17 irradiation

17
Irradiation
D. A. E. Ehlermann, Federal Research Centre for Nutrition,
Germany

17.1

Introduction

‘Genetically modified food’ has become the object of a heated debate by consumer activists and replaced irradiation’s leading role as a target. In this debate
the term irradiation is frequently confused with radioactive contamination, especially after the Chernobyl accident. The allegation is made that the nuclear industry needs food irradiation badly in order to find some use for the waste from
nuclear power stations. In addition, the historical involvement of the US Army
in research on food irradiation is used as proof of its link to nuclear weapons and
military purposes.
However, this chapter on the radiation processing of food by ionising energy,
i.e. on food irradiation, highlights the history of the subject which extends over
a hundred years. It elaborates the peaceful background, emphasises that radiation
processing is a non-nuclear technology and elucidates the physical principles of
the interaction between ionising radiation and matter. This basic information is
then used to elaborate the beneficial effects of ionising radiation by describing
its chemical, biological and microbiological action in the food environment.
These two sections will lead to the radiological and toxicological safety of
food processed by ionising radiation. The aim of the Nutrition Handbook for
Food Processors is covered in a section on nutritional adequacy and is followed
by a section summarising the evaluation of overall safety by national and international expert groups.
Radiation processing has already found its area of commercial application,
governments have approved the process, the food industry is using it and where
the irradiated product is available on the market consumers respond favourably.
Under the WTO-agreement with the associated Codex Alimentarius standards

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(1984) and recommended by the WHO*, it is a tool that helps resolve several
recent problems of food production, manufacturing and marketing. It can greatly
support food safety and environment conservation and therefore serve the consumer. In conclusion, there is a list of sources of further information; for detailed
literature the reader is referred to the monographs referenced.
Several concerns have been voiced, for instance about nutritional quality,
radiolytic products, toxicology, microbiology, occupational safety, environmental side-effects, deception of consumers, consumer acceptance, substitution for
good manufacturing practice, negligent hygienic practice, misuse and increased
prices. These are the main arguments of certain consumer organisations against
the legal clearance of this technology. They still influence the officials and politicians who are responsible for the regulation of food technologies. However, with
the information available in this chapter readers should be able to make their own
informed decisions. References given are restricted to textbooks, monographs and
survey or review articles only, but interested readers will use them to lead to more
detailed information.

17.2

The history of food irradiation

As early as in 1885 and 1886 ionising radiation was discovered and in subsequent years its bactericidal effects were described. The purpose of the first
patent on food irradiation (Appleby and Banks, 1905) was to bring about an
improvement in food and its general keeping quality. It was followed by an
invention of an ‘Apparatus for preserving organic materials by the use of X-rays’
(Gillett, 1918). However, radiation sources strong enough for industrial exploitation were not available before the 1950s. The following five decades were devoted
to the development of this technology to a state where it could be applied both
commercially and industrially as well as to an investigation into the health aspects
of food treated by ionising radiation.

This was done in a world-wide, concerted effort; the US Army and the US
Atomic Energy Commission were involved and stimulated by Eisenhower’s
initiative ‘Atoms for Peace’. The academia were led by the Massachusetts Institute of Technology and followed by university and government research establishments in many countries. Details are given by Diehl (1995). Radiation
sources, such as radioactive isotopes and machines, became available and were
strong enough for treating food at commercial throughput. A radiation processing industry developed so that everyday goods could be produced by using ionising radiation. Floor-heating pipes, automobile tyres, car parts, electrical wires
and cables, shrinkable food packaging, medical disposables (syringes, implants,
compresses, bandaging material, blood transfusion equipment) – all are manufactured using ionising radiation. Even astronauts prefer irradiated food in their
diets.
* The WHO Golden Rules for Safe Food Preparation list under ‘Rule 1 “Chose foods processed for
safety”: . . . if you have the choice, select fresh or frozen poulty treated with ionizing radiation . . .


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The world-wide first food irradiation facility became operational in Germany
in 1957 for spices, but had to be dismantled in 1959 when Germany banned food
irradiation. In 1974 in Japan the Shapiro Potato Irradiator was commissioned and
is the oldest food irradiation facility still in operation today. When in 1980 the
JECFI made a landmark decision and declared irradiated foods as safe and wholesome for human consumption, it led many governments to permit the radiation
processing of food. This did not result in commercial application of the process
in all countries. Nevertheless, the total amount of food treated by ionising radiation is increasing, about 200 000 tonne per annum at the time of writing, but is
still a very small volume compared to the total amount consumed. However, food
irradiation is a niche application, supplementing traditional methods of food
processing and serving specific purposes.
Two important classes of application, sanitary and phytosanitary, are increasingly recognised.
As recently as 1993, children died tragically after eating undercooked (‘rare’)
hamburgers. This was caused by Escherichia coli type O157:H7 (EHEC), an
emerging pathogen microorganism which is now considered to be ubiquitous.

