IMPI s Proceedings. 46th Annual Microwave Power Symposium (IMPI 46) June 20 22, Bally s Las Vegas Las Vegas, Nevada, USA

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1 IMPI s 46th Annual Microwave Power Symposium (IMPI 46) Focusing on food, agricultural and biological processes, food service and food safety 12 Proceedings June 22, 12 Bally s Las Vegas Las Vegas, Nevada, USA Presented by the International Microwave Power Institute PO Box 1140, Mechanicsville, VA Phone: +1 (804) info@impi.org PHOTO BY VILLE MIETTINEN

2 TEMPERATURE DEPENDENT DIELECTRIC AND THERMAL PROPERTIES OF WHEY PROTEIN GEL Jiajia Chen 1*, Krishnamoorthy Pitchai 2, John Diamond Raj 3, Sohan Birla 1, Ricardo Gonzalez 4, David Jones 1, Jeyamkondan Subbiah 1,2 1 Biological Systems Engineering, University of Nebraska-Lincoln 2 Food Science and Technology, University of Nebraska-Lincoln 3 Indian Institute of Crop Processing Technology, Thanjavur, India 4 ConAgra Foods, Inc., Omaha Dielectric and thermal properties of whey protein gel were measured between - to 1 C. The dielectric constant increased from - to 0 C, and then decreased from 0 to 1 C. The average thermal conductivity of the frozen and thawed sample were measured to be and W m -1 K -1, respectively. The latent heat of fusion and vaporization were kj kg -1 and kj kg -1, respectively. These properties are important input parameters in the microwave heat transfer model and thus can be used for validating the model with whey protein gel as model food. Keywords: whey protein gel, dielectric constant, dielectric loss factor, thermal conductivity, specific heat capacity. INTRODUCTION Microwave heating is rapid and offers convenience to consumers in this fast-paced world. However the biggest issue in domestic microwave ovens is the non-uniform heating, especially for the frozen food due to dramatic variation in properties between ice and water. Recent outbreaks and recalls associated with some of these frozen foods illustrate the urgency of improving microwave heating uniformity. Understanding how microwave interacts with the food will help food scientists in developing food products and packages that would provide better cooking performance in domestic microwave ovens in terms of heating uniformity (Bradshaw et al. 1997; Gunasekaran, Sundaram and Yang, Huaiwen. 07; Rakesh et al. 09; Ryynänen et al. 04). Coupled microwave and heat transfer model is a promising tool to understand the multiphysics process in microwave heating (Chen et al. 08; Geedipalli et al. 07; Pitchai et al. 12). Whey protein gel (WPG) is a model food that can be used for validating microwave heat transfer model. It has been widely used as model food to emulate the real food, as its properties match meat products (Tang et al. 07; Wang et al. 09a). WPG can be used to easily create different shapes and sizes to imitate physical characteristics of the real International Microwave Power Institute, 12 65

3 food (Tang et al. 07). It can be easily formulated to match the real food in its dielectric properties and thermal properties, which are critical parameters related to microwave heating. The dielectric and thermal properties are important parameters that influence microwave heating. Dielectric properties of materials are critically important in understanding the interaction of microwave electromagnetic energy with those materials. These properties, along with thermal and other physical properties, and the characteristics of microwave electromagnetic fields determine the absorption of microwave energy and consequent heating behavior of food materials in microwave heating (Nelson, S. O., and Datta, A. K. 01). The relative permittivity is a complex quantity, which is defined as ε*= ε '-jε", where the real component is the dielectric constant (ε ') and the imaginary component is the dielectric loss factor (ε"). The dielectric constant (ε ') is related to the capacitance of a substance and its ability to store electric energy, while the loss factor determines the ability of the material to dissipate electric energy as heat (Ryynänen 1995). Generally the dielectric properties of food material vary with composition, moisture, temperature and frequency (Ryynänen 1995). Several researchers have measured the dielectric properties of various food products, such as salmon fillets (Wang et al. 08), chicken breast muscle (Zhang et al. 07), egg whites and whole egg (Wang et al. 09b), whey protein gel (Wang et al. 03), macaroni and cheese dinner preparation, whey protein gel, ground whole wheat flour, and apple juice (Nelson, S.O. and Bartley P.G. 00, Nelson, S.O. and Bartley, P.G. 02). However, most of the food samples were measured only at room temperature. The properties in the frozen temperature range are needed to simulate microwave heating of frozen food. Thermal properties (thermal conductivity and specific heat capacity) are also important inputs to the heat transfer model. During heating and standing time, the thermal conductivity will influence the diffusion of thermal energy from hot spots to cold spots. In microwave heating process, specific heat capacity of food determines the heating rate. Especially for the frozen sample, the latent heat associated with phase change will influence the thawing process. The objective of this paper is to determine both the dielectric (dielectric constant and loss factor) and thermal properties (thermal conductivity and specific heat capacity) of whey protein gel as a function of temperature from frozen to cooked state. MATERIALS AND METHODS Sample Preparation Whey protein gel was formed by dissolving % whey protein powder (80% concentrate), 0.56% CaCl 2 and 0.5% guar gum (GuarNT, TIC GUMS, White Marsh, MD) in deionized water. The solution was stirred for 1.5 h for dispersion of whey protein powder completely. The solution was poured into the mold and was kept in 90 C International Microwave Power Institute, 12 66

