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Dielectric Properties. Electromagnetic Heating. Microwave and radiofrequency (RF) heating are used in many processes in industry and home Reheating Precooking Tempering Baking Drying Pasteurization Sterilization
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Electromagnetic Heating • Microwave and radiofrequency (RF) heating are used in many processes in industry and home • Reheating • Precooking • Tempering • Baking • Drying • Pasteurization • Sterilization • Electromagnetic heating processes related to dielectric properties of a material
Microwave Heating • Microwave heating is common in many food processes • Determination of dielectric properties becomes significant to understand the heating profiles of foods in a microwave oven and to develop equipment and microwaveable foods
Microwave frequency • Typical frequency in home microwave oven is 2450 MHz or 915 MHz for industrial use. • Interference with radar or other communication devices
Absorption of microwave energy in food involves primarily 2 mechanisms:1. ionic interaction2. dipolar rotation
Ionic Interaction (Ionic Conduction) Thermal Agitation of Molecules
Dielectric Properties of Food • Dielectric constant, ε’ • Ability of a material to store microwave energy • Dielectric loss factor, ε’’ • Ability of a material to dissipate microwave energy into heat • Parameter that measures microwave absorptivity • Dielectric constant and loss factor – important role in determining interaction of microwaves with food
Dielectric Constant Dielectric properties of water and high-moisture-containing foods such as fruits, vegetables, and meat are high because of dipolar rotation Dielectric Loss Factor
Dielectric Properties of Food • Microwave or radiofrequency heating ability of a product • Assessment of food quality Dielectric properties of foods depend on: • Moisture content • Temperature • Composition of the material • Also a function of frequency of oven
Free and bound water • Interaction of food components with water affect dielectric properties • Binding forces between protein and carbohydrate and water strong, smaller value of ε’ and ε’’ • Adjust moisture content in formulating microwaveable foods
Food Components • Carbohydrates • Fat • Protein • Moisture • Salt content
Microwaves • Microwaves are very short waves of electromagnetic energy that travel at the speed of light (186,282 miles per second). • Microwaves used in microwave ovens are in the same family of frequencies as the signals used in radio and television broadcasting. • Electromagnetic waves are, in themselves, stored energy in motion.
Microwaves penetrate and are absorbed by some substances • Microwaves penetrate and are absorbed by some substances, primarily food products. As the energy penetrates the food, its power is gradually absorbed, or lost, to each successive layer of molecules. • The rate of energy loss and depth of penetration vary with the depth, density, chemical properties and temperature of the food. • However, on the average, the power is cut in half about every three-quarters of an inch of penetration. Therefore, since the intensity of the electromagnetic field is less at the center of the food than at the surface, the molecules closer to the center of the food do not feel the full effect of the energy.
Microwaves possess 3 basic Characteristics: • Just as sunlight shines through a window, microwaves pass right through some materials. Materials such as glass, paper, and plastic are transparent to and generally unaffected by microwaves. • Microwaves are reflected by metal surfaces, much as a ball would bounce off a wall. The metal walls of the cooking space in microwave ovens actually form a cavity resonator.
Microwaves possess 3 basic Characteristics: • To illustrate this third characteristic, notice the cooked turkey below. The waves of microwave energy are cycling above and below a horizontal baseline. The half cycle below the baseline possesses negative properties, and the half cycle above the line is correspondingly positive. Basically, the effect of this wave, as it alternates between positive and negative, would be like a magnet flipping back and forth.
All liquids and food products, such as this turkey, are made up of molecules. These molecules have positive and negative particles, so they tend to behave like microscopic magnets. As the positive half cycle of the microwave penetrates the food, the negative particles of the molecules are attracted and attempt to align themselves with this positive field of energy. Then, when the microwave energy alternates to the negative half cycle, the opposite occurs -- The negative particles are repelled and the positive particles are attracted, causing a flipping motion (actually, this reaction is the movement of the particles within each molecule, so, technically, they reverse polarity). • This might be compared to a room full of people trying to run back and forth, from one side to the other. Obviously, there would be a lot of bumping, rubbing, agitation, and friction.
Microwave Cooking • Now, consider that the actual frequency of the RF energy used in microwave ovens is 2450 million cycles per second! Moreover, consider that within the course of one of those cycles, the molecules would actually change their direction (polarity) twice - once for the positive half-cycle and once for the negative half-cycle. This red-hot rate of vibration causes tremendous friction within the food, and - just as rubbing your hands together makes them warm - this friction produces heat.
Microwave Cooking • So the heat is produced directly in the food, but the food is not cooked, as is commonly believed, from the inside out. Actually, the cooking begins just beneath the outer surface and from there inward and outward, with the majority of the energy being expended in the outer layers. • The rate and degree of heating depend on the depth and density of the food, as well as its ability to conduct heat. Because the microwave energy is changed to heat as soon as it is absorbed by the food, it cannot make the food radioactive or contaminated. • When the microwave energy is turned off and the food is removed from the oven, there is no residual radiation remaining in the food. In this regard, a microwave oven is much like and electric light that stops glowing when it is turned off.
