III.D

Heat processing by direct and radiated energy

Dielectric

and infrared (IR or radiant) energy are two forms of electromagnetic energy

(Fig. 20.1) (see also high-intensity light/UV light (Chapter 9, section 9.4)). Each has a

much lower energy than gamma rays or X-rays (Chapter 7 and Chapter 9, section 9.5) and

is a form of non-ionising radiation that only produces thermal effects in foods (see section

20.1.4). Electromagnetic waves penetrate food and are then absorbed and converted to

heat. In contrast, ohmic (or resistance) heating uses the electrical resistance of foods to

directly convert electricity to heat (see also pulsed electric fields (Chapter 9, section 9.1)).

Dielectric and ohmic heating are direct methods in which heat is generated within the

product, whereas infrared heating is an indirect method in which energy is applied to the

surface of a food by radiation and then converted to heat. Radiated infrared energy is a

component of heat produced by conventional heaters, especially in baking ovens (Chapter

18, section 18.1), but this chapter describes the generation and use of infrared energy as a

main means of heating.

Dielectric and ohmic heating are used to preserve foods, whereas infrared radiation is

mainly used to alter the eating qualities of foods by changing the surface colour, flavour

and aroma. The main applications of these methods are shown in Table 20.1. The

advantages of dielectric and ohmic heating over conventional heating (Chapter 10) can be

summarised as:

Dielectric, ohmic and infrared heating

Abstract: This chapter describes the uses of dielectric and infrared electromagnetic

waves that penetrate food and are absorbed and converted to heat. They are compared

with ohmic heating, which uses the electrical resistance of foods to directly convert

electricity to heat. Each section explains the theory of heating and the equipment used.

The applications of dielectric heating to baking, dehydration, tempering and thawing

of foods, and the use of ohmic heating for aseptic processing are described. Finally the

effects of each type of heating on both foods and micro-organisms are discussed.

Key words: electromagnetic waves, dielectric heating, dielectric loss factor, radio

frequency heating, microwaves, magnetron, dehydration, baking, tempering and

thawing, ohmic heating, UHT processing, infrared heating.

1. There are differences in the terminology used to describe dielectric energy and in this chapter the term

`dielectric' is used to represent both radio frequency (RF) and microwave heating.

· rapid heating throughout the food without localised overheating or hot surfaces, which

results in minimum heat damage and no surface browning;

· heat transfer is not limited by boundary films and energy conversion efficiencies are

high;

· equipment is small, compact and suited to automatic control;

· there is no contamination of foods by products of combustion.

20.1 Dielectric heating

20.1.1 Theory

Microwave and RF energy are transmitted as electromagnetic waves and the depth to

which these penetrate foods is determined by both their frequency and the characteristics

of the food. Microwave energy has a range of frequencies from 300 MHz to 300 GHz

whereas RF energy has lower frequencies, from 1 to 200 MHz. However, because these

frequencies are also used for communications and navigation, an international agreement

has allocated the following bands for industrial, scientific and medical use:
· Microwaves: 915 MHz (range 902±928 MHz) and 2450 MHz (range 2400±2500 MHz).

· Radio frequency: 13.560 MHz (range 13.553±13.567 MHz), 27.120 MHz (range

26.957±27.283 MHz) and the seldom used 40.68 MHz (range 40.66±40.70 MHz).

The relationship between wavelength, frequency and velocity of electromagnetic waves

is shown in Equation 20.1:

Fig. 20.1 Electromagnetic spectrum (from Keiner 2007).

Table 20.1 Applications of dielectric, ohmic and infrared heating
Method of heating

Applications

Microwave

Cooking, thawing, melting, finish-drying, freeze drying, tempering,

pasteurisation, sterilisation, rendering, frying, blanching

Radio frequency

Drying, baking

Ohmic

UHT sterilisation, pasteurisation

Infrared

Drying, baking, frying, thawing, freeze drying, cooking, surface

pasteurisation

582 Food processing technology

v=f

20:1

where (m) wavelength, v (ms

ÿ1

) velocity and f

(Hz) frequency.

The velocity of electromagnetic waves is 3 10

m s

ÿ1

(Singh and Heldman 2001)

and using Equation 20.1, the calculated wavelength of microwaves is 0.328 m at

915 MHz and 0.122 m at 2450 MHz. Lower-frequency (and longer wavelength (Fig.

20.1)) waves have greater penetration depths. The energy in electromagnetic waves can

be considered to be in the form of photons that are discrete, very small quantities of

energy. When photons strike a target material, they are either absorbed or they pass

through the material. Different atoms in a material have electrons with different allowed

atomic energy states. For heating to take place, the energy in the photons must exactly

match the energy difference between these atomic energy states. If they are different, the

material is transparent to the electromagnetic wave (Ehlermann 2002). This is why, for

example, water in foods is heated but plastic and glass are not. The amount of energy

absorbed by foods from electromagnetic waves depends on a characteristic known as the

`dielectric loss factor' (

), a dimensionless number, which relates to the ability of the

food to dissipate electrical energy. The higher the loss factor, the more energy is absorbed

by the food (Table 20.2). The loss factor depends on the moisture content of the food, its

temperature, the presence of salts, and in some foods the structure of the food.

Water has a negatively charged oxygen atom separated from two positively charged

hydrogen atoms, which form an electric dipole (Chapter 1, section 1.1.1). When

alternating microwave or RF energy is applied to a food, dipoles in the water and other

polar components reorient themselves to the direction (or polarity) of the electric field in

a similar way to a compass in a magnetic field. Since the polarity rapidly alternates from

positive to negative and back again several million times per second (e.g. at the

Table 20.2 Dielectric properties of foods and packaging materials using microwaves at

2450 MHz (materials, except ice, at 20±25 ëC).
Material

Dielectric constant

Loss factor

Penetration depth

(

)

(

)

(cm)

Foods

Apple

63.4

Banana (raw)

0.93

Beef (raw)

0.87

Bread

0.005

1170

Brine (5%)

0.25

Butter

0.1

30.5

Carrot (cooked)

0.93

Cooking oil

2.6

0.2

19.5

Fish (cooked)

46.5

1.1

Ham

0.3

Ice

3.2

0.003

1162

Potato (raw)

16.7

0.93

Strawberry

75.1

36.7

Water (distilled)

9.2

1.7

Packaging materials

Glass

0.1

Paper

0.1

Polyester tray

0.02

195

Polystyrene

2.35

0.001

Adapted from Piyasena et al. (2003), Mudget (1982), Buffler (1993) and Mohsenin (1984)

Dielectric, ohmic and infrared heating 583

microwave frequency of 2450 MHz, the polarity changes 2.45 10

cycles s

ÿ1

), the

dipoles rotate to align with the rapidly changing polarity. The microwaves give up their

energy and the molecular movement creates frictional heat that increases the temperature

of water molecules. They in turn heat the surrounding components of the food by

conduction and/or convection.

The amount of heat absorbed by a food, the rate of heating and the location of `cold

spots' (points of slowest heating) depend on the food composition (e.g. moisture content,

ionic strength, density and specific heat), the shape and size of the food, the microwave

frequency used and the applicator design. The time that food is heated is also important

because its microwave absorption properties and the location of cold spots can change

with time.

Water in foods also has a degree of electrical conductivity due to dissolved salts that

form electrically charged ions. The charged ions move at an accelerated rate when an

electric field is applied (known as `ionic polarisation') to produce an electric current in

the food. Collisions between the ions convert kinetic energy to heat, and more

concentrated solutions (which have more collisions) therefore heat more quickly. At RF

frequencies, the conductivity of foods and hence the amount of energy absorbed increase

at higher temperatures, but at microwave frequencies the loss factor decreases at higher

temperatures and so reduces the amount of energy absorbed.

