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Dielectric Properties of foods Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency of the applied field Microwave heating: use very high frequency 2450 - 915 MHz & Transmit energy through space
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Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

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Page 1: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

Dielectric Properties of foods

Microwaves- like light - Electromagnetic waves

Heat transfer depends on 1. Degree of excitability of food molecules

2. Frequency of the applied field

Microwave heating: use very high frequency 2450 -

915 MHz & Transmit energy through space

Page 2: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

A dielectric material is an electrical insulator that

can be polarized by an applied electric field.

When a dielectric is placed in an electric field,

electric charges do not flow through the material

as they do in a conductor, but only slightly shift

from their average equilibrium positions

causing dielectric polarization.

If a dielectric is composed of weakly bonded

molecules, those molecules not only become

polarized, but also reorient so that their symmetry

axes align to the field.

Page 3: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

Non-ionizing (or non-ionising) radiation refers to any type

of electromagnetic radiation that does not carry

enough energy per quantum to ionize atoms or molecules—

that is, to completely remove an electron from

an atom or molecule.

•Instead of producing charged ions when passing through

matter, the electromagnetic radiation has sufficient energy

only for excitation, the movement of an electron to a higher

energy state.

Page 4: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

•RF (radio frequency)/Microwave/IR:

Electromagnetic energy (Show video)

https://www.youtube.com/watch?v=M2qz

CQ58_7Qhttps://www.youtube.com/watch?v=qT6EmMkKevY

• Ohmic or resistance heating: electrical

energy

P=VI

Page 5: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

Radio frequency and microwave heating are

electromagnetic heating techniques that are

applied for heating materials that are poor

electrical conductors.

Often, they are also poor conductors of heat.

These materials are known as dielectric

materials or dielectrics.

Dielectric heating is much more rapid.

Because electrically insulating materials are mostly

also poor conductors of heat.

Page 6: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

Near IR

eV-a unit of energy equal to the work done on an electron in accelerating it through a potential difference of one volt.

Page 8: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency
Page 9: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

The difference

between microwave

energy and other

forms of ionizing

radiation, such as X-

rays, Alpha, Beta and

Gamma rays, is that

microwave energy is

non-ionizing. In other

words, it does not alter

the molecular structure

of the item being

heated. The effects of

microwave energy are

strictly thermal and do

not cause cellular

change as with

ionizing radiation.

Microwaves

are a form of

radiant

energy. Other

common

forms are

radio waves,

visible light,

infrared heat

and

electricity. All

forms of

radiant

energy are a

part of

theelectroma

gnetic

spectrum.

Page 10: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

Table 1. Frequencies assigned for industrial, scientific, and medical use.

Frequency

Radio 13.56 MHz ±6.68 kHz

27.12 MHz ±160.00 kHz

40.68 MHz ±20.00 kHz

Microwaves 915 MHz ±13 MHz-industrial

2450 MHz ±50 MHz-home & ind.

5800 MHz ±75 MHz

24125 MHz ±125 MHz

GSM -3G phones – 2100 MHz?? (Mega- 106)

Page 11: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency
Page 12: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

Incident

ray

Air (ε’ =1)

Reflected rays

(40 <ε’<70)

Food

Transmitted

rays

Air (ε’ =1)ε’ =dielectric constant/permittivity

Refraction

Page 13: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

TV

Page 14: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency
Page 15: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency
Page 16: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

• When the frequency becomes higher:

• it becomes impossible for dipolar polarization to

follow the electric field in the microwave region

around 1010 Hz; in the infrared or far-infrared

region around 1013 Hz, ionic polarization loses

the response to the electric field;

• electronic polarization loses its response in the

ultraviolet region around 1015 Hz.

• In the wavelength region below ultraviolet,

permittivity approaches the constant ε0 in every

substance, where ε0 is the permittivity of the free

space

Page 17: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

• http://en.wikipedia.org/wiki/Dip

olar_polarization#Dipolar_pola

rization

• -FOR FURTHER READING

Page 18: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

Dielectrics- Analogy

•No material (especially tissue) is

either a perfect insulator or capacitor

More helpful to consider media that is

both Known as a dielectric

Page 19: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency
Page 20: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

tscoefficienarer −= ,,0

Page 21: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

Based on the above assumption (Biological materials

are also behaving like dielectrics(insulating

materials)) many theories have been developed.

Interaction of electromagnetic radiation with any type of materials is

explained by:

1−

'''

j−=

'''

j−=

( complex number (a + jb) where, ‘a’ is a real number, j = , jb imaginary portion )

Complex permeability

Complex permittivity

factorlossmagnetic

typermeabilimagnetic

typermeabilicomplex

−−=

−=

−=

''

'

factorlossDielectric

yPermitivit

ypermitivitcomplex

−−=

=

−=

''

'

Page 22: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

Above definitions are for absolute values of permeability or permittivity.

