfer coefficients. Processing time was shorter with microwave frying. Simulations revealed a varying convective heat transfer coefficient, which was in the range of W/m 2 K, during conventional frying. Higher convective heat transfer coefficient, 5 W/m 2 K, compared to conventional frying was observed during microwave frying with the simulations. This is suggested to be due to higher turbulence in microwave frying. Keywords Frying. Microwave. Modeling. Temperature profile Nomenclature A Area perpendicular to heat transfer in square meter c Speed of light in meters per second C p Specific heat capacity in joules per kilogram per kelvin D p Penetration depth in meters f Frequency in hertz I. Sensoy (*) : S. Sahin : G. Sumnu Department of Food Engineering, Middle East Technical University, Universiteler Mahallesi, Dumlupinar Bulvari, No:1, 68 Ankara, Turkey isensoy@metu.edu.tr h k m q q q T t x Convective heat transfer coefficient in watts per square meter per kelvin Thermal conductivity (watts per meter per kelvin) Evaporated moisture in kilograms Microwave heat absorption in watts per cubic meter Heat flux in watts per square meter Microwave surface heat absorption in watts per cubic meter Temperature in degree Celsius Time in seconds Distance from the surface in the x direction for the chicken meat sample in meters Greek letters ε Relative dielectric constant dimensionless ε Relative dielectric loss factor dimensionless ρ Density in kilograms per cubic meter λ Latent heat of vaporization in joules per kilogram Subscripts c Center i Initial oil Frying oil s Surface Introduction Deep fat frying is a popular and therefore important food process (Ahromrit and Nema 21; Gharachorloo et al. 21; Halder et al. 27). The frying process can be defined as a cooking and drying process by immersing a food product in edible oil or fat at a higher temperature than the boiling point of water (Barutcu et al. 29; Erdogdu and Dejmek 21; Halder et al. 27). Simultaneous heat and mass transfer

2 occur during frying (Alvis et al. 29;Farid and Kizilel 29; Oztop et al. 27a; Sahin et al. 1999). Immersing frying can be defined by four stages (Farkas et al. 1996). During the first stage, heat transfer is by convection and food surface heats up to the boiling point of water. In the second stage, surface water starts to boil and evaporate. Therefore, heat transfer between the oil and the food changes from natural convection to forced convection due to turbulence in the oil. This enhances the heat transfer coefficient. Dehydration of surface and high temperature cause crust layer formation in this stage. In the third stage, temperature in the internal region of the food increases slowly to boiling point of water. Physicochemical changes like starch gelatinization and protein denaturation happen in this stage. In addition, crust layer thickness increases and water vapor transfer at the surface diminishes. At the final stage, surface evaporation ceases and no bubbles are observed at the surface of the food (Alvis et al. 29). During frying, food is immersed into oil at a temperature of C which leads to intense vaporization of the water in the food and transport out through the surface (Ni and Datta 1999). As the water moves out, frying oil can move in to the material. During frying food absorbs substantial amount of oil which is affected by process conditions such as duration and temperature of the process, pretreatment of the food, and physicochemical characteristics of food and oil (Oztop et al. 27b). Most of the oil uptake happens during cooling process because drop of steam pressure inside the pores forces oil at the surface to the inside (Alvis et al. 29; Moreira et al. 1997). Improvement of the processing methods or exploring new processing methods is essential for the food industry. There are alternative frying methods such as vacuum frying and high pressure frying. Vacuum frying is carried in a closed system where the pressure below the atmospheric level results in reduced frying temperature due to water boiling point depression (Dueik and Bouchon 211). In high-pressure frying, the vapor released from the products naturally generates adequate pressure and known to take less time and results in longer lasting oil or fat with less energy use (Erdogdu and Dejmek 21). Microwave frying is also considered as an alternative method to conventional frying due to shorter time, lower temperature of processing, and convenience of use (Gharachorloo et al. 21). Shorter time and lower temperature of microwave heating compared to conventional frying yield less degradation of oil (Gharachorloo et al. 21). In addition, microwave frying have shown to reduce the oil uptake (Oztop et al. 27a). Microwave heating was simulated by several authors (Barringer et al. 1995; Campanone and Zaritzky 21; Chen et al. 27; Geedipalli et al. 27; Gunasekaran and Yang 27; Knoerzeretal.28; Rakesh et al. 29; Rakesh et al. 21; Salvi et al. 211). Modeling the frying process with fundamental equations or explanations remains a challenge due to complexity of the process, particularly the presence of rapid evaporation and bubbles (Alvis et al. 29; Erdogdu and Dejmek 21; Halderetal.27; Hubbard and Farkas 1999). Difficulties are due to both in formulation of the physical problem into mathematical equations as well as in numerical complexities of solving these equations (Halder et al. 27). In microwave frying, additional complexity arises due to the calculation of electromagnetic field distribution inside the oven cavity. Number