The other important electrical properties of the food, in addition to the loss factor, are

(a) the dielectric constant (

), a dimensionless number that relates to the rate at which

energy penetrates a food ± in practice most foods are able to absorb a large proportion of

electromagnetic energy and heat rapidly; and (b) the loss tangent (tan ), which gives an

indication of how easily the food can be penetrated by electromagnetic waves and the

extent to which it converts the electrical energy to heat. These terms are related using

Equation 20.2:

tan

20:2

The dielectric constant and the loss tangent are properties of the food and they influence

the amount of energy that is absorbed by the food as shown in Equation 20.3:

P 55:61 10

ÿ14

f E

20:3

where P (W cm

ÿ3

) power absorbed per unit volume, f (Hz) frequency and E

(V cm

ÿ1

) electrical field strength. There is therefore a direct relationship between the

properties of the food and the energy provided by the dielectric heater. Increasing the

electrical field strength has a substantial effect on the power absorbed by the food

because the relationship involves a square term.

The depth of penetration of electromagnetic waves is found from the loss factor and

the frequency of the waves:

tan

20:4

where x (m) the depth of penetration.

The electrical properties of the food also determine how energy is distributed through

the food, as represented by the attenuation factor (

) in Equation 20.5:

tan

ÿ 1

20:5

584 Food processing technology

where

ÿ1

) attenuation factor. Examples of the dielectric constants, loss factors

and penetration depths in selected foods are given in Table 20.2 and further details of heat

and mass transfer in microwave heating are given by Datta (2001) and Piyasena et al.

(2003).

It can be seen in Table 20.2 that electromagnetic waves penetrate foods that contain

small amounts of water to a much greater depth than moist foods, and that glass, paper

and plastic packaging materials have a low loss factor and are almost transparent to

microwaves. Microwave penetration increases dramatically when water changes phase to

ice (Fig. 20.2), because the molecules are less free to move or absorb energy from the

alternating electric field. Ice therefore has a lower loss factor than water and this has

important implications for dielectric thawing and tempering applications (section 20.1.3).

Continuous metal sheets reflect microwaves and very little energy is absorbed, but small

pieces of metal (e.g. wires) and metallised plastic (Chapter 25, sections 25.2.4 and 25.4.3)

absorb electromagnetic waves and heat very quickly.

When RF and microwave energy heat water in foods, it increases the vapour pressure

and causes movement of moisture from the interior to the surface and rapid evaporation

from the surface, therefore making this technology particularly suitable for dehydration

as well as heating (section 20.1.3; see also Chapter 16, section 16.1).

In contrast to conventional heating, where the maximum food temperature is that of

the heating medium and it is possible to predict the time±temperature history at the

slowest-heating point in a food (Chapter 10, section 10.2), this is less straightforward in

microwave heating. As food heats, microwave absorption increases, which increases the

rate of temperature increase and so further increases the rate of microwave absorption.

This `coupling' continuously generates heat to increase the food temperature and could

lead to runaway heating. Microwave equipment therefore needs to be turned on and off

(cycled) to keep the temperature within prescribed limits once the target temperature

has been reached. Also because microwave absorption is lower at lower temperatures,

the waves are able to penetrate further into the food. As it heats, the depth of

penetration falls and at higher temperatures, the surface can shield the interior from

further heating.

Since heat is generated throughout the food at different rates, the temperature

difference between the coldest and hottest points in the food increases with time. This is

in contrast to conventional heating, where the coldest point slowly approaches the surface

temperature, corresponding to the temperature of the heating medium. Because of their

widespread domestic use, there is a popular notion that microwaves `heat from the inside

out'. In fact, the food is heated while the surrounding cold air keeps the surface tem-

perature below that of locations within the food. Surface evaporation from unpackaged

Fig. 20.2 Variation in dielectric loss factor of water and ice (after Lewis 1990).

Dielectric, ohmic and infrared heating 585

LIVE GRAPH

Click here to view

food can further decrease the surface temperature. In some heating applications, such as

microwave-heated frozen foods, the surface could be the coldest location.

The shape, volume and surface area of foods can affect the amount and spatial pattern

of absorbed microwave energy, leading to overheating at corners and edges and focusing

of the energy. For example, a curved shape can focus microwaves and produce a higher

internal rate of heating than near the surface.

The moisture and salt contents of a food have a greater influence on microwave

processing than in conventional heat processing owing to their influence on the dielectric

properties of the food (high salt and moisture contents increase the efficiency of

microwave absorption and decrease the depth of penetration). Therefore, the interior part

of foods that have high salt or moisture contents is heated less, thus reducing microbial

destruction. The composition can also affect thermal properties such as specific heat and

thermal conductivity, and change the size and uniformity of temperature increases (e.g.

oil that has a low specific heat heats faster than water at the same level of absorbed

power). In multi-component frozen dinners, different foods heat at different rates.

Owing to the complexity of the system where the heating pattern depends on a large

number of factors (Table 20.3), sophisticated mathematical modelling and computer

simulations are used to predict the location of cold spots and the time±temperature history

at these locations to develop microbiologically safe processes for specific food and

equipment combinations. In calculating process times, the short come-up time in

microwave heating is not given the importance that is given in conventional heating

(Chapter 13, section 13.1). Software to simulate electromagnetic and heat transfer

properties has been described by Dibben (2000) and Zhang and Datta (2000) and

mathematical models are described by, for example, CampanÄone and Zaritzky (2005).

20.1.2 Equipment

Microwave heaters

The components of microwave equipment (Fig. 20.3) are a microwave generator (termed

a `magnetron'), aluminium tubes named waveguides, a stirrer (a rotating fan or

`distributor') and a metal chamber for batch operation, or a tunnel fitted with a conveyor

belt for continuous operation. Detailed descriptions of component parts and operation of

microwave heaters are given by Buffler (1993) and a number of commercial suppliers

(e.g. Anon 2007a).

Table 20.3 Summary of process factors in microwave heating
Factor

Examples

Food

Shape, size, composition (e.g. moisture, salt), multiple components

(e.g. frozen meals), liquid/solid proportion

Package

Transparency to microwaves, presence of metals (e.g. aluminium

foil)

Process

Power level, cycling, presence of hot water or air around the food,

equilibration time

Equipment

Dimensions, shape and other electromagnetic characteristics of the

oven, wave frequency, agitation of the food, movement of the food

by conveyors and turntables, use of stirrers

Adapted from Anon (2000a)

586 Food processing technology

The magnetron is a cylindrical diode (`di' meaning two and `ode' short for `electrode'),

which consists of a sealed copper tube with a vacuum inside. The tube contains copper

plates pointing towards the centre like spokes on a wheel. This assembly is termed the

`anode' and has a spiral wire filament (the cathode) at the centre. When a high voltage

(4000 V) is applied, the cathode produces free electrons, which give up their energy to

produce rapidly oscillating microwaves, which are then directed to the waveguide by

electromagnets. The waveguide reflects the electric field internally and thus transfers it to

the heating chamber. It is important that the electric field is evenly distributed inside the

heating chamber to enable uniform heating of the food. In batch equipment a stirrer is used

to distribute the energy evenly throughout the heating chamber, and/or the food may be

rotated on a turntable. Both methods reduce shadowing (areas of food which are not

exposed to the microwaves). It is important that the power output from the magnetron is

matched to the size of the heating chamber to prevent flash-over (unintended electrical

discharge around an insulator, or arcing or sparking between adjacent conductors).

