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Advances in Microwave Drying of Foods and Food Ingredients
APPLIED TECHNOLOGY Advances in Microwave Drying of Foods and Food Ingredients Y.J. Owusu-Ansah
pas Pilot
Plant Corp, 118 Veterinary Road Saskatoon, Saskatchewan S7N 2R4
Introduction The idea of microwave heating of foods has been around for over forty years. The vision of Dr. Percy Spencer, which stimulated his 1945 patent, that someday microwave heating would be a new integrated system for preparing foo~ is now a reality. It is estimated that there are over 60 million microwaves in American homes. Although heating is the fundamental process ~n the food industry and microwaves offer relatively better effiCIency and minimum time lag for heating foods, microwaves have not had compc: ble success in the food industry as in .dome.stic and catering food services. The growth of domestic utilization of microwave energy in food preparation has recently catalysed a cooperative working relationship betwee~ food engineers, scientists, technologists and microwave engineers to use the technology in the food industry. Currently some food industry operations such as meat thawing, pasteurization and even thermal sterilization (Anonymous, 1988) with microwaves are being practised to some extent. One area in food processing where microwaves seem to offer tremendous advantages is in food dehydration. In the so-called falling rate period of drying, microwave heating can accelerate the drying due to the more efficient heat transfer. In this article the use of microwaves in various food dehydration applications is reviewed. Pertinent issues such as available equipment, the economics of this technology in food dehydration and potential applications in some food and ingredients drying are presented.
Heating due to non-ionizing radiation depends on the penetrating power of the radiation. Among the non-ionizing radiation maximum penetration power is observed in the microwave region (Osepchuk, 1975). This penetrating feature is conveniently used in the various applications of microwaves in the food industry. It should, however, be noted that microwave penetration into products is determined by factors such as electrical and compositional properties of the material. Microwave is analogous to light in that it can be transmitted and reflected. Assuming we consider microwaves impinging on a hypothetical large surface of a piece of an apple (Figure 1). In general, some amount of the incident wave is reflected. Some of the wave is progressively attenuated and thus decreases in magnitude as it penetrates into the apple. If the power of the incident wave is represented as Pi, the reflected power represented as Pf and the residual power at a distance of d-cm from the surface is represented as Pd, then the attenuated power (pa) can be represented as follows: Pa
=
102/ AT
(1)
The degree of reflection and penetration of microwaves depends on the dielectric properties of the material. The dielectric property is in turn, described by two basic parameters; the relative dielectric constant (E) and the loss tangent (tan 0). The dielectric constant predominantly provides indication on the refection properties and wavelength in the material and is related to the wavelength in the material as follows: A
Characteristics and Theory of Microwaves Microwaves are part of the electromagnetic waves propagating between 300 MHz and 300 GHz. They generally have shorter wavelengths than FM radio or TV signals, but a longer wavelength than visible or infrared light. Being an electromagnetic wave, microwaves have electric and magnetic components, acting perpendicularly to each other. They also show monochromaticity and are highly polarized. The close proximity of microwaves to radio and TV waves posses a potential problem of interference between these waves. Therefore, for industrial, scientific a'nd medical (ISM) applications, specific frequencies known as ISM bands; 915 MHz (896 MHz in some European countries), 2450 MHz, 5800 MHz and 24125 MHz have been authorized for usage. The most popular frequencies used in North America are 915 and 2450 MHz.
Pi - (Pf + Pd)
=
~ -VE
where: A
=
AO =
wavelength in material wavelength in free space
(2)
For food material ,j E is large so that at 2450 MHz the wavelengths are small. If the incident wave is perpendicular to the surface of the material then the reflected power relative to the incident power may be approximately represented as: Pf
(1 - -V E)2
Pi
(1 + -vE)
(3)
Another important dielectric terminology used in microwave heating is the relative dielectric loss factor (E'). This factor measures the amount of energy dissipated when a material is subjected to an alternating current. The relative dielectric J, Inst. Can, Sci. Technol, Aliment, Vol. 24, No. 3/4, 1991
Table 1. Some conversion mechanisms for microwave heating (Adapted from White, 1973). Ionic Conduction Interface Polarization Dipole Rotation Entire Molecule Quantized Twisted Bend Dipole Stretching Ferroelectric Hysterisis Electric Domain Wall Resonance Electrostriction Piezoelectricity Gas Collision Frequency Resonance Multifactor Resonance Cyclotron Resonance Nuclear Magnetic Resonance Paramagnetic Resonance Ferromagnetic Resonance Ferrimagnetic Resonance Antiferromagnetic Resonance Spin Wave Resonance Magnetic Domain Wall Resonance Magnetostriction Piezo Magnetism
Fig. 1. Distribution of Microwave Incident on a Plane Surface of a Sliced Apple.
loss factor (E') is related to the dielectric constant as shown in equation 4. E' ~ E
tan 1>
(4)
Generally, the dielectric properties of a material are dependent on various factors such as temperature, physical structure, frequency, and chemical composition. Materials with high dielectric loss factors are termed lossy materials and these attenuate microwaves. When a dielectric material is introduced into an AC current containing a capacitor, the angular lead of the current over the voltage is reduced from the 90° exhibited for ideal capacitors. This reduction in angular lead of the current over the voltage is know as the loss tangent (tan D). The tan Dprovides indications on the penetration depth (the point at which the incident wave is reduced to approximately 36% of its magnitude) and is related to the half-power penetration depth D by the following equation (Osepchuk, 1975):
increases and consequently the penetration depth decreases. Also, the higher the dielectric constant, the lower the penetration depth. These parameters that seem to dictate the efficiency or even the propensity for microwave attenuation in materials clearly indicate that, depending on factors such as composition, temperature or moisture content of the material, microwave heating in a material could be a surface phenomenon rather than the conventional misconception of internal heating.
