Microwave Food Processing

1. Microwave Food
Processing
Presented by
Paththuwe Arachchige Maduni Jayahansi Wijepala
700042824
PhD Food Science & Technology
Course Supervisor: Dr. Raman Bhaskaracharya

2. History • Percy Spencer developed and patented the first microwave
oven after noticing that a magnetron was emitting heat-
generating microwaves during an experiment with radar in
1945.
• The first models were huge—about 6 feet in height and
weighing more than 750 pounds.
• This 1976 model was manufactured in Japan by Matsushita
Electric Industrial Co. for J.C. Penney and was sold to Jeff and
Jan Thompson for $261.35, including tax and an Assurance
Performance Plan.
• They donated it to the National Museum of American History in
2006.
• Unlike modern microwaves, this model features a door hinged
2
Richard Strauss, Smithsonian's National
Museum of American History (1976)

3. What are Microwaves?
• A microwave is a low-energy electromagnetic wave
• Wavelength in the range of 10 –1000 mm (0.01 to 1.0 m)
• Frequency in the range of 300 MHz – 30 GHz (3 x 106 HZ – 30 x 109 Hz)
• Above 30 GHz called mm wave, above 300 GHz called submm wave.
• Reflected by the metals, pass through the air/ vacuum, absorbed by food constituents

4. • Microwave, travels at the speed of light
(300,000 km/sec)
• Consists of two perpendicular oscillating fields:
an electric field and a magnetic field
• Microwave photon energy is relatively low
(1.24µeV)
• Affecting only kinetic molecular excitation.
4
• Mode of heat transfer: Radiation
• Household systems: 2450 MHz
• Industrial systems: 915 MHz and 2450 MHz
• Waves Have energy, but zero mass
(c = velocity of light,  = wavelength, f =
frequency)

5. Parts of the Domestic microwave oven

6. • Internal Parts
• High Voltage Transformer
• Magnetron
• Waveguide
• Cooling Fan
• Turntable
• Cooking Cavity
• External Parts
• Power Cord
• Door
• Control Panel
Name of the part Function
Transformer  Microwave requires high-voltage energy for instant cooking
 General home voltage is not adequate for the operation
 So, the microwave has a high-voltage transformer
 Consists of a series of capacitors and multiplies the power
supply
Magnetron  Consists of a vacuum tube and a magnet
 Convert high-voltage electricity into microwaves
Waveguides  Made from a hollow metal tube
 Transfers the microwave into the microwave cavity
 The inner wall of the microwave is reflective
Cooling fan/ Stirrer  Heating instruments have a chance of overheating
 It applies to microwave also
 A cooling fan inside the microwave cavity helps to reduce
overheating by distributing air in the cavity
Cooking cavity  Helps to keep the food to cook, grill or defrost
 The microwaves coming from the magnetron are harmful to
human beings
 So, the walls of the cooking cavity is very thick
Rotating Disc/ Turntable  Rotates the food at 360 degrees which help to cook the food
uniformly.
 Made from glass and easy to remove and clean
Control panel  Set the cooking time, the intensity of cooking, or functions such
as defrost or grill mode
Door  Allows user to place or remove the food
 Seal microwave cavity from all sides (Rubber seal on the edges)
 Prevent microwaves from getting out (metal mesh lining)
 Can see through and keep eye on the food inside the cavity

7. • Magnetron is a vacuum tube with central electron emitting cathode of highly negative
potential which is surrounded by a structured anode.
• The anode structure forms cavities, which are coupled by their fringing fields and have the
intended microwave resonant frequency (915 MHz/ 2450MHz).