In continuous tunnels (Fig 20.4) a different design of distributor is used to direct a

beam of energy over the trays or pouches containing the food as they pass on a conveyor.

The trays are precisely positioned in the tunnel and power levels of multiple microwave

generators are programmed to provide a custom-heating profile for that tray and product.

In an alternative design, known as a `leaky waveguide' applicator, slots are cut in the

waveguide to allow the controlled leakage of microwaves to give a uniform power

distribution over product widths of up to 3 m. In the `slotted waveguide' applicator, food

passes through a slot running down the centre of the waveguide (Brennan 2006).

Automatic control consists of monitors for insufficient power level delivered by a

Fig. 20.3 A microwave oven showing the magnetron (from Buffler 1993).

Dielectric, ohmic and infrared heating 587

generator, infrared surface temperature measurement of each tray as they are being

transported to the holding area, and monitoring of swelling of the top surface of

individual packages. This is due to internal steam generation during heating, and is

monitored to ensure that adequate heat has been received to produce enough steam for the

package to swell to a predetermined amount. Power settings for individual magnetrons

are stored and an alarm warns the operator if the power varies from the set values.

Temperature monitoring of microwave processing using thermocouples is difficult

because probes reflect and absorb microwaves, and cause electromagnetic field disturb-

ances that change the heating patterns. Instead, fibre-optic temperature probes are used,

which are transparent to electric and magnetic fields, accurate and have a fast response

time (Anon 2000a).

Because microwaves heat all biological tissues, there is a risk of leaking radiation

causing injury to operators. Within limits, the body can absorb microwave energy and the

blood flow removes heat to compensate for the temperature increase. However, damage

to the eyes is possible at an energy density of >150 mW cm

ÿ2

because they have

insufficient blood flow to provide adequate cooling. The permissible energy density at the

surface of microwave equipment is set at a maximum of 10 mW cm

ÿ2

in Europe and the

USA (Ehlermann 2002). Chambers and tunnels are sealed to prevent the escape of

microwaves, interlocked doors cut the power supply when opened to prevent accidental

leakage, and in continuous equipment there are energy trapping devices at the conveyor

entry and exit points.

Microwave heaters are very efficient in energy use because moist foods absorb most of

the microwave energy, and metals reflect microwaves so that neither the metal of the

chamber nor the air is heated. Power outputs of continuous industrial equipment range

from 500 W to 15 kW in the 2450 MHz band and 25±120 kW in the 915 MHz band.

Radio frequency heaters

There are several designs of RF applicators (Fig. 20.5):
· The `through-field' design is the simplest and consists of two electrodes at different

voltages that form a parallel plate capacitor, supplied by a high-voltage generator (Fig.

20.5a). Food is placed or conveyed between the plates, and this design is used for

Fig. 20.4 Continuous microwave finish drying equipment (after Decareau 1985).

588 Food processing technology

relatively thick pieces of food (e.g. in drying chambers or RF units at the end of bakery

tunnel ovens). Because the food is an electrical component of the heater, variations in

the amount of food passing between the plates, its temperature and moisture content,

each cause a variation in the power output of the generator. This is a valuable self-

controlling feature: for example, the loss factor of a food falls as the moisture content

is reduced and the power output correspondingly falls, so reducing the possibility of

burning the food.

· In the `fringe-field (or `stray-field') design (Fig. 20.5b), a thin layer of material passes

over bars, rods or plates that are connected to either side of the voltage generator and

Fig. 20.5 Designs of conventional radio frequency applicators: (a) through-field applicator; (b)

fringe-field applicator; (c) staggered through-field applicator (adapted from Jones and Rowley

1997).

Dielectric, ohmic and infrared heating 589

have alternating polarity. The product makes complete contact with the electrodes

which ensures that there is a constant electric field in the product between the bars.

· The `staggered through-field' design (Fig. 20.5c) has bars arranged above and below

the product, and is used for foods of intermediate thickness (e.g. biscuits) (Jones and

Rowley 1997).
There are two methods of producing and transmitting power to RF applicators: (1)

conventional RF equipment in which the applicator is part of the RF generation circuit

and may be used to control the amount of power supplied by the generator ± the position

of the RF applicator plates is adjusted to keep the power within set limits; and (2) 50 ohm

() technology, where the RF generator is separated from the applicator and connected

using a high-power coaxial cable. The frequency of the generator is set at 13.56 or

27.12 MHz and an impedance of 50 . An impedance-matching network transforms the

impedance of the RF applicator to 50 and is used to adjust the power supplied. The RF

applicator can then be designed for optimum performance as it is not itself part of any

tuning system. International Electromagnetic Compatibility Regulations (Anon 2005)

limit the electromagnetic disturbance that can be emitted by dielectric equipment, and the

fixed operating frequency of 50 technology makes it easier to control the frequency to

meet the regulations. The use of a matching network also enables advanced process

control to give on-line information on the condition of the food (e.g. average moisture

content), which can be used to control the RF power, conveyor speed and temperature of

air in the applicator. Further information is given by Anon (2007b) and Jones and Rowley

(1997).

A simple method to calculate the amount of radio frequency energy needed for a

particular process is described by Anon (1999):

863

20:6

where E (kW) energy supplied, m (kg h

ÿ1

), mass flow rate of product,

(ëC) final

product temperature,

(ëC) initial product temperature, c

(kJ

ÿ1

) specific

heat.

There are a number of additions to the calculated amount of energy required:

· 1 kW is added for each 1.4 kg of water to be evaporated per hour in drying

applications.

· An additional 10±20% of energy required is added to account for surface cooling,

depending on the surface area : volume ratio of the product.

· It is assumed that the equipment is 65% efficient in the use of energy supplied, and an

additional correction is needed to calculate the actual power requirement.

20.1.3 Applications

The high rates of heating and absence of surface changes have led to studies of dielectric

heating of a large number of foods. The most important industrial applications are baking,

dehydration, tempering and thawing. Other applications, which involve bulk heating of

foods with higher moisture contents (e.g. blanching), are less successful. This is due to

the low depth of penetration in large pieces of food and to evaporative cooling at the

surface, which results in survival of micro-organisms. Microwave pasteurisation and

sterilisation are now used commercially for the production of ready meals. These

applications are discussed briefly in this section and details are given by Schubert and

590 Food processing technology

Regier (2005), Piyasena et al. (2003), Datta and Anantheswaran (2001), Zhao et al.

(2000) and Rosenberg and Bogl (1987).

Baking

Conventional ovens operate effectively when products have relatively high moisture

contents, but the thermal conductivity falls as baking proceeds and considerable time is

needed to bake the centre of the product adequately without causing excessive changes to

the surface colour. RF or microwave heaters are located at the exit to tunnel ovens

(Chapter 18, section 18.2.2) to reduce the moisture content and to complete baking

without further changes in colour. This reduces baking times by 30±50% and hence

increases the throughput of the ovens. RF or microwave finishing (removing the final

moisture) improves baking efficiency for thin products such as breakfast cereals,

babyfoods, biscuits, crackers, bread sheets to be made into breadcrumbs, crispbread and

sponge cake. Meat pies, which require a good crust colour in addition to pasteurisation of

the filling, can be baked in about one-third of the time required in conventional ovens by

combined RF and conventional baking (Jones 1987). Other advantages include: savings

in energy, space and labour costs; close control of final moisture contents (typically

2%) and automatic equalisation of moisture contents as only moist areas are heated;

separate baking and drying stages allow control over the internal and external product

colour and moisture content; and improved product texture and elimination of `centre

bone' (a fault caused by dense dough in the centre of biscuits). The use of dielectric

heating alone is less successful for baking. It causes undesirable qualities in bread, owing

to the altered heat and mass transfer patterns and the shorter baking times. These produce

insufficient starch gelatinisation, microwave-induced changes to gluten and too-rapid gas

and steam production. As a result, microwave-baked breads have no crust and have a

tougher, coarser, but less firm texture (Yin and Walker 1995). However, more recently

crust-less bread has been produced (i.e. without having to remove the crust), which gives

savings of 35% in raw materials. The RF technology also permits automatic control of

moisture levels, to produce bread that has lower moisture content (<38%) which increases

the shelf-life and reduces evaporation of volatile flavourings. The technology permits

significant space savings, and can increase production by 40% because of shorter baking

times compared with conventional ovens.