There is always the threat of such emerging hazards in modern, industrial food
production. Such risks can only be fought by further improvement of good manufacturing practices and the application of ‘Hazard Analysis and Critical Control
Point (HACCP)’. Adherence to such procedures and improvement of hygienic
concepts can only reduce or limit the hazard but never eliminate it. For this
reason, supplementary methods, in addition to good practices, help suppress such
residual risks to a tolerable, acceptable level. Ionising radiation is such a tool,
now legal in the USA and helping to make hamburgers, fresh or deep-frozen,
far safer for the consumer. Many other pathogen microorganisms are a threat
to society, causing death and illness, damages and economic losses. Other examples are Campylobacter and Salmonella in poultry, Salmonella in eggs,
Listeria in cheese and sprouts. Governments increasingly recognise the value of
radiation processing of food in fighting such threats to health and hygiene.
The threat to plant production (i.e. phytosanitary aspects) is less widely feared
but many areas that are very productive in fruit and vegetables have suppressed
several of the original pests. Such areas have strict quarantine controls on imports
that might carry insects or pests capable of proliferation. The USA is the leading
country in exploitation of ionising radiation for insect elimination: an X-ray
facility for treating fruit on Hawaii is now operational and allows for the direct
transport of fruit to mainland areas such as California. Also, other countries have
strict quarantine regulations; they include Australia, Japan and South Africa
where ionising radiation can play a valuable role. Certification systems presently
under development will help facilitate international trade.

17.3 The principles of irradiation
Processing by ionising radiation is a particular kind of energy transfer: the portion
of energy transferred per transaction is high enough to cause ionisation. This kind


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Table 17.1

Types of particle

Particle

Description

electron

An elementary corpuscle carrying one unit of positive or negative electrical
charge. The positively charged electron is called a positron.
A charged particle, identical to the nucleus of a helium atom, composed of
two neutrons and two protons. It carries two positive elementary units of
charge.
A charged particle, identical to an electron or a positron but emitted from a
radioactive nucleus.
A particle or photon emitted from a radioactive nucleus.
Fast-moving charged particles in an electric or magnetic field, usually
generated by high-energy electrons impinging on a high-atomic-number
absorber (e.g. tungsten); also called Röntgen-rays. They are generated by
braking radiation (bremsstrahlung).

alpha

beta
gamma
X

Wavelength [cm]
104

102

100

10–2

10–4

10–6

10–8

10–10

10–12

10–14

10–16

108

1010

Radio waves
Infrared
Visible light

Ultraviolet
X-radiation
Gamma radiation
Cosmic radiation
ionising radiation

100

102

104

106

Photon energy [eV]

Fig. 17.1 Range of energies (electromagnetic spectrum): ionising radiation is characterised by the ability to split molecular bonds and to transfer electrons; this energy limit
is indicated by the vertical, dashed line beginning in the range of ultraviolet light.

of radiation was discovered because the emitting radioactive material caused ionisation in the surrounding air. From the multitude of atomic particles known, only
gamma rays from nuclear disintegration and accelerated electrons are useful for
food processing (Table 17.1). Electrons may be converted into X-rays by stopping them in a converter or target (Fig. 17.1). Other particles such as neutrons


Irradiation
Electrons

375

Photons

1

1

4
4
5
3

3
2

Fig. 17.2 Interaction with matter (photon versus electron): 1) primary incident radiation,
2) Compton electrons caused by photon interaction, 3) secondary electrons and final
energy transfer, 4) irradiated medium, 5) finite depth of penetration for electrons.

are unsuitable because induced radioactivity is produced. The same may occur at
elevated energy levels with electrons and X-rays; for this reason the electron
energy is limited to a maximum of 10 MeV and the nominal energy of X-rays is
limited to 5 MeV. Gamma rays of cobalt-60 have photon energies of 1.17 MeV
and 1.33 MeV and cannot induce radioactivity; caesium-137 is not available in
commercial quantities but gamma rays of 0.66 MeV are emitted from it. This
means that gamma rays from available isotope sources are incapable of inducing
radioactivity.
Whether in the form of particles or as electromagnetic waves, the primary high
energy is broken into smaller portions and converted into a ‘shower’ of secondary
electrons (Fig. 17.2). These electrons finally interact with other atoms and molecules knocking out electrons from their orbits or transferring them to other
positions (Fig. 17.3). This means that an elementary negative charge is removed
and a positively charged atom or molecule, i.e. an ion, is left behind. If an electron has been transferred then orbital electrons are no longer paired and free

radicals are created. Both ions and free radicals are very reactive, in particular in
an aqueous medium such as in food, leading finally to chemical reaction products that are stable. The effects caused by corpuscular or electromagnetic radiation are essentially equal; the difference is in the dose distribution along the
penetration line into matter. Corpuscles have a definite physical range in matter,
they are slowed down by several processes of collision and finally stopped. They
have no energy beyond their range. Electromagnetic waves are attenuated exponentially and do not have a defined physical range.
A schematic diagramme of irradiation facilities (Fig. 17.4) helps to understand
the simplicity of the irradiation process: the goods are brought by a transport
system into the irradiation cell which essentially is a concrete bunker shielding


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Incident

Scattered

Ionisation

Secondary
electron
Orbital
electrons

Fig. 17.3 Principal diagram of ‘ionisation’: whether photon or electron, the incident particles interact with the orbital electrons and are scattered, an orbital electron is removed
gaining kinetic energy as a secondary electron; in this way an ionised atom/molecule is
left behind and a cascade of secondary electrons causes further ionisation or formation of
free radicals.