8. How it works?
• Magnetron generates electromagnetic waves and
through waveguide releases the microwaves to the
cooking cavity
• The metal walls of the cavity reflect the microwaves
• The cavity becomes a “Resonance Cavity”
• When microwaves pass the food they do not bounce off
• The microwaves penetrate the food
• Penetration depth is higher in lower frequencies
• If there is no rotating table uniform cooking is difficult
• Energy in the microwaves converts to kinetic energy
and molecular-level vibration occurs inside the food
• The faster the molecules vibrate the hotter the food
becomes
• Glass, ceramic, or plastic containers are used in
8

9. Resonance Cavity
Path of microwaves
Magnetron Waveguide Cavity Opposite wall of the
cavity
• The metal wall reflects the microwaves
• The original wave and the reflective wave travel in opposite directions
• The resultant wave is called the stationary wave
• The target is to achieve the stationary wave with the highest amplitude

11. • Frequency of household microwave oven - 2450 MHz
(2.45*109Hz)
• Speed of microwaves - 3 * 108 m/s
• The wavelength – 12.2 cm
• The distance between the metal walls of the microwave
oven should be proportionate to half of the wavelength
of stationary wave
• This cavity design is more energy-efficient and safe for
the users
• This is called “Resonance Cavity”

12. How do microwaves generate heat?
Dipolar Rotation Ionic Conduction
Heating through the interaction of
electromagnetic radiation with the polar
molecules of foods (i.e. H2O)
Heating through the interaction of
electromagnetic radiation with the ions and ionic
species in the food
Dielectric properties create polarization within
the food matrix when exposed to an external
electromagnetic field
A free ion or ionic species moves translationally
through space, attempting to align with the
changing electric field (Back and forth vibration)
Polar molecules rotate continuously to align with
the ever-oscillating electromagnetic field
Leads to friction generation between the ions or
ionic species
Friction generation between the molecules leads to heat generation. Heat transfer happened
through conduction, convection and radiation resulting volumetric heating
Food warms up
In both cases, the more polar and/or ionic a species, the more efficient the rate of heat

13. At the microwave frequency of 2450 MHz, the
polarity changes 2.45 billion cycles per second

14. Factors affect the Microwave Heating
1. Dielectric properties and Penetration depth of the food matrix
• When a material is subjected to an electromagnetic field the molecules get stressed and because of
inter-molecule friction heat is produced. This is known as dielectric heating.
• Foods: “Lossy dielectric materials” (Ability to store and dissipate energy)
• Dielectric properties describe the ability of the food matrix to
– Absorb
– Transmit Microwaves (electromagnetic energy)
– Reflect
• Dielectric properties can be defined as,
– Permittivity of the food (describes a material’s ability to absorb, transmit and reflect electromagnetic
energy)
’ – Dielectric constant (The ability of a material to store microwave energy due to induced polarization/
ability to obstruct microwave as it passes the food)
” – Dielectric loss (The ability of a material to dissipate microwave energy into heat via dipolar rotation
and ionic conduction)

15. • Ionic Conduction
– Ionic conduction is the conductive migration of the dissolved ions in the applied electromagnetic
electromagnetic field.
– Translational movement (Forth and back) due to effort in aligning with the oscillating electro-
electro-magnetic waves
– Ionic collision leads to heat generation
– Ionic conduction results production of heat as per the Joule’s law:
Q = I2Rt
• Penetration Depth
– The penetration depth has been defined as the depth into a sample where the electric field
strength (Power density) has fallen by approximately 36.7%.
– Thus, as wavelength increases (or processing frequency decreases), penetration depth
increases
𝑃𝐷 =
0 𝜀′
2𝜋𝜀″

16. 2. Microwave frequency
– Shorter the frequency higher the penetration
• Ice: 30cm at 915 MHz and 10cm at 2450 MHz
• Center heating more prominent at 915 MHZ
3. Physical properties of food
– Penetration depth depends on the state
• 10cm in ice vs 1cm in water at 2450 MHz
– The dielectric loss factor increases with the increase in
temperature of the material due to ionic conduction
• So reduced heat transfer to cold areas happen
• Leads to non-uniform heating
– Density of the food
• Higher the density lower the dielectric constant
• Higher the density lower the rate of heating
– Shape of the food
• Round is the best shape
• Overcooking in squared corners
– Mass
• The temperature rise inversely proportional to the mass of
food
𝑳𝒐𝒔𝒔 𝒕𝒂𝒏𝒈𝒆𝒏𝒕 𝒕𝒂𝒏 𝜹 =
𝜺"
𝜺′
Loss tangent: Material’s capacity to be penetrated
by an electromagnetic field and subsequently
dissipate electromagnetic energy as heat
Large values of dielectric loss tangent will result in surfa
heating of the material