Mechanisms of Microwave Heating Food systems generally have high dielectric constants due to water. They also have relatively high loss tangents (0.1-1.0) that make them good materials for microwave attenuation. Microwaves are used in food processing due to the spontaneous heat they generate. Some of the mechanisms by which heat is generated by microwaves are shown in Table 1. For microwave heating in foods however, only ionic conduction and dipolar rotation are of primary interest. In ionic conduction, ionized compounds randomly collide with non-ionized groups when subjected to an electric field. The kinetic energy of these ions are transmitted as heat dur-
POWER SUPPLY
mOB VOLTAGE
Ir-;'f~TRAN~S~MI~SSI~ON~~ MICROWAVES S~ON
APPLICATOR
'-'-"'-;-"",-,....J
D = 0.189 AO
yE
y'-----
(5)
MICROWAVE GENERATOR
..j 1 + tan 2 1> - 1
for small tan D ~ 1, eqn 5 can be simiplified to: D
=
0.269 AO
Y-E
(6)
tan 1>
It can be appreciated from the preceeding equations that as the loss tangent increases the amount of attenuated energy Can. Inst. Food Sci. Technol. J. Vol. 24, No. 3/4, 1991
CONTROLS AND 8AFE1T DEVICES
Adapted trom eosllllo, 1973
Fig. 2. Some Essential Components of Microwave Equipment.
AT / 103
Table 2.
A summary of microwave drying processes in food and food ingredient industry (Adapted from Ontario Hydro Publication).
Dryer type
Product
Advantage over conventional systems
Booster (Microwave-hot air)
Pasta
• Floor space reduced by 2/3 to 4/5: eg, 36 m - 8.2 m for pasta equipment • Cleanup time reduced: eg, 24 h - 6 h (pasta) • Product quality superior: eg, less starch slough-off and better bite (pasta), breaking of surface and surface hardening prevented (pasta), enhanced colour • Drying time reduced: eg, 8 h - 1.5 h (pasta), onethirtieth of original time (egg yolk powder), 8 h - 6 min (dried milk) • Infestation reduced: eg, 90% (onions) • Energy savings in final drying: eg, 30% (onions), by 20-25% (pasta) • Moisture-levelling control of output • Higher yield • Versatility in scheduling • Lower equipment cost
Egg yolk powder Dry milk for babies Onions
Tomato paste Chocolate powder Rice cakes Snack food Seaweed Bacon bits Microwave vacuum
Fruit juice powder
• Continuous system: savings in labour cost, energy, operating costs • Product reconstitutes quickly and easily • Savings in cost/kg product versus freeze-dried or spray-dried product • Superior retention of flavour • Drying times much shorter: eg, 40 min for orange juice powder; from 2 h to 0.5 h for soybeans • Conditioner unnecessary but required in heated air dryers (soybeans) • Safe from fire startup (grains) or dust explosions • Improvement in product quality: ego higher germination rates for seed grains; seeds remain whole • No blowing air to carry dust, clean • Improved efficiency: 48% greater • Better flexibility for production • Better versatility: different crops using same equipment • Quiet
Grains: eg, wheat soybean, rice, rye
Cottonseed Yeast Peanuts Pecans Corn Fruit Tomatoes Peppers Seasonings Protein preparations Meat extracts Plan extracts Instantly soluble vegetable powders Microwave freeze
• Faster: eg, drying time reduced from 12h 6h or less (coffee) • Lower cost than conventional freeze-drying: eg, 47% lower using micro-wave/radiant energy
Coffee Beef Slices Vegetable pieces Fruit Mushrooms Chicken Shrimp, lobster fish slices
• Lower energy cost: eg, 25% lower • More than doubles production • Lower capital and operating costs
ing such collisions. The heating rate due to ionic conduction could be expressed as: p
-p.-
E2 qn p.
(7)
Vp. where:
E Electrical field Pp. Power V p. ~ Volume of material q the electrical charge on each of the ions p. ~ level of mobility of the ions n ~ the number of charges
The conductivity
(u,)
may be expressed as:
conductivities of each ion. In the case of dipolar rotation, the randomly orientated dipolar compounds undergo alignment and disorientation cycles at a rate equal to the frequency of the applied field. This build up and decay of orientation genera,tes kinetic energy which is converted to heat. Dipolar rotation IS dependant on frequency and temperature. For dipole rotational heating the conductivity (uo) and the heating rate could respectively be expressed by the following equations; 00 ~
(8)
104/ AT
For materials containing different types of ions in a specific volume, the total conductivity is the sum of the individual
2
11"
f
E
tan {j
(9)
where: f is the frequency of the field. J. Inst. Can. Sci. Technol. Aliment. Vo!. 24, No. 3/4, 1991
Fig. 3. A Batch Pilot Plant Microwave Vacuum (MIVAC) Drying Unit
Fig. 4. A Continuous Microwave Vacuum Dryer (GIGIVAC Minor).