Beam handling system

(10 MeV electrons)

Radioactive source
(Co 60)

3
2
1

10
11

6

9
12

Irradiated food
product

8
7

Fig. 17.4 Schematic diagram of irradiation facilities: the product to be irradiated has to
pass through the irradiation zone; the design details largely depend on the physical properties of the type of radiation used and may be adapted to the packaging and handling
requirements of the goods.


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the environment and the workers from the radiation. A tunnel system allows free
access for the goods but prevents radiation leakage; fences and detectors prevent
unintentional access of anything or anyone when the radiation is ‘on’. Machine
sources (accelerators) emit the radiation uni-directionally, gamma sources
(radioactive isotopes) emit it in all directions. This means that for electron and
X-ray processing the goods pass just before the beam exit window and for gamma
processing the goods are piled and moved around the source to absorb as much
as possible of the emitted energy. When it is not needed a machine source is
simply switched off; for radioactive isotopes the frame with the source must be
moved to a safe position which is usually a deep water pool. The design of irradiation facilities is widely standardised; the safety-features are offically approved
and authoritative control is well established.

17.4

The effects of irradiation on food

There is a vast literature on the effects of ionising radiation on food and food
components; for the nutritional aspects of the subject a very few references are
sufficient (Diehl, 1995; Molins, 2001). Early textbooks even today are still relevant (Elias and Cohen, 1977, Josephson and Peterson, 1983) and in later years
there has been an updating of details (WHO, 1994).
The interaction of ionising radiation with matter takes place by means of a
cascade of secondary electrons carrying enough kinetic energy to cause ionisation of atoms and molecules and the formation of free radicals. Besides these
direct effects and primary chemical reactions chain reactions of secondary and
indirect transitions take place. In systems as complex as food and for biological
systems usually high in water content most primary reactive species are formed
by the radiolysis of water and the pathways of further reactions largely depend
on composition, temperature, dose rate and relative reactivities. Only for a few
very simple single-component models have the full pathways of reactions been

identified; for highly complex systems a complete picture has not yet been
achieved. Nevertheless, some aspects of the picture are beginning to emerge,
especially with regard to the main components, i.e. carbohydrates, lipids and
proteins. The effects of radiation on micronutrients, in particular on vitamins, are
complex and are also dependent on overall composition; some macronutrients
may protect micronutrients from radiolysis. Minerals and trace elements are
not studied because they cannot be affected by radiation processing of food.
However, the toxicological and nutritional consequences are discussed in further
sections of this chapter.
Biological effects include the beneficial use of irradiation for sprout inhibition, ripening delay and insect disinfestation. Microbiological effects include the
use of irradiation for the suppression of pathogen microorganisms and the reduction of other, spoilage-causing microorganisms. For both procedures, the principal reaction is irreversible radiation damage to the DNA disabling essential
functions of the cell. Such DNA changes are irrelevant with regard to food and


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nutrition. There was previous concern as to whether irradiation and recycling
these irradiated microorganisms could cause mutations that were capable of survival and were more toxic or vigorous as their precursors. It has been shown that
this is not the case and that ‘no special microbiological problems’ are introduced
(World Health Organization, 1981). The storage of irradiated food is important
in order to avoid growth of microorganisms or recontamination.

17.5

The safety of irradiated food

Irradiated food does not become radioactive and this is now accepted even by
opponents of the procedure. The limitation of allowable isotope sources to cobalt60 and caesium-137 and the limitation of the maximum energy of electrons to

10 MeV and of the maximum nominal energy for X-rays (bremsstrahlung or
braking radiation) to 5 MeV provides adequate safeguards. Even if the nominal
energy for X-rays is increased to 10 MeV the theoretically induced radioactivity
would be much less than the natural activity there already is in food due mainly
to the presence of potassium-40. Furthermore, it would be very difficult to
measure such sparse induced activity in the presence of the natural radioactivity.
It can be generally stated that the safety record of the radiation processing industry is slightly higher than that of other branches. There have been only a few accidents related to radiation exposure or radioactive contamination and the reason
for all of them was a conscious violation of safety rules or non-adherence to prescribed procedures that included bridging safety circuits.
From the beginning of systematic studies in the late 1940s it was recognised
that irradiated food needed careful toxicological study before the technology
could be applied to food manufacturing and processing. It is useless to question
why the word ‘radiation’ carries such a negative image and causes considerable
suspicion, not only among lay persons, but also among many scientists. In such
a situation, governments and food control authorities were well advised to restrict
the application of the new technology. However, further results have become
available and the final judgement has been stated by the World Health Organization (1981) as: ‘Irradiation of any commodity . . . presents no toxicological
hazard’. This means that governments and authorities are responsible for the consequences and recognise the radiation processing of food as safe and as simply
one among several other technologies. There have been thorough chemical
studies, leading to the principle of ‘chemiclearance’ and classes of food that are
chemically similar have been compared. It was also standard procedure to feed
the food under consideration to animals and to look for possible effects on factors
such as longevity, reproductive capacity, tumour formation, growth, unusual
behaviour, haematological and biochemical indices, chromosomal abnormalities
and genetic defects. These studies are very numerous and difficult for the nonspecialist to follow; expert reviews are available elsewhere (Diehl, 1995). There
have been also several publications reporting negative effects; however, a thorough follow-up always revealed deficiencies in the experimental organisation or


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in the final evaluation and validation of the results. This is not the place to discuss
such findings as increased polyploidy in malnourished children and the reasons
why those experiments have been dismissed by expert bodies; full details and
arguments can be found elsewhere (Diehl, 1995). It is sufficient to state that the
validation of competent expert bodies (World Health Organization, 1981, 1994)
always resulted in the ‘green light’ for food irradiation, and finally for any food
at any dose (World Health Organization, 1999).