17. 4. Food composition
– Moisture Content
• High moisture content, high loss factor,
• high MW absorption
• High moisture content, lower penetration
• Loss factor of ice < water
– Fat content
• Less specific heat compared to protein and carbohydrates
• Higher loss factor
• Rapid heating
– Sugar content
• Polar nature due to hydroxyl groups
• High dielectric constant
– Protein content
• Polar nature due to hydroxyl groups
• High non-uniform temperature distribution
• Low heating rate (Molecules are larger)

18. Microwave vs conventional heating
Conventional Heating Microwave Heating
Source Vessel Food surface
Inside
Heats up the entire mixture layer by layer
Interact directly with the food matrix
Efficient penetration of the heat through the mass
Rate of heating depends on the thermal
conductivity
Mainly depend on the chemical properties
(polarity and presence of ions)
Slow and non-uniform heating Rapid and uniform heating
Superficial heating Volumetric heating

20. Application of Microwaves in food industry
1. Microwave Drying
• Complex process of heat and mass transfer based on
volumetric heating
• Vapour is generated inside a food item and then spread
through internal pressure gradient
• Microwaves are penetrable and inside and outside are heated
at the same time (Volumetric heating)
• The liquid moisture is intensively evaporated and transported
toward the surface
• Drying consists of three stages
– Heating up
– Constant rate drying
– Falling rate drying

21. Different Microwave drying techniques
Drying Technique Mechanism
Hot-air microwave
drying (Uniform and
fast drying)
• 100kw microwave can dramatically remove interior moisture content of
material, and 50kw hot air specifically dries the surface parts of material.
• Fast and Uniform drying
Microwave-far
infrared combination
drying
• Both Infrared and MW heating mechanism is volumetric heating.
• In which, the heating rate is high, and the moisture gradient and
temperature gradient are in the same direction, which supports the
moisture diffusion.
• Thus, the drying rate and the quality of the drying product would
significantly increase.
Microwave-
convective drying
(MCD)
• Internal heat generated by the microwave increase pressure within the
pores and drives the moisture to the surface.
• The convective air takes the moisture by evaporation, and sometimes
evaporative cooling occurs at the surface.
Vacuum-microwave
drying (MVD)
• Vacuum enables the products to be dried at a lower product temperature
• This technique is reported to be used successfully for the dehydration of
grapes, cranberries, bananas and tomatoes, carrots and garlic, kiwifruit,
apple and pear.

22. Flavonoid content and polyphenols were higher in MVD than conventional hot
air drying
Guobin et al.
(2019)

23. • Effect of Microwave Drying on Food Quality Attributes
– Nawirska-Olszanska, Stepien, and Biesiada (2017)- At the power of 100 W, the content of total polyphenols,
bioactive compounds (chlorophyll a+b, carotenoids), and antioxidative properties of pumpkin slices dried by
microwave were greater than that using the power of 250 W.
– Wodjdyio, Figiel, Lech, Nowicka, and Oszmianski (2013)- When sour cherry fruits were processed using
vacuum-microwave drying antioxidants, phenolic compounds, and anthocyanins were lower in high
temperatures than in low temperatures.
– Sarimeseli (2011)- The rehydrating capacity of coriander leaves processed by microwave decreased as the
microwave power increased (180- 900 W).
– Aydogdu et al., (2015); Horuz & Maskan, (2015) – Eggplant slices and pomegranate arils dried by microwave
drying produced a more porous structure than those dried by rapid hot-air drying.
– Zielinska, Sadowski, and Blaszczak (2015) – Hardness, chewiness, and gumminess of blueberries which were
dried using microwave-vacuum drying were several times lower than that of hot-air drying.
– Han et al., (2016) – When drying time, discoloration, rehydration ratio, and energy consumption were
considered during combined microwave and far-infrared ray drying, 15 minutes preheating in a microwave and
drying at 100°C in a far-infrared dryer resulted comparatively higher quality sea tangles.

