Definition Instrumentation Advantages Disadvantage Applications

1. Department of dairy science and food technology
IAS, BHU
Topic-Microwave heating
Presented by,
Kunwar Pratik Singh
MSc. Food Technology
Presented to-
D.S Bunkar Sir

2. INTRODUCTION
• Microwave : very short
wave
• Alternating current signals
with frequencies between
300MHz to 300GHz.
Microwave heating is a processwhereby microwavesproducedby magnetronsare directed toward reactants or
heating medium, which absorb the electromagnetic energy volumetricallyto achieve self-heating uniformlyand
rapidly.

3. Microwave heating is a multiphysicsphenomenon that involves electromagnetic waves and heat transfer;
any material that is exposed to electromagnetic radiation will be heated up.
The rapidly varying electric and magnetic fields lead to four sources of heating. Any electric field applied to
a conductive material will cause current to flow.
In addition, a time-varying electric field will cause dipolar molecules, such as water, to oscillate back and
forth.
A time-varying magnetic field applied to a conductive material will also induce current flow. There can also
be hysteresislossesin certain types of magnetic materials

4. • Non ionizing.
• Approved for applications in food (2450MHz and
915MHz).
• Reflected by metals and pass though air.
• Absorbed by several food constituents.
• Microwave frequencies are close to radio wave
and overlap the radar range they can interfere
with communication process.
MICROWAVES
What are microwaves?
A microwave is a low energy
electromagnetic wave with a
wavelength in the range of 0.001
– 0.3 meters and a frequency in
the range of 1,000 – 300,000 MHz
(Figure 1). Laboratory (and
household) microwave
instrumentation almost
exclusively operate with
microwaves at a frequency of
2450 MHz (or 12.2 cm
wavelength).

5. HISTORY
• 1888- Heinrich Hertz was the first to demonstrate the existence of radio waves by
building a spark gap radiometer that produced 450MHz microwaves, in the UHF region.
• 1894- Jagdish Chandra Bose publicly demonstrated radio control of a bell using
millimeter wavelengths and conducted research into propagation of microwaves.
• The first patent, describing an industrial conveyor belt microwave system was issued in
1952 (Percy Spencer), however its first application started 10 years later.
• First major application- finish drying of potato chips, precooking of poultry and bacon,
tempering of frozen food and drying of pasta.

6. MICROWAVE HEATING - PRINCIPLE
DIPOLE ROTATION
• Food materials contain polar molecules (dipoles)
such as water, have a random orientation.
• When an electric field is applied, molecules orient
themselves according to polarity of field.
• The polar molecules rotate to maintain alignment
with rapidly changing polarity.
• Molecules oscillating at such frequencies generate
intermolecular friction with surrounding medium.
• With increasing temperature, the molecules try to
align more rapidly.
IONIC POLARIZATION
• When an electric field is applied to food solution
containing ions, the ions move at an accelerated
pace due to their inherent charge.
• The resulting collision between the ions causes
conversion of kinetic energy of moving ions to
thermal energy.
• Due to more frequent ionic collision and therefore
exhibit an increase in temperature
DIPOLE ROTATION IONIC POLARIZATION

7. MECHANISM OF MICROWAVE HEATING
• Microwaves penetrate materials and release their energy in the
form of heat as the polar molecules vibrate at high frequency to
align themselves with the frequency of microwave field.
• Microwave interact directly with the object being heated.
• Primary effect of microwave heating: intermolecular frictional heat.
• Secondary effect of microwave heating: conduction and convection.

8. How do microwaves generate heat?
In the case of microwaves, the electric field is primarily responsible for generation of heat, interacting with
molecules via two modes of action: dipolar rotation and ionic conduction (Figure 3). In dipolar rotation, a
molecule rotates back and forth constantly, attempting to align its dipole with the ever-oscillating electric
field; the friction between each rotating molecule results in heat generation.
A microwave, like any other electromagnetic wave, travels at the speed of light (300,000 km/sec) and
consists of two perpendicular oscillating fields: an electric field and a magnetic field (Figure 2).
Microwave photon energy is relatively low (0.03 – 0.00003 kcal/mol), affecting only kinetic molecular
excitation.

