Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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See also: Browning: Nonenzymatic; Enzymatic –

Biochemical Aspects; Emulsifiers: Organic Emulsifiers;

Uses in Processed Foods; Homogenization;

Pasteurization: Principles; Vitamins: Overview

Further Reading

Cunningham FE (1995) Egg product pasteurization. In:

Stadelman WJ and Cotterill OJ (eds) Egg Science and

Technology, 4th edn. Binghampton: Haworth Publish-

ing.

Hyde A (1999) Heat transfer processes in breweries – part

3. Ferment 12: 30–34.

Lewis MJ and Heppell NJ (2000) Continuous thermal pro-

cessing of foods: pasteurization and UHT sterilization.

Gaithersburg: Aspen.

Zufall C and Wackerbauer K (2000) The biological impact

of flash pasteurization over a wide temperature interval.

Journal of the Institute of Brewing 106: 163–167.

Pasteurization of Viscous and

Particulate Products

G Tucker, Campden & Chorleywood Food Research

Association, Chipping Campden, UK

Background

0001 For food products of high viscosity, the choice to

pasteurize in a continuous system has distinct advan-

tages over one where the process takes place in the

container (see Figure 1). This is because the heat

transfer from a heating or cooling medium to a vis-

cous food is far from ideal, but careful design and

selection of the heat exchange system can overcome

this limitation. Of principal concern are the econom-

ics of food production, which become attractive in

continuous systems for high volumes and long

production runs. Further economic benefits can be

realized by minimizing the product’s exposure to the

adverse effects of high temperatures, long processing

times and high shear preparation methods, resulting

in improved product quality, reduced processing

costs, increased safety, and increased plant through-

put. This chapter considers both continuous and in-

container processing systems.

Continuous Processing Considerations

0002 A challenge in the design of heat-transfer equipment

are the so-called prepared food products, such as

tomato products, soups and sauces, and dessert prod-

ucts. These products are normally of high viscosity as

well as of complex composition. Also, in most cases,

the particle content is significant. With respect to the

rheological or flow behavior, the products are typic-

ally non-Newtonian, showing, in many cases, quite

extraordinary behavior.

0003The microbiological demands of a pasteurization

process are still to achieve commercial sterility, i.e.,

the end product must be free from pathogenic and

spoilage organisms capable of growth under nor-

mal storage and distribution conditions as well as

being free from toxins. Depending on the rheological

properties of the product and the possible presence of

particles, the design and choice of equipment can vary

significantly from case to case.

Flow Behavior

0004In the design and choice of heat exchangers, the flow

behavior of the product has to be taken into consider-

ation. The flow behavior will affect, for instance, the

residence time distribution and hence the design of

heat exchangers and holding cells to obtain the suffi-

cient thermal treatment. The basic difference between

laminar (streamline) and turbulent flow is well

known (see Figure 2), as is the effect on the velocity

profile from heating or cooling of the product. For

example, the maximum velocity in laminar flow is

theoretically twice the mean velocity, and in turbulent

100

150

Temp.C

20 40 60 80 100 120

Minutes

Continuous

processing

In-container

processing

fig0001Figure 1 Time/temperature graph for comparison of in-

container and continuous processes.

PASTEURIZATION/Pasteurization of Viscous and Particulate Products 4395

flow around 10% higher. For viscous products, how-

ever, the flow conditions are nearly always laminar.

0005 For liquid foods that have a complex flow behavior

the velocity profiles in pipe-flow depend on the flow

behaviour index. This index can range from 1.0 in

simple materials such as water, milk, or fruit juice, to

0.7 in starch-based sauces, to 0.3 in tomato paste, and

in theory can go to 0.0. With decreasing flow behav-

ior index, the velocity profile increases in flatness,

which means in practice that the maximum velocity

decreases from twice the mean velocity. Maintaining

the factor of two for the calculation of the necessary

holding cell length thus creates overcooking of the

product, but it is a safe assumption and one that

most of the industry adopts.

0006 A further complication to the viscous flow behavior

arises with additives that give elastic properties, e.g.,

xanthan or gellan gum. These are sometimes used to

enhance the particle-carrying properties for a carrying

fluid in continuous processing. The so-called yield

value, which normally is a measure of the product’s

willingness to flow by itself, e.g., from a storage tank,

is also a measure of the particle carrying abilities. A

significant yield value, typical of paste-like products,

also adds to the flatness of the velocity profile.