17.6

The nutritional adequacy of irradiated food

Most food preservation and decontamination procedures, including irradiation,
cause some loss in the nutritional value of foods. Further losses generally occur
during storage and during preparation for consumption (e.g. in cooking). The specific chemical changes brought about in foods by irradiation include some that
alter the nutritional value, but the magnitudes of the changes are small when compared with those that result from other procedures currently in use. This has led
most expert groups to conclude that reduction in the nutritional quality of foods
resulting from the widespread use of irradiation is an insignificant part of the total
diet as a whole (Elias and Cohln 1977; Advisory Committee on Irradiated and
Novel Foods, 1986). One expert group concluded that ‘irradiation of food . . .
introduces no special nutritional problems’ (World Health Organization, 1981).
This conclusion emphasises the word ‘special’, recognising that there might be
particular problems with some individual food products. Most expert groups also
recommend that the nutrient content of irradiated foods should continue to be
monitored while such foods are being introduced.
A problem with many of the literature reports on the effects of irradiation on
food constituents is that the studies have used laboratory ‘model’ experiments,
often with pure or relatively pure target substances and irradiated in such media
as water or buffers. Whilst these studies are ideal for investigating the chemistry

of the radiation-induced changes, it is very difficult to extrapolate from them to
the situation in real foods. In real foods, many of the other components present,
usually in large quantities, interact, quench and otherwise interfere with the
reactions of the radiolysis-derived products. Consequently, the magnitude of
the changes that occur in specific components in a food matrix is generally
much lower than the magnitude of those observed in simpler laboratory studies
(Josephson et al, 1979).
In general, the nutritional values of the macronutrients in foods (e.g. the carbohydrate, lipid and protein components) are very little affected by ionising radiation. Some of the micronutrients, including some vitamins and polyunsaturated
fatty acids, are more sensitive but their sensitivity is very dependent on the nature
of the food. At the 1 kGy dose level, which is in excess of insect disinfestants
applications, virtually no nutrient depletion is usually measurable although there
have been reports of rise and fall in ascorbic acid (vitamin C) levels made in conflicting publications. At the 10 kGy level, the vitamins ascorbic acid, thiamine


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(vitamin B1) and pyridoxine (vitamin B6) are generally the most sensitive to
change but the extent varies considerably and depends on the specific food.
Certain minerals and trace elements are essential for health but their irradiation at the energies employed in food processing does not result in any change
(Harris and von Loeseke, 1969).

17.7

Vitamins

Some vitamins are well known for their sensitivity to the effects of ionising radiation. Their inactivation (i.e. loss of biological activity) results predominantly
from reactions with free radicals and other reactive species generated by the radiolysis of water in foods. Since these reactive molecules interact with a wide
variety of food components, the exact effect of irradiation on a particular vitamin

depends not only on the chemical nature of the particular vitamin, but also varies
greatly with the nature of the food itself. In vitro studies, in which dilute solutions of vitamins have been irradiated, may indicate sensitivities that are never
seen in foods, where substantial ‘quenching’ by competitor molecules usually
occurs (Goldblith, 1955).
Reactivity of individual vitamins varies according to their chemical nature
(World Health Organization, 1994). The most important with respect to food irradiation, include the water soluble vitamins: ascorbic acid (vitamin C); thiamine
(vitamin B1); riboflavin (vitamin B2); niacin (vitamin B5); biotin (vitamin B10);
folic acid (pteroylglutamic acid); pyridoxine (vitamin B6); pantothenic acid;
cyanocobalamin (vitamin B12); and the fat soluble vitamins: retinol and some of
its derivatives (vitamin A); calciferol and some of its derivatives (vitamin D);
tocopherols (vitamin E); naphthaquinone derivatives (vitamin K).
Among the fat-soluble vitamins the ranking by decreasing sensitivity to radiation is:
E >> A >> D >> K
Carotenoids have a similar sensitivity to vitamin A. However, this is no strict
order as sensitivity is largely affected by the protective properties of the other
main components of a particular food. For this reason, conflicting findings from
the published literature are easily explained by the experimental conditions,
sometimes using low concentrations of a single vitamin in a solvent which does
not resemble a real food. Such findings always need expert interpretation. Among
the water-soluble vitamins B1 (thiamine) is the most sensitive. However, it is
notable that radiation-sterilised pork and beef still retains more thiamine than a
heat-sterilised equivalent. The most contradictory results have been obtained with
vitamin C. One main explanation is whether only ascorbic acid or ascorbic and
dehydroascorbic acid was determined, or whether the results are reported as ‘total
vitamin C’. Vitamin C is also very sensitive to storage conditions and natural
variability might even mask irradiation effects. The following sections discuss
particular vitamins.


