Microwave (and RF) Heating in Food
Processing Applications
Juming Tang, Ph.D.
Professor of Food Engineering
Department of Biological Systems Engineering
Washington State University, Pullman WA
OUTLINE
1. Microwave and Radio Frequency (RF)
Heating Principles
2. Established Applications in Food
Processing
3. Development of Microwave Sterilization
Technology for Pre-packaged Foods
SPECTRUM
Electromagnetic Spectrum
The wavelength
MW
RF
l = c/f
with c = 3.108m/s
f
13.56 MHz(+/- 0.05%)
915 MHz
27.12 MHz (+/- 0.6%)
2450 MHz Domestic Ovens
40.68 MHz (+/- 0.05%)
10 12 18 GHz (plasma)
Very restricted bands
Can be used for ISM
applications
Generation of microwave
Magnetron
Waveguide
Magnetron
Microwave cavity (applicator)
Domestic Microwave Oven (2450 MHz)
Microwave Cooking of Meat Patties (APV system)
RF Heating Systems
Food
Plate Electrodes in RF Heater
Industrial Applications of RF Technology
- typical up to 85 kW per module
Food Dryer
Source: Strayfield, UK
Dielectric heating of foods
- Product temperature in electromagnetic field:
T
C p
= 5.563 10 11 f E 2 ''
t
Loss factor
- Dielectric property:
Dielectric constant
- Contributions of dipole rotation and ionic conduction:
Multi-mode Systems
Electric field pattern for a loaded microwave cavity at 2.4295 GHz.
(Chan and Reader, 2000)
Single - mode Systems
TM010 cavity resonator, 915 MHz (adopted from Regier and Schubert)
How Does RF or Microwaves Heat Foods?
Alternating Electric field
Space charge polarization
Orientation polarization
Mechanism of Dielectric Heating
RF
Ref: Tang et al., 2002, Scientific Press, New York
MW
Penetration Depth (mm)
RF
Ref: Wang et al.,2003. J. Food Eng. 57: 257-268.
Microwaves
2. Industrial Microwave (MW) and Radio
Frequency (RF) Technologies
-- Better quality
-- Reduced process times
-- New products?
MW COOKING DESSERTS
Cooking of desserts in pots
Microwave Tunnel
22 000 Pots / hour
Power : 144 KW.
MW COOKING/PASTEURIZATION OF VEGS
Nutripack (a French Company) solution with valve Valvo-PackTM
The valve allows a thermal treatment and the cooling of the product in the packing without
counter-pressure.
The valve is adhesive and is installed on the upper part (heat sealing film, cover) of a
packing (plastic, metal, paper, cardboard) that has been previously pierced.
Opening limit : 10 mbar 3 mbar
Closing limit : 5 mbar 2 bars
A similar concept developed by MicVac, Sweden
From their website:
http://www.micvac.com/
MicVac develops, markets and sells two methods
for processing and packaging convenience food.
Both solutions (for pasteurised and unpasteurised
foods respectively) use a valve that offers several
benefits for both the producer and the consumer.
The MicVac method is based on the principle of
cooking and pasteurising food using microwaves
directly in the sealed consumer package. The film
cover on the package has a valve that opens
during cooking, releasing the overpressure in the
form of steam and oxygen. After cooking, the
valve closes and a natural vacuum is created in
the package another of the products many
advantages.
MW COOKING/PASTEURIZATION
EXAMPLES OF PRODUCTS
Wokingham, UK
RF Finish Drying of Bakery Products
50kW RF Post Baking Unit - Courtesy of Foxs Biscuits, UK
Moisture profile at exit of an industrial bakery
oven, and after RF drying for 5 mins
Moisture % BDW
Before RF
After RF
At Packing
2.05
1.85
1.65
1.45
1.25
Working Side
Middle
Non-working Side
RF Post-Baking
Virtually Eliminates
Checking!