Choice of Heat Exchanger

0007 The choice of optimal heat exchanger depends to a

great deal on the flow conditions. Fluids with low

viscosities and no particles are preferably treated in

a plate heat exchanger (see Figure 3). This is the most

economic option. The food product and heating (or

cooling) media flow in alternate channels to provide

good heat-transfer characteristics. The plates are 0.5–

1.25 mm thick, separated by 3–6 mm, and sealed with

gaskets.

0008For fruit juices with pulp and fibers up to 5 mm in

length, special types of plates are available with wide

gaps. Even with foods of higher viscosities, the plate

heat exchanger can be utilized as long as the pressures

developed are not too high, which can cause failure of

the gaskets. Attempts to pasteurize foods with par-

ticles can result in blocked flow channels and in-

creased pressure until the gaskets fail.

0009For fruit juices with fibers up to 15 mm in length, a

multitube tubular heat exchanger (see Figure 4)is

preferable to the plate heat exchanger. Also, foods

of moderate to high viscosity with only small particles

(< 5 mm) will flow through a multitube exchanger

Turbulent flow

012

Laminar flow

fig0002 Figure 2 Velocity profiles for turbulent and laminar (stream-

line) flow showing the ratio of maximum velocity to average

velocity.

fig0003Figure 3 Plate heat exchanger.

fig0004Figure 4 Multitube tubular heat exchanger.

4396 PASTEURIZATION/Pasteurization of Viscous and Particulate Products

without problems. The typical inside diameter of each

tube is 14 mm, and these can be grouped in bundles of

four, seven, 12, 19, 27, or 37 tubes. Other combin-

ations of tube diameters and tube bundle numbers are

available, depending on the equipment supplier. Most

commercial tubular heat exchangers are 6 m long.

0010 However, if the fluid is significantly viscoelastic, i.e.,

exhibits a large yield value, often in combination with

a high viscosity, there is a risk of maldistribution across

the tubes of a multitube exchanger. In the worst case,

the product flow will stop in some of the tubes, causing

overcooking of parts of the product and subsequent

cleaning problems. Examples of such products are hot

break tomato paste or a stiff dessert pudding. In these

cases, the concentric tube is the best choice, which has

only one product channel and eliminates this risk. At

the same time, the narrow gap and the two service

medium channels surrounding the product channel

provide efficient heat transfer (see Figure 5).

0011 Finally, if large particles are present, the monotube

is the optimal choice of tubular heat exchanger (see

Figure 6). The drawback with a monotube compared

to multi- or concentric tubes is a reduced thermal

efficiency due to the thicker product layer. This is

illustrated in Figure 7, which shows the cross-

sectional flow areas of each tube type. However, to

a great extent, the particles present in the product will

work as ‘internal mixers’ and hence will promote heat

transfer.

0012If the food viscosity is high, and there is the chance

of product fouling the heated or cooled surfaces of a

tubular heat exchanger, the scraped surface heat ex-

changer must be employed (see Figure 8). In principle,

a scraped surface heat exchanger is a monotube

equipped with a rotating internal scraper. The scraper

keeps the heating surface free from any deposits and

also promotes turbulence. Hence, this type of heat

exchanger is ideal for products of a very high viscos-

ity, possibly also containing large particles.

Heat-transfer Correlations

0013Design models for heat exchangers are normally

based on empirical correlations of the dimensionless

Nusselt, Prandtl, and Reynolds numbers. The derived

equation is basically of the form Nu ¼f(Re, Pr). By

using dimensionless numbers, only a limited number

of experiments have to be performed in which the

product and heating/cooling medium flow rates and

the product physical properties are varied in order to

cover a large range of Reynolds as well as Prandtl

numbers. The physical properties are changed prefer-

ably by changing the temperatures of the fluids in-

volved. Decreased and increased temperatures can

normally vary the viscosity of the product signifi-

cantly.

0014The definition of the Nusselt (Nu), Prandtl (Pr)and

Reynolds (Re) numbers is as follows:

Nu ¼



Pr ¼





Re ¼





fig0005 Figure 5 Concentric tubular heat exchanger.

fig0006 Figure 6 Monotube tubular heat exchanger.

Concentric tube

Heating or cooling media

Product

Mono tube Multi tube

fig0007Figure 7 Cross-sectional tube designs for tubular heat

exchangers.

PASTEURIZATION/Pasteurization of Viscous and Particulate Products 4397

where a is the individual heat transfer coefficient

(W/m

1

); d

is the hydraulic diameter (m); l is

the thermal conductivity of liquid (W/m

1

); c

is the specific heat of liquid (J/kg

1

); m is the

dynamic viscosity of liquid (Pa.s); v is the velocity

(m.s

1

) and r is the density (kg.m

3

). A comparison

of tubular heat exchangers with plate heat exchangers

shows that the thermal performance, as read off in a

typical Nu–Pr–Re graph, is better for the plate heat

exchanger, mainly due to the complex corrugated

pattern including a large number of contact points

in the product channel (see Figure 9).