Microwave Drying Equipment and Application in the Food Industry
Microwave Vacuum Dryers
Foods by the nature of their composition can be heated at a very fast rate by microwaves. It is this fast heating that ultimately makes microwaves attractive for food processing operations and processes. Generally for all industrial microwave applications, the essential components of the equipment include: the microwave generator (magnetron), an applicator and the controls and safety mechanisms (Figure 2). The various modes in which microwaves are used for drying foods and food ingredients in the industry comprise of Booster (microwave-convection) Dryers, Microwave Vacuum Dryers and Microwave Freeze Dryers.
Booster (Hot Air-Microwave) Dryers Booster dryers consist of conventional dryers operating in conjunction with microwaves. A description of types of these dryers for drying pasta and onion has been presented elsewhere (Smith 1979). For the pasta application, the dryer (manufactured by Microdry Corporation) consists of a conventional hot air predryer on top of a microwave-hot air dryer. The final stage is an equilibration stage. The microwave system consists of 30 kW units propagating in a 6 m long multimode cavity with a 2 m wide conveyer. The product enters the predryer at a nominal moisture of 30% and is reduced to about 18% in 35 min. It then enters the microwave-hot air stage and the moisture content is dropped from 18% to about 13% in about 12 min. The introduction of the microwave at the moisture content of 18% is strategic. At this point microwave penetration depth is not seriously affected by the moisture content. Secondly, the drying seem to be approaching the falling range where conventional systems perform poorly. The microwaves, therefore, enhance the drying by accelerating moisture movement within the product to the surface. In the final stage the product is maintained in an environment of 70-80% without any heat or air flow. This treatment prevents surface cracks which otherwise would have occurred due to thermal stress. During the equilibration time of 1 h the product loses about 1% moisture. Examples of products for which such dryers are used are presented in Table 2. A summary of the advantages for using such dryers over conventional systems is also presented. The economics of these dryers is discussed it' another section. Can. Inst. Food Sci. Technol. J. Vo!. 24, No. 3/4, 1991
Microwave vacuum dryers are used in the drying of heat sensitive products or produce. A batch pilot plant unit (MIVAC) is shown in Figure 3. A continuous pilot plant type (GIGIVAC) and the distribution of power in the cavity are shown in Figures 4 and 5, respectively. In both units the equipment consists of a power supply, vacuum pumps, a condenser, wave guides and the drying chamber. The continuous unit has a product collecting chamber. Industrial units of the MIVAC manufactured by Aeroglide Corp. have been used in drying peanuts, yeast, corn and various produce. The GIGIVAC was developed and built by a company presently known as IMI-ZWAG in France. The equipment operates at a frequency of 2450 MHz. Microwave energy is introduced into the drying chamber via primary resonant cavities operating at atmospheric pressures. The microwave beams are polarized at this point and are then transferred through wave guides and windows into the drying chamber. A multiple microwave power source is used. The drying chamber is made up of a stainless steel cylindrical drum and the design ensures that the power distribution is as shown in Figure 5. Such a power distribution allows thermoplastic
Fig. 5. The Power Distribution with Respect to Load in the GIGIVAC Minor.
AT / 105
Table 3. Comparison of average specific energy consumption in some dryers used in the food industry (Adapted from Aigeldinger, 1989).
Dryer type Spray dryer Drum dryer Vacuum band dryer Microwave vacuum dryer Freeze dryer
Consumption/453.6 kg/h Evaporation Steam Electricity (KG)
(KWh)
Investment (%)
771-907 635-771 272-408 136-181 227-318
95 65 36 110 230
100 90 170 140 900
materials to be effectively dried, cooled and removed from the conveyor belt which carries products into the dryer. In the case of liquid materials the dryer operates essentially as foam mat dryer, but the better heat transfer in a foamy environment makes drying comparatively faster in these units. Drying residence times of 40 min are very common with such units. The drying process retains the flavour and nutrients in the product. In most cases the product obtained is comparable in quality to freeze dried products and some microwave dried products show better rehydration properties than freeze dried ones. A summary of the products dried in microwave vacuum dryers and the advantages of using these dryers are presented in Table 2. The economics of the process are discussed in another section.
Microwave Freeze Dryer Due to the inherently slow process of drying materials in conventional freeze dryers attempts have been made to incorporate microwaves into these dryers. Initial attemps to develop microwave freeze dryers were plagued with corona (glow) discharge problems. This is usually a purplish or bluish glow that occurs in the chamber due to excessive field strength resulting from voltage breakdown. Generally glow discharge never occurs at pressures above 50 Torr but since lower pressures are needed to sublimate water in freeze drying this became an associated problem in microwave types. These problems have been solved and there are industrial microwave freeze drying units for coffee, beef slices, mushrooms and various other products. The microwave freeze dryer used by Nestle for drying coffee may be considered as a "booster microwave freeze dryer" because it combines infrared and microwave energy and this seems to improve the production economics.