Dehydration

The main disadvantages of hot-air drying are the low rates of heat transfer, caused by the

low thermal conductivity of dry foods, and damage to sensory characteristics and

nutritional properties caused by long drying times and overheating at the surface (Chapter

16, section 16.1.1). Microwave and RF drying overcome the barrier to heat transfer

caused by the low thermal conductivity, by selectively heating moist areas while leaving

dry areas unaffected. This improves moisture transfer during the later stages of drying by

heating internal moisture and thus increasing the vapour pressure and the rate of drying.

For example, the loss factor for free water is higher than that for bound water, and both

are higher than the dry matter components. Dielectric heating reduces product shrinkage

during the falling rate period, prevents damage to the food surface, and eliminates case

hardening. Tohi et al. (2002) found a correlation between the capacitance of foods and

moisture content, which enables automatic control of drying conditions without sampling

the material during the process. Other advantages include energy savings by not having to

heat large volumes of air, and minimal oxidation by atmospheric oxygen. However, the

use of microwave drying by itself has limitations: the inherent non-uniformity of the

Dielectric, ohmic and infrared heating 591

microwave electromagnetic field and limited penetration of the microwaves into bulk

products compared with RF energy, lead to uneven heating; also microwaves and RF

units have higher cost and smaller scales of operation compared with traditional drying

methods. Non-uniform field strength can be partly overcome by keeping the food in

constant motion to avoid hot-spots (e.g. using a spouted or fluidised bed dryer (Chapter

16, section 16.2.1)) or using pulsed microwaves. However, these factors restrict

microwave drying to either finishing of partly dried or low-moisture foods, or their use in

`hybrid' dryers in which microwaves are used to increase the rate of drying in

conventional hot-air dryers (Vega-Mercado et al. 2001, Garcia and Bueno 1998) (Fig.

20.4). For example, in pasta drying the fresh pasta is pre-dried in hot air to 18% moisture

and then in a combined hot-air and microwave dryer to lower the moisture content to

13%. Drying times are reduced from 8 h to 90 min with a reduction in energy

consumption of 20±25%, bacterial counts are 15 times lower, there is no case hardening,

the drying tunnel is reduced from 36±48 m to 8 m, and clean-up time is reduced from 24

to 6 person-hours (Decareau 1990). In grain finish drying, microwaves are cheaper and

more energy efficient than conventional methods and do not cause dust pollution. The

lower drying temperature also improves grain germination rates. Zhang et al. (2006) have

reviewed the advantages and limitations of microwave drying of fruits and vegetables.

Combined microwave±vacuum drying has been used for heat-sensitive products that

are difficult to dry using hot air (e.g. fruits that have high sugar contents) but it has high

costs owing to the need to maintain the vacuum over long drying periods (Gunasekaran

1999). In conventional freeze drying (Chapter 23, section 23.1) the low rate of heat

transfer to the sublimation front limits the rate of drying. Microwave freeze drying

overcomes this problem because heat is supplied directly to the ice front, which can

reduce the drying time by 50±75% compared to conventional freeze drying (Cohen and

Yang 1995). However, careful control over drying conditions is necessary to prevent

localised melting of the ice. Because of the difference in loss factors of ice and water

(Table 20.2), any water produced by melting ice heats rapidly and causes a chain reaction

leading to widespread melting and an end to sublimation. Accelerated freeze drying using

microwaves has been extensively investigated but the process remains expensive and is

not widely used commercially. It is reviewed by Zhang et al. (2006) and further details

are given in Chapter 23 (section 23.1.2).

Thawing, melting and tempering

During conventional thawing of frozen foods (Chapter 22, section 22.2.5), the lower

thermal conductivity of water, compared with ice, reduces the rate of heat transfer and

thawing slows as the outer layer of water increases in thickness. Microwaves and RF

energy are used to rapidly thaw small portions of food and for melting fats (e.g. butter,

chocolate and fondant cream) (Jones 1987). However, difficulties arise with larger (e.g.

25 kg) frozen blocks, such as egg, meat, fish and fruit juice, that are used in industrial

processes. Because water heats rapidly once the ice melts, thawing does not take place

uniformly in the large blocks, and some portions of the food may cook while others

remain frozen. This is overcome to some extent by reducing the power and extending the

thawing period, or by using pulsed microwaves to allow time for temperature

equilibration.

A more common application is `tempering' frozen foods, in which the temperature is

raised from around ÿ20 ëC to ÿ3 ëC and the food remains firm but is no longer hard.

After frozen food has been tempered, it is more easily sliced, diced or separated into

pieces (Chapter 3, section 3.1.2). Tempering is widely used for meat and fish products,

592 Food processing technology

which are more easily boned or ground at a temperature just below the freezing point. If

frozen foods are tempered but not allowed to melt, they require much less energy. For

example, the energy required to temper frozen beef from ÿ17.7 to ÿ4.4 ëC is 62.8 J g

ÿ1

whereas 123.3 J g

ÿ1

is needed to raise the temperature a further 2.2 ëC (Decareau 1990).

The lower energy cost of tempering gives a good return on investment in dielectric

equipment. Production rates range from 1 to 4 t h

ÿ1

of meat or 1.5±6 t h

ÿ1

of butter in

equipment that has power outputs of 25±150 kW. The advantages over conventional

tempering in cold rooms include the following:
· Faster processing (e.g. meat blocks are defrosted in 10 min instead of several days).

Tempering can also take place when the food is required with little loss or spoilage in

the event of a process delay.

· The costs of operating a tempering room are eliminated and savings are made in

storage space and labour.

· No drip losses or contamination, which improves product yields and reduces nutri-

tional losses. There is also better control over defrosting conditions and more hygienic

defrosting because products are defrosted in the storage boxes, leading to improved

product quality.

Other applications

Compared with conventional heating, microwave rendering of fats improves the colour,

reduces fines by 95% and costs by 30%, and does not cause unpleasant odours (Decareau

1985). Microwave frying is not successful when deep baths of oil are used, but can be

used with shallow trays in which the food is rapidly heated (Chapter 19, section 19.2).

There is less deterioration in oil quality and more rapid frying. Pretreating potatoes with

microwaves before frying has also been shown to reduce the formation of acrylamide

(Belgin et al. 2007). Other commercial microwave applications include heating bacon or

meat patties in foodservice applications and setting meat emulsions in microwave

transparent moulds to produce skinless frankfurters and other sausage products (Decareau

1990).

Microwave blanching has been extensively investigated, but the higher costs, com-

pared with steam blanching (Chapter 11), have restricted its use to products that are more

difficult to blanch by conventional methods. Microwave blanching of peanuts causes off-

flavours, but control of the processing conditions can limit this (Schirack et al. 2006).

