In conventional thermal processing systems, heat energy is transferred through conduction and convection from a hot medium to a cooler product that may result in large temperature gradients and uneven temperature distribution with the product. Heat exchangers typically utilize pressurized steam from petroleum-fired boilers with less than 25 – 30 percent of the energy conversion. Five electro-magnetic heating techniques, including infrared (IR), microwave (MW), dielectric or radio-frequency (RF), ohmic (OH), and magnetic induction heating (MI) can heat foods faster and more efficiently and consequently produce higher quality product with better nutritional attributes Except for magnetic induction heating, heat is generated within the product as a result of the transfer of electro-magnetic energy directly into the product. This initiates volumetric heating due to frictional interaction between water molecules and charged ions. These methods offer a considerable speed advantage, particularly in solid foods and high efficiency of energy conversion ranging from 60 percent, up to almost 100 percent.
Infrared (IR) radiation encompasses the portion of the electro-magnetic spectrum bordering on visible light and microwaves. IR can be classified into three regions, namely, near infrared (NIR), mid-infrared (MIR), and far-infrared (FIR), corresponding to the spectral ranges of 0.75 – 1.4, 1.4 – 3, and 3 – 1000 μm, respectively. The amount of the IR radiation on any surface has a spectral dependence because energy coming out of an emitter is composed of different wavelengths and the fraction of the radiation in each band, dependent upon the temperature and emissivity of the emitter.
As food is exposed to IR radiation, it is absorbed, reflected, or scattered. In general, the food substances absorb FIR energy most efficiently through the mechanism of changes in the molecular vibrational state. Water and organic compounds, such as proteins and starches, which are the main components of food, absorb FIR energy at wavelengths greater than 2.5 μm.
Because IR’s penetrating powers are limited, it can be considered as surface treatment in both liquid and solid foods. Recently, IR radiation has been widely applied to various operations in the food industry, such as dehydration, frying, and surface pasteurization, as well as in domestic applications, such as grilling and baking. Electrical IR heaters are popular because of installation controllability, their ability to produce a prompt heating rate, and cleaner form of heat. They also provide flexibility in producing the desired wavelength for a particular application. In general, the operating efficiency of an electric IR heater ranges from 40 – 70 percent, while that of gas-fired IR heaters ranges from 30 – 50 percent. Efficacy of IR heating depends on the IR power level, peak wavelength, and bandwidth of IR heating source; type, depth, moisture content, and temperature of food sample.
Microwave (MW) frequency waves are generated through a magnetron applicator at frequencies between 300 MHz and 300 GHz and, essentially, the interaction with the food material and water molecules causes the food to heat itself. Commercially available MW frequency heating include 915, and 2450 MHz. Domestic MW ovens commonly used for pre-heating and preparing ready-to-eat (RTE) products operate at 2450 MHz, while most MW industrial heating processes use 915 MHz and have larger penetration depth and more uniform temperature distribution within the product.
Simply placing a processed sample in a MW heating system and expecting it to be heated efficiently is rarely fruitful due to complexity of interaction of electromagnetic energy with food matrix. Non-uniformity of heating, difficulty to track cold spots, unpredictable energy coupling and unpredictable microbial inactivation along with high equipment cost are major issues when using MW technology that delayed technology commercialization.
However, rapid internal heating leaves the possibility of selective heating of materials through differential absorption and self-limiting reactions. This presents opportunities and benefits not available from conventional heating and provides an alternative MW technology for a wide variety of products and processes. This includes defrosting, drying, blanching, pasteurization and sterilization of pre-packed products and pumped products in continuous flow. In addition, MW processing systems can be energy efficient up to 65 -70 percent. Heat can be instantly turned on and off — this constitutes important preconditions for controllability of operation along with improvements in quality over conventionally processed products.
The future of MW processing of foods appears to be strongest for special applications and it will probably be of limited usefulness as a general method of producing process heat. Solid-state MW sources (SS-MW) have been introduced for power generation and heating. High degree of control and SS semi-conductor nature lead to the advantages over magnetron-powered systems: higher repeatability; precise control over power levels and energy doses; higher yields; homogeneous energy distribution; efficient use of generated energy, low voltage electronics and smaller size. Despite SS-MW process advantages, widespread use has yet to catch on because of cost and lack of testing data.