Economics of Microwave Drying Generally, microwave drying of foods or food ingredients at high moisture content (over 20% moisture) is not comparatively economical. At high moisture contents conventional heating methods more effectively remove water than microwaves. This is because although water has high dielectric constant and would absorb microwaves easily, it also has a very high specific heat. Therefore, considerable amount of microwave energy would be needed to significantly raise the temperature for dehydration if the bulk of water is high. Cost comparison of microwaves with conventional processes can more effectively be done on specific products or process basis. An attempt is made here to draw general cost comparisons. Smith (1988) graphically represented the cost advantages for using microwave-hot air combinations for processing.
106/ AT
Table 4. Processing cost comparison for conventional and microwave freeze drying. (Adapted from Sunderland, 1982). Cost Capital Costs Yearly Depreciation Costs (14%) Yearly Maintenance Cost (1%) Yearly Taxes and Insurance (2 1/2%) Total Yearly Cost of Depreciation, Maintenance, Taxes and Insurance. Cost of Depreciation, Maintenance Taxes and Insurance/kg of frozen food processed. Energy Cost/kg at $0.05/kwh Labor Costs/kg Total Costs/kg of Frozen Processed Cost/kg of Water Removed (assuming frozen food is 75% water)
Conventional US$ 131,000
Microwave US$ 165,400
18,340
23,156
1,310
1,654
3,275
4,135
22,925
28,945
0.146/kg 0.083/kg 0.064/kg 0.291/kg
0.073/kg 0.063/kg 0.064/kg 0.291/kg
0.388/kg
0.265/kg
Essentially, in a plant using conventional dryers, if the profit was $71/h, it was estimated that doubling the thermal capacity of the plant conventionally would increase the profit to $225/h. However, by adding microwave dryers to the plant, the profit increases to $506/h. A comparison of the average specific energy comsumption for steam and electricity for some dryers was made by Aigeldinger (1989). According to these energy consumption data, (Table 3) microwave vacuum drying is less economical than spray drying, drum drying and vacuum belt drying but considerably more economical than freeze drying. It must be noted that the cost advantage over freeze drying was based on energy only. Microwave vacuum dryers normally cost more than freeze dryers. A more exhaustive cost comparison of the continous microwave vacuum dryer (GIGIVAC) and freeze dryer for producing orange juice powder (Meiser, 1978) indicates that it costs about 3.6 times more on a freeze dryer to produce the same quantity of powder in a microwave vacuum dryer. Detailed production cost comparison of a conventional freeze dryer and a microwave freeze dryer has been made by Sunderland (1982). According to this cost comparison, (Table 4), it costs US $0.388 to remove a kg of water in a conventional freeze dryer compared to US $0.265 for a microwave freeze dryer. The comparisons seem to indicate that for specific applications microwave dryers are more economical especially when introduced at the appropriate points in the process.
Some Potential Applications of Microwave Drying In this comptetive world improvements on old processes in the food industry is of paramount importance. One food processing area that could use some improvement is the process for making gelatin. Conventional processing of gelatin requires several operations; concentration, cooling, extrusion, drying, cutting, coarse grinding, fine grinding, seiving and packaging. The concentration is normally done on multiple effect evaporators, but reverse osmosis is being practised. The J. Inst. Can. Sci. Techna/. Aliment. Vol. 24, No. 3/4, 1991
operations used in the conventional process do not only require high capital equipment, but also large infra-structural cost. The dryers alone are normally over 120 meters long, posing infrastructural problems especially if one is considering expansion or increase in production. Using a combination of reverse semosis and a continuous microwave dryer, the processing of gelatin could be reduced to few operations; concentration, microwave drying, fine grinding and packaging. The microwave dryer is compact so batteries of them could be housed in a small area. The final products are not as dense as the extruded material from the conventional process, therefore, coarse grinding is not necessary. The process is relatively simple and very cost effective considering the savings that would be made by eliminating the various operations. Another potential application of microwaves is in the area of drying encapsulated flavors. Spray dried encapsulated flavors tend to have air pocket entrapped with the oils in the capsules. The proximity of oxygen with these oil droplets increases the potential for oxidation. Drying encapsulated flavors in a microwave vacuum dryer tends to solve this problem and more stable flavors are produced. Evaluations are required to determine the role of microwave drying in these potential applications.
Can. Inst. Food Sci. Technol. J. Vo!. 24, No. 3/4. 1991
Referencess Aigeldinger, j.c. 1989. Energy saving continuous vacuum dehydration with contact or microwave heating. Food Marketing and Technology 3 (3):53. Anonymous 1985. Processing with microwaves. Food Engineering Int'l. 10(10): 46. Bosisio, R.G. 1973. Microwave instruments for measuring properties of materials. Trans. of IMPI 1:#9. Meisel, N. 1978. Microwave vacuum drying by the Gigivacprocess for continuous manufacture of instantly soluble fruit powders. IMI Process Bulletin. Ontario Hydro Publication. Microwave Applications in the Food and Beverage Industry. pp. 22-25. Osepchuk, j.M. 1975. Basic principles of microwave ovens. Trans. of IMPI 4:5. Smith, F.j. 1979. Microwave-Hot air drying of pasta, onions and bacon. The Microwave Newsletter. 12 (6):6. Smith, F.j. 1988. Microwave processing is increasing but it needs special knowledge. Research and Development 30 (1):54. Sunderland, j.E. 1982. An economic study of microwave freeze drying. Food Technol. 36(2):50. White j.R. 1973. Why Materials Heat. Trans. IMPI 1:4:40.
