Radio frequency processing and Microwave heating in food processing

RADIO FREQUENCY
IN FOOD
PROCESSING
PRAKASH.S.P
M.Tech Food Technology
1

 Introduction
 Definition
 Radio frequency heating
 History
 RF heating principle
 RF heating mechanism
 Application of Radio Frequency in food industry
 Conclusion
 References
1 2

 Radio frequency heating - a part of innovative techniques based
on electromagnetic heating non-thermal methods - providing high
quality foods economically.
 Radio frequency (RF) heating - for food application because of
its higher penetration depth, heat distribution and low energy
consumption.
 RF heating in current scenario is the preferred method in modern
industries for product cooking, baking, drying, pasteurization,
disinfect food and other applications
1 3

 Electro heating can be sub-divided into either direct electro
heating where electrical current is applied directly to the food (e.g.
ohmic heating (OH)) or (e.g. microwave
(MW) or radio frequency (RF) heating) where the electrical energy
is firstly converted to electromagnetic radiation which
subsequently generates heat within the product.
1 4

 RF heating is a combination of dipole heating and electric
resistance heating resulting from the movement of dissolved ions
present in the food.
 RF with electromagnetic waves in 30-300MHz spectrum - added
advantage of uniform heating in homogeneous foods - high
penetration depth used to pasteurize or sterilize liquid products.
 The mechanism of RF heating is that the molecules within a
product placed in an RF environment re-orient themselves (27
million times/s at 27 MHz) continuously in response to the applied
field. This response initiates volumetric heating within the entire
product due to frictional interaction between the molecules, thus
selectively heats only the product and not the air or equipment
surrounding it.
1 5

 The potential use of RF technology for food processing was
recognized after World War II.
 These early efforts employed RF energy : the cooking of
processed meat products, heating of bread, dehydration and
blanching of vegetables.
 By 1960-(Jason and Sanders)- the application of RF energy to
foods focused on the defrosting of frozen products, which resulted
in several commercial production lines.
 In 1974( Demeczky) studied the juices (peach, quince and
orange) sealed in bottles and carried on a conveyer belt through
an RF applicator had better bacteriological and organoleptic
qualities than the juices treated by conventional thermal methods
suggesting potential applications in heat processing for
preservation of foods..
1 6

CONT..,
 Commercial applications for RF energy in the food industry was
post-bake drying of cookies and snack foods which began in
the late 1980s.
 1990s, area of RF pasteurization was studied with attempts
made to improve energy efficiency and solve technical
problems such as run-away heating.
 The most recent focus is on drying, post-baking, pasteurization
of juices especially apple juices and other high value produce,
waste water treatment in diary industries, disinfection in
agricultural produce especially walnut, red and black pepper,
poultry and fish industries along with the future expansion of the
technology
1 7

 The RF portion of the electromagnetic spectrum occupies a
region between 1 and 300 MHz although the main frequencies
used for industrial heating lie in the range 10–50 MHz, within
the latter range only selected frequencies namely 13.56 ±
0.00678, 27.12 ± 0.16272 and 40.68 ± 0.02034 MHz are
permitted for industrial, scientific and medical applications.
 In general food materials are poor electric insulators. They
have the ability to store and dissipate electric energy when
subjected to an electromagnetic field.
1 8

 This relative complex permittivity ɛ*, is composed of two
properties ɛ (called the dielectric constant) and ɛ” (called the
dielectric loss factor), are the component responsible for dielectric
heating. This dimensionless number measures the ability of the
material to interact with the electric field of RF.
 The dielectric constant is the measure of the food material’s
ability to store electromagnetic energy; whereas dielectric loss
factor is the material’s ability to dissipate electromagnetic
energy.
1 9

 In a radio-frequency heating system, the RF generator creates
an alternating electric field between two electrodes.
 The material to be heated is placed in between the electrodes
where the alternating energy causes polarization of the
molecules, thus continuously reorient themselves to face
opposite poles.
 The energy absorption involves primary two mechanisms -
dipolar relaxation and ionic conduction.
 Water in the food is often the primary component responsible
for dielectric heating. Due to their dipolar nature, water
molecules try to follow the electric field associated with
electromagnetic radiation as it oscillates at very high
frequencies, thus producing heat.
1 13

