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Innovative Methods of Energy Transfer L. E. MCBEE Rose Acre Farms, Seymour, Indiana 47274 ABSTRACT Energy is utilized in many forms for processing egg products and other foods. Energy in the form of heat has commonly been used to kill microorganisms and pasteurize eggs. Transfer of energy by convection and conduction is limited by the properties of the egg product. Energy transfer by radiation is being used to advantage in the development of innovative methods to kill or inactivate microorganisms. A review of the electromagnetic spectrum reveals underutilized forms of energy with unique properties. Specific frequencies and method of application are selected for their ability to focus energy toward the destruction of microorganisms and the production of safe food products for the public. (Key words: pasteurization, radiation, eggs, microwave, radio waves) 1996 Poultry Science 75:1137-1140 INTRODUCTION The pasteurization of egg products is mandatory in the U.S. The purpose of pasteurization is to provide consumers with safe egg products, free of pathogenic microorganisms, especially Salmonella. The early work by Owen Cotterill and associates at the University of Missouri-Columbia, Garibaldi, Lineweaver, and others at the USDA Western Regional Laboratory, and many pioneering researchers established the basis upon which the USDA developed the pasteurization standards currently in use. These standards were designed to minimize the risk of Salmonella while maintaining the functional properties of egg products. Presently, these standards are being reevaluated for their adequacy in meeting current concerns of the public regarding other pathogens, new product formulations, and alternative processing methods. Leaders in the egg products industry have continued to seek improved processing methods to provide greater margins of safety to the consumer, improve functional properties, and reduce costs of operations. Today we have learned of developments indicating that pasteurization of shell eggs will become a reality. Food engineers and technologists are adapting unique concepts for the development of improved methods of pasteurizing eggs and egg products. All of these methods involve the transfer of energy with the goal of eliminating Salmonella and other pathogens. The objective of the following presentation is to review energy transfer principles, discuss selected recent developments in egg pasteurization, and stimulate Received for publication August 16, 1995. Accepted for publication February 6, 1996. thought about innovative methods of pasteurizing eggs and egg products. THERMODYNAMICS Food processing operations directly involve mass and energy. The food product may be considered a mass to be mixed, blended, separated, heated, cooled, or transformed into a desired form with specific attributes. All of these processes involve the transfer of mass and energy by the input of work. Energy is usually considered to be heat; however, there are many forms of energy used in food processing. A review of thermodynamics principles reveals limitations and areas of opportunity for development of innovative food processing methods. First Law of Thermodynamics The total amount of energy in the universe is constant. Energy can be neither created or destroyed but only changed from one form to another as shown in the equation: E2 = Ei + (q - w) where: Ei and E2 = total energy of a system; q = heat; and w = work. In order to transform a food product from one energy state to another it is necessary to apply energy in the form of heat with work. Second Law of Thermodynamics Any spontaneous change that occurs in the universe must be accompanied by an increase in the entropy of the universe. This law may also be applied to a closed system 1137 1138 MCBEE or to relationships between defined interactive systems by the following equation: AG = AH - T AS (for a system) where: AG = change in energy; AH = change in heat content; T = temperature; and AS = change in entropy. If this equation is applied to changes in biological systems, we observe that the growth of large ordered molecules from smaller molecules will have a negative AS and under isothermal conditions will require the input of energy. This equation