AT / 107
pas Pilot
Plant Corp, 118 Veterinary Road Saskatoon, Saskatchewan S7N 2R4
Introduction The idea of microwave heating of foods has been around for over forty years. The vision of Dr. Percy Spencer, which stimulated his 1945 patent, that someday microwave heating would be a new integrated system for preparing foo~ is now a reality. It is estimated that there are over 60 million microwaves in American homes. Although heating is the fundamental process ~n the food industry and microwaves offer relatively better effiCIency and minimum time lag for heating foods, microwaves have not had compc: ble success in the food industry as in .dome.stic and catering food services. The growth of domestic utilization of microwave energy in food preparation has recently catalysed a cooperative working relationship betwee~ food engineers, scientists, technologists and microwave engineers to use the technology in the food industry. Currently some food industry operations such as meat thawing, pasteurization and even thermal sterilization (Anonymous, 1988) with microwaves are being practised to some extent. One area in food processing where microwaves seem to offer tremendous advantages is in food dehydration. In the so-called falling rate period of drying, microwave heating can accelerate the drying due to the more efficient heat transfer. In this article the use of microwaves in various food dehydration applications is reviewed. Pertinent issues such as available equipment, the economics of this technology in food dehydration and potential applications in some food and ingredients drying are presented.
Heating due to non-ionizing radiation depends on the penetrating power of the radiation. Among the non-ionizing radiation maximum penetration power is observed in the microwave region (Osepchuk, 1975). This penetrating feature is conveniently used in the various applications of microwaves in the food industry. It should, however, be noted that microwave penetration into products is determined by factors such as electrical and compositional properties of the material. Microwave is analogous to light in that it can be transmitted and reflected. Assuming we consider microwaves impinging on a hypothetical large surface of a piece of an apple (Figure 1). In general, some amount of the incident wave is reflected. Some of the wave is progressively attenuated and thus decreases in magnitude as it penetrates into the apple. If the power of the incident wave is represented as Pi, the reflected power represented as Pf and the residual power at a distance of d-cm from the surface is represented as Pd, then the attenuated power (pa) can be represented as follows: Pa
=
102/ AT
(1)
The degree of reflection and penetration of microwaves depends on the dielectric properties of the material. The dielectric property is in turn, described by two basic parameters; the relative dielectric constant (E) and the loss tangent (tan 0). The dielectric constant predominantly provides indication on the refection properties and wavelength in the material and is related to the wavelength in the material as follows: A
Characteristics and Theory of Microwaves Microwaves are part of the electromagnetic waves propagating between 300 MHz and 300 GHz. They generally have shorter wavelengths than FM radio or TV signals, but a longer wavelength than visible or infrared light. Being an electromagnetic wave, microwaves have electric and magnetic components, acting perpendicularly to each other. They also show monochromaticity and are highly polarized. The close proximity of microwaves to radio and TV waves posses a potential problem of interference between these waves. Therefore, for industrial, scientific a'nd medical (ISM) applications, specific frequencies known as ISM bands; 915 MHz (896 MHz in some European countries), 2450 MHz, 5800 MHz and 24125 MHz have been authorized for usage. The most popular frequencies used in North America are 915 and 2450 MHz.
Pi - (Pf + Pd)
=
~ -VE
where: A
=
AO =
wavelength in material wavelength in free space
(2)
For food material ,j E is large so that at 2450 MHz the wavelengths are small. If the incident wave is perpendicular to the surface of the material then the reflected power relative to the incident power may be approximately represented as: Pf
(1 - -V E)2
Pi
(1 + -vE)
(3)
Another important dielectric terminology used in microwave heating is the relative dielectric loss factor (E'). This factor measures the amount of energy dissipated when a material is subjected to an alternating current. The relative dielectric J, Inst. Can, Sci. Technol, Aliment, Vol. 24, No. 3/4, 1991
Table 1. Some conversion mechanisms for microwave heating (Adapted from White, 1973). Ionic Conduction Interface Polarization Dipole Rotation Entire Molecule Quantized Twisted Bend Dipole Stretching Ferroelectric Hysterisis Electric Domain Wall Resonance Electrostriction Piezoelectricity Gas Collision Frequency Resonance Multifactor Resonance Cyclotron Resonance Nuclear Magnetic Resonance Paramagnetic Resonance Ferromagnetic Resonance Ferrimagnetic Resonance Antiferromagnetic Resonance Spin Wave Resonance Magnetic Domain Wall Resonance Magnetostriction Piezo Magnetism
Fig. 1. Distribution of Microwave Incident on a Plane Surface of a Sliced Apple.
loss factor (E') is related to the dielectric constant as shown in equation 4. E' ~ E
tan 1>
(4)
Generally, the dielectric properties of a material are dependent on various factors such as temperature, physical structure, frequency, and chemical composition. Materials with high dielectric loss factors are termed lossy materials and these attenuate microwaves. When a dielectric material is introduced into an AC current containing a capacitor, the angular lead of the current over the voltage is reduced from the 90° exhibited for ideal capacitors. This reduction in angular lead of the current over the voltage is know as the loss tangent (tan D). The tan Dprovides indications on the penetration depth (the point at which the incident wave is reduced to approximately 36% of its magnitude) and is related to the half-power penetration depth D by the following equation (Osepchuk, 1975):
increases and consequently the penetration depth decreases. Also, the higher the dielectric constant, the lower the penetration depth. These parameters that seem to dictate the efficiency or even the propensity for microwave attenuation in materials clearly indicate that, depending on factors such as composition, temperature or moisture content of the material, microwave heating in a material could be a surface phenomenon rather than the conventional misconception of internal heating.
