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by Tara McHugh

Radio Frequency Processing of Food

adio frequency (RF) heating is a ionic conduction whereby the ions oscil- coaxial probe, and the resonant cavity commonly used food processing late forward and backward in the food method. It is important to remember that Rtechnology that has been applied for creating friction and heat. dielectric properties are dependent on drying and baking as well as thawing of The electromagnetic characteristics temperature and frequency as well as the frozen foods. Its use in pasteurization, as of the food are important in determining density, structure, composition, and well as for sterilization and disinfection process parameters. The two important moisture content of the food. of foods, is more limited. This column will properties are permeability and permit- review various RF heating applications in tivity. Permeability has a very small Radio Frequency Versus Microwave food processing, as well as the basic contribution to dielectric heating and is RF heating has several advantages over principles of this technology. therefore not usually considered. microwave heating. RF heating is simpler Permittivity is the parameter most com- than microwave heating in that it gener- The Basic Science monly used to describe the dielectric ates an electrical field between a pair of RF processing uses dielectric heating to properties of foods. It relates the reflec- electrodes in one direction in contrast thermally process foods using electro- tion of electromagnetic waves at with microwave heating, which transmits magnetic waves. RF wavelengths cover interfaces and the attenuation of the in all directions. Microwave equipment the range of the electromagnetic spec- wave energy within the food. Dielectric also requires a waveguide to transmit trum from 3 kHz to 300 MHz. Only properties of many foods have been stud- microwaves. In these ways, construction frequencies of 13.56 MHz, 27.12 MHz, and ied by researchers and can be found in of large RF heating systems is simpler 40.68 MHz are permitted for use in the the literature. The three most popular and its application to continuous pro- United States. During RF processing, methods to measure dielectric properties cesses more straightforward. Another dielectric materials are placed in an are transmission line, open-ended difference is that RF processing uses alternating electric field. RF energy is generated by a triode valve and applied Radio frequency processing is an effective method of processing flour, grains, spices, and more.Photo courtesy of Radio Frequency Co. to the food via a pair of electrodes. Dielectric heat energy is generated in the food by molecular friction as high-fre- quency electric fields alternate. Unlike conventional heating, dielectric heating is fast and volumetric. RF processing can be used to process any dielectric material and most food products are dielectric. Foods contain polar molecules such as water, and these molecules have dipole moments, positive and negative charge centers that do not coincide. When placed in an electric field, polar molecules align with the field via the dipole moment phenomenon. In this way, polar molecules rotate continu- ously to align with the field as the field is alternated. Friction is created between the molecules, converting electromag- netic energy into heat and increasing the temperature of the food. Dissociative ions in foods also produce heat through

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Radio Frequency Processing of Food continued...