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17.7.1 Vitamin A (retinol)
Dry retinol and dietary precursors, such as b-carotene, are relatively radiationtolerant, with little inactivation brought about by doses up to about 20 kGy
(Lukton and MacKinney, 1956). Even doses as high as 200 kGy only reduced bcarotene levels in tomatoes by about 10 to 20% (Lukton and MacKinney, 1956),
depending on whether or not oxygen was present. Irradiation of carrot purée at
20 kGy caused no more than a 5% loss. Changes in vitamin A activity in fruits
given low doses for disinfestation or to delay ripening were well below this level
of loss, e.g. mangoes (Thomas and Janave, 1975), papayas and strawberries
(Beyers et al, 1979).

17.7.2 Vitamin B
This section discusses the following vitamins: B1 (thiamine), B2 (riboflavin), B5
(niacin), B6 (pyridoxine), B10 (biotin) and B12 (cyanocobalamin).
Irradiation of thiamine causes deamination and destruction of the pyrimidine
ring (Groninger and Tappel, 1957) with loss of biological activity (Ziporin et al,
1957). Thiamine is relatively radiation sensitive in some foods.
Low disinfestation doses of 0.25–0.35 kGy, delivered to cereal grains resulted
in losses of thiamine of 20–40% (Diehl, 1975). In cooked pork chops, irradiated
at 0.3 and 1.0 kGy (the dose range proposed for Trichina control), losses
were 5.6 and 17.6% (Fox et al, 1989). It is calculated that loss of thiamine in the
American diet, due to irradiation of pork chops and roasts, would be 1.5% at
1 kGy.
When radiation doses as high as 25 kGy were used, raw fish retained nearly
40% of total thiamine (Brooke et al, 1966), and treatment of clams, at 45 kGy,
led to no detectable loss of thiamine (Brooke et al, 1964). Following a major US
study of the potential nutritional and toxicological effects of radiation sterilisation on chicken breasts, Black et al (1983) concluded that g-irradiation, at doses
of 45–68 kGy, reduced thiamine levels to a similar level as that produced by heat
sterilisation.

As a consequence of its relative chemical inertness, riboflavin is the vitamin
most resistant to irradiation in the majority of foodstuffs. Sometimes levels of
riboflavin in foods have been found to rise following irradiation, most probably
due to release from binding to proteins, e.g. in pork meat (Fox et al, 1989) and
onions (Le Clerk, 1963).
Although slightly less stable to irradiation than is riboflavin in simple aqueous
solution, niacin has substantial radiation tolerance in foods. As has been observed
with riboflavin, niacin levels in some foods rise on radiation, e.g. in pork
and chicken (Fox et al, 1989) and in bread made from irradiated flour (Diehl,
1980).
In general, radiation-induced losses of pyridoxine in foods have been found
to be small, similar to or slightly greater than losses of thiamine. Losses induced
by radiation sterilisation of poultry and liver, at doses up to 55 kGy, were less
than those induced by sterilisation by heat (Richardson et al, 1961). The converse


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was true for cabbage. Most studies have found little pyridoxine loss in foods
irradiated at realistic doses and little further loss on subsequent storage.
Biotin is very radiation resistant in foods. Sterilising doses of gamma and electron beam irradiation did not significantly reduce levels in poultry (Black et al,
1983), or in eggs, at doses up to 50 kGy (Kennedy, 1965). Its relative stability to
irradiation is reduced in the presence of oxygen more so than that of the other
vitamins (Watanabe et al, 1976).
Most studies have indicated that little or no loss of vitamin B12 occurs during
food irradiation, e.g. in various seafoods at doses up to about 4.5 kGy (Brooke et
al, 1964); in pork irradiated at doses up to about 7 kGy (Fox et al, 1989); and in
poultry, in a study comparing the nutritional effects of preservation by freezing

with sterilisation by heat and with sterilisation by irradiation (Thayer et al, 1987).

17.7.3 Vitamin C (ascorbic acid)
Ionising radiation initially induces oxidation of ascorbic acid to dehydroascorbic
acid (Barr and King, 1956). This reaction (in which the biological activity of
the vitamin is retained) has been found to occur in many studies of irradiated
fruits and vegetables. Further irradiation eventually leads to losses of activity as
biologically non-functional products are formed.
Low doses of g-radiation, used to delay sprouting of potatoes, reduced ascorbic but not dehydroascorbic acid levels. However, during subsequent storage,
ascorbate levels rose, so that the differences between irradiated and non-irradiated potatoes disappeared (Schrieber and Highlands, 1958). Similarly, losses of
ascorbic acid in orange and lemon juices, irradiated at 16 kGy, were accompanied by neat stoichiometric increases in dehydroascorbic acid (Romani et al,
1963).
Losses of about 16% ascorbic acid occurred in 3 kGy irradiated freeze dried
apples. Tomatoes lost between about 8 and 20% according to the state of ripeness
of the fruit (Maxie and Sommer, 1968). In other studies, virtually no losses of
vitamin C (nor of the B vitamins riboflavin, niacin or thiamine) were detected in
mangoes, papayas, lychees or strawberries, irradiated at 2 kGy (Beyers et al,
1979). No vitamin C losses were detected in grapefruits irradiated at up to about
1 kGy (Moshonas and Shaw, 1984).
Overall, the most likely changes occurring in low dose irradiated fruit and vegetables seem to be the conversion of a proportion of ascorbate to dehydroascorbate, and, sometimes, a small reduction in total vitamin C level. This reduction
may then be reversed in intact fruits and vegetables as metabolism continues.