of heating methods are used to prepare foods (Rakesh et al. 21). Customized heating profiles suitable for a particular process can be designed by combining different heating modes (Rakesh et al. 21). Combining microwave with convection and radiation heating are examples which found to be effective technique (Rakesh et al. 21). Microwave frying combines microwave and convection heating. This manuscript compares microwave frying with conventional frying via experimental data and numerical simulation, which lacks in literature. Materials and Methods Raw Material Sunflower oil was used as frying medium and chicken breast meat as a food sample. Sunflower oil and chicken breast meat were bought from a local market. Meat was kept in the freezer ( 18 C) until use and thawed in the refrigerator (4 C) before use. Experimental Procedure Chicken breast meats were cut in cm dimensions with a custom-made cutter. Then 1.1-cm slices were cut by a knife to obtain a rectangular prism with the dimensions of cm. Sample weights were checked to have uniform range of 11.5±1 g. Conventional frying was done on a Bunsen burner and temperature of the oil kept constant during frying by adjusting the flame level manually. A domestic microwave oven (BOSH HMT 982, BSH Ev Aletleri Sanayi ve Ticaret A.Ş., Istanbul, Turkey) was used for microwave frying. Microwave oven was used at 296-W power level. IMPI 2-L test was used to determine the power level of microwave (Buffler 1993). Conventional frying was performed by using a steel container containing 75 ml oil. Microwave frying was performed by using a glass container containing 75 ml oil. Oil was discarded after a maximum of 4-h use. Oil was heated until the temperature of 18±1 C before insertion of the single food sample for both frying methods. Experiments were specifically designed to be as close as possible to real cases. Therefore, the turn table

3 was used to enhance uniformity during microwave frying and samples let float freely in the frying oil for both frying methods. The temperature of both chicken slice and oil temperature were recorded by insertion of fiber optic temperature probes (FISO Technologies Inc., Quebec, Canada) in to the geometric center of chicken sample and oil for both microwave and conventional frying. Position of the fiber optic probes were checked after each frying process to ensure the probes were not displaced. Data were recorded with a signal conditioner (FTI-1, FISO Technologies Inc., Quebec, Canada). Drying rates (dm/dt) of the samples, which were used in simulations, were determined by frying the samples for different time intervals:,.5, 1, 1.5, and 2 min for microwave frying and, 1, 2, 4, and 5 min for commercial frying. Moisture content of the samples was determined by drying the samples at 15 C oven for 48 h. All experiments were conducted in triplicate. Numerical Simulation Numeric simulation was conducted on the one fourth of the sample due to symmetry. Size of the sample was assumed constant during frying. Governing Equations and Boundary Conditions for Conventional Frying Temperature distribution was obtained by a solution of energy balance equation for conventional frying in three ρc ¼ kr2 T ð1þ With the initial condition: T ¼ T i at t ¼ ð2þ Boundary conditions are given below. x coordinate is presented as a representative for all the coordinates. At surface, convection and phase change was considered: qj x¼xs ¼ ht ð s T oil Þ l dm ð3þ A dt At the center, symmetry boundary condition ¼ ð4þ x¼xc During simulations convective heat transfer coefficient was determined by trial and error until the experimental data were matched. Literature values were used as starting point. Governing Equations and Boundary Conditions for Microwave Frying For microwave frying, a local heat generation term due to absorbed electromagnetic microwave power was included in the conventional energy balance ρc ¼ kr2 T þ q ð5þ Absorbed microwave power was calculated using the Lambert s law as given for the x direction only. q ¼ q e ð x=d pþ ð6þ where penetration depth was defined as sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 11 D p ¼ c 1=2 ; 1 þ " 2pf " 1AA ð7þ Lambert s law assumes power absorption as an exponential decay, and it is valid only for semi-infinite geometry with dimensions much larger than the wavelength or for large geometries where microwave penetration depth is lower than thickness of the sample (Campanone and Zaritzky 21; Oliveira and Franca 22). Turntable is used in microwave oven for enhancing uniformity. In addition, presence of oil and free floating food sample in the frying medium makes solution of Maxwell equations for microwave absorption almost impossible. Therefore, for simplicity Lambert s law prediction was used with the expense of accuracy. Boundary conditions for the energy equation (Eq. 5) for microwave frying were same as in conventional frying. Simulation Geometry and mesh were formed with geometry drawing and meshing capabilities of a software (ANSYS Workbench 12., ANSYS Inc., USA). Heat transfer equations were solved using a computational fluid dynamic program (FLUENT in ANSYS Workbench), which can solve continuity, momentum transfer, and energy transfer equations. Heat generation due to microwave heating and the latent heat loss at the surface due to evaporation were included in the solution with the inclusion of user-defined functions. Time step was set as.1 s. The simulations were run on a personal computer with Windows 32 bit operating system (Intel Core 2.4 GHz 2. GB memory). Thermal and dielectric properties used in the simulations are presented in Table 1. Table 1 Thermal and dielectric properties of chicken Thermal and dielectric properties Source Specific heat, C p kj/kg K (Siripon et al. 27) Thermal conductivity, k.593 W/m K (Siripon et al. 27) Dielectric constant, ε 49. (Lyng et al. 25) Dielectric loss factor, ε 16.1 (Lyng et al. 25)