Mechanisms of Microwave Heating Food systems generally have high dielectric constants due to water. They also have relatively high loss tangents (0.1-1.0) that make them good materials for microwave attenuation. Microwaves are used in food processing due to the spontaneous heat they generate. Some of the mechanisms by which heat is generated by microwaves are shown in Table 1. For microwave heating in foods however, only ionic conduction and dipolar rotation are of primary interest. In ionic conduction, ionized compounds randomly collide with non-ionized groups when subjected to an electric field. The kinetic energy of these ions are transmitted as heat dur-
POWER SUPPLY
mOB VOLTAGE
Ir-;'f~TRAN~S~MI~SSI~ON~~ MICROWAVES S~ON
APPLICATOR
'-'-"'-;-"",-,....J
D = 0.189 AO
yE
y'-----
(5)
MICROWAVE GENERATOR
..j 1 + tan 2 1> - 1
for small tan D ~ 1, eqn 5 can be simiplified to: D
=
0.269 AO
Y-E
(6)
tan 1>
It can be appreciated from the preceeding equations that as the loss tangent increases the amount of attenuated energy Can. Inst. Food Sci. Technol. J. Vol. 24, No. 3/4, 1991
CONTROLS AND 8AFE1T DEVICES
Adapted trom eosllllo, 1973
Fig. 2. Some Essential Components of Microwave Equipment.
AT / 103
Table 2.
A summary of microwave drying processes in food and food ingredient industry (Adapted from Ontario Hydro Publication).
Dryer type
Product
Advantage over conventional systems
Booster (Microwave-hot air)
Pasta
• Floor space reduced by 2/3 to 4/5: eg, 36 m - 8.2 m for pasta equipment • Cleanup time reduced: eg, 24 h - 6 h (pasta) • Product quality superior: eg, less starch slough-off and better bite (pasta), breaking of surface and surface hardening prevented (pasta), enhanced colour • Drying time reduced: eg, 8 h - 1.5 h (pasta), onethirtieth of original time (egg yolk powder), 8 h - 6 min (dried milk) • Infestation reduced: eg, 90% (onions) • Energy savings in final drying: eg, 30% (onions), by 20-25% (pasta) • Moisture-levelling control of output • Higher yield • Versatility in scheduling • Lower equipment cost
Egg yolk powder Dry milk for babies Onions
Tomato paste Chocolate powder Rice cakes Snack food Seaweed Bacon bits Microwave vacuum
Fruit juice powder
• Continuous system: savings in labour cost, energy, operating costs • Product reconstitutes quickly and easily • Savings in cost/kg product versus freeze-dried or spray-dried product • Superior retention of flavour • Drying times much shorter: eg, 40 min for orange juice powder; from 2 h to 0.5 h for soybeans • Conditioner unnecessary but required in heated air dryers (soybeans) • Safe from fire startup (grains) or dust explosions • Improvement in product quality: ego higher germination rates for seed grains; seeds remain whole • No blowing air to carry dust, clean • Improved efficiency: 48% greater • Better flexibility for production • Better versatility: different crops using same equipment • Quiet
Grains: eg, wheat soybean, rice, rye
Cottonseed Yeast Peanuts Pecans Corn Fruit Tomatoes Peppers Seasonings Protein preparations Meat extracts Plan extracts Instantly soluble vegetable powders Microwave freeze
• Faster: eg, drying time reduced from 12h 6h or less (coffee) • Lower cost than conventional freeze-drying: eg, 47% lower using micro-wave/radiant energy
Coffee Beef Slices Vegetable pieces Fruit Mushrooms Chicken Shrimp, lobster fish slices
• Lower energy cost: eg, 25% lower • More than doubles production • Lower capital and operating costs
ing such collisions. The heating rate due to ionic conduction could be expressed as: p
-p.-
E2 qn p.
(7)
Vp. where:
E Electrical field Pp. Power V p. ~ Volume of material q the electrical charge on each of the ions p. ~ level of mobility of the ions n ~ the number of charges
The conductivity
(u,)
may be expressed as:
conductivities of each ion. In the case of dipolar rotation, the randomly orientated dipolar compounds undergo alignment and disorientation cycles at a rate equal to the frequency of the applied field. This build up and decay of orientation genera,tes kinetic energy which is converted to heat. Dipolar rotation IS dependant on frequency and temperature. For dipole rotational heating the conductivity (uo) and the heating rate could respectively be expressed by the following equations; 00 ~
(8)
104/ AT
For materials containing different types of ions in a specific volume, the total conductivity is the sum of the individual
2
11"
f
E
tan {j
(9)
where: f is the frequency of the field. J. Inst. Can. Sci. Technol. Aliment. Vo!. 24, No. 3/4, 1991
Fig. 3. A Batch Pilot Plant Microwave Vacuum (MIVAC) Drying Unit
Fig. 4. A Continuous Microwave Vacuum Dryer (GIGIVAC Minor).