0015 From systematic laboratory tests and from experi-

ence of commercial plants, it has been found, how-

ever, that to completely model the heat transfer of

liquid food products, several more parameters have

to be included. Such parameters include, for instance,

particle content, particle shape, and size as well as the

type of particles. In addition, the softness or hardness

of the particles can effect the heat transfer by their

influence on the laminar boundary layers at the

exchanger surfaces.

Particulate Processes

0018Flow rates of particulates and carrier liquids should

be balanced to give even thermal processing of the

product. This relies on controlling the density of both

phases to prevent sedimentation or flotation of the

particulates. The limiting step will be the heat transfer

to the center of the fastest and largest of the particu-

lates, which should be determined by experimenta-

tion in the measurement of residence time in the

holding tube and of thermal conductivity. Once deter-

mined, an estimation of the pasteurization level can

be obtained using a mathematical procedure to solve

conduction-based heat transfer equations. Before

production of the food product commences, a full

microbiological validation of the process safety

should be conducted.

Batch Processing Considerations

0019Batch systems fall into two main categories, the most

common being where the viscous food is pasteurized

in its container, and the alternative is to carry out the

processing within a large container (e.g. a closed

vessel) and fill into individual containers. The advan-

tage of the former method is that both food and

container are pasteurized during the process, which

allows the integral package to be commercially sterile

and the potential shelf life extended. The disadvan-

tage is that the containers need to be strong enough to

withstand high temperatures (80–100



C) and the

swings in pressure differential.

0020Irrespective of the method used, heat transfer from

the media to the product thermal center is poor, be-

cause of the viscous boundary layers that develop on

Nu / φ (Pr)

0.1

100

1000

10 100 1000 10 000 100 000

Tubular heat exchanger

Plate heat exchanger

fig0009Figure 9 Comparison of Nu–Pr–Re correlations for plate and

tubular heat exchangers.

1 Cylinder

2 Rotor

3 Blade

fig0008 Figure 8 Scraped surface heat exchanger.

4398 PASTEURIZATION/Pasteurization of Viscous and Particulate Products

the inside surface of the containers. The effect is to

form an insulating layer that impairs heat transfer if

there is no mixing inside the containers. This results

in lengthy processing times to achieve pasteurization

and so few companies will process viscous foods

without agitation.

In-container Systems

0021 The traditional package is the metal can processed in

a steam atmosphere, although glass jars, pouches and

flexible plastic or aluminum trays can now be suc-

cessfully processed in steam or hot water. These con-

tainers require an air overpressure to counteract the

natural expansion of the gases present in the head-

space and those released from the food as it rises in

temperature. The desired effect is to push the lid or

sides back to their original position and therefore

minimize the stress on the seals. Examples of viscous

foods pasteurized in their containers are fruit pie

fillings, tomato salsas and cook-in-sauces.

0022 The first equipment (introduced by FMC Corpor-

ation in the 1930s) to increase heat transfer to this

type of food was the reel & spiral cooker-cooler. This

imposes rotation of cans around their central axis and

allows heating times to be reduced substantially (e.g.

from 90 minutes to 15 minutes). Cans travel on a

helical reel through the steam heating and water

cooling sections, and in doing so rotate at high

speeds. The continuous operation of the cooker-

cooler allows a high throughput to be achieved and

favors high volume production.

0023 More recently, end-over-end container rotation has

become popular, where the containers are constrained

in baskets that rotate. Being a batch system, this

offers greater production flexibility and is not re-

stricted to cylindrical metal cans. However, heat

transfer enhancement is not as great as with the

cooker-cooler and so the throughput is lower and

operating costs higher. Sauce products in plastic

pouches are processed in steam and air mixtures,

using low rotation speeds (2 to 10 rpm) to induce

mixing without damaging the delicate pouch.

In-vessel Systems

0024 The vessel acts in a similar way to a heat exchanger in

that it raises the food temperature to that required for

pasteurization. A typical vessel size is around 800 to

1 000 kg, and comprises a hemispherical steam-

jacketed base with cylindrical sides above. A hinged

lid is usually present to reduce heat loss and prevent

foreign objects falling into the food. With viscous

foods it is essential that the food is well-mixed, other-

wise laminar boundary layers develop and the food

burns on to the heated surface. Horizontal agitators

with scraped surface blades offer the most effective

mixing, although recirculating pumps and vertical

mixing blades are alternatives. Examples of viscous

foods pasteurized this way are fruit preparations,

confectionery, cook-in-sauces and tomato products.

