Transcript Dielectric Properties

Electromagnetic Properties
Electromagnetic Heating
• Microwave and radiofrequency (RF) heating are
used in many processes in industry and home
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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 interaction
2. dipolar rotation
Ionic Interaction (Ionic Conduction)
Thermal Agitation
of Molecules
Dipolar Rotation
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 redhot 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 nonionizing 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 highvoltage 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
Magnetron Tube
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
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Question
How do microwave ovens work and are they harmful in any way?
Answer
Microwave 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
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Type of food
Degree of accuracy
Frequency
Reflection of transmission type
Methods
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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