also requires that heat must flow from higher to lower temperature. ENERGY TRANSFER APPLICATIONS Conduction Energy is conducted through a gas by intermolecular transfer of kinetic energy from more energetic to less energetic molecules when they collide. Energy conduction in liquids is similar to that in gases. The thermal conductivity of liquids decreases with increasing molecular weight. Energy is conducted in solids by two mechanisms: migration of free electrons and lattice vibration. In conductive metals, there is a rapid transfer of thermal energy by electron transfer, whereas in nonmetallic materials, the lattice vibration transfers energy more slowly (Keith and Bohn, 1993). Food products are complex and variable blends of solids, semi-solids, and liquids having quite slow thermal transfer. This slow thermal transfer brings in the factor of interface resistance. There is no single theory or set of data that fully describes interfacial resistance at surfaces for food processing. Liquid egg products are usually homogeneous fluids, so fluids containing particulates will not be discussed in this presentation. Convection When heating fluids with conventional heat exchangers the convection mode is most active. Convection is a combination of conductive energy transfer due to molecular motion and the macroscopic motion of fluid parcels. The motion of the fluid may be due to density gradients or an external force such as pumps or fans. Irrespective of the details of the mechanism, the rate of heat transfer by convection between a surface and a fluid can be calculated from the relationship: q c = he A AT where: q c = rate of heat transfer by convection, Watts (British thermal units per hour); A = heat transfer area, square meters (square feet); AT = difference between the surface temperature T s and a temperature of the fluid J_at some specified location, degrees Kelvin (Fahrenheit); h c = average convection heat transfer coefficient over the area A, Watts per square meter per degrees Kelvin (British thermal units per hour per square foot per degrees Fahrenheit). The rate of heating can be accelerated by increasing the heat transfer area, increasing the surface temperature, or improving the heat transfer coefficient; however, there is a limit to the surface temperature imposed due to the coagulation temperature of egg. Area is restricted by economic and physical considerations. To improve the rate of heat transfer, improvement must be accomplished by the design of the heat transfer surface. Electromagnetic Spectrum The electromagnetic spectrum (Figure 1) may be used to categorize most energy sources. High frequency radiation, greater than about 10 - 7 m, is characterized by its high energy level and ability to penetrate materials easily. Ultraviolet, visible and infrared radiation will penetrate only thin layers of selected material. Within the group comprised of microwave, radio wave and electric power, penetration decreases with increasing wavelength. Radiation Ionizing radiation has long been promoted as an effective means of destroying microorganisms but has suffered from poor acceptance by the general public. Currently, foods are being treated with high energy electron beams from a linear accelerator (Rice, 1993). Wong and Herald (1995) compared thermal (57 C for 3.5 min) and irradiation (2.5 to 3.3 kGy for 81 s) treatments of liquid egg white and found both to destroy Salmonella typhimurium at 10 7 cfu/mL. Irradiated liquid egg white exhibited better functional properties, improved resistance to microbial growth, and reduced required storage energy by 67% for refrigerated vs frozen storage. Electric Power Ohmlc Heating. It is well known that the passage of an electric current will increase the temperature of materials due to the electrical resistance of the material. This process, using 60 Hz alternating current, has been offered for use by a leading food equipment manufacturer. It has the advantage of rapid heating of both fluid and solid particles in the same process stream. Electroheating. In electroheating the electrical power is conducted through the product as in ohmic heating; however, with frequencies in the range of 100 to 450 KHz there is minimal electrolysis of metallic electrode materials into the food product as