Dielectric heating is also called microwave heating or high frequency heating. Dielectric energy is form of electromagnetic energy. They are transmitted as wave which penetrate food and are then absorbed and converted into heat.
Principle: the majority of food contain substantial proportion of water. The molecular structure of water consist of negatively charged oxygen atom separated from positively charged hydrogen atom and this form an electric dipole. When a microwave or radio frequency electric field is applied to a food, dipoles in water and in some ionic components such as salt attempt to orient themselves to the field (in a similar way to a compass in a magnetic field). Since the rapidly oscillating electric field changes from positive to negative and back again several million times per second, the dipoles attempt to follow and these rapid reversals create frictional heat. The increase in temperature of water molecules heats surrounding component of food by conduction and or convection. The depth of penetration of microwaves energy is determined by
1. Dielectric constant
2. Loss factor of food
Microwave generator (magnetron)
1. Aluminum tubes (wave guides)
2. Metal chambers (for batch operation)
3. Tunnel fitted with conveyor belt (for continuous operation)
Because microwave heat all biological tissue, there is risk of leaking radiation, causing injury to operators. Particularly to eyes which have insufficient blood flow to provide sufficient cooling. Chambers and tunnels are therefore sealed to prevent escape of microwaves. Radio frequency heating also operates using similar principle of microwave heating but at lower frequencies. Radio frequencies are mostly used for heating or evaporating moisture from product.
Application of microwave heating:
Thawing: Microwave energy are used to rapidly thaw small portion of food and for melting fats (butter, chocolate, fondant, cream). However difficulties arise with larger frozen blocks (egg, meat, fish, fruit juice) used in industrial process. Water has higher loss factor than ice and as a result, heating rapidly once the ice melts. In large blocks, thawing doesn’t take place uniformly and some portion of food may cook while other remain frozen.
Tempering: during microwave tempering, temperature is raised from -20°C to -3°C. Here food remains firm but is no longer hard. After frozen food has been tempered, it is more easily sliced, diced or separated into pieces. Tempering is used widely for meat and fish products which are more easily boned or ground at temperature just below freezing point and for butter and other edible fats. The advantage of microwave tempering over conventional tempering in cold room are,
1. Processing is faster (Meat blocks defrosted in 10 minutes instead of several days in cold room)
2. Greater flexibility in operation
3. Cost of operating a refrigerated tempering room is eliminated
4. Savings in storage space and labor
5. Better control over defrosting condition and hence improved quality
6. More hygienic defrosting because products are defrosted in the storage boxes
7. No drip loss or contamination which improves product yield and reduces nutritional losses.
Dehydration: The conventional hot air drying have disadvantage of low rate of heat transfer caused due to low thermal conductivity of dry foods. It also cause damage to sensory characteristics and nutritional properties caused by long drying time and overheating at surface. Case hardening and oxidation of pigments and vitamins are also major drawbacks of hot air drying. Microwave and radio frequency energy overcome the barrier to heat transfer caused by low thermal conductivity. Microwave dehydration have following qualities.
1. Prevents damage to the surface
2. Improves moisture transfer during later stage of drying and eliminates case hardening
3. Radiation selectively heats moist areas while leaving dry areas unaffected.
4. It is not necessary to heat large volume of air and oxidation by atmospheric oxygen is minimized.
Freeze drying: in conventional freeze drying, the low rate of heat transfer to the sublimation front limits the rate of drying. Microwave freeze drying overcome this problem because heat is supplied directly to the ice front. However, careful control over drying conditions is necessary to prevent localized melting of ice. Any water produced in drying food heats rapidly, owing to the higher loss factor and causes a chain reaction leading to widespread melting and end to sublimation.
Baking: Efficiency of baking is improved by microwave finishing for thin products such as breakfast cereals, baby foods, biscuits, crackers, crisps and sponge cake. Advantage of microwave baking are,
1. Increases production up to 50 %
2. Saves energy, space and labor cost
3. Close control of final moisture content (typically ± 2 %) and automatic levelling of moisture contents as only moist areas are heated.
4. Improved product texture and elimination of “center bone” (dense dough in center of cookies.
5. Improved taste and flavors due to shorted heating period at high temperature.
Sterilization: sterilization using microwave has significant reduction in thermal processing time which results in fresher like taste and texture and improves appealing character of food and also increase retention of nutrients. Other advantages of microwave sterilization technology are;
1. Instantaneous turn on and off of process allows more precise control, better energy usage and clean working environment in food processing facilities.
2. Use of post packaging processing could benefit manufacturers in reusing spoilage, eliminating refrigeration cost and provide safe food for consumers
3. Develop new product which have not been possible due to limitation of severe heat in conventional heating
Sterilization by microwave heat is achieved in laminated pouches made from polypropylene / EVOH or PVDC/polypropylene in multitherm process. During sterilization by microwave heating, pouches are submerged in a medium that has higher dielectric constant that the product and heating is by microwaves instead of steam. The product passes liquid bath, heated at up to 90°C and final sterilization temperature reaches more than 130°C before cooling.
Microwave rendering: microwave rendering of fat improves color, doesn’t cause unpleasant odor and reduces cost by 30 %.
Microwave frying: In microwave, frying can be done in shallow trays. But is not successful when deep bath of oil is used. Doughnuts are cooked without using oil in microwave.
Current industrial application of microwave heating:
Microwave oven is operated at 2450 MHz and 915 MHz with 10 – 200 KW heating capacities are used for processing of bacons and tempering deep frozen meat for patties. Commercial system performing microwave pasteurization / sterilization are also available.
Safety: microwave energy is changed to heat, as soon as it is absorbed by the food. It cannot make food radioactive or contaminated.
Shelf life of processed food: shelf life of product is determined by its microbiological safety and sensory attributes. Microwave sterilization can achieve same reduction of bacterial population as conventional retorting. Product intended for microwave sterilization are usually packaged in plastic trays or pouches. The ability of plastic to withstand oxygen permeation will affect the organoleptic or sensory acceptance of product during storage. Normal shelf life of microwave sterilized product, prepackaged in plastic container or pouches is 2 – 3 years or longer. With innovative plastic technology coming to the market, the new generation of plastic may increase expected shelf life of product even longer.
Storage of microwave processed food: microwave sterilized food can be stored at ambient temperature in the common household microwave prior to consumption. The product do not require refrigeration cutting down cost for food processors and distributers.
Name : Pratiksha Shrestha
Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.