Microwave Drying Equipment and Application in the Food Industry
Microwave Vacuum Dryers
Foods by the nature of their composition can be heated at a very fast rate by microwaves. It is this fast heating that ultimately makes microwaves attractive for food processing operations and processes. Generally for all industrial microwave applications, the essential components of the equipment include: the microwave generator (magnetron), an applicator and the controls and safety mechanisms (Figure 2). The various modes in which microwaves are used for drying foods and food ingredients in the industry comprise of Booster (microwave-convection) Dryers, Microwave Vacuum Dryers and Microwave Freeze Dryers.
Booster (Hot Air-Microwave) Dryers Booster dryers consist of conventional dryers operating in conjunction with microwaves. A description of types of these dryers for drying pasta and onion has been presented elsewhere (Smith 1979). For the pasta application, the dryer (manufactured by Microdry Corporation) consists of a conventional hot air predryer on top of a microwave-hot air dryer. The final stage is an equilibration stage. The microwave system consists of 30 kW units propagating in a 6 m long multimode cavity with a 2 m wide conveyer. The product enters the predryer at a nominal moisture of 30% and is reduced to about 18% in 35 min. It then enters the microwave-hot air stage and the moisture content is dropped from 18% to about 13% in about 12 min. The introduction of the microwave at the moisture content of 18% is strategic. At this point microwave penetration depth is not seriously affected by the moisture content. Secondly, the drying seem to be approaching the falling range where conventional systems perform poorly. The microwaves, therefore, enhance the drying by accelerating moisture movement within the product to the surface. In the final stage the product is maintained in an environment of 70-80% without any heat or air flow. This treatment prevents surface cracks which otherwise would have occurred due to thermal stress. During the equilibration time of 1 h the product loses about 1% moisture. Examples of products for which such dryers are used are presented in Table 2. A summary of the advantages for using such dryers over conventional systems is also presented. The economics of these dryers is discussed it' another section. Can. Inst. Food Sci. Technol. J. Vo!. 24, No. 3/4, 1991
Microwave vacuum dryers are used in the drying of heat sensitive products or produce. A batch pilot plant unit (MIVAC) is shown in Figure 3. A continuous pilot plant type (GIGIVAC) and the distribution of power in the cavity are shown in Figures 4 and 5, respectively. In both units the equipment consists of a power supply, vacuum pumps, a condenser, wave guides and the drying chamber. The continuous unit has a product collecting chamber. Industrial units of the MIVAC manufactured by Aeroglide Corp. have been used in drying peanuts, yeast, corn and various produce. The GIGIVAC was developed and built by a company presently known as IMI-ZWAG in France. The equipment operates at a frequency of 2450 MHz. Microwave energy is introduced into the drying chamber via primary resonant cavities operating at atmospheric pressures. The microwave beams are polarized at this point and are then transferred through wave guides and windows into the drying chamber. A multiple microwave power source is used. The drying chamber is made up of a stainless steel cylindrical drum and the design ensures that the power distribution is as shown in Figure 5. Such a power distribution allows thermoplastic
Fig. 5. The Power Distribution with Respect to Load in the GIGIVAC Minor.
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Table 3. Comparison of average specific energy consumption in some dryers used in the food industry (Adapted from Aigeldinger, 1989).
Dryer type Spray dryer Drum dryer Vacuum band dryer Microwave vacuum dryer Freeze dryer
Consumption/453.6 kg/h Evaporation Steam Electricity (KG)
(KWh)
Investment (%)
771-907 635-771 272-408 136-181 227-318
95 65 36 110 230
100 90 170 140 900
materials to be effectively dried, cooled and removed from the conveyor belt which carries products into the dryer. In the case of liquid materials the dryer operates essentially as foam mat dryer, but the better heat transfer in a foamy environment makes drying comparatively faster in these units. Drying residence times of 40 min are very common with such units. The drying process retains the flavour and nutrients in the product. In most cases the product obtained is comparable in quality to freeze dried products and some microwave dried products show better rehydration properties than freeze dried ones. A summary of the products dried in microwave vacuum dryers and the advantages of using these dryers are presented in Table 2. The economics of the process are discussed in another section.
Microwave Freeze Dryer Due to the inherently slow process of drying materials in conventional freeze dryers attempts have been made to incorporate microwaves into these dryers. Initial attemps to develop microwave freeze dryers were plagued with corona (glow) discharge problems. This is usually a purplish or bluish glow that occurs in the chamber due to excessive field strength resulting from voltage breakdown. Generally glow discharge never occurs at pressures above 50 Torr but since lower pressures are needed to sublimate water in freeze drying this became an associated problem in microwave types. These problems have been solved and there are industrial microwave freeze drying units for coffee, beef slices, mushrooms and various other products. The microwave freeze dryer used by Nestle for drying coffee may be considered as a "booster microwave freeze dryer" because it combines infrared and microwave energy and this seems to improve the production economics.