9. In ionic conduction, a free ion or ionic species moves translationally through space, attempting to align with
the changing electric field. Like in dipolar rotation, the friction between these moving species results in heat
generation, and the higher the temperature of the reaction mixture, the more efficient the transfer of energy
becomes. In both cases, the more polar and/or ionic a species, the more efficient the rate of heat
generation.
Because microwaves interact directly with the contents of a reaction mixture, energy transfer occurs more
efficiently than with conventional heating techniques (Figure 4). Conventional heating techniques rely on
thermal conductivity, where heat is transferred first from source to vessel, and then from vessel to solution.
This is a slow and inefficient method of heat transfer, where differing thermal conductivitiescomplicate
temperature control abilities and lengthen achievement of thermal equilibrium.

10. CONVENTIONAL HEATING V/S MICROWAVE HEATING
✓ Heating start from
surface of material only
✓ Electric or thermal
source of heating
✓ Long processing time
✓ High energy
consumption
✓ Product quality and
quantity can be affected
✓ Uniform heating
✓ Heating takes place by
microwaves
✓ Short and instant
heating
✓ Moderate to low energy
consumption
✓ Higher product quality
and quantity possible
CONVENTIONAL HEATING MICROWAVE HEATING

11. MICROWAVE OVEN
MAGNETRON
TRANSMISSION
SECTOR
STIRRER
OVEN CAVITY
1
2
3
4

12. COMPONENTS
• Basic structure of microwave heating device are:
1. Power supply and control: control power to be fed to the magnetron as well as
the cooking time.
2. Magnetron: power source
3. Wave guide: rectangular metal tube which directs microwave generated from the
magnetron to the cooking cavity. Also, prevent direct exposure of the magnetron to
any spattered food which would interfere with function of the magnetron.
4. Stirrer: distribute microwaves from the waveguide and allow more uniform
heating of food.

13. 5. Turntable: rotates food products through the fixed hot and cold spots inside the
cooking cavity and allows the food products to be evenly exposed to microwaves.
6. Cooking cavity: space inside which the food is heated when exposed to microwaves.
7. Door and choke: allows the access of food to the cooking cavity. These are specially
engineered that they prevent microwaves from leaking through the gap between the
door and cooking cavity.
Magnetron: It is a cylindrical diode with a ring of resonant cavities that acts as a anode structure. The cavity is the
space in the tube which becomes excited in a way that makes at a source for the oscillation of microwave energy .
The Magnetron is a vacuum valve in which the electron, emitted by the cathode, turn around under the action of a
continuous electric field produced by the power supply and of a continuous magnetic field. The movement produces
the electro-magnetic radiation.

14. APPLICATIONS
• BAKING
• CONCENTARING
• COOKING
• CURING
• DRYING
• FINISH DRYING
• FREEZE DRYING
• PASTEURIZATION
• ENZYME INACTIVATION
• ROASTING
• THAWING
• TEMPERING
1.Baking: for internal heating microwave, for external heating hot air
(electric coil) or infrared for crust formation.
2.Concentrating: concentration of heat sensitive fluids and slurries at
relatively low temperature in relatively short time.
3.Cooking: it cooks relatively larger pieces without high temperature
gradients between surface and interior (for continuous cooking of meals).
4.Curing: effective for glue-line curing of laminates (as in package) without
direct heating of the laminate themselves.
5.Drying: microwave selectively heats water with little direct heating of
most solids. Drying is uniform throughout the product, drying at relatively
low temperature.

15. Heating Food
When you place food in a microwaveoven and press the "start" button, electromagnetic waves oscillate within
the oven at a frequency of 2.45 GHz. These fields interactwith the food, leadingto heat generationand a rise in
temperature.
The efficiency of microwaveheating depends upon the material properties.
For example, if you place foods with varying water contentin a microwaveoven, they will heat up at different
rates. A dinner platemay come out with some food on it that is very hot while the rest of it is still cold.
Furthermore, the position of food relative to each other will also affect the electromagnetic field within the
oven. That is why most microwaveovens have turntables to rotate the food and promote even heating.

16. ADVANTAGES AND DISADVANTAGES
ADVANTAGES
The main advantage of a microwave oven over the conventional oven (electric and gas oven) is its high thermal
efficiency in converting the energy in electricity into heat in the food. Other advantages are:
1.Speedy: microwave cookers heat food more quickly than any other conventional oven (shortening of
processing time often by 70-85% and more).
2.Clean: with microwave cooking there is no risk of the food burning onto the cooker walls or they do not
become hot in the way that the surfaces of conventional oven do. In addition, most foods are cooked covered
and so remain in their containers (higher quality of product).
3.Smell free: because food is contained within the cooker cavity (and usually also in a covered dish), smells are
kept to a minimum.