described by Reznik (1988). Reznik and Knipper (1994) and Knipper and Reznik (1995) improved the process describing equipment and optimum conditions for pasteurization of egg products utilizing 5,000 to 37,000 V at up to 12 A/cm2 at 150 to 450 KHz. This process is currently approved by the USDA for the production of extended shelf life refrigerated egg products. Pulsed Electric Power. In biology, cell membrane selective permeability is an advantage to the cell by SYMPOSIUM: SECOND O. J. COTTERILL EGG AND EGG PRODUCTS SYMPOSIUM 1A 1 nm 1 m 1 km Wavelength, j Q - 1 4 - i s - I : - n - i o -9 -8 -: -6 -5 -4 -.» -: -1 o i : 3 4 *<m) » i * t 1 1 I . .I I I I 1 i 1 I 1 I I Frequency, Ms* 1 ) 1 ]Qn 1139 5 6 7 1 1 1 1 I I I I I I I I I I 1 I I I I I I I I F 21 20 19 is 17 16 15 M 13 12 11 10 9 s 7 6 5 4 3 2 1 1 I I I I Ivisibl Radio waves H*— X-rays—*| « Thermal Cosmic rays Gamma radiation rays Electric power -Hertzian waves* (a) Wavelength, X (m) 10" 7 1 Frequency, y d " ) Ultraviolet 10- 4 T 10" Near Intermediate infrared infrared io» Far infrared llJiJ* (b) FIGURE 1. a) Electromagnetic spectrum, b) thermal radiation portion of the electromagnetic spectrum, (Kreith and Bohn, 1993). preserving the interior of the cell from exogenous aggression, but a strong limitation to experimental manipulation of the cytoplasm for genetic modification. Electropulsation, i.e., submitting cells to strong shortlived electric field pulses, was pioneered by Neumann and Rosenheck (1972) to create permeable cell membranes. Teissie et al. (1992) described a flowthrough device for electropermeating cells. Under the best of conditions many cells do not remain viable after electroporation. A photomicrograph of electroporated cells show orientation of pores within the electric field and different forms of pores with different pulse conditions (Tekle et al, 1992). In food processing, it is desired to select treatment conditions of greatest detriment to the cell structure of microorganisms leading to death of the cells. Castro et al. (1993) determined that membrane destruction occurs when a differential membrane potential exceeds - 1 V in many cellular systems. An external field strength of - 10 KV is damaging to Escherichia coli (Sale and Hamilton, 1967). This effect on cell walls was described as "dielectric breakdown" by Zimmerman (1986). Dunn and Pearlman (1987) revealed a method and apparatus for preserving fluid food products, such as fluid egg products, by exposure to controlled, pulsed, high 1140 MCBEE Bialod, D., 1985. Development of technology for industrial applications of radio frequencies, in: Radio Frequency/ Radiation and Plasma Processing-Industrial Applications and Advances. Technomic Publishing Co., Inc., Lancaster, PA. Castro, A. J., G. V. Barbosa-Canovas, and B. G, Swanson, 1993. Microbial inactivation of foods by pulsed electric fields. J. Food Process. Preserv. 17:47-73. Decareau, R. V., 1985. Microwaves in the Food Processing Industry. Academic Press, Inc., Orlando, FL. Dunn, J. E., and J. S. Pearlman, 1987. Methods and apparatus for extending the shelf-life of fluid food products. U.S. Patent 4,695,472. Dunn, J., A. Bushnell, P. Hall, S. Cheng, T. Ott and W. Clark, 1995a. Pulsed electric field pasteurization. Presented at IFT Annual Meeting, Anaheim, CA. Dunn, J., A. Bushnell, T. Ott, and W. Clark, 1995b. Pulsed light for food processing. Presented at IFT Annual Meeting, Anaheim, CA. Heinz, V., and D. W. Knorr, 1995. Inactivation of Bacillus subtilus endospores by ultra-high-pressure in combination with other treatments. Presented at IFT Annual Meeting, Anaheim, CA. Huang, F., 1989. Method of treating liquid egg and egg white with microwave energy to increase refrigerated shelf life. U.S. Patent 4,853,238. Knipper, A. J., and D. Reznik, 1995. Producing extended refrigerated shelf life food without high temperature heating. U.S. Patent 5,415,882. Kreith, F., and M. S. Bohn, 1993. Principles of Heat Transfer. West Publishing Company, St. Paul, MN. Lentz, R. R., P. S. Peapack, G. R. Anderson, J. DeMare, and T. R. Peck, 1993. Method of processing food utilizing infrared radiation. U.S. Patent 5,382,441. Neumann, E., and K. Rosenheck, 1972. Permeability changes induced by electric impulses in vesicular membranes. J. Membr. Biol. 10:279-290. Reznik, D., 