17.7.4 Vitamin D (calciferol)
Although the presence of water increases sensitivity, the D vitamins are relatively
stable to ionising radiation in their normal lipid-rich food environments. At doses
of up to 15 kGy, cholecalciferol is more radiation resistant than is vitamin A or


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vitamin E. Irradiation resistance of vitamin D in fish oils was even greater than
in solvents, such as iso-octane, presumably due to the presence of tocopherols
and other naturally occurring antioxidants (Knapp and Tappel, 1961).
17.7.5 Vitamin E (tocopherols)
Vitamin E is the most radiation-sensitive of the fat-soluble vitamins. A sterilising dose for beef (30 kGy) reduced beta-tocopherol levels by about 60% in air,
but not significantly in nitrogen. Alpha- and gamma-tocopherols decreased similarly in irradiated chicken breast (Lakritz and Thayer, 1992).
Oats irradiated at a dose of 1 kGy had lost only 5% tocopherol after 8 months
storage in nitrogen, but nearly 60% when stored in air (Diehl, 1979). Diehl (1980)
reported a near 20% loss following 1 kGy irradiation of hazelnuts.

17.8

Carbohydrates

Apart from water, the major constituents of most foods are carbohydrates, proteins and lipids. Irradiation of low molecular weight food carbohydrates, such
as glucose, mannose, ribose and lactose results in the formation of low levels
of radiolytic products mostly derived from reaction of hydroxyl radicals (OH°),
generated from water, with the sugar. A predominant reaction is the oxidation of
hydroxyl groups, often with loss of a neighbouring hydroxyl group. Products such
as 2-deoxy-gluconolactone and gluconic acid are formed, and the pH value of
simple sugar solutions falls (von Sonntag, 1980). Carbohydrates irradiated in the
solid state are generally more resistant than those irradiated in solution.
Irradiation of high molecular weight carbohydrates (starch, pectin, cellulose,
carrageenans, etc.) sometimes causes major changes in the physical properties of
the foods that contain them. Properties such as viscosity, mechanical strength,
swelling and solubility are likely to change in such a way as to reduce their functionality in a food, but sometimes change to improve their effectiveness for a
particular function.
Irradiation of lignocelluloses, in woody materials, has been shown to increase

their subsequent biodegradability by microorganisms such as Flavobacterium
species (Bhatt et al, 1992). The limited breakdown that occurs increases their
susceptibility to the microorganism’s hydrolytic exoenzymes. The properties of
gums, such as Karaya gum (Le Cerf et al, 1991) change greatly on irradiation
with, for example, very large increases in solubility, falls in viscosity and loss of
water-swelling properties.

17.9

Lipids

The irradiation of unsaturated fatty acids in foods predominantly results in the
formation of alpha and beta unsaturated carbon compounds (Nawar, 1983).


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Further reaction, and the addition of oxygen, leads to the formation of a hydroperoxyl radical.
R

CH

CH
O

CH

R1

O

Then formation of a hydroperoxide:
R

CH

CH
O

CH

R1

OH

The hydroperoxides are generally unstable in foods and breakdown to form
mainly carbonyl compounds, many of which have low odour thresholds, and contribute to the rancid notes often detected when fat-rich (and particularly unsaturated fat-rich) foods are irradiated (Hammer and Wills, 1979; Wills, 1981). For
example, irradiation of whole egg and egg yolk powder resulted in the generation of lipid hydroperoxides (Katusin-Razem et al, 1992). In the absence of air,
their formation was limited by available oxygen. Interestingly, destruction of
carotenoids was strongly correlated with hydroperoxide formation. Irradiation in
the presence of oxygen leads to accelerated autoxidation (Diehl, 1995), but the
end products are similar to those found following long storage of unirradiated
lipids (Urbain, 1986).

17.10

Proteins

Many studies of the nutritional effects of irradiation on proteins have been made
with generally only small or insignificant changes found. For example, irradiation of fish and meat meal, eggs, wheat and wheat gluten (Kennedy and Ley,
1971) showed little change in nutritive value in feeding studies after irradiation
at doses up to 10 kGy. The biggest changes were in wheat gluten (7%). At 50 kGy
larger losses occurred, but were largely reversed by supplementation of the diets
with methionine.

17.11

The wholesomeness of irradiated food

The definition of ‘wholesomeness’ (in the sense of being sound, healthy, clean
and otherwise fit for human consumption) requires some elaboration as it does
not occur in food laws and regulations. It was originally introduced in the 1950s
in the US, expanded by the FDA and others and developed in food and inspection acts. At the same time the studies on the safety of irradiated food for consumption were begun on a large scale and in international cooperation. During


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the procedure the terminology of wholesomeness was unanimously accepted to
mean ‘safety for consumption’ under any relevant aspect and comprises the following features: radiological safety, toxicological safety, microbiological safety,
and nutritional adequacy.
The main contribution to the judgement of wholesomeness was made by the
FAO/IAEA/WHO Joint Expert Committee on Food Irradiation (World Health
Organization, 1965, 1970, 1977, 1981). At the time of the Committee’s foundation in 1961 it was concluded that ‘general authorization of the commercial use
of radiation for the treatment of food is premature’. Based on the work of the
International Project in the Field of Food Irradiation (IFIP, founded in 1970 and
concluded in 1981) and on the international work coordinated through IFIP the

World Health Organization concluded finally (1981) that:
. . . the irradiation of any food commodity up to an overall average dose
of 10 kGy presents no toxicological hazard; hence, toxicological testing
of foods so treated is no longer required.
and
. . . the irradiation of food up to an overall average dose of 10 kGy
introduces no special nutritional or microbiological problems.
Several national advisory groups have endorsed those findings and many
national governments that had previously banned food irradiation introduced permission legislation for irradiated food. In addition, the European Commission
asked their Scientific Committee on Food for advice, the JECFI conclusions were
expressedly endorsed and a list of foods for clearance was proposed. However,
at the time of writing no resolution of this issue has been achieved.