 Secondary mechanism of heating is through the
in the food that generates heat.
 The amount of heat generated in the product is determined by the
frequency, the square of the applied voltage, dimensions of the
product and the dielectric loss factor of the material, which is
essentially a measure of the ease with which the
.
 RF power is produced when electricity is applied to an RF
generator whose signal is amplified and delivered to a parallel
electrode system (RF cavity), in which the selected material is
placed.
1 14

 The RF heating mechanism in meat and fish product processing
have significant effect on nutrition, quality, quantity and storage of
the produce.
 Wang et al., (2012) reported that the RF heating at 27MHz and
applying an alternating current of 6kW can be used to process
pre-packaged heterogeneous foods (meat lasagna) to retain
product quality better compared to conventional method of
heating.
1 16

 RF heating of meat and fish products; eg: fish meal -diets for
aquaculture and animal production- high protein content and in
vivo digestibility.
 The RF process operated in the 70-90ºC range and with high
efficiency (50%), which give the uniform and deep penetration of
RF waves resulting in rapid and homogeneous heating provide
greater than 99.999% reduction in infection levels for Salmonella
sp., and for Escherichia coli O157:H7 Simultaneously, the natural
flora was reduced by greater than 99.9% adding quality attribute
to the treated products
1 17

carried at 25MHz providing at higher
temperature of 65˚C L. innocuacells microorganisms are
completely inactivated and the visual quality of the caviar
products treated by RF was good and acceptable compared to
the conventional Pasteurization processes affect the texture,
color, and flavor of these foods
1 18

 RF energy - potential selective killing effect on microorganisms
along with the rapid heating and high penetration of RF
energy
 Used a 2 kW, 27.12 MHz, RF applicator - to find best conditions
to inactivate surrogates of both Listeria and E. coli cells in milk
under continuous laminar flow conditions.
 Maximum microbial inactivation the RF treated samples are found
to be similar in its color and flavor property to the natural juices
with nil loss in ascorbic acid or enzymatic browning in RF
treated juices.
 The inactivation of Bacillus subtilis spores in soybean milk by
radio-frequency flash heating at 28MHz by heating up to 115ºC
for 0.4sec.
1 19

 RF drying - used for post-bake drying of cookies, crackers and
pasta.
 Cookies and crackers, fresh out of the oven, have a non-
uniform moisture distribution which may yield to cracking
during handling.
 RF heating can help even out the moisture distribution after
baking, by targeting the remaining moisture pockets.
 Recent development - hybrid drying systems involving radio
frequency to cater for the special needs of heat-sensitive food
stuffs.
 The use of RF technology in high value and highly perishable
commodities like walnut for the purpose of drying as well as
disinfection of the agricultural commodity.
1 20

1 22
RF Post-Baking
Virtually
Eliminates
Checking!

 A large-scale treatment for conveyor walnuts was designed - 25
kW, 27MHz RF system to achieve an average walnut surface
temperature of 60˚C and minimum temperature of 52˚C for 5 min.
 The treatment caused 100% mortality of fifth-instar navel orange
worm larvae, the most heat tolerant target pest, in both unwashed
and air dried walnuts, and was effective over a relatively wide
range of walnut moisture contents (3–7.5%).
 . The
kernel color, peroxide values and fatty acid values of treated
walnuts were similar to untreated controls after 20 days at 35˚C
simulating 2 years of storage under commercial conditions at 4˚C.
1 23