The radio frequency (RF) band of the electro-magnetic spectrum covers a broad range of high frequencies, typically in MHz range (1MHz<f ≤300MHz). The permitted frequencies for industrial applications are 13.56, 27.12 and 40.68 MHz (RF). During RF heating, the product to be heated forms a “dielectric” between two metal capacitor plates, which are alternatively charged positively and negatively by a high-frequency, alternating electric field. Processing parameters of RF heating are output power, RF frequency, power absorption/coupling, heat capacity, dielectric properties of foods that depend mainly on frequency, temperature, water content, chemical composition, geometry and shape of food, and applicator.
Due to larger depth of the penetration, RF heating is suitable for large-size products and food trays. It was demonstrated that RF energy has adequate penetrations to inactivate heat resistant bacteria spores in food prepackaged in 6-lbs capacity polymeric trays. As a unique benefit of RF sterilization, the total time of RF processing in 6-lbs capacity trays can be reduced to one-third the time required in conventional canning processes to achieve approximately the same level of thermal sterility for bacterial spores. RF is also known for high-energy efficiency between 50 – 60 percent. Promising applications of RF dielectric heating in the food industry include blanching vegetables, thawing frozen foods, post-baking snack foods, meat processing, and pasteurizing and sterilizing liquid foods and pre-packed foods. Additionally, RF heating is showing value for insect control in walnut processing.
Ohmic heating (OH) relies on direct ohmic conduction losses in a medium. The internal heating of food occurs when an alternating electrical current is passed through due to its electrical resistance. OH requires the direct contact between the electrodes and the food product to be heated and cant be use for packaged products. The major benefits OH technology claims include reducing heating time by 90 percent, uniform heating of liquids and liquids with particles with faster heating rates, reduced problems of surface fouling, no residual heat transfer after the current is shut off, low maintenance costs (no moving parts), and high-energy conversion efficiencies up to 97 – 100 percent. OH technology is suitable for use in applications such as the processing of low-acid particulate products, meat cooking, stabilization of baby foods, and pasteurization of milk. Commercial ohmic heating systems are now available from a number of suppliers.
Magnetic Induction (MI) heats metal heat exchanger structures within the product flow stream. Inductively generated heat is transferred passively to the food and, unlike microwave or ohmic heating, is not produced in the food itself. Therefore, the FDA considers MI a conventional thermal process. Since MI generates heat within metals, elaborate, even flow-driven rotary heat exchange surfaces become possible. Surfaces can be patterned to process foods of any viscosity or particulate composition. MI heat exchangers made of passive metal (such as 316 stainless), provide cheap, low-heat per-unit-surface-area heat exchangers with energy conversion efficiency up to 95 percent that potentially eliminate bake-on from high-protein/high-carbohydrate foods.
The induction coil and the heat exchanger do not have direct contact. The absence of direct utility contacts allows the MI heat exchanger to slip in and out of the induction coil with simple detachment of sanitary clamps. Therefore, swapping out heat exchangers for different food types or for cleaning becomes simple. MI can also heat makeup and cleaning water completely, eliminating the need for a boiler. MI heating can replace any industrial process currently serviced by steam. MI heating is all electric and can be used for products in continuous flow or in metal packaging. MI allows zero carbon footprint food processing, if desired. However limited research of MI has been conducted in private labs in pilot scale with a low level of commercialization attempts.
Electro-magnetic heating technologies offer local, all electrical heating solutions, with zero carbon footprints, and are highly-efficient processing alternatives to steam with superior process control and optimal food quality. Despite these advantages, and the fact that electro-magnetic techniques are available for many years, commercialization has been slow due to difficulties to implement expected benefits and lack of information on added benefits to convince food processors. Further studies are needed to investigate temperature distributions, develop appropriate process conditions with minimal quality losses and design industrial system.
About The Author
Tatiana Koutchma is a research scientist at Agriculture and Agri-Food Canada, focusing on research related to novel processing technologies and development of new processes for industry. She is an internationally recognized expert of novel processing technologies including high pressure, ultraviolet light, and other advanced thermal and non-thermal methods.