Microwaves are produced inside the oven by an electron tube called a magnetron. The microwaves are reflected within the metal interior of the oven where they are absorbed by food. Microwaves cause water molecules in food to vibrate, producing heat that cooks the food.MW are electromagnetic waves generated by magnetrons and klystrons. Frequency 300MHz and 300GHz Wavelength from 1mm to 1m industrial heating purposes the available frequencies are 915 and 2450MHz MW giving up their energy to the material, with a consequential rise in temperature Two imp mechanisms are: Ionic polarization: conversion of kinetic energy of the moving ions into thermal energy Dipole rotation: rotation of polar molecules leads friction with surrounding medium and heat is generated ENERGY EFFICIENCY: During microwave heating, electrical energy is first converted into microwave energy The microwave then interacts with foods and is converted into heat TWO EFFICIENCIES: Microwave generation efficiency Microwave absorption efficiency ENERGY CONSUMPTION Specific energy consumption was defined as the total energy supplied divided by the amount of water removed during drying Overheating could increase the energy consumption due to high moisture loss from the overheated region CONVERSION OF MICROWAVE ENERGY INTO HEAT MW Heating is a consequence of the interactions between microwave energy and a dielectric material PD= 55.61 X 10-14 f’ E2 ἐ tanᵹ where, PD Power dissipation W/cm3 f ‘ frequency in Hz E electric field in v/cm (V/m) ἐ relative dielectric constant tanᵹ: loss tangent MICROWAVE OVEN GENERALLY CONSISTS OF THE FOLLOWING BASIC COMPONENTS (i) power supply and control: it controls the power to be fed to the magnetron as well as the cooking time (ii) magnetron: it is a vacuum tube in which electrical energy is converted to an oscillating electromagnetic field. Frequency of 2450 MHz has been set aside for microwave oven for home use Conti….. (iii) waveguide: it is a rectangular metal tube which directs the microwaves generated from the magnetron to the cooking cavity (iv) stirrer: it is commonly used to distribute microwaves from the waveguide and allow more uniform heating of food (v) cooking cavity: it is a space inside which the food is heated when exposed to microwaves Conti…. (vi) turntable: it rotates the food products through the fixed hot and cold spots inside the cooking cavity and allows the food products to be evenly exposed to microwaves (vii) door and choke: it allows the food to the cooking cavity. they prevent microwaves from leaking through the gap between the door and the cooking cavity FACTORS AFFECTING MICROWAVE HEATING Dielectric properties Temperature and frequency Shape and size of food items DIELECTRIC PROPERTIES Penetration of microwave energy inside a material is a function of its dielectric properties.

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Presenter by:
LAVANYA V.
M.tech
Department of PFE
CAE, raichur.
MICROWAVE HEATING

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 MW are electromagnetic waves
generated by magnetrons and
klystrons.
 Frequency 300MHz and 300GHz
 Wavelength from 1mm to 1m
 industrial heating purposes the
available frequencies are 915 and
2450MHz
INTRODUCTION

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Electromagnetic waves spectrum

5. MICROWAVE HEATING
 CONVERSION OF ELECTRICAL ENERGY TO
MICROWAVE ENERGY TO HEAT FOODS
(MAGNETRON)
 Microwave heating is based on the material’s ability to
absorb electromagnetic radiation and convert it to heat
 Heat is generated directly inside food materials,
causing a much faster temperature
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6. MICROWAVE HEATING MECHANISM
 MW giving up their energy to the material,
with a consequential rise in temperature
Two imp mechanisms are:
 Ionic polarization: conversion of kinetic
energy of the moving ions into thermal energy
 Dipole rotation: rotation of polar molecules
leads friction with surrounding medium and
heat is generated 6

7. ENERGY EFFICIENCY, CONSUMPTION,AND CONSERVATION
DURING MICROWAVE HEATING
ENERGY EFFICIENCY:
 During microwave heating, electrical energy is first
converted into microwave energy
 The microwave then interacts with foods and is
converted into heat
TWO EFFICIENCIES:
1. Microwave generation efficiency
2. Microwave absorption efficiency
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8. GENERATION EFFICIENCY
 Generation efficiency=100x microwave output power
input electrical power
ABSORPTION EFFICIENCY
 Absorption efficiency=100x Thermal energy absorbed by foods
microwave output power x heating duration
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9.  Combine both generation efficiency and absorption
efficiency, we can get the total thermal efficiency
expressed as:
Thermal efficiency=100 X Thermal energy absorbed by foods
input electrical energy
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10. ENERGY CONSUMPTION
 Specific energy consumption was defined as the total
energy supplied divided by the amount of water
removed during drying
 Overheating could increase the energy consumption
due to high moisture loss from the overheated region
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11. CONVERSION OF MICROWAVE ENERGY INTO HEAT
 MW Heating is a consequence of the interactions
between microwave energy and a dielectric material
PD= 55.61 X 10-14 f’ E2 ἐ tanᵹ
where,
PD Power dissipation W/cm3
f ‘ frequency in Hz
E electric field in v/cm (V/m)
ἐ relative dielectric constant
tanᵹ: loss tangent
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12. Basic structure of a microwave oven
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13. MICROWAVE OVEN GENERALLY CONSISTS
OF THE FOLLOWING BASIC COMPONENTS
 (i) power supply and control: it controls the power
to be fed to the magnetron as well as the cooking time
 (ii) magnetron: it is a vacuum tube in which
electrical energy is converted to an oscillating
electromagnetic field. Frequency of 2450 MHz has
been set aside for microwave oven for home use
13