4 conventional oven for 1.5 h to form the desired shape of the gel. The whey protein gel was then stored at 4 C in a refrigerator until used for measurement of dielectric and thermal properties. Measurement Procedure Measurement of dielectric properties The dielectric properties were measured using an open-ended coaxial high temperature probe and network analyzer (Agilent PNA-L 5230C) in the frequency range of 300 to 3000 MHz at 5 MHz interval. Air, open short and room temperature deionized water were used to calibrate the system. A cylindrical test cell (22 mm inner diameter and 100 mm height) made of stainless steel was designed to control and maintain the sample temperature during the measurements (Fig.1). The sample size meets the requirement of Agilent s recommendation that the sample diameter to be greater than mm and the thickness to be greater than / mm. Initially, the refrigerated sample was placed on the test cell and silicon oil (Pure Silicone Fluid cst, Clearco Products Co.) in the jacket of the test cell from a circulation bath (9012A11B, PolyScience) was used to freeze the sample. Due to heat loss, the sample touching the probe never reached the desired frozen temperature. Therefore, cylindrical shape whey protein gel sample was cored and kept overnight in a freezer at -80 C. In the frozen temperature range, the surface temperature of the sample was measured with a K- type thermocouple. Immediately, the dielectric properties of the sample were collected. The data of the frozen sample were collected at different temperature points until the temperature reached near 0 C. Then, the sample was placed in the test cell and silicon oil circulation was used to adjust the temperature in the thawed state. A K-type thermocouple was used to monitor the sample temperature as shown in Fig. 1. From 0 to 10 C, the dielectric properties were measured for every 5 C whereas from 10 to 1 C, the dielectric properties were measured at every 10 C interval. The results were reported at key microwave frequencies (915 and 2450 MHz) as a function of temperature. International Microwave Power Institute, 12 67

5 Fig. 1 Schematic of dielectric measurement test cell. Measurement of thermal properties Thermal conductivity of whey protein gel was measured using KD-2 Pro thermal properties analyzer (Decagon Devices, Inc., Pullman, WA) connected with a singleneedle (KS-1 6 cm sensor). The accuracy of this apparatus is ±5%. The WPG sample formed in cylindrical tubes (3 cm diameter and 10 cm height) was kept in two freezers maintained at - and -4 C. The thermal conductivity in frozen temperature range was collected at these two temperatures. From 10 to 1 C, the temperature of the sample was controlled by the circulation of the silicon oil. The specific heat capacity of whey protein gel was measured by Mettler Differential Scanning Calorimeter (Mettler Toledo, DSC 822) from - to 1 C at the heating rate of 2 C/min. About 10 mg of fresh WPG sample was filled in a 40 µl pan (Al-crucibles) and was covered with a lid. The pans of sample and reference were placed in the DSC for measuring. Liquid nitrogen was used to control the frozen temperature. All measurements were performed in triplicate. RESULTS AND DISCUSSIONS Dielectric Properties Dielectric Constant Fig. 2 shows dielectric constant of whey protein gel from 300 MHz to 3000 MHz at four temperatures of -6,, 50 and 100 C. The dielectric constant value was quite different for frozen sample compared with thawed sample. At all temperature value of dielectric constant decreased as frequency increased. At high frequencies, the dielectric constant values fluctuate slightly Dielectric Constant Temperature, C Frequency, MHz International Microwave Power Institute, 12 68