17. Other advantages –
1. Easy to use: once controls and cooking techniques are mastered, microwave cookers are extremely easy to use.
2.Cool: unlike conventional ovens, microwave cookers do not produce external heat and so can be used anywhere that is
convenient such as a dining room.
3.Higher capacity: due to shorter residence time
4.Less space requirement by up to 50-90% against other methods
5.Better hygiene of working environment
6.Easier and faster maintenance
7.Savings of electric energy in comparison with conventional methods are frequently within the range of 25-50%.
8.Waste elimination and lower consumption of fossil fuels, causing lowering of environmental stress.
4.Less washing up: it is often possible to microwave food in serving containers or on the plate from which it is to be
eaten. This is reducing the kind of washing up required when saucepans and metal oven dishes are used.
5.Thawing: thawing can be done quickly in a microwave cooker, saving hours in the fridge or kitchen and removing, the
need for too much forward planning.
6. Nutritionally sound: many foods retain more nutrients than when cooked conventionally, as cooking time is so short,
and there is little or no added water, particular examples are fish, vegetable.

18. DISADVANTAGES
1.Because of speed, and the way in which microwave energy cooks, food cooked in a microwave oven will not be
brown, so no crust formation or browning in case of bread or meat (in such cases microwave with grilling can be
used).
2.High initial cost.
3.Short cooking time does not allow flavors to develop and this makes food unacceptable.

19. NON-UNIFORM TEMPERATURE DISTRIBUTION
• Even though microwave heating is volumetric and hence more uniform compared to many
traditional heating methods, non-uniform temperature distribution is one of the major problem
associated with the microwave heating.
• Because of uneven temperature distribution, few regions of material get heated very rapidly,
where as the remaining regions get heated to a lesser extent.

20. DEVELOPMENT OF METHODODLOGY FOR ASSESSING THE
HEATING PERFORMANCE OF DOMESTIC MICROWAVE OVENS
C. James, M. V. Swain, S. J. James, M. J. Swain (2002)
• Microwave heating of liquid (water, sauce), solid (mashed potatoes) and multicomponent food ( mashed
potato and sauce).
• The mean temperatures at hot and cold spots were found to be 83.9 and 61.7℃, respectively for water
whereas for multicomponent food, the hot and cold spots corresponding to 91.8 and 36.7℃, respectively.
• Thus temperature distribution is found to be less uniform for multi component food.

21. SIZE AND SHAPE EFFECT ON NON UNIFORMITY OF TEMPERATURE
DISTRIBUTIONS IN MICROWAVE HEATED FOOD MATERIALS
R. S. Vilayannur, V. M. Puri, R. C. Anantheswaran (1998)
• Three different shapes such as, brick, cylinder and hexagonal prism with three different volumes were
studied to determine non-uniform temperature distribution of potato samples.
• For brick shaped samples, the hot spot occurred at the corner whereas the cold spot occurred at the
geometric centre.
• For cylinder shaped products, the hotspot occurred at the centre whereas for hexagonal prism samples,
the hotspot was found to be at the boundary regions. It is also reported that the hexagonal prism shaped
products provided more uniform temperature distribution than cylinder and brick shaped product.

22. CONCLUSION
Microwave energy is unique energy sources that may allow shorten processing time, saving in energy, labor and
space and often better quality products. Advances in technology concentrating, focusing and controlling microwave
energy has increased the feasibility of developing microwave processing for the food and dairy industry. Microwave
processing is expected to grow beyond our expectation due to increasing consumers demands for newer type of
convenience foods having more nutritional value and better sensory quality in the recent years. There is a great
potential for the combination ovens because they are more effective than either oven alone in the manufacture of
shelf stable packed foods. Advances in microwave oven design and narrowing gap in cost between microwave and
thermal processing will provide and incentive for the development of newer microwave processes.
Microwave food processing design development will require additional research on mechanisms of microwave
heating of foods, particularly in the areas of energy coupling and propagation modes, and further development of
quantitative electro physical and electrochemical models as an aid to microwave process design

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24. THANK YOU