Economics of Microwave Drying Generally, microwave drying of foods or food ingredients at high moisture content (over 20% moisture) is not comparatively economical. At high moisture contents conventional heating methods more effectively remove water than microwaves. This is because although water has high dielectric constant and would absorb microwaves easily, it also has a very high specific heat. Therefore, considerable amount of microwave energy would be needed to significantly raise the temperature for dehydration if the bulk of water is high. Cost comparison of microwaves with conventional processes can more effectively be done on specific products or process basis. An attempt is made here to draw general cost comparisons. Smith (1988) graphically represented the cost advantages for using microwave-hot air combinations for processing.
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Table 4. Processing cost comparison for conventional and microwave freeze drying. (Adapted from Sunderland, 1982). Cost Capital Costs Yearly Depreciation Costs (14%) Yearly Maintenance Cost (1%) Yearly Taxes and Insurance (2 1/2%) Total Yearly Cost of Depreciation, Maintenance, Taxes and Insurance. Cost of Depreciation, Maintenance Taxes and Insurance/kg of frozen food processed. Energy Cost/kg at $0.05/kwh Labor Costs/kg Total Costs/kg of Frozen Processed Cost/kg of Water Removed (assuming frozen food is 75% water)
Conventional US$ 131,000
Microwave US$ 165,400
18,340
23,156
1,310
1,654
3,275
4,135
22,925
28,945
0.146/kg 0.083/kg 0.064/kg 0.291/kg
0.073/kg 0.063/kg 0.064/kg 0.291/kg
0.388/kg
0.265/kg
Essentially, in a plant using conventional dryers, if the profit was $71/h, it was estimated that doubling the thermal capacity of the plant conventionally would increase the profit to $225/h. However, by adding microwave dryers to the plant, the profit increases to $506/h. A comparison of the average specific energy comsumption for steam and electricity for some dryers was made by Aigeldinger (1989). According to these energy consumption data, (Table 3) microwave vacuum drying is less economical than spray drying, drum drying and vacuum belt drying but considerably more economical than freeze drying. It must be noted that the cost advantage over freeze drying was based on energy only. Microwave vacuum dryers normally cost more than freeze dryers. A more exhaustive cost comparison of the continous microwave vacuum dryer (GIGIVAC) and freeze dryer for producing orange juice powder (Meiser, 1978) indicates that it costs about 3.6 times more on a freeze dryer to produce the same quantity of powder in a microwave vacuum dryer. Detailed production cost comparison of a conventional freeze dryer and a microwave freeze dryer has been made by Sunderland (1982). According to this cost comparison, (Table 4), it costs US $0.388 to remove a kg of water in a conventional freeze dryer compared to US $0.265 for a microwave freeze dryer. The comparisons seem to indicate that for specific applications microwave dryers are more economical especially when introduced at the appropriate points in the process.
Some Potential Applications of Microwave Drying In this comptetive world improvements on old processes in the food industry is of paramount importance. One food processing area that could use some improvement is the process for making gelatin. Conventional processing of gelatin requires several operations; concentration, cooling, extrusion, drying, cutting, coarse grinding, fine grinding, seiving and packaging. The concentration is normally done on multiple effect evaporators, but reverse osmosis is being practised. The J. Inst. Can. Sci. Techna/. Aliment. Vol. 24, No. 3/4, 1991
operations used in the conventional process do not only require high capital equipment, but also large infra-structural cost. The dryers alone are normally over 120 meters long, posing infrastructural problems especially if one is considering expansion or increase in production. Using a combination of reverse semosis and a continuous microwave dryer, the processing of gelatin could be reduced to few operations; concentration, microwave drying, fine grinding and packaging. The microwave dryer is compact so batteries of them could be housed in a small area. The final products are not as dense as the extruded material from the conventional process, therefore, coarse grinding is not necessary. The process is relatively simple and very cost effective considering the savings that would be made by eliminating the various operations. Another potential application of microwaves is in the area of drying encapsulated flavors. Spray dried encapsulated flavors tend to have air pocket entrapped with the oils in the capsules. The proximity of oxygen with these oil droplets increases the potential for oxidation. Drying encapsulated flavors in a microwave vacuum dryer tends to solve this problem and more stable flavors are produced. Evaluations are required to determine the role of microwave drying in these potential applications.
Can. Inst. Food Sci. Technol. J. Vo!. 24, No. 3/4. 1991
Referencess Aigeldinger, j.c. 1989. Energy saving continuous vacuum dehydration with contact or microwave heating. Food Marketing and Technology 3 (3):53. Anonymous 1985. Processing with microwaves. Food Engineering Int'l. 10(10): 46. Bosisio, R.G. 1973. Microwave instruments for measuring properties of materials. Trans. of IMPI 1:#9. Meisel, N. 1978. Microwave vacuum drying by the Gigivacprocess for continuous manufacture of instantly soluble fruit powders. IMI Process Bulletin. Ontario Hydro Publication. Microwave Applications in the Food and Beverage Industry. pp. 22-25. Osepchuk, j.M. 1975. Basic principles of microwave ovens. Trans. of IMPI 4:5. Smith, F.j. 1979. Microwave-Hot air drying of pasta, onions and bacon. The Microwave Newsletter. 12 (6):6. Smith, F.j. 1988. Microwave processing is increasing but it needs special knowledge. Research and Development 30 (1):54. Sunderland, j.E. 1982. An economic study of microwave freeze drying. Food Technol. 36(2):50. White j.R. 1973. Why Materials Heat. Trans. IMPI 1:4:40.
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