17.12

Current and potential applications

Some of the benefits of food irradiation are listed in Table 17.2. Food safety will
present great challenges for all involved, including governments and industry
(Loaharanu, 2001; Osterholm and Potter, 1997). Globalisation brings food from
previously unavailable sources to markets that were previously unreachable. Production conditions are not always acceptable and the rules and regulations that
are already in place do not always accord with targets and measurements. This
means that attempts at harmonisation are indispensable (Mortarjemi et al, 2001).
New hazards are emerging which means that appropriate and coordinated action
must be taken. Food security is still the main issue in developing areas, but in
industrialised countries the problem of food safety is paramount and the main
aspects are hygienic quality and, in particular, microbial contamination (Doyle,
2000). Processing of food by ionising radiation is a perfect tool (Molins et al,
2001), supplementing traditional methods and in some applications is the only

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Table 17.2

Some of the benefits of food irradiation

Benefit
•
•
•
•
•
•
•
•
•

improves microbiological safety
reduces chemical treatment
facilitates international trade regarding food safety and quarantine security
improves availability and quality of tropical products previously unavailable
fresh food remains fresh, raw food remains raw
can be applied to solid foods for pasteurisation
can be applied in the frozen state; there is no need for warming-up
leaves no residue
can be applied to pre-packed food

procedure available. Irradiation will not be at all effective if it displaces good
practices and is most effective if used as the final critical control point in an
overall HACCP-concept.
Under the SPS-agreement (Sanitary and Phytosanitary) food safety is the first
aspect and quarantine for plant products is the second. Here again, radiation processing is a perfect tool to achieve quarantine and at the same time conserve the
environment, thus avoiding the use of ozone-layer depleting chemicals. It contributes to occupational safety by avoiding the use of toxic fumigants. Both uses
are spreading, the volume of treated goods is increasing; governments and competent international bodies are developing harmonised protocols. The potential of
such applications in the future is high despite the fact that at present world-wide
only about 250 000 tonne per annum are irradiated (Loaharanu, 2001).

17.13

Consumer attitudes and government regulations

It is widely said that consumers reject food irradiation and any irradiated product
on the market will be turned down. Table 17.3 lists common objections to irradiation. In most countries, however, irradiated food products are not on the market
and the consumer has no decision left to buy or to abstain and so it appears that
there is no consumer demand. On the other hand, the food industry is reluctant
to bring irradiated products on the market or to be identified with food irradiation. There have even been advertising campaigns of consumer activists publishing names of companies who guarantee that their products are not treated by
ionising radiation.
This is a vicious circle which can only be broken by strong arguments which
has occurred in the US where a number of people, in particular children, died
because they ate undercooked, ‘raw’ hamburgers. The reason is a nearly unavoidable infection of raw, minced meat by Escherichia coli, including type O157:H7
(also called EHEC). E. coli microorganisms are deadly and can spread despite
tight hygienic measures; an effective safety measure is radiation processing to


Irradiation
Table 17.3

387

Some arguments against food irradiation

Argument
•
•
•
•
•
•
•
•
•
•
•

allows lax food hygiene
does not remove toxins
spores are not killed
causes loss of nutrients
freshness is apparent rather than real
spoilage may flourish without warning signs
impairs sensory quality
increases costs
harms the environment
endangers personnel
needs restructuring of the total logistics

fight any residual risk. ‘Red meat’ irradiation has been legal in USA since 1999

and has been applied on a commercial scale since the middle of the year 2000.
An increasing number of food suppliers now rely on hamburger patties for domestic use and for institutional catering which should be stamped with the words:
‘irradiated for your safety – serve with confidence’.
The consumer appreciates the availability of such products and the choice
between irradiated products and those which are not irradiated. The indications
are that the well-informed consumer will respond favourably to the irradiated
product once it becomes available to the market as well as being open-minded
and ready for pertinent, trustworthy information. Scientific and sociological
studies back these observations. Activists against food irradiation play guardians
for an ‘under-age population’.
There have been other studies on the market place, for instance on the sale of
irradiated fruit from the Hawaiian islands in mainland USA where strict quarantine regulations against Mediterranean fruit fly are in place. Consumers responded
favourably to such tests and now the product is on the commercial market; in
Hawaii a facility dedicated to fruit irradiation has been established.
From this it can be inferred that alleged consumer resistance to irradiated food
either does not exist or that it can be overcome (anon, 1998). Such resistance is
created by certain opponents and is taken over by a timid food industry. In the
history of food irradiation there have been many ill-founded claims, misunderstandings, half-truths, and intentional distortions. Much controversial information
has been published and the discussions contain more emotion than fact. However,
the professional view is that the benefits by far outweigh any potential, still
unidentified, risk.
From this position of alleged consumer resistance, reinforced by the loud
voices of consumer activists, politicians and governments are very cautious when
it comes to regulating food irradiation. The Codex Alimentarius (1984) in its standard on food irradiation does not restrict by individual food, nor by groups or