0025Once pasteurized, the food can be filled either hot

or cold into the containers. A hot-fill process will only

require a short hold time at high temperature to

ensure the inside container surfaces are pasteurized.

This is usually achieved in a raining water tunnel

pasteurizer, although it is possible to omit this step if

the food’s acidity is high (pH < 3.8), the filling tem-

perature above 95



C and the containers pre-warmed

or of low heat capacity. The shelf life of a viscous food

of low pH will be many months if hot-filled, and

determined by its chemistry. A cold-fill process will

not guarantee commercial sterility of the container,

and as such requires far greater attention to hygiene

in order to minimize the introduction of microbial

contamination during filling. Most cold-filled prod-

ucts are sold chilled and have shelf lives up to 14 days.

0026Heat transfer issues are similar to those for con-

tinuous processing in terms of transferring heat from

steam or hot water to the core of a viscous food.

Boundary layer generation on the inside walls of a

food container limit the heating effect, which be-

comes more detrimental as the food viscosity in-

creases. Agitation of the food container can

overcome this resistance to some extent but it is im-

portant to match the rotational conditions to the

product viscosity, otherwise the benefits will not be

realised. Food containers can be rotated either axially

in a continuous canning system or end-over-end in a

batch retorting system. The continuous system offers

greater efficiency for long production runs whereas

the batch system offers greater flexibility.

Thermal Process Validation

0027Although pasteurization of the food is the desired

condition, the food is referred to as commercially

sterile. A pasteurization process usually operates to

6 log reductions of the target organism (further

details on pasteurization treatments can be found

in the CCFRA guidelines), and this differs from fully

sterilized foods where the intention is to achieve at

least 12 log reductions in Clostridium botulinum

spores. The lower target log reductions for

pasteurization are because of the reduced risks asso-

ciated with the target microbial species when com-

pared with the lethal botulinum toxin. The following

equation is used to calculate the process or F-value

from heat resistance data on the target pathogenic or

spoilage organisms. When the F-value is divided by

PASTEURIZATION/Pasteurization of Viscous and Particulate Products 4399

the decimal reduction time (D

) this gives the number

of log reductions of surviving spores.

F ¼ D

ðlog N

=NÞ

where N

final

is the final number of organisms after a

specific time-temperature history; N

initial

is the initial

number of organisms; D

is the decimal reduction

time at a fixed temperature (T) to reduce the number

of organisms by a factor of ten (minutes).

0028 To prove that the pasteurization process has

achieved the F-value it will be necessary to conduct

validation studies using an approved method. Various

methods can be selected, and their choice depends on

the costs and on the nature of the food and the process

type. Temperature measurements provide the cheapest

method but are not appropriate for all foods, and so

microbiological methods are required as alternatives.

0029 In a validation using temperature measurements,

the heat resistance data for the target organism is used

to convert the measurements to log reductions. For a

viscous food, the position within a container that

heats slowest is usually the geometric center and so

the probes are placed at the centers of several contain-

ers. To validate a process for a food containing par-

ticulates, a large food chunk or piece is usually

attached to the end of a temperature probe. While

this will work for incontainer processes, this method

cannot be used for continuous processes.

Microbiological Methods for Process

Validation

0030 These are often referred to as direct methods, but they

in fact rely on measuring the achieved log reductions

for a process using a non-pathogenic organism and

converting this to an F-value using the same equation.

If there are no surviving organisms then it is only

possible to conclude that the process achieved greater

than e.g. 6 log reductions for a 10

initial loading. In

this situation, there will be uncertainty as to whether

the organisms died as a result of the process, during

transportation to or from the factory, or if the spores

germinated during the come-up time making them

more susceptible to destruction at milder tempera-

tures than for the heat resistant spores. Hence, con-

trolling how these tests are performed is critical and

the expertise to conduct a test using encapsulated

spores or organisms tends to be restricted to a limited

number of microbiology laboratories. A microbio-

logical method can be conducted using organisms

distributed evenly throughout a food product or con-

centrated in small beads.

0031 The inoculated pack method is also known as

the count reduction method and involves inoculat-

ing the entire food with organisms of known heat

resistance. For ease of handling, the organisms are

usually in the spore form. It is essential that some

organisms survive the heat process in order that the

containers can be incubated and the surviving organ-

isms counted. The average thermal process received

by a container can be calculated using the equation

for F-value. If the product is liquid it is relatively easy

to introduce the organisms but for solid products it is

necessary to first mix the organisms in one of the

ingredients to ensure that they are dispersed evenly

throughout the container. Typical levels of the inocu-

lum are between 10

and 10

organisms per con-

tainer. An alternative is to use a gas-producing

organism and estimate the severity of the process by

the number of blown cans.