6 Fig. 2 Frequency dependence of dielectric constant of whey protein gel at four temperatures. The temperature dependence of dielectric constant at two typical microwave heating frequencies (915 and 2450 MHz) is shown in Fig.3. It shows that in the frozen temperature range, the dielectric constant increased rapidly from low values of around 4 at - C to the highest value of 60 at 0 C which is in thawed state. In the thawed temperature range, the dielectric constant at both frequencies decreased gradually as the temperature increased from 0 to 1 C. For the entire temperature range, the dielectric constant at 915 MHz was always slightly higher than that of 2450 MHz. The phenomenon contributing to the frequency dependence of the dielectric properties is the polarization arising from the orientation with the imposed electric field of molecules which have permanent dipole moments (Nelson 1991) MHz 2450 MHz 50 Dielectric Constant Temperature, C Fig. 3 Temperature dependence of dielectric constant of whey protein gel at frequency of 915 and 2450 MHz. Dielectric Loss Factor Fig. 4 shows dielectric loss factor of whey protein gel from 300 to 3000 MHz at temperatures of -6,, 50 and 100 C. It shows that, higher temperature sample has higher dielectric loss factor value. From low frequency of 300 MHz to high frequency of 00 MHz, the dielectric loss factor of thawed sample decreased with the increasing frequencies. After 00 MHz, the dielectric loss factor of thawed sample was steady at about 16. Within the whole frequency range, the dielectric loss factor of frozen sample did not change much. International Microwave Power Institute, 12 69

7 Dielectric Loss Factor Temperature, C Frequency, MHz Fig. 4 Frequency dependence of dielectric loss factor of whey protein gel at four temperatures. The dielectric loss factor of whey protein at 915 and 2450 MHz increased with increase in temperature as shown in Fig. 5. In the frozen temperature range, the dielectric loss factor of both frequencies increased sharply from the lowest value of about 0.27 at - C to about 18 at 0 C. Within the thawed temperature range, the dielectric loss factor at 2450 MHz did not change much with a value of around 16 in the temperature range of 0 to 1 C, while the dielectric loss factor of 915 MHz increased from 19.2 to 41.3 with the temperature increasing from 0 to 1 C. In the entire temperature range, the dielectric loss factor at 915 MHz was higher than that at 2450 MHz. International Microwave Power Institute, 12 70

8 MHz 2450 MHz 35 Dielectric Loss Factor Temperature, C Fig. 5 Temperature dependence of dielectric loss factor of whey protein gel at 915 and 2450 MHz. The dielectric constant and loss factor of whey protein gel showed the same trend as compared to the whey protein gel data measured by Nelson, S.O. and Bartley, P.G. (02). The dielectric properties of material are governed by free water dispersion, bound water dispersion, and ionic conduction (Feng et al. 02). The gradual decrease in the dielectric constant with increasing temperature appears reasonable for the gel containing about 80% water, which is dominated by free water dispersion and ionic conduction (Feng et al. 02). According to the relaxation mechanism, ionic conduction is the dominant factor for dielectric loss factor when frequencies are lower than about 1 GHz. Therefore, the increase in the loss factor is mainly attributable to the increase in ionic conduction (Nelson, S.O. and Bartley, P.G. 02) with increasing temperature at 915 MHz. Thermal Conductivity The thermal conductivity of frozen sample and thawed sample are different from each other. In the frozen temperature range, the thermal conductivity did not change much with an average value of about W m -1 K -1. The thermal conductivity of thawed sample also did not change much with an average value of about W m -1 K -1. The thermal conductivity values of frozen sample and thawed sample are close to the values of ice and water (Young, 1992), respectively, as gel contains about 80% water. The thermal conductivity difference between frozen and thawed sample will influence the temperature distribution of food during microwave heating. Specific Heat Capacity International Microwave Power Institute, 12 71

9 The specific heat capacity of WPG as a function of temperature is shown in Fig.6. As shown in Fig.6, there are two peaks at just above 0 and 110 C, which correspond to latent heat of fusion and vaporization, respectively. By integrating the peaks, the latent heat of fusion and vaporization were calculated as and kj kg -1, respectively. The latent heat will delay the temperature rise during microwave heating, which will also tremendously influence heating pattern of frozen food Cp, KJ/(Kg K) Temperature, C Fig. 6 Specific heat capacity of whey protein gel. CONCLUSIONS In this study, dielectric and thermal properties of whey protein gel were determined and reported as a function of temperature and frequency. These properties are critical input parameters to a microwave heat transfer model, which can be used by food scientists in developing novel food products that would minimize non-uniform heating during cooking in a domestic microwave oven. Better uniform heating leads to better food quality and safety. ACKNOWLEDGEMENTS This study is sponsored by ConAgra Foods, Inc. Partial support for this study was provided by the USDA CSREES NIFSI grant (Project number: ). International Microwave Power Institute, 12 72

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