microwaves- like light - electromagnetic waves
TRANSCRIPT
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
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.
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.
•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
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.
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.
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.
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)
Incident
ray
Air (ε’ =1)
Reflected rays
(40 <ε’<70)
Food
Transmitted
rays
Air (ε’ =1)ε’ =dielectric constant/permittivity
Refraction
TV
• 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
• http://en.wikipedia.org/wiki/Dip
olar_polarization#Dipolar_pola
rization
• -FOR FURTHER READING
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
tscoefficienarer −= ,,0
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
−−=
=
−=
''
'
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
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
spacefreethetorelativeareaboveallBut
factorLoss
tconsdielectriccomplex
tconsDielectric
........*
tan
''
tan'
−=
−−=
−=
• 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.
• 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.
Applications in microwave heating
smallasneglectedfactorLoss
tconsDielectric
surfacefromreflectedenergyofFractionP
P
r
r
−−−−−
=
−−=
=
+
−
)(
tan..
...
''
'
2
1
1
'
'
)/(
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)
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
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
Ohmic heating
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.
Na+ Cl-
Alternating
Electric Field
105oCO=
H+
H+
Rotation
Ionic Dipolar
• Another good website to read
• http://www.leonardo-
energy.org/webfm_send/172
• RT Toledo Refer -question
Q-7
END OF THE LESSON
Q7
• 1. Write one MCQ question (with four
answers) from the last lesson dielectric
heating.
Time: 10 Mins,
Marks: 10
https://onlinelibrary.wiley.com/doi/full/10.11
11/j.1541-4337.2007.00024.x
Infrared heating of foods- for self learning