24. Qiushan Guo., et al. (2017)

25. 2. Microwave Heating
• Conventional heating- Heat conduction and convection
• Microwave heating: Microwave convection and radiation based on volumetric, instantaneous heating
• Microwave penetration depends on the food composition
• The hot spot and cold spot are found due to the standing wave
• Uneven heating is an intrinsic feature of microwave heating
• Achieving heating uniformity is a challenge
• Temperature distribution of the food item should be considered when optimizing the process conditions

26. • Effect of Microwave Heating on Food Quality Attributes
– Chang et al. (2011) – The thermal denaturation and shrinkage of collagen were higher in beef
semitendinosus muscle heated with a microwave than using a water bath.
– Dolinsky et al. (2015) – Microwave cooking with water significantly reduces the polyphenol content of kale
(23.4%), tomato (21.9%), and green beans (22.9%) compared to steaming.
– Hefnawy (2011) – The anti-nutritional factors of lentils were significantly reduced after microwave treatment.
– Trypsin inhibitor (93.9%)
– Tannins (34.4%)
– Phytic acid (39.2%)
– Yarmand et al. (2013) – The compression force of the camel muscle (microwave-cooked) was more than
twice than that of raw meat due to less solubilization of collagen.
– Perez-Juan et al. (2012) – Marinating with 10% injected brine solution (5.6% salt, 4% Sodium Lactate, 5%
Lactose, 0.5% Ascorbate) can enhance the tenderness of muscles from Friesian cows cooked by
microwave.
– Pellegrni et al. (2010) – When compared with boiling and steaming microwave heating is the best cooking
method for preserving the colour of both fresh and frozen broccoli, brussels sprouts, and cauliflower.
– Perez-Juan et al. (2012) – Semitendinosus muscles from Friesian cows cooked by microwaving at 654 W
showed lesser lightness, higher redness, and lesser yellowness than those at 182 W.

27. 3. Microwave Sterilization
• Improve the safety and extend the shelf life of foods
• Mechanisms
– Selective heating: Microbial cells can reach a higher temperature than
the surrounding fluid
– Electroporation: The electrical potential across the cell membrane can
generate pores in cells
– Cell membrane rupture: Cell membrane rupture leads to leakage of
cellular material
– Magnetic field coupling: Components of the cells such as protein or
DNA coupled with the magnetic field and can be destroyed
• Microwave Assisted Thermal Sterilization (MATS)
– Quickly immersing packaged foods in pressurized hot water and then
heating with microwaves (915 MHz for 10-15 mts)
– Recognized by USFDA (2016)
– Nutrients and flavours remain intact than retorting
27

29. Effect of Microwave Sterilization on Quality Attributes of Food Products
• Piasek et al. (2011) – Aronia (Chokeberry) subjected to microwave sterilization resulted in a total
loss of anthocyanins ranging from 39.7% to 59.1% which was lower (66.1% to 99.8%) than that of
treated by thermal processing at 100°C
• Marszalek et al. (2015) – Strawberry puree was subjected to microwave and conventional heating.
• Chen et al. (2016) – The activity of wheat germ lipase decreased by 60% and 100% respectively at
45°C and 60°C.
• De La Vega-Miranda et al. (2012) – When the microwave sterilization time is less than 1 minute the
texture of the grape tomato, jalapeno pepper, and coriander remained without any significant
difference.
Component loss Microwave heating
(90°C for 10 S)
Conventional heating
(90°C for 15 mts)
Polyphenols 5.7% 14%
Total anthocyanins 19.2% 60.2%
Vitamin C 3.4% 61.7%