0032Encapsulating spores or organisms in an alginate

bead allows the organisms to be placed at precise

locations within a container or within the food par-

ticulates. The alginate bead can be made up with a

high percentage of the food material so that the

heating rate of the bead is similar to that of the

food. This method has been used for continuous pro-

cesses where the food contains particulates that re-

quire evaluation at their centers, and conventional

temperature sensing methods cannot be used. Large

numbers of alginate beads are used to determine the

distribution of F-values that can occur in continuous

processes as a result of the distribution of particle

residence times. Estimating the exact number to use

in a test is not straightforward because it depends on

the F-value distribution, which is not known until

after the test is conducted and the results analyzed.

The number of organisms used will be greater than

for an inoculated container test and can be of the

order of 10

per bead. It is also important that not

all are destroyed by the heat process otherwise it is

not possible to estimate an F-value.

General Conclusions

0033When pasteurizing a viscous food, the limits to heat

transfer are to overcome the development of bound-

ary layers that impair heat transfer rates at the heated

surfaces. Some degree of agitation is advantageous

irrespective of whether it is a continuous heat ex-

change process or one processed in a container. If

particulates are introduced then this imposes a further

limitation to heat transfer rates. The concepts of high

temperature short time (HTST) treatments do not

apply easily to viscous foods either with or without

particulates. Hence, most thermal processes will be

considerably longer than those for foods of lower

viscosity. Minimizing the thermal impact is a chal-

lenge to food processing companies operating in a

market where the consumer demands products of

4400 PASTEURIZATION/Pasteurization of Viscous and Particulate Products

increased viscosity or consistency, but high quality is

also essential.

See also: Heat Transfer Methods; Pasteurization:

Principles

Further Reading

Bolmstedt U (2000) New Food 3(2): 15–18.

CCFRA (1992) Pasteurisation Heat Treatments, CCFRA

Technical Manual No. 27. Chipping Camden, UK:

CCFRA.

Department of Health (1994) Guidelines for the Safe

Production of Heat Preserved Foods. London: HMSO.

Holdsworth D (1997) Thermal Processing of Packaged

Foods. London: Blackie Academic & Professional.

Richardson P (ed.) (2000) Thermal Technologies in Food

Processing. Cambridge: Woodhead.

Tucker G and Bolmstedt U (1999) Liquid Foods Inter-

national 3(3): 15–16.

Other Pasteurization Processes

G J Swart, Dairy Belle, Olifantsfontein, South Africa

C M Blignaut and P J Jooste, University of the

Orange Free State, Bloemfontein, South Africa

This article is reproduced from Encyclopaedia of Food Science,

Background

0001 Conventional pasteurization processes usually

employ continuous heat-transfer mechanisms. The

primary aims of these processes are to destroy patho-

genic organisms in liquid foods such as milk and to

extend the shelf-life of the product for a limited

period of time. (See Heat Transfer Methods.)

0002 In addition to the conventional processes, other

methods exist that employ alternative heating

methods, no heating at all, or methods that are not

generally regarded as true pasteurization processes.

These processes may, under certain circumstances,

have the same effect on foods as the conventional

pasteurization techniques. It is with this in mind that

the following processes have been included in this

article, namely pasteurization by irradiation, micro-

wave pasteurization, ohmic heating, ‘pasteurization’

by blanching, as well as lesser-known versions of the

pasteurization process, namely ‘cold’ pasteurization,

extrusion pasteurization, thermization, and chemical

preservation using hydrogen peroxide.

Pasteurization by Irradiation

0003Foods are irradiated for different reasons. When the

aim of food irradiation is the inactivation of certain

spoilage microorganisms that may be present in foods

without necessarily leading to sterilization, the pro-

cess is called ‘radurization.’ The ‘radicidation’ of

foods, however, is aimed at the inactivation of patho-

genic nonspore-forming bacteria and, in some cases,

will also inactivate toxigenic fungi, viruses, and para-

sites. Both radurization and radicidation have an

effect comparable to that of heat pasteurization. To

understand the process better, the following aspects

are dealt with in this discussion, namely the basic

theoretical principles, factors influencing the process,

uses, product suitability, nutritional implications,

and, finally, the acceptability and current status of

the process.