Industrial microwave pasteurisation and sterilisation systems have been reported for

30 years starting with batch processing of yoghurt in cups and continuous processing of

milk (Anon 2000a) and now focusing on ready-to-eat meals in a few companies

worldwide (e.g. Anon 2006). RF pasteurisation and sterilisation are feasible but are not

yet used commercially. The lack of widespread commercial operations may be due to the

greater complexity, expense and non-uniformity of heating which makes it difficult to

ensure sterilisation of the whole package. Pandit et al. (2007) describe a computer-vision

system (Chapter 2, section 2.3.3) to identify cold-spots in microwave sterilised foods.

Microwave and RF heating for pasteurisation and sterilisation require less time to reach

the process temperature, especially for solid and semi-solid foods that heating by

conduction (Datta and Hu 1992). Other advantages are that equipment can be turned on

or off instantly, the product can be pasteurised after packaging so eliminating post-

pasteurisation recontamination, and processing systems can be more energy efficient.

Microwave pasteurisation of packed complete pasta meals, soft bakery goods and peeled

potatoes is reported by Brody (1992).

Dielectric, ohmic and infrared heating 593

The temperatures reached by dielectric and conventional heating are similar (Fig.

20.6a), but the F

values (time±temperature histories) for cumulative volume fractions of

food at each temperature are very different (Fig. 20.6b). Conventional heating shows a

larger spread of F

values, which indicate non-uniformity of temperatures and long

processing times that cause over-processing of the surface parts of the food.

The equipment consists of a pressurised microwave tunnel up to 25 m long, through

which food passes on a conveyor in microwave-transparent, heat-resistant, laminated

pouches or trays that have shapes specifically adapted for microwave heating. Poly-

propylene with an ethylene vinyl alcohol (EVOH) barrier or a polyethylene terephthalate

(PET) film has been used (Chapter 25, section 25.2.4). Because metal reflects micro-

waves, packages that have a metal component can change the food temperature distri-

bution. In some applications, metals have been added to the package to redistribute the

microwave energy to increase the uniformity of heating. The packs are positioned in the

tunnel so that they receive a predetermined amount of microwave energy that is

optimised for that type of package.

The process consists of heating, holding for the required period for pasteurisation or

sterilisation, and cooling the packs in the tunnel. In the Multitherm process, the

microwave-transparent pouches are formed and filled from a continuous reel of film but

are not separated. This produces a chain of pouches that passes through a continuous

hydrostat system, similar to a small hydrostatic steam steriliser (Chapter 13, section

13.1.3). The pouches are submerged in a medium that has a higher dielectric constant

than the product and heating is by microwaves instead of steam.

The design of the equipment can influence the location and temperature of the

slowest-heating point in the food, which makes it difficult to predict microbial

destruction. The process may therefore include an equilibration stage before holding the

heated product to equilibrate the temperatures and avoid non-uniform temperature

distribution within the product. Other methods used to improve the uniformity of heating

include rotating the packs and using pulsed microwaves. The process operates auto-

matically, with computer control of delivered power, temperature, pressure, conveyor

speed and process cycle time. Other processes use a combination of microwaves and hot

air at 70±90 ëC, followed by an equilibration stage where the slowest heating parts of the

Fig. 20.6 (a) Temperatures reached by dielectric and conventional heating and (b) F

values for

dielectric and conventional heating for cumulative volume fractions of food (from Anon 2000a).

Microwave 3.5 min at 2 W cm

ÿ3

, conventional 40 min at 121 ëC.

594 Food processing technology

LIVE GRAPH

Click here to view

packs reach 80±85 ëC within 10 min. The packs are then cooled to 1±2 ëC and have a

shelf-life of 40 days at 8 ëC. Details of a procedure for the microwave pasteurisation of

fruits in syrup to inactivate pectinesterase are reported by Brody (1992).

20.1.4 Effect on foods and micro-organisms

The effects of electromagnetic energy on food components are similar to those found

using other methods of heating, although the more rapid heating results in shorter

processing times and hence fewer changes to nutritional and sensory properties. The

process therefore has the benefits of bacterial destruction with reduced damage to sensory

and nutritional properties (Fig. 20.7). These changes are reviewed by Ehlermann (2002)

and are described further in Chapter 10 (sections 10.3 and 10.4). As in conventional

heating, heat-sensitive vitamins (e.g. ascorbic acid) undergo losses and, for example,

Watanabe et al. (1998) found that appreciable losses (30±40%) of vitamin B

occurred

in raw beef, pork and milk after microwave heating.

In pasteurisation and blanching applications, the high rates of heat transfer for a

specified level of microbial or enzyme destruction result in reduced losses of heat-

sensitive nutrients compared with conventional methods (e.g. there is no loss of carotene

in microwave-blanched carrots, compared with 28% loss by steam blanching and 45%

loss by water blanching), although Mirza and Morton (2006) found no difference in the

colour of carrots that were blanched by four different methods, including microwaves.

Ramesh et al. (2002) found reduced losses of nutrients after microwave blanching of

vegetables, but results for other foods are highly variable and, for these, microwave

heating offers no nutritional advantage over steaming. Changes to foods in other types of

processing (microwave or RF frying, baking, dehydration, etc.) are similar to

conventional methods and are described in the relevant chapters.

Similarly, the energy absorbed from electromagnetic waves can raise the temperature

of the food sufficiently to inactivate micro-organisms (e.g. Fujikawa et al. 1992). There is

disagreement over possible non-thermal effects of electromagnetic energy on micro-

organisms (i.e. effects such as ionisation that are not related to lethality caused by heat).

Microwaves correspond to an energy range of 1 eV±1 meV, whereas binding energies of

Fig. 20.7 Quality parameters for microwave and conventional heating (F

represents accumulated

lethality) (from Anon 2000a).

Dielectric, ohmic and infrared heating 595

LIVE GRAPH

Click here to view

electrons to atoms are >4 eV (Ehlermann 2002) and microwaves are therefore not capable

of ionisation. Anon (2000a) has reviewed research into these non-thermal effects and

reports that studies are inconclusive and only thermal effects are presumed to exist. All

changes are caused by heat alone and microbial inactivation is therefore the same as in

conventional heat processing (Chapter 10, section 10.3) (i.e. bacteria are more resistant

than yeasts and moulds to thermal inactivation by microwave heating, and bacterial

spores are more resistant than vegetative cells). There have been many studies of the

effect of microwave heating on pathogenic micro-organisms: Bacillus cereus,

Campylobacter jejuni, Clostridium perfringens, pathogenic Escherichia coli, Enterococ-

cus, Listeria monocytogenes, Staphylococcus aureus, and Salmonella are each reported to

be inactivated by microwave heating (Heddleson et al. 1994). The effect of microwave

pasteurisation on E. coli in fruit juices is reported by CanÄumir et al. (2002). However,

non-uniform heating may enable survival of pathogens when measured temperatures

indicate that they would be lethal (e.g. survival of pathogens at the surface of poultry due

to lower temperatures at the product surface than the measured internal temperature)

(Schnepf and Barbeau 1989). The effects of microwaves on micro-organisms and

enzymes are described by Anatheswaran and Ramaswarmy (2001).

20.2 Ohmic heating

Also termed `resistance heating', `electroconductive heating' or `Joule heating', this is a

process in which an alternating electric current is passed through a food, and the electrical

resistance of the food causes the power to be translated directly into heat (see also pulsed

electric field processing (Chapter 9, section 9.1)). As the food is an electrical component

of the heater, it is essential that its electrical properties are matched to the capacity of the

heater. The concept of direct heating in this way is not new, but it was developed into a

commercial process during the 1980s±1990s. The process can be used for UHT

sterilisation of foods, and especially those foods that contain larger particles that are

difficult to sterilise by other methods. It is in commercial use in Europe, the USA and

Japan for aseptic processing of high-added-value ready meals, stored at ambient or chill

temperatures (see Chapter 13, section 13.2), pasteurisation of particulate foods for hot

filling, and preheating products before canning (Fryer 1995).