 RF heating to control brown rot on peaches and nectarines, the
RF treatment at 27.12 MHz, with 17mm distance between fruit
and upper electrode and 18 min exposure time. The results
indicated that RF treatment conducted for 22.5 min had a
significant effect on brown rot control in peaches artificially
inoculated with M. fructicola., when the exposure time was
reduced to 15 min, the incidence of brown rot was only
reduced in all the varieties tested.
 RF heating to inactivate Salmonella Typhimurium and
Escherichia coli O157:H7 on black and red pepper spice by RF
heating for 50s resulted in 2.80 to 4.29 log CFU/g reductions of
S. Typhimurium and E. coli O157:H7 in black peppers and RF
heating of red peppers for 40 s reduced pathogens by 3.38 log
CFU/g to more than 5 log CFU/g without affecting the color
change, provided 27.12MHz RF heating system.
 RF heating has the potential for novel thermal process to
control food borne pathogens in spices.
1 24

 Overcoming many heat transfer problems
 Rapid heating and require less time for desired process time.
 The ease of targeting the product, not the air surrounding it, tends to
drive the moisture from inside out, and hence leveling the moisture,
 Decreasing the processing time,
 Avoiding overheating and dehydration on the surface of the product.
 They can be turned on or off instantly and can be provided with
temperature measurement systems (thermocouple).
 RF heating is restricted by limited understanding of RF photons
interact with materials and by high cost of its source electronics.
Subject to fluctuations of electrical costs is another drawback of RF
heating.
1 25

 The major obstacle in RF technology ; the factors such as product
dielectric properties, size, shape, position between the RF
electrodes and the electrode configuration may affect temperature
uniformity thus providing non uniform heating of food produce.
 The RF heating system should be provided such that all
generators and applicators must be properly shielded and
specially designed to meet the product specific requirements.
 The lack of in-depth information on area such as its impact on all
aspects of product quality is another factor of concern, the recent
focus are only on meat product but this needs to be replicated
across a wider range of food commodities.
1 26

INDUSTRIAL
APPLICATIONS OF
RF TECHNOLOGY
1 28Food Dryer

 Appetisers snack
 Meringue
 Puff pastry
 Breakfast cereal
 Bread crouton
 Sponge product
 Pretzel
 Short bread
 Profiterole
1 29

 The RF heating technology is in research for many decades, the
industrial application, still limited towards heterogeneous food
stuff like meat, disinfection of agricultural commodities and post
baking, where as there is a large potential source available in
utilizing RF technology.
 The study on material factors which include dielectric properties
of the host material and contaminants, material geometry and
thermal sensitivity of the host material and RF process factors like
frequency, electric and electromagnetic field intensity, RF power
provides a greater challenge to the engineers to develop a large
scale production unit for the industrial application.
1 30

 RF radiation is a very attractive processing method to provide
safe and high quality food products because of rapid and uniform
heating patterns and large penetration depth.
 Promising applications of RF dielectric heating existing in the food
industry include blanching vegetables, thawing frozen foods,
post-baking snack foods, meat processing, pasteurizing and
sterilizing liquid foods and pre-packed foods.
1 31

 Punidadas Piyasena , Chantal Dussault , Tatiana Koutchma , H. S.
Ramaswamy & G. B. Awuah (2003), “Radio Frequency Heating of
Foods: Principles, Applications and Related Properties—A Review,”
Critical Reviews in Food Science and Nutrition, 43:6, 587-606, DOI:
10.1080/10408690390251129.
 Sona Jojo and R. Mahendran, (2013),”Radio frequency heating and its
application in food processing: a review” International Journal of
Current Agricultural Research Vol. 1, No.9, PP. 042-046.
1 32

 Microwave heating, which uses electromagnetic energy in the
frequency range 300-3000 MHz, can be used successfully to heat
many dielectric materials.
 Microwave heating is usually applied at the most popular of the
frequencies namely 915 MHz and 2450 MHz.
 The way in which a material will be heated by microwaves depends
on its shape, size, dielectric constant and the nature of the microwave
equipment used.
Introduction

 Microwave heating rate can be varied depending
on dielectric properties of food.
 The dielectric properties describe the ability of a
material to absorb, transmit, and reflect
electromagnetic energy.
 Dielectric properties can be defined as:
ε =ε '− ε "
 where ε is the dielectric properties
ε’ the dielectric constant
ε” the dielectric loss factor
The dielectric constant (ε’) is associated to the
material’s capability to store electric energy (for
vacuum ε’=1), while the dielectric loss factor (ε”)
is related to dissipation of electric energy due to
different mechanisms

PRINCIPLE OF MICROWAVE HEATING
 The heat is generated in the food materials because of the moisture
content in the food.
 Microwave heating works on the following mechanisms
A)Ionic Polarization
B) Dipole Rotation
 Microwave heating used the dipole molecules in the food to generate
heat.