14. Conti…..
 (iii) waveguide: it is a rectangular metal tube which
directs the microwaves generated from the magnetron
to the cooking cavity
 (iv) stirrer: it is commonly used to distribute
microwaves from the waveguide and allow more
uniform heating of food
 (v) cooking cavity: it is a space inside which the food
is heated when exposed to microwaves
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15. Conti….
 (vi) turntable: it rotates the food products through
the fixed hot and cold spots inside the cooking
cavity and allows the food products to be evenly
exposed to microwaves
 (vii) door and choke: it allows the food to the
cooking cavity. they prevent microwaves from
leaking through the gap between the door and the
cooking cavity
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16. FACTORS AFFECTING MICROWAVE HEATING
 Dielectric properties
 Temperature and frequency
 Shape and size of food items
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17. DIELECTRIC PROPERTIES
 Penetration of microwave energy inside a material
is a function of its dielectric properties.
 The permittivity of foods describes how the food
materials interact with electromagnetic radiation.
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18. CONTI….
 Dielectric constant and Dielectric loss factor
decrease with the increase in microwave frequency
 Penetration depth of MW is proportional to wave
length, sq.root of dielectric constant & inversely to
loss factor
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19. TEMPERATURE AND FREQUENCY
 The temperature increase in foods during
microwave heating depends on both the
dielectric and thermo physical properties of
foods
 power absorption and radiation penetration
during microwave heating are more effective at
lower frequencies than at higher ones
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20. SHAPE AND SIZE OF FOOD ITEMS
 Microwave heating is significantly dependent
on the size and shape of foods.
 In larger samples, a large temperature
gradient occurs from the surface toward the
center during microwave heating.
 In smaller food items, microwave heating is
more uniform
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21. APPLICATION OF MICROWAVE HEATING IN THE FOOD
INDUSTRY
 Microwave Dehydration
 Microwave pasteurization
 Microwave sterilization
 Microwave cooking
 Microwave baking
 Microwave thawing
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22. MICROWAVE DEHYDRATION
 Drying is a complex process involving
simultaneous coupled heat and mass transfer
Stages during drying of food materials:
 Increasing rate at the beginning
 Constant rate period in the middle
 Falling rate period at the end
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23. Conti….
 MW –assisted combination drying is a rapid dehydration
technique that can be applied to specific foods,
particularly to fruits and vegetables
 It reduce the drying time
 MW has also been as an energy source in vacuum drying
to improve energy efficiency and product quality
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MICROWAVE DRYER

25. MICROWAVE PASTEURIZATION
 MW heating has been used for HTST pasteurization
of foods, particularly thick food items.
 microwave heating system used for in- package
pasteurization of ready-to-eat meats
 It inactivates the bacteria at the temp of 650C,
750C,and 850C,respectivly.
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26. MICROWAVE THAWING
 Microwave thawing include a high thawing speed,
uniformity of thawing , flexibility, and less floor
space requirement
 Significantly reduce the processing time and
weight loss
 It Causes a greater moisture loss
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27. MICROWAVE BAKING
 Backing is a new technology in the bakery industry
 Microwave can penetrate into products and thus
rapidly heat the products from the inside
 Microwave backing cannot induce browning and
crust on the surface of breads and cakes
 It increase the quality of bakery products
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Microwave puffing system for snack
foods

29. MICROWAVE COOKING
 It help us heat and cook almost anything from a cup
of coffee to full diners.
 Reduce cooking time and achieve a uniform
temperature profile
 Microwaves cause molecules in food to vibrate. This
creates heat that cooks the food fast.
 Surface temperature of foods during microwave
cooking may easily reach 1000C 29

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MICROWAVE COOKING

31. MICROWAVE STERILIZATION
 inactivation of harmful bacteria from food
 ability to heat solid, semi-solid and meal
combination food products
 more rapid heating and preservation of sensory
and nutritional quality
 Reduction in the thermal processing time
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32. THE ADVANTAGES AND DISADVANTAGES OF MICROWAVE
HEATING
ADVANTAGES
 It is fast and easy
 It takes less time to heat the food
 provides a convenient way to thaw, cook and reheat foods
 uniform cooking
 Cook large pieces of meat at medium power for longer
periods
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33. DISADVANTAGES :
 The waves usually destroys the vitamins and
minerals found in most fresh foods
 It does not cook to perfection any dish but merely
warms it up to the degree you desire
 Microwaves can at times destroy some taste and
flavour in some dishes
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34. SAFE MICROWAVING
 Avoid direct contact with the microwave’s heat
 Use only microwave safe utensils like glass,
ceramic, plastic, or paper.
 Be sure to cover, rotate, stir and re-arrange food to
promote even cooking
 Do not reheat your food for long period of time
 Safe only if the container remains cool
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35. CONCLUSION
 Heat the material internally
 Food materials with higher moisture content absorb
more MW energy
 If the piece of the material is smaller than the wave
length, its centre will be overheated
 Use only microwave safe utensils
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