Ohmic heating has higher energy conversion efficiencies than microwave heating

(>90% of the energy is converted to heat in the food). Another important difference is

that microwave and radio frequency heating have a finite depth of penetration into a food

whereas ohmic heating has no such limitation. However, microwave heating requires no

contact with the food, whereas ohmic heating requires electrodes to be in good contact.

This means that in practice the food should be liquid or have sufficient fluidity to allow

both good contact with the electrodes and to pump the product through the heater.

The advantages of ohmic heating are as follows:

· The food is heated rapidly (>1 ëC s

ÿ1

) throughout the bulk of the food (i.e. volumetric

heating) for example, from ambient to 129 ëC in 90 s (Ruan and Chen 2002). The

absence of temperature gradients results in even heating of solids and liquids if their

resistances are the same, which cannot be achieved in conventional heating.

· There are no hot surfaces for heat transfer, as in conventional heating. Therefore heat

transfer coefficients do not limit the rate of heating, and there is no risk of surface

fouling or damage to heat-sensitive foods by localised over-heating.

596 Food processing technology

· Liquids containing particles are not subject to shearing forces that are found in for

example scraped surface heat exchangers (Chapter 13, section 13.2.3), and the method

is suitable for viscous liquids because heating does not have the problems associated

with poor convection in these materials.

· It has a lower capital cost than microwave heating and it is suitable for continuous

processing, with instant switch-on and shutdown.

Further details are given by Ruan et al. (2004), Rahman (1999) and Sastry (1994).

Ohmic heating has been used commercially to pasteurise milk, liquid egg and fruit

juices, to process viscous liquids, such as apple sauce and carbonara sauce, to produce

high-quality whole fruits for yoghurt, and low-acid products that contain particles,

including ratatouille, pasta in tomato or basil sauce, beef bourguignon, vegetable stew,

lamb curry and minestrone soup concentrate (Ruan et al. 2004, Ruan and Chen 2002).

However, three factors limit the widespread commercial uptake of the process: (1)

differences in the electrical conductivities of the liquid and solid components of multi-

component foods and variations in conductivity with increasing temperature, which can

cause irregular and complex heating patterns and difficulties in predicting the heating

characteristics; (2) a lack of data on the critical factors that affect the rate of heating

(section 20.2.1); and (3) a lack of accurate temperature-monitoring techniques to profile

heat distribution and locate cold-spots during the process. This risks under-processing

and the consequent survival of pathogenic spores in low-acid foods. Advances in

magnetic resonance imaging (MRI) are being used to address the last issue and are

reviewed by Ruan et al. (2004).

20.2.1 Theory

Foods and other materials have a resistance (known as the `specific electrical resistance')

that generates heat when an electric current is passed through them. Electrical

`conductivity' is the inverse of electrical resistance and is measured in a food using a

multimeter connected to a conductivity cell. The relationship between electrical

resistance and electrical conductivity is found using:

1=RL=A

20:7

where (S m

ÿ1

) product conductivity, R () measured resistance, L (m) length of

the cell and A (m

) area of the cell.

Conductivity measurements are made in product formulation exercises, process

control and quality assurance for foods that are heated electrically. Data on electrical

conductivity of foods (Table 20.4) are relatively scarce, but it has a much greater range

than thermal conductivity (Chapter 10, Table 10.2). For example, it can vary from

S m

ÿ1

for copper to 10

ÿ8

S m

ÿ1

for an insulating material such as wood. Foods that

contain water and ionic salts are more capable of conducting electricity (they have a

lower resistance). In composite foods, the conductivity of particles is measured by

difference (i.e. the product conductivity minus the carrier medium conductivity).

Unlike metals, where conductivity falls with temperature, the electrical conductivity

of a food increases linearly with temperature (Wang and Sastry 1997, Reznick 1996). It

can also vary in different directions (e.g. parallel to, or across a cellular structure), and

can change if the structure changes (e.g. gelatinisation of starch, cell rupture or air

removal after blanching). It can be seen in Table 20.4 that the conductivity of

vegetables is lower than muscle tissue, and this in turn is considerably lower than for a

Dielectric, ohmic and infrared heating 597

sauce or gravy. The salt content of a gravy is typically 0.6±1% and from the data (5b) in

Table 20.4 the conductivity of the beef is about a third of that of the gravy. This has

important implications for processing of particles (section 20.2.3): if in a two-

component food consisting of a liquid and particles in which the particles have a higher

conductivity, they are heated at a higher rate. This is not possible in conventional

heating owing to the lower thermal conductivity of solid foods, which slows heat

penetration to the centre of the pieces (Chapter 10, section 10.1.2) (Fig. 20.8). Ohmic

heating can therefore be used to heat sterilise particulate foods under UHT conditions

without causing heat damage to the liquid carrier or over-cooking of the outside of

particles. Furthermore, the lack of agitation in the heater maintains the integrity of

particles and it is possible to process larger particles (up to 2.5 cm) that would be

damaged in conventional equipment.

The rate of heating also depends on the density, the pH, thermal conductivity and

specific heat capacities of each component, the way that food flows through the

equipment and its residence time in the heater, in addition to the electrical conductivity of

the components. Each of these may change during processing and hence alter the heating

characteristics of the product (Larkin and Spinak 1996).

In two-component foods the heating patterns are not a simple function of the relative

conductivities of the particles and liquid carrier. For example, when a particle that has a

lower conductivity than the liquid is heated the liquid heats faster, but if the density of the

particle is higher, the heating rate may exceed that of the liquid (Sastry and Palaniappan

1992). If two components have similar conductivities, the lower moisture solid portion

heats faster than the carrier liquid. The calculation of heat transfer is therefore very

complex, involving the simultaneous solution of equations for changes in electrical fields,

thermal properties and fluid flow, and is beyond the scope of this book. Details are given

by Fryer (1995) and Sastry and Li (1996). Mathematical models are described by

Salengke and Sastry (2007), Samprovalaki et al. (2007) and Ye et al. (2004) and are

reviewed by Ruan et al. (2004). A simplified theory of heating is given below.

The resistance in an ohmic heater depends on the specific resistance of the product,

and the geometry of the heater:

R R

L=A

20:8

where R () total resistance of the heater, R

( m

ÿ1

) specific resistance of the

product, L (m) distance between the electrodes and A (m

) area of the electrodes.

The resistance determines the current that is generated in the product:

Table 20.4 Electrical conductivity of selected foods at 19 ëC
Food

Electrical conductivity

(S m

ÿ1

)

1 Potato

0.037

2 Carrot

0.041

3 Pea

0.17

4 Beef

0.42

5 Starch solution (5.5%)

(a) with 0.2% salt

0.34

(b) with 0.55% salt

1.3

4.3

From Kim et al. (1996)

598 Food processing technology

20:9

where V (V) voltage applied and I (A) current.

The available three-phase power sources in most countries have 220±240 V per phase

at a frequency of 50 Hz, and to make the best use of the power the geometry of the heater

and the resistance of the product have to be carefully matched. If the resistance is too

high, the current will be too low at maximum voltage. Conversely, if the resistance is too

low, the maximum limiting current will be reached at a low voltage and again the heating

power will be too low.

Every product has a critical current density and if this is exceeded there is likely to be

arcing (or flash-over) in the heater. The current density is found by:

I=A

20:10

where I

(A cm

ÿ2

) current density. The minimum area for the electrodes can therefore

be calculated once the limiting current density and maximum available current are

Fig. 20.8 Heat penetration into solid pieces of food by (a) conventional heating and (b) ohmic

heating (adapted from Fryer 1995).

