Microwave heating is used in many food processing applications like reheating, precooking, tempering, baking, drying, pasteurization and sterilization. Microwaves are electromagnetic waves with frequencies between 300 MHz to 300 GHz that travel at the speed of light. Microwave ovens operate at 2.45 GHz. Microwaves penetrate foods and are absorbed, causing molecules to vibrate and generate heat. The dielectric properties of foods like water content, temperature and composition determine how microwaves interact with them. Microwave heating has advantages like short cooking times and lower nutrient loss but also disadvantages like difficulty controlling heat and risk of overheating closed containers. It can both positively and negatively impact the nutrient content of foods.

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Microwave Heating of Food
Faizan Ahmad
Assistant Professor
Department of Post Harvest Engineering & Technology
Faculty of Agricultural Sciences
Aligarh Muslim University
Aligarh (India) 202 002

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Microwave Heating
 Microwave and radiofrequency (RF) heating are used in many processes in
industry and home
 Reheating
 Precooking
 Tempering
 Baking
 Drying
 Pasteurization
 Sterilization
 Microwave heating of food depends on dielectric properties of a material/foods

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What are microwaves?
 Microwaves are electromagnetic waves like light waves or radio waves.
Their frequencies (wavelengths) are in the range from 300 MHz (λ = 1m) up
to 300 GHz (λ = 1 mm).
 Microwaves travel at the speed of light (186,282 miles per second).
 Microwave ovens operate at frequencies of about 2.45 GHz, i.e. λ = 12.23
cm.
 All wave energy changes polarity from positive to negative with each cycle
of the wave. In microwaves, these polarity changes happen millions of times in
every second.
 Electromagnetic waves are, in themselves, stored energy in motion

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What are microwaves? Cont….
 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 waves of

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Characteristics of Microwaves
 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.
 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.

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Microwave Oven
Components of Microwave oven :
 Magnetron (heart of a microwave oven)
 wave guide
 Turntable and base plate
 Power supply/Capicator
 stirrer
Cooking Cavity

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How does microwave cooks food?
 Inside the strong metal box, there is a microwave generator called a magnetron.
When you start cooking, the magnetron takes electricity from the power outlet and
converts it into high-powered, 12cm (4.7 inch) radio waves.
 The magnetron blasts these waves into the food compartment through a
channel called a wave guide.
 The food sits on a turntable, spinning slowly round so the microwave cooks it
evenly.

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How does microwave cooks food?
The microwaves bounce back and forth off the reflective metal walls of the food
compartment, just like light bounces off a mirror. When the microwaves reach the
food itself, it penetrate inside the food. As they travel through it, they make the
molecules inside it vibrate more quickly.
 Vibrating molecules have heat so, the faster the molecules vibrate, the hotter
the food becomes. Thus the microwaves pass their energy onto the molecules in
the food, rapidly heating it up.

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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

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Dielectric Properties of Food Cont….
 Dielectric properties of
water and high-moisture-
containing foods such as
fruits, vegetables, and meat
are high because of dipolar
rotation
Dielectric Constant
Dielectric Loss Factor

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Dielectric Properties of Food
 Dielectric properties of food depends on:
 Moisture content
 Temperature
 Composition of the material
 Also a function of frequency of oven

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Advantages and disadvantages of Microwave heating
Advantages:
 Cooking time is short
 Destruction of nutrients is less
 No physical change of foods
 Melting process is easy
 Sterilization effect exists
 There is no flame, then treatment is easy
 Microwaves heat and cook your food from the inside out rather than
going through your food, from the outside in
 Microwave ovens work quickly and save energy as compared to other
conventional cooking methods

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Advantages and disadvantages of Microwave heating
Disadvantages:
 Constraint with metal container
 Heat force control is difficult
 Water evaporation
 Closed container is dangerous because it could be burst
 Surface toasting is impossible

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Positive Effects of Microwave heating
on foods
 Using a microwave oven can prevent the loss of important vitamins and
minerals in your food. Vitamin C and potassium are often lost in liquid used to
boil food. Cooking broccoli in your microwave oven may prevent the loss of
glucosinolate, a compound in broccoli which may be responsible for its cancer-
fighting properties, and which is lost in boiling.

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Negative Effects of Microwave heating on foods
 Microwaves Zap Food Nutrition
- diminishes the nutrient content in the food. if food is heated at a
high temperature and used for a long time.
- The correct combination of time and temperature can help preserve
most nutrients while also improving the foods’ taste, texture and
colour.
 Microwaves Destroy Vitamin B-12 (raw beef, pork and milk)
- 30 to 40 % loss of the vitamin when the foods received microwave
exposure.
- health benefits of vitamin B-12 are instantly neglacted once heated
in a microwave

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Negative Effects of Microwave heating on foods Contd.
 Microwaves Create Carcinogens In Food
- plastic containers used to heat these microwave meals have been
found to release the carcinogens along with other harmful toxins into
your food.
Microwaves Can Change the Makeup of Your Blood
- In a Swiss clinical study, researchers found that blood changes in
individuals who consumed microwaved milk and vegetables.
- red blood cells decreased while white cell levels increased, along
with cholesterol levels.

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THANKS FOR YOUR ATTENTION!