Basic Theoretical Principles

0004The underlying principle of food irradiation (and

therefore also irradiation pasteurization) is based on

chemical changes caused in foods by a form of energy

called ‘ionizing radiation.’ Different kinds of radi-

ation are included in this concept. Only a few of

these are suitable for use in the treatment of foods,

namely X-rays, g-rays (from

Co or

137

Cs), and elec-

tron beams (cathode rays and b-rays). Nonionizing

radiation, such as ultraviolet radiation, is essentially

absorbed at the food surface and does not have effect-

ive penetration properties. Although limited work has

been done with ultraviolet radiation, research in this

regard concentrates more on extending the shelf-life

of fruit and vegetables by controlling fungal spoilage,

such as fusarium rot (caused by Fusarium solani)

charcoal rot (caused by Marcrophomina phaseolina)

and soft rot (caused by Rhizopus stolonifer). Ultra-

violet lamps are also used to inhibit the formation of

bacterial slimes on surfaces. Of the two radiation

treatments, processes using ionizing radiation are

more important in the context of the present assign-

ment. (See Irradiation of Foods: Basic Principles.)

0005In the case of either radurization or radicidation,

an irradiation dose of less than 10 kGy will have the

desired effect. For radurization the doses required

vary within the 1–5 kGy range. Sometimes, even less

will suffice, e.g., beer radurization at 500 Gy. An

acceptable end product, however, is not necessarily

guaranteed. For the radicidation of foods, the

radiation doses vary according to the targeted organ-

ism. To control Salmonella contamination, for

example, doses of between 2 and 6.5 kGy have been

recommended to reduce the contamination level

three- to sevenfold. An estimated sevenfold reduction

of other pathogenic nonspore-forming foodborne

PASTEURIZATION/Other Pasteurization Processes 4401

bacteria, such as Shigella, Mycobacterium, Escheri-

chia, Staphylococcus, Streptococcus, and other

species, may also be obtained using doses ranging

from 5 to 8 kGy. It should always be remembered

that the suggested doses may be influenced by the

nature of the food to be irradiated. It is, therefore,

essential to take note of the factors that generally

influence irradiation processes. (See Escherichia

coli: Occurrence; Mycobacteria; Shigella; Staphylo-

coccus: Properties and Occurrence.)

Factors Influencing the Process

0006 The success of any form of irradiation depends on the

influence and interaction of certain factors. These

should also be taken into account when deciding on

pasteurization by way of irradiation, since it could

influence the postirradiation properties of the food.

These factors include:

0007 the resistance of some organisms to radiation;

0008 the rate at which radiation is applied;

0009 the water content of the food to be irradiated;

0010 the influence of temperature on subsequent

effects in the irradiated food;

0011 the presence of oxygen and additives during

irradiation;

0012 the possibility of combining irradiation with

other treatments to obtain the desired effect;

0013 the penetration ability of the selected type of

ionizing radiation within stated radiation limits;

0014 the elemental composition of the food (as a

whole) to be irradiated;

0015 the magnitude and composition of the initial

microbial population present in the food to be

irradiated;

10.

0016 the susceptibility of the product (such as fruit and

vegetables) to radiation damage;

11.

0017 cost factors with regard to the process.

Since the above-mentioned factors influence the end

result, attention should be given to the applications of

the process, product suitability, and nutritional impli-

cations that may result from irradiation.

Uses, Product Suitability, and Nutritional

Implications

0018 In order to understand the use of radurization and

radicidation, it is essential to take into account the

fact that these treatments should supplement normal

food-processing practices. Radurization especially is

used solely to prolong the shelf-life of products. As

such, it has shown promise with fresh products such

as fish, meat, poultry, vegetables, fruit, baked goods,

etc. (See Irradiation of Foods: Applications.)

0019 Radicidation is usually applied to products in

which radurization is not normally able to completely

eliminate pathogenic nonspore-forming bacteria.

Such foods include fruits and fishery products (e.g.,

shellfish and fishmeal), although it is necessary to

mention that radicidation of shellfish has little effect

on the presence of viruses. Irradiation of meat prod-

ucts, such as poultry and pork at radiation levels up to

7 kGy, have, however, been successful in controlling

pathogens, such as Salmonella, Campylobacter, and

Listeria species. These pathogens cannot be con-

trolled by good manufacturing practice alone. (See

Campylobacter: Properties and Occurrence; Listeria:

Properties and Occurrence.)

0020With regard to nutritive value, it has been shown

that normal nutritional values are retained in pro-

teins, lipids, and carbohydrates after irradiation.

The nutritional availability of minerals can, however,

be altered by the treatment, and a small amount of

vitamins may be destroyed. These losses, however,

compare favorably with those incurred during con-

ventional processes. These aspects should have a posi-

tive influence on the acceptability and current status

of radurization and radicidation. (See Irradiation of

Foods: Processing Technology.)