Sponge Products
Reduces Mold Spores
Increases Shelf Life
Increases Production
Appetisers snack
Meringue
Puff pastry
Breakfast cereal
Bread crouton
Sponge product
Pretzel
Short bread
Profiterole
RF DRYING OF BAKERY
PRODUCTS
Manufacturers using RF
Heating and Drying
Kraft (Nabisco) (USA)
United Biscuits (UK)
Kelloggs (Keebler) (USA)
Foxs Biscuits (UK)
Weetabix (UK, Canada)
Arnotts (Australia/ New
Zealand/ Indonesia
National Biscuits (S.Africa)
Safeway (USA)
Ferrero (Italy)
Dancake (Portugal)
Ulker (Turkey)
Pepperidge Farms (USA)
M&M Mars (USA)
Parle (India)
Britannia (India)
Crown (Korea)
Horizon Biscuits (UK)
United Biscuits/Danone (Lu
France, Bagley Argentina)
Tong Yang (Korea)
Morinaga (Japan)
Dare Foods (Canada/USA)
Barilla (Italy)
ETI (Turkey)
Ralston Purina (Bremner) (USA)
Griffin (New Zealand)
Approximately 500 RF food processing systems
sold over 37 years
3. Development of Microwave Sterilization
Technology for Pre-packaged foods
(~50 years)
All past MW sterilization systems used 2450
MHz
Conventional Retorting Relies on Heat
Convection and Conduction in Foods
Retort
Convection
Target: C. botulinum spores
A Multi-mode 2450 MHz Microwave Cavity
2450 MHz microwave heating system for food in plastic
pouches at US Army Natick Soldier Center in 1970
Microwave pasteurization equipment with module
open to show alternating placement of magnetrons
(Berstoff, 1992)
Microwave pasteurization line for prepared meals
(Berstoff, 1992)
Microwave sterilization unit with external covering removed
(overpressure of 2.5 bar,OMAC, 1992)
Continuous microwave sterilization system
(OMAC, 1992)
Otsuka plant
in Japan (2008)
TOPS FOODS
Olen, Belgium (2000)
The 2nd generation Microwave Sterilization
System2000
The 3d Generation of Microwave Technology at
Tops Foods, 2004
Sterilized products produced by TOPS Foods
MW pasteurized, processed to 80C
shelf-life 35 days at 7C
MW sterilized, shelf-life 1 year
at room temperature
4. Research on Microwave Sterilization
Technologies at Washington State University
4. Engineering and Scientific Challenges in
Developing MW Sterilization Processes
1. Technology/systems to provide stable
electromagnetic field patterns with high
energy efficiency
Single mode, high coupling efficiency
2. FDA Approval
Stability of system and processes
Scientific base/means for process development
Food Safety
3. High quality foods
4. Scaling-up and economic viability
MW STERILIZATION CONSOTIUM (2001-)
Washington State University
Microwave Company: Ferrite Component, NH
Food Packaging Companies:
Rexam Container, MO
Graphic Packaging, CO
Food Processing Companies:
Kraft Foods, ILL
Hormel, MN
Masterfoods (MARS USA), CA
Ocean Beauty Seafood Inc., WA
Technical Consultant: NPA
Supported by DoD DUST Program and US Army Natick Soldier
Center
Development of Microwave Sterilization Technology
Computer simulation
System development
Microbial studies
Process development
FDA Approval
Scaling-up
Industrial implementation
0 phase shift
cavity
waveguide
WSU 915 MHz Single-Mode 10 kW Pilot Scale Sterilization
System (2001-2008)
Steps involved in identification of cold and hot spots with mashed
potato sample
Computer vision to measure
M-2 color of processed
trays.
M-2 Kinetics
Fo vs M-2 yield relationship
Checking repeatability of
hot and cold spots with different
tray sizes and level of salt
Identification of cold/ hot location
For different tray size.
Verification of location
Using fiber optics probes
77 min heating time (steam at 247
F)
MW sterilization of salmon
April 15, 2003
(Run-1)
140
circulating water tempture
120
parameters
100
80
temperature of salmon
60
40
MW heating: 2.4 min;
Total processing time: 10 min.
F0=8.4
20
0
0.00
2.00
4.00
6.00
time [min]
8.00
10.00
12.00
Change of Shear Force in Pink Salmon Fillet Heated at 121C
170
Shear force (N)
150
130
110
90
70
50
30
0
20
40
60
80
Cooking time (min)
100
120
140
Eggs
Mashed Potato
Microwave sterilized salmon and fried rice
Maximum and minimum C values for food in a 10 oz tray after
a F0=6 process, water temp=121 C
Process time
(min)
Max cook
value
Min Cook
value
Ideal
Retort
6
33+4
39
212
39
93
Microwave
9+4
58
50
C values for food in a 10 oz tray after a F0=6 process, water
temp =125 C
Process time
(min)
Ideal
2.4
Retort
27+4
Microwave
3.9+4
Max cook
value
Min Cook
value
22
22
279
78
36
32
56
Sensory and self-life studies at US Army
Natick Soldier Center
50 trays chicken
were produced for
Natick in January
2004
Chicken product:
Natick evaluation,
shelve study and
taste panels
Hedonic score
(1-9 scale)
Chicken Breast, OVERALL Acceptability
o
Averages over 6 months storage at 100 F
8
7
6
5
4
3
2
1
0
y = -0.0796x + 6.7086
R = 0.6015 (Microw ave)
2
y = -0.0865x + 5.4943
R2 = 0.946 (Retort)
Microwave
Retort
Months in 100o F Storage
Linear (Microwave)
Linear (Retort)
WSU 2nd generation system developed for FDA approval
- four single mode cavities, 40 kW max MW power
Potential applications for microwave
sterilization/pasteurization technologies
- new processes
- new products
- new markets
For more information
http://www.microwaveheating.wsu.edu/
MW and RF Technologies are much more
complicated than conventional methods.
Successful development of new applications
requires extensive R&D efforts based on good
understanding of MW and RF heating principles
and the systems.
Pilot-scale studies are necessary for scaling up to
industrial processes
Thank You