MW : non-ionizing radiation • As illustrated by the frequency spectrum at top, microwaves used in microwave ovens, similar to microwaves used in radar equipment, and telephone, television and radio communication, are in the non-ionizing range of electromagnetic radiation. Non-ionizing radiation is very different from Ionizing radiation • Non-ionizing radiation is very different. Because of the lower frequencies and reduced energy, it does not have the same damaging and cumulative properties as ionizing radiation. Microwave radiation (at 2450 MHz) is non-ionizing, and in sufficient intensity will simply cause the molecules in matter to vibrate, thereby causing friction, which produces the heat that cooks the food.
Ionizing radiation • the ionizing range of frequencies includes X-rays, gamma rays, and cosmic rays. Ionizing radiation is the sort of radiation we associate with radioactive substances like uranium, radium, and the fall-out from atomic and thermonuclear explosions.
Microwave Oven • The heart of every microwave oven is the high voltage system . Its purpose is to generate microwave energy. The high-voltage components accomplish this by stepping up AC line voltage to high voltage, which is then changed to an even higher DC voltage. This DC power is then converted to the RF energy that cooks the food.
Magnetron Tube • The nucleus of the high-voltage system is the magnetron tube . • The magnetron is a diode-type electron tube which is used to produce the required 2450 MHz of microwave energy. It is classed as a diode because it has no grid as does an ordinary electron tube. A magnetic field imposed on the space between the anode (plate) and the cathode serves as the grid. While the external configurations of different magnetrons will vary, the basic internal structures are the same. These include the anode, the filament/cathode, the antenna, and the magnets
Microwave Cooking • A microwave is probably used more often for reheating leftovers or frozen foods. Unlike a conventional oven that must be preheated, a microwave doesn’t waste energy heating the air inside the oven. Only the food gets heated. • Plastic, ceramics and glass also do not absorb microwave radio waves. For this reason, some microwaveable foods come with a reflective “browning sheet” to intensify heat in a specific area in order to brown the bottom of a pizza, for example, or the top of a pastry.
Microwave Cooking • In microwave cooking, the radio waves penetrate the food and excite water and fat molecules pretty much evenly throughout the food. • No heat has to migrate toward the interior by conduction. • There is heat everywhere all at once because the molecules are all excited together. • There are limits, of course. Radio waves penetrate unevenly in thick pieces of food (they don't make it all the way to the middle), and there are also "hot spots" caused by wave interference, but you get the idea. • The whole heating process is different because you are "exciting atoms" rather than "conducting heat."
What’s Cooking • When food in a microwave absorbs radio waves, the energy translates into atomic motion, which becomes heat. • In other words, microwave radio waves excite the atoms that make up food. This results in evenly and quickly cooked food, all things being equal. • In reality, some types of food do not allow equal penetration of radio waves, resulting in “cold spots.” This is a concern with poultry, meat and eggs, where bacteria can survive in the uncooked areas.
Danger! • Despite their small size, they carry a huge amount of energy. One drawback of microwaves is that they can damage living cells and tissue. This is why microwaves can be harmful to people—and why microwave ovens are surrounded by strong metal boxes that do not allow the waves to escape. • Microwaves can be very dangerous, so never fool around with a microwave oven. • Microwaves are also used in cellphones (mobile phones), where they carry your voice back and forth through the air, and radar.
Simply Said • QuestionHow do microwave ovens work and are they harmful in any way?AnswerMicrowave ovens produce electromagnetic radiation of exactly the right wavelength to excite water molecules. When water molecules become excited, they heat up. Since most of our food contains a fair amount of water, we can heat up our food by selectively heating up the water inside the food. Microwave radiation also passes through glass and plastic, which allows it to travel through tupperware and heat up the food inside. However, microwave radiation does not penetrate very deep into the food itself, so if you put something big into the microwave oven for a short amount of time, it'll be hot on the outside but still cool in the middle. To heat up something big like a turkey breast, the heat has to diffuse from the surface to the inside.
Microwave ovens can definitely be harmful if used improperly. Microwave radiation can pass through plastic and glass, but it'll reflect off of metal. If you put a metal object (such as a metal bowl or fork) into the microwave oven, this can cause the microwaves to reflect back to the source that produces them (called the 'magnetron'), and can result in considerable damage to the oven. (The metal wiring in the glass window of the door keeps the microwaves from leaving the oven, but doesn't reflect them back to the source.)
Effects of Composition of Foods on Dielectric Properties • Carbohydrate, fat, moisture, protein and salt contents are major food components • Presence of free and bound water, surface charges, electrolytes, nonelectrolytes, and hydrogen bonding in food product • Physical changes during processing, moisture loss and protein denaturation • Dielectric behavior important for food technologists and engineers to improve quality of microwave foods, to design microwaveable foods, and to develop new microwave processes
Use of dielectric properties • Quality control of foods • Freshness of fish and meat • Evaluate frying oil quality • Determine moisture content of grains and seeds or agricultural products • Detection of pollutants in water at microwave frequency of 2.685 GHz
Measurement of dielectric properties • Type of food • Degree of accuracy • Frequency • Reflection of transmission type • Methods • Transmission line • Coaxial probe • Cavity perturbation • Free space transmission
Dielectric measurement • A microwave signal is generated at the frequency of interest • Signal is directed through the sample • Changes in the signal caused by the sample are measured • From these changes the dielectric constant and loss factor are determined