1988. Apparatus and method for electrical heating of food products. U.S. Patent 4,739,140. Other Energy Reznik, D., and A. Knipper, 1994. Method of electroheating liquid egg and product thereof. U.S. Patent 5,290,583. Berlin and Hoover (1995), Roberts and Hoover (1995), Rice, J., 1993. E-B ionization zaps salmonella. Food Process. July, Aleman et al. (1995), and Heinz and Knorr (1995) recently p 12. presented their findings regarding the application of high Roberts, C. M., and D. G. Hoover, 1995. Tolerance of Bacillus pressure for inactivating various bacteria and preserving eoagulans 7050 spores to combinations of high hydrostatic food products. pressure, heat, acidity, and nisin. Presented at IFT Annual Pulsed broad spectrum light with peak emission Meeting, Anaheim, CA. Sale, A.J.H., and W. A. Hamilton, 1967. Effect of high electric between 400 and 500 n m provided high levels of microbial fields on microorganisms. I. Killing of bacteria and yeast. kill on simple surfaces (Dunn et al, 1995b). A method of Biochim. Biophys. Acta 163:37-43. processing food using a filtered source of infrared Tekle, E., P. B. Chock, and R. D. Austumian, 1992. Electric field radiation for deep heating was described by Lentz et al. induced asymmetric breakdown of cell membranes, in: (1993). Charge and Field Effects in Biosystems-3. Birkhauser, Boston, MA. REFERENCES Teissie, J., S. Sixou, and M. P. Rols. 1992. Large volume cell electropermeabilization and electrofusion by a flow process. Aleman, G. D., E. Y. Ting, A Hawes, D. F. Farkas, and J. A. Torres, in: Charge and Field Effects in Biosystems-3. Birkhauser, 1995. Inactivation of yeasts in pineapple juice by constant Boston, MA. and pulsed ultra high pressure. Presented at Institute of Food Technologists Annual Meeting, Anaheim, CA. Wong, Y. C, and T. J. Herald, 1995. A comparison study of thermal and irradiated pasteurization on the functional, Allen, M. J., S. F. Cleary, A. E. Sowers, and D. D. Shillady, 1992. Charge and Field Effects in Biosystems-3. Birkauser, Boston, physical and microbiological properties of liquid egg white MA. during refrigerated storage. Presented at IFT Annual Meeting, Anaheim, CA. Berlin, D. L., and D. G. Hoover, 1995. Effect of high hydrostatic pressure on viable and noncultural but viable pathogenic Zimmerman, U., 1986. Electric breakdown, electropermeabilizaspecies of Vibrio. Presented at IFT Annual Meeting, tion and electrofusion. Rev. Physiol. Biochem. Pharmacol. Anaheim, CA. 105:175-256. voltage electric field treatment. This technology is currently being marketed by PurePulse Technologies tinder the CoolPure trademark. Dunn et al. (1995a) presented data on the efficacy of multiple, short duration, high strength electrical field pulses for providing nonthermal antimicrobial effects and pasteurization of pumpable foods. Radio Waves. Radio frequency energy waves create heat within the product stream itself rather than by conduction from heated surfaces. This method results in the ability to reduce heating time and can heat large quantities with relatively small processing systems. Bialod (1985) discussed the development and market characteristics of radio frequency energy applications. High frequency installations frequently operate at 27 or 13 MHz. The three major components of the radio wave system, generator, applicator, and load, must be matched for optimum efficiency. Although calculating the electrical characteristics of the system is possible, much experimentation is required for optimum efficiency. Microwaves. Microwave heating typically uses frequencies from 300 to 3,000 MHz, although the ISM frequencies permitted are 433, 915, and 2,450 MHz. The microwave ovens in home use and many commercial installations use 2,450 MHz. Greater depth of penetration in food products can be obtained using the 915 MHz frequency. Few installations use the 433 MHz frequency. A comprehensive review of microwave technology and its application to food processing was presented by Decareau (1985). Huang (1989) patented the use of microwaves to heat egg white and whole egg at temperatures up to 83.6 C without significant gelation. He claimed the treatment provided thermal kill, biological damage, and alteration of cell membranes and metabolic function to microorganisms.