Acceptability and Current Status of the Process

0021Irradiation of foods is the one process for which

safety aspects have been considered very thoroughly.

These include aspects such as microbiological impli-

cations, effects on nutritional value, and the possible

production of toxic substances, carcinogens, and

radioactivity in treated foods. It may be concluded

that treated foods can be considered safe on all counts

(especially at the dose levels required by radurization

and radicidation), provided that:

0022approved doses are applied;

0023the foodstuffs to be irradiated do not possess ab-

normal levels of elements that could be rendered

radioactive; and

0024acceptable packaging materials are used.

0025A limiting factor with regard to both radurization

and radicidation processes is the possible develop-

ment of objectionable changes in the properties of

some foods, such as flavor, odor, color, texture, per-

ceived freshness, etc. Negative effects of irradiation

can, however, be counteracted by using the treatment

in association with other processes. Lower doses of

radiation (radurization) accompanied by a refriger-

ation process are proving to be technically and

economically feasible.

0026It should be mentioned that irradiation can also

have a product-enhancing effect, e.g., the enhanced

flavor of brandy. Research with regard to this aspect

of the process is continuing. Table 1 demonstrates the

current interest in low-dosage irradiation and fields

4402 PASTEURIZATION/Other Pasteurization Processes

of application. It clearly shows the wide interest (both

positive and negative) generated by the process.

0027 In conclusion, it should be stated that the current

status of the process depends on individual countries

and their health standards. Before commercial ex-

ploitation of the process by a manufacturer, the gov-

ernment of the country involved should approve or

have approved it. Although many countries permit

food irradiation, doubts still exist in some quarters

with regard to the safety of the process. (See Irradi-

ation of Foods: Legal and Consumer Aspects.)

Microwave Pasteurization

0028 Microwave processing of foods has been the focus of

much research since the 1940s. As a food-processing

method, however, it became of practical interest in

the 1960s with the development of multikilowatt

conveyorized ovens. The microwave principle only

came into general use in the 1980s, when microwave

heating applications started to increase. One of

theseapplications waspasteurizationusing microwave

energy. To understand the process better, this article

concentrates on the basic theoretical principles of the

process as well as its applications and acceptability.

Basic Theoretical Principles

0029 Microwaves are electromagnetic radiant energy

waves with wavelengths of between 0.025 and

0.75 m and frequencies of approximately 2450 and

915 MHz in the case of food applications. Most coun-

tries have standardized on 915 or 896 MHz for indus-

trial use. The energy level for a specific foodstuff is

chosen according to the amount of energy lost in the

process due to packaging, for example, since this

adversely affects the penetration capability of the

microwaves. If a high penetration level is required, a

microwave frequency with a lower loss factor should,

therefore, be selected.

0030Another important aspect is the fact that micro-

wave energy has an effect on dipolar molecules such

as water molecules. Application of microwave energy

causes these molecules to oscillate, causing intermo-

lecular friction and, ultimately, producing heat.

Heating is, therefore, achieved inside the foodstuff

being irradiated, depending on the type of applicator

and the foodstuff being treated. All in all, the treat-

ment facilitates control and the prevention of over-

processing in the case of pasteurization.

Applications and Acceptability

0031Microwave pasteurization as a process has been ap-

plied especially to milk and milk products. The Bach

process (using a dual-frequency application) claims

great success with regard to cultured milk products.

In the case of fluid milk, patents exist for a microwave

heat exchanger used in the pasteurization of milk. In

spite of the fact that different processes, such as

tbl0001 Table 1 Current interest in irradiation processes from differing perspectives

Type ofinterest Authorandyear of publication Title of paper Irradiation dose

applied

Source

Research Ma CY et al. (1990) Gamma irradiation of shell eggs; internal

and sensory quality, physicochemical

characteristics and functional properties

0.97, 2.37,

2.98 kGy

Canadian Institute of Food

Science and Technology

Journal 23(4/5): 226–232

Research Huhtanen CN (1990) Gamma radiation inactivation

of enterococci

Varying Journal of Food Protection

53(4): 302–305

Research Stevens C et al. (1990) Effect of ultraviolet radiation on mold rots

and nutrients of stored sweet potatoes

Varying Journal of Food Protection

53(3): 223–226

Research Harris T et al. (1989) Poultry meat irradiation – effect of

temperatures on chemical changes and

inactivation of microorganisms

0.5 –10.0 kGy Journal of Food Protection

52(1): 26–29

Research Dempster JF et al. (1985) Effect of low-dose irradiation (radurization)

on the shelf-life of beefburgers stored

at 3



1.03, 1.54 kGy Journal of Food Technology

20: 145–154

Research Modi NK et al. (1990) Effects of irradiation and temperature on

the immunological activity of

staphylococcal enterotoxin A

9.4, 12.2 kGy International Journal of

Food Microbiology

11: 85–92

Research Paster N et al. (1985) Preservation of a perishable pomegranate

product by radiation pasteurization

2.0, 4.0 kGy Journal of Food Technology

20: 367–374

Overview Various (1989) Food irradiation: a most versatile twentieth

century technology for tomorrow

Food Technology July:

75–97

General Lacey RW and

Dealler SF (1990)

Food irradiation: unsatisfactory preservative British Food Journal 92(1):

15–17

Reproduced from Pasteurization: Other Pasteurization Processes, Encyclopaedia of Food Science, Food Technology and Nutrition, Macrae R, Robinson RK

and Sadler MJ (eds), 1993, Academic Press.

PASTEURIZATION/Other Pasteurization Processes 4403

the high-temperature/short-time, multitherm, and in-

pack continuous process, have been developed, the

feasibility of microwave pasteurization of milk on an

industrial scale has not really been established.

0032 Microwave pasteurization has the following

advantages and disadvantages:

0033 Amino acids may be altered during the process,

and this aspect should be looked into in more

detail.

0034 Microwaves are successful in pasteurization be-

cause of their heat-generating action. The effect

it has on microorganisms as such, however, needs

to be resolved more fully.

0035 Microwave-pasteurized products retain the

properties of the fresh product.

0036 The process shows promise as a high-efficiency,

low-energy process. This limits bacterial build-

up in usually restricted flow areas of the system.

0037 The design of the system is of paramount import-

ance for successful processing.

0038 The effectiveness of heating is related to applica-

tor design and can be either more or less even

than conventional methods.

0039 The operating costs are high.

0040 Magnetrons have a relatively short lifespan and

are essential in the conversion of electrical energy

to microwave energy.

0041 Depending on the design, less factory space may

be required.

10.

0042 Safety is of great importance, since damage can

be caused to the eyes and other tissues that

absorb microwaves.

11.

0043 Both small static microwave ovens (home pas-

teurization) and complex, moving-belt micro-

wave tunnels (industrial application) may be

used. A schematic diagram of a microwave

system for processing foods under pressure is

given in Figure 1.

12.

0044Microwaves are not necessarily effective in

killing bacteria, especially in higher-density

products. Manual mixing has been suggested to

facilitate more uniform heat distribution.

Research in this field is continuing.

13.

0045Heat-sensitive products can be treated to great

advantage using microwave technology.

0046In conclusion, many questions relating to micro-

wave pasteurization and microwave technology in

general still remain to be answered. Continuing

research should, however, make more general appli-

cations a reality.

Blanching

0047Although blanching is not traditionally seen as a pas-

teurization process, some authors deem it to be a

‘kind’ or ‘type’ of pasteurization. While pasteuriza-

tion, however, has as its main function the destruction

of pathogenic microorganisms, it also destroys cer-

tain natural food enzymes in the process. Blanching,

however, is applied primarily to inactivate natural

enzymes in fruits and vegetables to be processed. Its

secondary effect is that it reduces the microbial load.

Both pasteurization and blanching are similar in that

they employ temperatures below 100



0048A process involving acid blanching and the add-

ition of ethylenediaminetetraacetic acid to a canning

brine has been a recent topic of research. The results

have shown potential with regard to the control of

spoilage and botulinal toxigenesis in canned prod-

ucts. This draws a further parallel between blanching

(a preparatory method) and pasteurization (a pro-

cessing method). (See Canning: Principles.)

0049With this in mind, the reader is referred to the

article on canning for further information.

Ohmic Heating

0050Ohmic heating is a new food-processing operation

which offers a major advantage in the continuous

processing and particulate food products.

Basic Theoretical Principle

0051Ohmic heating occurs when heat is internally gener-

ated by the passage of an electrical current through a

food product and is in this way similar to microwave

heating. In the case of ohmic heating, the penetration

depth, however, is virtually unlimited, and heat pene-

tration is more effective. The process depends on the

electrical conductivity of the product. Fats, sugars,

and syrups are, therefore, not suitable for this type

of processing, while foods that contain dissolved

ionic salts conduct electricity sufficiently and are,

Inlet air lock

Microwave section

Cooling section

Pouches

fig0001 Figure 1 Schematic diagram of a microwave system for

processing foods under pressure. From Decareau RV (1985)

Microwaves in the Food Processing Industry. London: Academic

Press, with permission.

4404 PASTEURIZATION/Other Pasteurization Processes