Dielectric, ohmic and infrared heating 599

known. As resistance is determined in part by the area of the electrodes (Equation 20.8),

the distance between the electrodes can be calculated. It is important to recognise that the

design of the heater is tailored to products that have similar specific electrical resistances

and it cannot be used for other products without modification.

The rate of heating is found using Equation 20.11:

Q mC

20:11

and the power by:

P VI

20:12

and

P RI

20:13

Assuming that heat losses are negligible, the temperature rise in a heater is calculated

using:

Lmc

20:14

where (ëC) temperature rise,

(S m

ÿ1

) average product conductivity throughout

temperature rise, A (m

) tube cross-sectional area, L (m) distance between electrodes,

m (kg s

ÿ1

) mass flowrate and c

(J kg

ÿ1

ëC

ÿ1

) specific heat capacity of the product.

In conventional heaters, turbulence is needed to create mixing of the product and

maintain maximum temperature gradients and heat transfer coefficients (Chapter 10,

section 10.1.2). In ohmic heating, the electric current flows through the product at the

speed of light and there are no temperature gradients since the temperature is uniform

across the cross-section of flow. The flowrate of product is negligible compared with the

velocity of the electric current, but if the flowrate is not uniform across the cross-

sectional area, the very high rates of heating mean that slower moving food will become

considerably hotter. It is therefore important to ensure that uniform (or `plug') flow

conditions are maintained in the heater (also Chapter 1, section 1.3.4). Kim et al. (1996)

give details of experimental studies, which confirm that this takes place. Similarly, the

type of pump that is used should provide a continuous flow of material without pulses, as

these would lead to increased holding times in the tube and uneven heating. A high

pressure is maintained in the heater (up to 400 kPa for UHT processing at 140 ëC) to

prevent the product from boiling.

20.2.2 Equipment and applications

As described in section 20.2.1, the design of ohmic heaters must include the electrical

properties of the specific product to be heated, because the product itself is an electrical

component. This concept is only found elsewhere in RF heating and requires more

specific design considerations than those needed when choosing other types of heat

exchangers.

The factors that are taken into account include the following:

· The type of product, its electrical resistance and change in resistance over the expected

temperature rise, its composition, shape size, orientation, specific heat capacity,

thermal conductivity and density. For liquid carriers, the additional properties are

viscosity and added electrolytes.

600 Food processing technology

· Temperature rise (determines the power requirement) and rate of heating required.

· Flowrate and holding time required.
Early ohmic heater designs used DC power, which caused electrolysis (corrosion of

electrodes and product contamination) and also required expensive electrodes. The use of

mains power at 50 Hz reduces the risk of electrolysis and minimises the complexity and

cost. Alternatively, higher frequencies (>100 kHz) or carbon electrodes may be used to

reduce electrolysis. To be commercially successful for aseptic processing, ohmic heaters

must have effective control of heating rates and product flowrates that avoids electrolysis

or product scorching, and be cost effective. The layout of an ohmic heating system is

shown in Fig. 20.9.

The heater consists of a vertical tube containing a series of pure carbon cantilever

electrodes (supported from one side) that are contained in a PTFE housing and fit across

the tube. The tube sections are made from stainless steel, lined with an insulating plastic

such as polyvinyidene fluoride (PVDF), polyether ether ketone (PEEK) or glass. Food is

pumped up through the tube and an alternating current flows between the electrodes and

through the food to heat it to the required process temperature. The system is designed to

maintain the same impedance between the electrodes in each section, and the tube

sections therefore increase in length between inlet and outlet because the electrical

conductivity of the food increases as it is heated. Food then passes from the heater to a

holding tube where it is held for sufficient time to ensure sterility and is then cooled and

aseptically packaged (Chapter 13, section 13.2, and Chapter 25, section 25.2.7).

Typically, a heater tube of 2.5 cm diameter and 2 m length could heat several thousand

litres per hour (Reznick 1996). Commercial equipment is available with power outputs of

Fig. 20.9 Layout of an ohmic heating system (after Parrott 1992).

Dielectric, ohmic and infrared heating 601

75 and 300 kW, which correspond to throughputs of 750 and 3000 kg h

ÿ1

respectively

(Fryer 1995). The process is automatically controlled via a feed-forward system (Chapter

27, section 27.2), which monitors inlet temperature, product flow rate and specific heat

capacity, and continuously adjusts the power required to heat the product (Dinnage

1990).

Ohmic heating has been used to process various combinations of meats, vegetables,

pasta and fruits when accompanied by a suitable carrier liquid. It may be necessary to

pretreat components of the food to make them more homogeneous (e.g. pregelatinising

starch in the carrier liquid, homogenising sauces, especially those that contain fats and

heat-sensitive proteins to produce a uniform material, blanching vegetables to expel air

and soaking foods in acids or salts to alter the electrical resistance of particles (Zoltai and

Swearingen 1996)).

In operation, a small amount of carrier liquid is used to suspend the particles as they

pass through the heater. The bulk of the carrier liquid is sterilised by conventional plate or

tubular heat exchangers and is then injected into the particle stream as it leaves the

holding tube. This has the advantage of reducing the capital and operating costs for a

given throughput (Dinnage 1990). The combined product is then aseptically packaged

(Chapter 13, section 13.2.3). Ohmic heating costs were found by Allen et al. (1996) to be

comparable to those for freezing and retort processing of low-acid products. The almost

complete absence of fouling in ohmic heaters means that after one product has been

processed, the plant is flushed through with a base sauce and the next product is

introduced. At the end of processing, the plant is flushed with a cleaning solution.

The process is suitable for foods that contain up to about 60% solids. In contrast to

conventional UHT processing of particulate foods, where sufficient amounts of the liquid

component are required for heat transfer into the particles, in ohmic heating a high solids

content is desirable for two reasons: there is faster heating of low-conductivity particles

than the carrier liquid; and a high solids content creates plug-flow conditions in the heater

tubes. To obtain high solids concentrations, the particles should be pliable and small, or

their geometry is varied to reduce the void spaces between particles. If lower solids

concentrations are processed, they require a higher-viscosity carrier liquid to keep the

particles in suspension and maintain plug-flow conditions. In all products the viscosity of

the sauce or gravy carrier liquid should be carefully controlled, and this may involve

using pregelatinised starches to prevent viscosity changes during processing.

The density of the particles should also be matched to the carrier liquid: if particles are

too dense or the liquid is not sufficiently viscous, the particles sink in the system and

become over-processed. Conversely, if the particles are too light they float, which leads

to a variable product composition and the risk of under-processing. It is almost impos-

sible to determine the residence time or heating profiles of particles that float or sink.

To ensure sterility, it is necessary to ensure that the coldest part of the slowest heating

particle in the food has received sufficient heat (see Chapter 10, section 10.1.2, and

Chapter 13, section 13.1). It is not easy to measure heat penetration into particles,

whereas it is relatively easy to measure the temperature of the carrier liquid. The process

must therefore demonstrate that solid particles have been heated to an equal or greater

extent than the liquid when they enter the holding tube. This is achieved by adjusting the

electrical properties of each component (e.g. by control of salt content in the

formulation). This is more difficult for non-homogeneous particles such as fatty meat

pieces, in which the different components have different electrical resistances. Com-

plexities increase if, for example, salt leaches out of the particles into the surrounding

sauce and causes changes to the electrical resistance and hence the rate of heating of both

602 Food processing technology

components. The presence of fats or other poorly conductive materials (e.g. pieces of

bone, nuts or ice) in particles means that they will not be heated directly, and the slower

heating by conduction creates a cold spot within the particle (Larkin and Spinak 1996). If

this happens, the surrounding food may also be under-processed and there is a risk of

growth of pathogenic bacteria. Further details are given by Anon (2000b).