30. 4. Microwave Blanching
• Conventional blanching: leads to loss of weight and nutritional value
• Hot water and steam blanching are common
• Microwave blanching: an alternative to conventional blanching
• Microwave needs little or no added water for blanching
• Can reduce nutrient loss by leaching as compared to hot water immersion
Pradeep et al.
2013)

31. 5. Microwave Baking
• Baking Process:
– Expansion of dough and initiation of moisture loss
– Expansion and rate of moisture loss becomes maximal
– Rise in product height and decrease in rate of moisture loss
• Issues associated with microwave baking
– Low volume
– Lack of colour development
– Lack of crust formation
– More dehydration
– Rapid staling
• Chemat et al. (1999) introduced a modified microwave (Microwave- Ultra-violet combined) oven
• Halagen lamp- microwave combination heating
– Halogen lamp: generates near-infrared radiation (high frequency and low penetration)- Leads to
crust formation and browning

32. 6. Microwave Tempering
• Tempering is defined as the thermal treatment of frozen foods to raise the temperature from below -
18ºC to temperatures just below the melting point of ice (approximately -2ºC).
• At these temperatures, the mechanical product properties are better suited for further machining
operations (e.g. cutting or milling).
• The time for conventional tempering strongly depends on the low thermal conductivity of the frozen
product and takes long time for the larger food pieces such as blocks of butter, fish, fruits or meat.
• By using microwaves (mostly with 915 MHz due to their larger penetration depth) the tempering
time can be reduced to minutes or hours and the required space is diminished to one sixth of the
conventional system.
• Another advantage is the possibility to use the microwaves at low air temperatures, thus reducing or
even stopping microbial growth.

33. Advantages of microwave processing
• Fast volumetric heating
• Reduced energy consumption (Cook only the food, no effect on temperature rise in the compartment)
• Lower operational cost
• Higher drying rate and short drying time
• Preserve colour
• Microwave sterilization can be effectively used to ensure microbial safety
• Exposure for a short time: Preserve antioxidants and bio-active components compared to
conventional heating
• Reduce anti-nutritional factors
• Cooking without addition of extra oil is possible
• Rapid defrosting of frozen foods
• Microwaves are non-ionizing radiation, so they do not have the same risks as x-rays or other types of
ionizing radiation

34. Challenges and limitations
• High initial cost for industrial-level microwave dryers
• Product texture may be affected
• Significant losses in nutrients could happen if cooking with massive water
• The non-uniformity of the microwave field leads to non-uniform processing of foods
(Cold and hot spots)
• Metal containers/ films can not be used
• No crust formation
• Non-availability of actual temperature profiles
• Safety concerns
– Acrylamide formation
– Possible leakage of microwaves can cause burns

35. Joana et al. (2019)

36. Regulatory and safety considerations
• The Food and Drug Administration (FDA) has regulated the
manufacture of microwave ovens since 1971.
• Microwave oven manufacturers are required to certify their products
and meet safety performance standards created and enforced by
the FDA to protect the public health.
• A Federal standard (21 CFR 1030.10) limits the amount of
microwaves that can leak from an oven throughout its lifetime to 5
milliwatts (mW) of microwave radiation per square centimeter at
approximately 2 inches from the oven surface.
• The FDA requires that all ovens have a label explaining precautions
for use

37. Conclusion and Future Trends
• Microwave sterilization and pasteurization
– Effectively destroy pathogens
– Significantly reduced processing time
– Without serious damage to overall quality
• The use of microwave heating for blanching, cooking, and baking has a great effect on the
preservation of nutritional quality
• Microwave drying requires less energy than conventional dehydration techniques
• Microwave processing with combined conventional heating applications for uniform heating are
widely investigated
• In terms of nutrients and sensory attributes conventional heating technologies are still far ahead of
microwave processing
• Investigation of parameters that can influence the effectiveness of microwave heating is important
– Dielectric, physical and chemical properties of foods

38. References
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