Ionic Polarization:-
It occurs when ions in solution move to the electric field.
Ions carry an electric charge, due to this their is formations of an
kinetic energy.
Kinetic energy of an ions produce heat.
More the concentration of solution more will be the collision [i.e
waves to sample] and more will be the kinetic energy released as a heat.
Ionic polarization generally occur in fluids or liquid sample.
Microwave heating mechanism

Dipole Rotation:-
This heating mechanism depends on the existence of polar molecules.
Water is an polar molecule present in foods ,will rotate according to the
alternating electromagnetic field.
The water molecule is a “dipole” with one positively charged end and
one negatively charged end similar to the action of magnet, these
“dipoles” will orient themselves when they are subject to
electromagnetic field.
The polarity of field dependent on rate of microwave frequency.
The rotation of water molecules would generate heat for processing.
Microwave heating also dependent on the physical state of an material

Figure 1: The structure of water molecules (left) and permanent dipole (right)
Figure (2) When there is no external
electric field
Figure 3 When there is an external electric field show the
direction of dipoles influenced by external electric field

A] When microwaves are of low frequency their will be no formation of
heat by water molecule because the permanent dipole will immediately
follow the directions of electric field.
B] When we used microwave of high frequency their will be no formtion of
heat because since electric field changes its direction too fast, dipole won't
be able to follow.
Fig:- At lower frequency Fig:- At high frequency

C] When we used moderate frequency of microwave their will formation of
heat by water molecule because the permanent dipole changes a little
behind the electric field due to which heat is formed.
Fig:- At moderate frequency

MICROWAVE EQUIPMENT
A commercial microwave system consist of
1.A magnetron
2.A wave guide
3.A treatment chamber
In microwave equipment heating source is generally a Magnetron tube
is used.
The equipment operate at 2450 MHZ ,Although is sometimes used
when greater penetration at 915 MHZ is desired.
Power output is usually between 500 and 1500W. The oven cavity has
metallic walls.

Magnetron:-
It is an empty tube with magnets and the copper wire which provide
electric current to heating chamber.
The magnetron is based on the motion of electron under electric and
magnetic fields.
The direction of the electric field is from the positive electrode to the
negative electrode.
The magnetron can produce oscillations at microwave frequency.
Magnetron are of 3 types:-
A. Slot
B. Vane
C. Hole and slot
Fiber - Optic probes are used to monitor the temperature inside the
microwave equipment.

Figure:-A) Slot B) Vane C) Hole and slot

Wave guide:-
The wave guide directly help to transfer the energy to the processing
chamber internally.
A rotating fan :is used in batch heating oven, or food material itself is
rotated on a round table so that all parts of the food material are exposed to
the microwaves.
Fig- A Wave Guide

MICROWAVE HEATING
A) Batch Heating
 In the batch process, the magnetron-based microwave ovens are used for
the heating purpose.
 The food sample is placed in the oven for a predetermined time to achieve
a target temperature.
 The power level is normally adjusted to achieve a certain desired
temperature difference in a given time frame.

CONTINUOUS - FLOW HEATING
 The raw fluid is pumped (peristaltic pump) through Teflon or glass
helical coils placed inside one or several microwave ovens connected in
series for heating .
 Outside the microwave oven, the fluid then passes through a holding
section to allow a predefined holding time followed by chilling in some
form of a tubular heat exchanger.
 Thermocouples are used for gathering sample temperatures at the entry
and exit sections.
 While fiber-optic probes are used to monitor the temperature inside the
Oven.