20.2.3 Effect on foods and micro-organisms

Ohmic heating is an HTST process and therefore has similar benefits to other methods of

rapid heating that destroy micro-organisms before there are adverse effects on nutrients

or chemicals that produce required organoleptic qualities (see Chapter 10, section 10.3

for details of D- and z-values). It also causes similar changes to foods as does

conventional heating, such as starch gelatinisation, melting of fats and coagulation of

proteins (Chapter 10, section 10.4). Ohmic heating also increases diffusion of material

from solid particles to the carrier liquid, which may be due to electroporation (formation

of pores in cell membranes due to electrical potential across the membrane, resulting in

leakage), membrane rupture caused by the voltage drop across the membrane, and cell

lysis, disrupting internal components of the cell. These effects may contribute to

microbial destruction (see also Chapter 9, section 9.1.3). Losses of material from cells

only alter the nutritional value if the liquid is not consumed, in for example blanching. A

study by Mizrahi (1996) showed that solute losses had a similar pattern in hot-water

blanched and ohmic blanched beets, and were proportional to the surface : volume ratio

and the square root of processing time. However, it was not necessary to slice the beets

before ohmic processing, and this, together with the shorter blanching time, reduced

solute losses in ohmic blanching by a factor of ten compared with hot water blanching.

20.3 Infrared heating

The main commercial applications of IR energy are drying low-moisture foods (e.g.

breadcrumbs, cocoa, flours, grains, malt, pasta products and tea) and in baking

applications (e.g. pizzas, biscuits) or roasting ovens (Chapter 18, section 18.2.1) for

products such as coffee, cocoa and cereals. The technology has also been used to fry or

thaw foods, and for surface pasteurisation of bread and packaging materials. The main

advantages are a reduction in roasting or baking time and savings in energy compared

with traditional processes. However, it is not widely used as a single source of energy for

drying or baking larger pieces of food because of the limited depth of penetration.

Radiant energy is also used in vacuum band driers and cabinet driers (Chapter 16, section

16.2.1), in accelerated freeze driers (Chapter 23, section 23.1.2), in some domestic

microwave ovens to brown the surface of foods; and to heat-shrink packaging films

(Chapter 26, section 26.3).

20.3.1 Theory

Infrared energy is electromagnetic radiation (Fig. 20.1) that is emitted by hot objects.

When it is absorbed, the radiation gives up its energy to heat materials. The rate of heat

transfer depends on:
· the surface temperatures of the heating and receiving materials;

· the surface properties of the two materials; and

· the shapes of the emitting and receiving bodies.

Dielectric, ohmic and infrared heating 603

The amount of heat emitted from a perfect radiator (termed a `black body') is

calculated using the Stefan±Boltzmann equation:

Q AT

20:15

where Q (J s

ÿ1

) rate of heat emission, (W m

ÿ2

ÿ4

) Stefan±Boltzmann constant

5.73 10

ÿ8

, A (m

) surface area, and T (K ëC 273) absolute temperature.

This equation is also used for a perfect absorber of radiation, again known as a black

body. However, radiant heaters are not perfect radiators and foods are not perfect

absorbers, although they do emit and absorb a constant fraction of the theoretical

maximum. To take account of this, the concept of `grey bodies' is used, and the Stefan±

Boltzmann equation is modified to:

Q "AT

20:16

where emissivity of the grey body (a number from 0 to 1) (Table 20.5).

Emissivity varies with the temperature of the grey body and the wavelength of the

radiation emitted. The amount of absorbed energy, and hence the degree of heating,

varies from zero to complete absorption. This is determined by the components of the

food, which absorb radiation to different extents, and the wavelength of the radiated

energy. The wavelength of infrared radiation is determined by the temperature of the

source. Higher temperatures produce shorter wavelengths that have a greater depth of

penetration.

Some radiation is absorbed by foods and some is reflected back out of the food. The

amount of radiation absorbed by a grey body is termed the `absorptivity' () and is

numerically equal to the emissivity (Table 20.5). Radiation that is not absorbed is

expressed as the `reflectivity' (1 ÿ ). There are two types of reflection: that which takes

place at the surface of the food and that which takes place after radiation enters the food

structure and becomes diffuse due to scattering. The net rate of heat transfer to a food

therefore equals the rate of absorption minus the rate of emission:

Q "AT

ÿ T

20:17

where T

(K) temperature of the emitter and T

(K) temperature of the absorber.

It can be seen from Equation 20.17 and sample problem 20.1 that the temperature of

the food has a significant effect on the amount of radiant energy that is absorbed.

Table 20.5 Approximate emissivities of materials in food processing
Material

Emissivity

Burnt toast

1.00

Dough

0.85

Water

0.955

Ice

0.97

Lean beef

0.74

Beef fat

0.78

White paper

0.9

Painted metal or wood

0.9

Unpolished metal

0.7±0.25

Polished metal

< 0.05

From Earle (1983) and Lewis (1990)

604 Food processing technology

20.3.2 Equipment

Industrial radiant heaters are required to reach operating temperatures quickly to enable

good process control and to transfer large amounts of energy (SkoÈldebrand 2002). Quartz

or halogen lamps fitted with tungsten or nichrome electric filaments heated to 2200 ëC

produce near IR radiation with wavelengths of 1.1±1.3 m and medium wave IR (Table

20.6). Ceramic IR heaters heat up to 700 ëC and produce far IR radiation. Products are

either conveyed through a tunnel or beneath banks of radiant heaters.

20.3.3 Effect on foods and micro-organisms

The rapid surface heating changes the flavour and colour of foods due to Maillard

reactions and protein denaturation, and also seals moisture and flavour or aroma

compounds in the interior of the food. These changes are similar to those that occur

during baking and are described in Chapter 18 (section 18.3). IR heating also has similar

effects on micro-organisms to those described during baking.

Sample problem 20.1

A 12 kW oven operates at 210 ëC. It is loaded with a batch of 150 loaves of bread

dough in baking tins at 25 ëC. The surface of each loaf measures 12 cm 20 cm.

Assuming that the emissivity of dough is 0.85, that the dough bakes at 100ëC, and that

90% of the heat is transmitted in the form of radiant energy, calculate energy

absorption at the beginning and end of baking and the percentage of radiant energy

absorbed by the surfaces of the loaves at the end of baking.

Solution to sample problem 20.1

Area of dough 1500:2 0:12

3:6 m

From Equation 20.17, energy absorbed at the beginning of baking,

Q 0:85 5:73 10

ÿ8

3:6 483

ÿ 298

8159:8 W

and the energy absorbed at the end of baking,

Q 0:85 5:73 10

ÿ8

3:6 483

ÿ 373

6145:6 W

Radiant energy emitted 12 000 0:9 W

10 800 W

Percentage of energy absorbed by the bread 6145:9=10 800

0:57 or 57%

Dielectric, ohmic and infrared heating 605

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Dielectric, ohmic and infrared heating 609

Document Outline

Front Matter
Table of Contents
Part I. Basic Principles
Part II. Ambient-Temperature Processing
Part III. Processing by Application of Heat
Part III.A. Heat Processing Using Steam or Water
Part III.B. Heat Processing Using Hot Air
Part III.C. Heat Processing Using Hot Oils
Part III.D. Heat Processing by Direct and Radiated Energy
20. Dielectric, Ohmic and Infrared Heating
Part IV. Processing by Removal of Heat
Part V. Post-Processing Operations
Part VI. Appendices
Index

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