Practically use relative values

valuevacuumspacefree

valueabslative

factorlossDielectriclative

ypermitivitcomplexreallative

tconsdielectricrealrelativeypermitivitreallative

−−

−=

−−−=

−−−=

−−−−−=

/Re

Re

Re

''

}tan{Re'

No foods magnetically interact with microwaves

Therefore µ = 1 for the above equation

The term PERMITIVITY is not in use now & use

dielectric constant

Page 23: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

spacefreethetorelativeareaboveallBut

factorLoss

tconsdielectriccomplex

tconsDielectric

........*

tan

''

tan'

−=

−−=

−=

Dielectric constant and loss factor play

a critical role in microwave interaction

Both are dependent upon wave

frequency and temperature for a given

food

See dielectric food map –pge 49

Both are

dimension

less

Page 24: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

spacefreethetorelativeareaboveallBut

factorLoss

tconsdielectriccomplex

tconsDielectric

........*

tan

''

tan'

−=

−−=

−=

Page 25: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

• Some other materials, such as metal and

foil, tend to reflect microwave energy. The

use of metal utensils in microwave ovens

should be avoided. Why?

• Metal utensils do not allow complete

penetration of the food by the microwaves,

so cooking efficiency is greatly reduced.

• If the cooking load is not large enough to

absorb the microwave energy, the oven

could be damaged by an arc between the

metal utensils and the cavity interior or

door assembly.

Page 26: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

• The life of the magnetron tube can be

shortened by extended periods of back-

feeding R. F. energy, which raises the

magnetron tube's filament temperature.

• Because metal reflects microwave energy,

the metal walls of the cooking cavity are

not affected by microwaves and do not get

hot.

Page 27: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

Applications in microwave heating

smallasneglectedfactorLoss

tconsDielectric

surfacefromreflectedenergyofFractionP

P

r

r

−−−−−

=

−−=

=

+

)(

tan..

...

''

'

2

1

1

'

'

Page 28: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

)/(

Re

)(

/(

*56.5

''

3

2''*104

cmVFieldElectricE

factorLossdielectriclative

GHzfrequency

WvolumeunitperabsorbedPower

f

P

P

cm

Ef

v

v

−==

−−−−

−=

−−−−=

=−

For materials having high loss factors, microwave energy does not penetrate

deeply, Power decrease rapidly with penetration depth

For lower loss factors, penetrate deeply, less energy dissipation (ice, glass,

plastics-very little energy is absorbed)

Page 29: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

Transmission

•Transmission = 1- Pr - Pa

•Pr –Reflection

• Pa-fraction absorbed

Penetration depth- dp

22

'0 112

2

'

"

−+=

d p

cmMHzat 76.224502

20 =

lengthwavespacefree −−−−0

"

'

0

"

2=

−−−−

d p

ofvalueslowFor

Page 30: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

Ohmic heating (sometimes also referred to as Joule

heating, electrical resistance heating, direct electrical

resistance heating, electro heating, and

electroconductive heating) is defined as a process

wherein (primarily alternating) electric currents are

passed through foods or other materials with the

primary purpose of heating them. The heating occurs in

the form of internal energy generation within the

material.

When a Material is placed between two electrodes

and an electric current is passed (Frequency; 60Hz

to 100MHz)

Ohmic heating

Page 31: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

Ohmic heating

Page 32: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

Acknowledgement

Juan L. Silva

http://www.msstate.edu/org/silvalab/Dielectric,%20Ohmic,%20and%20Infrar

ed%20Heating.pdf

Ref.

1. R.T. Toledo, Food process Engineering pp 249-252

2. C R Buffler, 1993. Microwave cooking & Processing, pp. 1-14,47-60

3. http://www.leonardo-energy.org/webfm_send/172

Home work

1. The dielectric constant (ε’) of beef at 23oC and 2450MHz (2.45GHz)is 28, and

the loss factor (ε”) is 0.2. The density is 1004kg/m3, and the specific heat is

3250J/kg.K. The dielectric constant (ε’) of potato at 23oC and 2450MHz

(2.45GHz)is 38, and the loss factor (ε”) is 0.3. The density is 1010kg/m3, and

the specific heat is 3720 J/kg.K.

When 250g of potatoes were kept in a 811W microwave oven and heated for

1 minute, the temperature raised from 23oC to 38.5oC. Calculate the expected

temperatures of 0.5kg of beef and 0.5kg potatoes heated for 100s

simultaneously in the same oven.

Page 33: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

Na+ Cl-

Alternating

Electric Field

105oCO=

H+

H+

Rotation

Ionic Dipolar

Page 34: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

• Another good website to read

• http://www.leonardo-

energy.org/webfm_send/172

• RT Toledo Refer -question

Page 35: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

Q-7

Page 36: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

END OF THE LESSON

Page 37: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

Q7

• 1. Write one MCQ question (with four

answers) from the last lesson dielectric

heating.

Time: 10 Mins,

Marks: 10

Page 38: Microwaves- like light - Electromagnetic waves · Microwaves- like light - Electromagnetic waves Heat transfer depends on 1. Degree of excitability of food molecules 2. Frequency

https://onlinelibrary.wiley.com/doi/full/10.11

11/j.1541-4337.2007.00024.x

Infrared heating of foods- for self learning