4 Convective heat transfer coefficient (h) of the oil during both conventional and microwave frying and, the microwave surface heat absorption (q ) during microwave frying were determined by trial and error until the simulations matched the experimental data. There was an on and off time for microwave heating. The heat transfer coefficient was determined for the off time where there was only convective heating. The obtained heat transfer coefficient was used during the on time, and q determined by trial and error until the simulation results were matched with the experimental data. The end results of the simulated data were compared with the experimental data by using the root mean squared error (RMSE). Experimental data points had higher time intervals than simulation data points. Therefore, only the simulation data corresponding to same time intervals with the experimental data were used in RMSE calculations. Results and Discussion Microwave frying gave a faster heating rate due to internal heat absorption compared to conventional frying (Fig. 1). During microwave frying, the center temperature reaches to 1 C in about 3 s while in conventional frying it takes more than 16 s. The results indicate that microwave frying can be an alternative process for reducing the processing time and probably reducing the oil consumption as well. Simulation was matched with the experimental data by using a heat transfer coefficient that changes with time for conventional frying (Fig. 2a). The heat transfer equations which gave matching results for the experimental data were demonstrated below. h ¼ 58 3t W=m 2 K h ¼ 4 þ 3t W=m 2 K for the first 3 s ð8þ (a) Temperature ( o C) (b) Temperature ( o C) Oil 4 Conventional Frying 2 Simulation Food Bioprocess Technol 6 Oil 4 Microwave Frying 2 Simulation Fig. 2 Experimental (with ± standard deviations) and simulated temperature profiles of the geometric center of the chicken breast sample with the oil temperature a during conventional frying process; b during microwave frying for the last 14 s ð9þ This indicates that heat transfer coefficient varied during commercial frying. It started as 4 W/m 2 K and reached a Temperature ( o C) Microwave Frying 1 Conventional Frying Fig. 1 Center temperatures of the chicken breast samples during microwave and conventional frying processes (with ± standard deviations) Moisture content (g water/ g dry solid) Conventional Frying Microwave Frying Fig. 3 Moisture content of the chicken samples as function of time during conventional and microwave frying (with ± standard deviations)

5 maximum of 49 W/m 2 K and dropped to 16 W/m 2 Kat the end of frying. This result is expected because during deep frying convective heat transfer coefficient can range from 9 to 1.1 W/m 2 K and strongly coupled with the bulk movement of the oil during frying (Farkas and Hubbard 2; Hubbard and Farkas 1999; Yildiz et al. 27). At the beginning of frying, heat transfer is by natural convection and as the frying progress due to evaporation turbulence enhances the heat transfer. Toward the end of frying, evaporation ceases and this reduces the heat transfer coefficient. Halder et al. (27) and Hubbard and Farkas (1999) stated that deep fat frying can be divided in two phases: boiling phase and non-boiling phase. In addition, the boiling phase can be broken in two stages: first, surface boiling stage where sudden loss of free moisture at the surface enhances heat transfer and crust start to form; second, falling rate stage where crust thickens, vapor mass transfer decreases, and heat transfer decreases (Halder et al. 27; Hubbard and Farkas 1999). Convective heat transfer coefficient for the microwave frying were determined by using the experimental data when there was only convective heating which was during the microwave off time (Fig. 2b). Due to short heating time, variation of the heat transfer coefficient with time was not considered for microwave frying simulations. Constant heat transfer coefficient values were used in the trials during the off time. Microwave simulations gave an average convective heat transfer coefficient of 5 W/m 2 K. This is a higher value when compared to conventional frying. Moisture loss was higher in microwave frying when compared to conventional frying (Fig. 3). This may lead higher turbulence in the oil and hence higher heat transfer coefficient. RMSE values for conventional frying and microwave frying, which were.59 and 2.49 C, respectively, demonstrates a good fit for the simulations. RMSE value was higher for microwave heating because the transition between the on and off times during microwave heating was set to be very sharp in the simulations compared to the experimental data. Conclusion Microwave frying gave a shorter frying time, which can be used to reduce the processing time. Convective heat transfer coefficients obtained from the simulations showed a higher value for microwave frying which may be due to higher turbulence during microwave frying. Heat transfer coefficient can vary with time during frying as there are many stages during the process. 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