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LWT - Food Science and Technology 42 (2009) 1204–1212

Contents lists available at ScienceDirect

LWT - Food Science and Technology
journal homepage: www.elsevier.com/locate/lwt

Dielectric properties of egg whites and whole eggs as influenced by thermal treatments
Jian Wang a, Juming Tang a, *, Yifen Wang b, Barry Swanson c
a

Department of Biological Systems Engineering, WashingtonState University, Pullman, WA 99164-6120, USA Department of Biosystem Engineering, Auburn University, Auburn, AL 36849, USA c Department of Food Science and Human Nutrition, Washington State University, Pullman, WA 99164-6376, USA
b

a r t i c l e i n f o
Article history: Received 14 September 2007 Received in revised form 16 February 2009 Accepted 17 February 2009 Keywords: Dielectricproperties Denaturation Radio frequency Microwave frequency Egg white Whole egg

a b s t r a c t
Effects of cooking on dielectric properties of liquid whole eggs and liquid egg whites were studied in connection with radio frequency and microwave heating processes to preserve shelf-stable products. Dielectric measurements were made using an open-ended coaxial probe method over a temperature range of 20and 120  C at radio frequencies 27 and 40 MHz, and microwave frequencies 915 and 1800 MHz. Thermal denaturation of liquid egg whites and whole eggs influenced the dielectric constants and dipole loss component of eggs, as reflected by changes in loss factors above 60  C. In addition, loss factor of liquid whole eggs was generally smaller than that of egg whites and larger than the loss factor ofegg yolk. Ionic conductivity was a dominant factor determining the dielectric loss behavior of egg products at radio frequencies, whereas dipole water molecules played an increasing role with an increase in microwave frequencies. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Thermal processing is a common commercial preservation method to produce shelf-stable low acid (pH > 4.6) foodproducts. To achieve commercial sterility, foods are hermetically packed in containers and thermally processed at high temperatures (up to 121  C or above) to inactivate spores of toxin-producing and heat resistant microorganism, Clostridium botulinum type A and B (Mudgett, 1986). For foods, especially solid and semisolid, conventional retort processes are time-consuming due to the slow heattransfer within foods. During a conventional sterilization process, the surface of the packaged food is exposed to high temperature much longer than the inner part, resulting in overcooking of the food. Dielectric heating, including radio frequency (RF) and microwave heating, can potentially reduce processing time and improve heating uniformity because the heat is volumetrically generated inside thefood by conversion of alternating electromagnetic (EM) energy to thermal energy (Cathcart, Parker, & Beattie, 1947; Kenyon, Westcott, Case, & Gould, 1971; Kinn, 1947). But engineering design of dielectric heating is hindered by a lack of

information about the dielectric properties of selected foods as functions of food composition, temperature, and EM wave frequencies (Zhao, Flugstad, Kolbe,Park, & Wells, 2000). This knowledge is needed in analyzing electromagnetic field distributions in microwave or RF sterilization systems. The dielectric properties of a food describe its ability to store and dissipate electrical energy in response to an alternating EM field (Mudgett, 1986). Dielectric properties are normally described by complex permittivity, 3c:

3c ¼ 30 À j300 ¼ 30 30r À j300 rÀ

Á

(1)

pffiffiffiffiffiffiffi where j ¼ À1. In above equation, the real part 30 , referred to as dielectric constant, describes material’s ability to store electric energy in an alternating field, while the imaginary part 300 , referred to as loss factor, determines the property of the material to convert electric energy to thermal energy; 30 is the permittivity of free space (¼8.854 Â 10À12 F/m),...
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