moisture levels, lower drying temperatures, and smaller equipment footprints. In addition, RF is highly energy efficient for removing final moisture from dry baked products. Traditional baking is carried out in forced- air convection ovens where the heat is transferred to the surface of the dough by convection, conduction, and irradiation, and then from the surface toward the inside by heat conduction. The dough has low heat conductivity; therefore, the traditional baking process is slow and can require as long as 30 min. In RF processing of bread, dough pieces are placed inside pans made of heat-resist- ant, RF compatible, food-grade polymers and introduced into the RF baking oven with con- trolled-temperature air flow and exhaust fans to evacuate excess moisture released during the baking process. Total baking times are reduced to 8–10 min for commercial bread loaves, and the bread does develop a crust when it comes out of the RF oven. Other tech- niques, such as infrared, can be used to form Radio frequency processing is an energy-efficient approach to removing moisture from dry baked goods like pita chips. crusts if desired. In breads, RF processing Photo courtesy of Radio Frequency Co. has also been shown to reduce mold growth and staling during storage. lower frequencies than microwave, resulting as moisture content. • Defrosting. Traditional defrosting meth- in longer penetration depths that make it use- Parallel plate “through-field” RF systems ods suffer from intrinsically slow heat ful to process bulk food materials that have are the most commonly used in the industry; transfer. The larger the size of product, the larger dimensions. RF processing also has the however, there are two other configuration longer the time needed. This provides oppor- ability to dynamically control final product types. “Fringe-field” and “staggered tunities for bacteria to grow, results in high moisture content. Oscillator circuits can be through-field” systems, which contain rod- or drip losses, and causes adverse changes in dynamically matched to load automatically, controlling the power absorbed by the load. Radio frequency heating provides more uniform field distributions In this way, RF driers can automatically draw than microwave heating and as a result is slightly more efficient. more power if the moisture in the product is too high. Finally, RF heating provides more tube-shaped electrodes, can be used to treat the product quality, leading to significant eco- uniform field distributions than microwave thin or intermediate thickness products. nomic loss. heating and as a result is slightly more Several companies manufacture RF pro- RF rapidly generates heat volumetrically, efficient. cessing equipment for the food industry. A and defrosting is achieved in minutes rather few of the leaders are Radio Frequency Co., than hours/days, even for large product Equipment Design Millis, Mass.; Stalam Co., Nove, Italy; and blocks. It can also be performed directly There are two design types of RF heating Strayfield Co., Theale, Berkshire, United inside packaging. The heating process results equipment. They are the free-running tradi- Kingdom. in a significant reduction of drip losses while tional oscillator system and the Crystal minimizing product deterioration due to bac- Oscillator Source Matched Impedance Commercial Applications terial growth, making it ideal for many Generator (COSMIG) 50 ohm system. • Drying and Baking. Post-baking RF drying is defrosting processes. Traditional oscillator-based RF systems are common in cookie, cracker, cereal, and snack • Sterilization and Pasteurization. simple to use and lower in cost than newer food manufacturing. Surface cracking often Sterilization processes like retorting are gen- COSMIG 50 ohm systems. The newer sys- occurs during traditional drying processes erally carried out by treating products with tems, although more costly, allow for precise due to moisture gradients in the product. RF drastic heat treatments, resulting in loss of frequency and power control with feedback drying solves this issue because the energy product quality. Pasteurization is a relatively on forward and reflected power, thus penetrates evenly, minimizing moisture gradi- gentle heat treatment carried out at tempera- enabling precise control of processing ents. The use of RF also avoids discoloration tures below the boiling point of water. Both parameters such as power and heating rate and flavor damage due to thermal buildup. It traditional sterilization and pasteurization to obtain exact final product properties such enables faster line speeds, more consistent methods use saturated steam or pressurized pg 74 08.16 • www.ift.org hot water to heat the packaged food. The cereal grains, protein supplements, spices, foods), final RF-treated food quality and temperature rise of the product is slow and seeds, and pet foods, insuring their food safety, and advances in mathematical model- nonuniform, frequently resulting in overheat- safety. This is particularly important for ing all support increased implementation of ing of the outside layers of the food, which ingredients that are added to foods after the RF technology and additional FDA approvals results in quality losses. The ability of RF to cooking “kill step,” such as cookie dough of RF processing for specific food applica- rapidly generate heat volumetrically within inclusions for ice creams and cold-pressed tions like sterilization. Challenges related to the product minimizes product deterioration. health bars. avoiding dielectric breakdown (arching) and The RF equipment requires less space com- • Disinfestation. Insect infestation is thermal runaway heating from hot spots are pared to other machines based on another major problem. Chemical fumigants being addressed by researchers and equip- conventional heating technology that have (like methyl bromide) are commonly used for ment manufacturers. And specific new long heating times and require large product postharvest pest control, but they have envi- packaging materials tailored for RF process- volumes under processing at any given time. ronmental concerns and regulatory issues. ing continue to be developed. RF processing Because no thermal energy is wasted into the Alternative thermal treatments frequently of foods has a very bright future. FT environment, the RF process is more effi- result in product quality deterioration. RF cient, economic, and environmentally processing allows for rapid and uniform heat- friendly. RF pasteurization is used in meat ing of many substrates. Recent research has Tara McHugh, PhD, Contributing Editor processing. Liquid pasteurization applica- confirmed the viability of RF for disinfestation • Research Leader, USDA Agricultural Research Service, Albany, Calif. tions are also being explored. of fruits and nuts. • [email protected] One of the most prevalent applications for RF pasteurization is in the treatment of dry RF and the Future ingredients. In a dry state (5% and 15% mois- Continued expansion of applications of RF REFERENCES ture content) microbes are considered processing of foods is anticipated during the Awuah, G. B., H. S. Ramaswamy, and J. Tang. 2014. “dormant” and are difficult to kill. RF is very years ahead. Ongoing research on dielectric Radio Frequency Heating in Food Processing: Principles and Applications. Boca Raton: CRC Press. effective with dry ingredients like flour, properties of foods (including heterogeneous

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