Figure :- Continuous-Flow Heating

Bakery product
1)Cake:-
Microwave baking of Cake batter was baked in a bench-scale
microwave oven.
Cake baked in the microwave oven at 250 W for 4-5 min showed
improved textural properties as compared to cake baked in the
convective oven.(at 103°C).
2)Biscuits and Cookies:-
microwave heating used for final baking of cookies for more uniform
moisture distribution.
microwave oven at 617.3 and 745.5 W for 30 or 40 sec respectively
used for manufacturing of cookies and biscuits.

Meat Products
Precooked bacon:-
Microwave cooking gives an better yield and better quality of an
bacon .
The microwave oven works at 150kw to 240kw for preparation of
precooked bacon slices.
Poultry processing:-
For this processing microwaves of about 50kw to 80kw required
power required for cooking of poultry meat for different parts such
as legs, thigh etc
Meat patties:-
This product is famous in the Sweden, 2450 MHz unit and 30 kw
power required for processing meat patties

 Microwave blanching of fruits and vegetables is still
limited. Some of the advantages include speed of operation
and no additional water required.
 Lower leaching of vitamins and other soluble nutrients, and
waste water is eliminated or greatly reduced.
 Provides a fresh vegetable flavor, the combination with
initial water or steam blanching provides an economic
advantage
 Enables a finish blanching of the center sections more
quickly
 Uniformity is also more rapidly accomplished in microwave
ovens
56

 Pre-packed foods like yoghurt or pouch-packed meals as
well as continuous pasteurization of fluids like milk
 The possibly high and nearly homogeneous heating rates,
also in solid foods (heat generation within the food) and
the corresponding short process times
 Initially 2450 MHz microwaves has been used.
 However, even more uniform heating of foods
(pasteurization) was achieved using 915 MHz microwave
radiation, and it could be due to greater penetration
depths of 915 MHz microwaves than 2450 MHz
microwaves
 When microwave heating at 915MHz was used to
sterilize pouches containing cooked macaroni and
cheese, there was no significant change observed in the
texture of product or loss of flavor
57

 In microwave drying, heat is generated directly in the interior the material,
making possible higher heat transfer and thus a much faster temperature rise
than in conventional heating.
 Microwave drying has the advantages of achieving fast drying rates and
improving the quality of some food products.
 Drying methods using microwave can be divided into four categories;
1. Microwave Drying (MD)
2. Microwave-Assisted Freezing Drying (MFD)
3. Microwave-Assisted Vacuum Drying (MVD)
4. Microwave- Assisted Hot Air Drying (MHD)
5. Microwave-Enhanced Spouted Bed Drying (MSD)
 Microwave energy combined with other drying methods can improve the
drying efficiency as well as the quality of food products which is far better
than that achievable by microwave drying only or by other conventional
methods only
58

 Microwave Applications in Thermal Food Processing, chapter
1,Mohamed S. Shaheen, Khaled F. El-Massry, Ahmed H. El-
Ghorab and Faqir M. Anjum
 Microwave (and RF) Heating in Food Processing
Applications,Juming Tang, Ph.D., Department of Biological
Systems Engineering, Washington State University, Pullman WA
 Microwave Sterilization Technology, Dr. Juming Tang, Professor
of Food Engineering, Dept. of Biological Systems Engineering,
Washington State University, Pullman
 Potentials of Microwave Heating Technology for Select Food
Processing Applications - a Brief Overview and Update,Pradeep
Puligundla1,4, Seerwan A Abdullah2, Won Choi3, Soojin Jun3,
Sang-Eun Oh4 and Sanghoon Ko1*1Department of Food Science
and Technology, Sejong University, 98 Gunja-dong, Gwangjin-gu,
Seoul 143-747, Korea
 Microwave food processing—A review,S. Chandrasekaran, S.
Ramanathan, Tanmay Basak ,Department of Chemical
Engineering, Indian Institute of Technology Madras, Chennai 600
036, India
59

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