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Table 17.4

Legislation concerning food irradiation

European Union clearances
only for ‘dried aromatic herbs, spices and vegetable seasonings’: Austria, Denmark,
Finland, Germany, Greece, Ireland, Luxembourg, Portugal, Spain, Sweden
‘dried aromatic herbs, spices and vegetable seasonings’ and other specified items:
Belgium, France, Italy, Netherlands, United Kingdom
Non-EU countries in Europe
Clearance: Croatia, Czech Republic, Hungary, Norway, Poland, Russian Federation,
Switzerland, Turkey, Ukraine, Former Yugoslavia
Other countries with clearances
Asia/Pacific: Australia, Bangladesh, China, China, Republic of (Taiwan), Indonesia,
India, Iran, Japan, Korea, Pakistan, Republic of, Philippines, Thailand, Vietnam
Africa (including Middle East): Egypt, Ghana, Israel, South Africa, Syrian Arab
Republic
Latin America (Middle and South): Argentina, Brazil, Chile, Costa Rica, Cuba, Mexico,
Uruguay
North America: Canada, United States of America

classes, most countries have preferred to regulate by this approach (Table 17.4).
A very few have adopted Codex Alimentarius completely (namely Brazil, which
has removed any upper dose limit, Ghana, Mexico, Pakistan, Turkey and ASEAN
member states). On the other hand, the USA prefers ‘permit as petitioned’ and
requires documentation in addition to Codex Alimentarius and WHO evidence
(the US regulatory system is explained in much detail elsewhere (Looney at al,
2001)).
At present, some 53 countries have regulations on food irradiation; this varies
widely and conflicts with international trade. The International Consultative

Group on Food Irradiation (ICGFI) holds an inventory of regulations by country
and by item ( and provides other useful information. In
some cases, the minimum or the maximum dose or both are regulated; in other
cases an ‘average’ is regulated. Under the aspects of food irradiation technology
only the upper and the lower dose limits are of interest because they are related
to the effectiveness of the treatment. An average dose is of interest under rather
rare circumstances: a liquid being stirred after irradiation.
The idea of ‘overall average dose’ originated from toxicological considerations (World Health Organization, 1981), a concept which is totally unsuitable
for regulatory purposes. Regulating the average requires the food inspector to
execute a certain integration over a prescribed extent of the sample. This problem
has been resolved by more recent regulations in the Netherlands and in the United
Kingdom: the regulated reference value is the average of the ‘batch’ and the
minimum and the maximum dose values are strictly bound to this set value.
However, the common regulation for all EU-members falls back to crude average


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limits. As such details vary widely and are sometimes contradictory, international
trade is severely hampered. For this reason regional harmonisation efforts have
been undertaken (1993/1999 Asia/Pacific; 1996 Africa; 1997 Latin America and
Caribbean; 1998 near east) and are now being implemented by ASEAN-members.
Imports and exports are permitted in most regulations; the mutual conditions,
however, are not coordinated. In particular, the European Union requires that the
irradiation facility in the exporting country must be registered with and inspected
by the EU authorities.

17.14 World Trade Organization, Codex Alimentarius

and international trade
The World Trade Organization (WTO) agreement has replaced the former GATT
and the standards of Codex Alimentarius have become the indisputable reference
for trade in food. Most countries have adopted the rules of WTO and disputes
between signatories must be settled in a WTO conciliation procedure. Such disputes have already arisen in the European Union because of its regulations on
‘hormone beef’ and on ‘dollar bananas’. Irradiated food might become the next
case. The specifications in the Codex Alimentarius Standard on Irradiated Food
and its associated Code of Practice (1984) are not restricted to any class or group
of food. Such specifications are presently under revision, and the upper dose limit
will be removed in order to follow the latest development (World Health
Organization, 1999).
The Joint FAO/WHO Codex Alimentarius Commission (CAC) was created in
1962 with the intention to facilitate international trade in food by world-wide harmonisation and the Codex Alimentarius has become a collection of accepted and
internationally recognised standards. With the emergence of WTO these standards
have become the only technical reference; there are particular references under
the agreements on Technical Barriers to Trade (TBT) and on Sanitary and Phytosanitary (SPS) measures which are an integral part of WTO. In this situation,
even the existence of any regulation that does not cover all food but restricts permission to a particular list may be considered as a TBT and a violation of the
WTO rules. The SPS agreement explicitly refers to processing by ionising radiation as one of the generally acceptable tools for achieving sanitary and phytosanitary purposes. Only a few countries have initiated the legal procedures to
convert to the new framework (as described for Brazil in section 17.13). The
European Union in particular has issued directives on food irradiation that are at
variance with the rules of WTO underwritten by all EU-Member States. For
instance, Germany is bound by a parliamentary vote to object to and block any
clearance of food irradiation beyond spices: ‘We strictly object to any expansion
of the (EU) “positive list” [which presently only contains spices] because we
deem irradiation of any further products as unnecessary.’
As can be seen from the possible applications and from reports from many
countries on their food safety and food security needs it is obvious that there is


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a technological need for processing food by ionising radiation. Negating such
needs lacks arguments founded on sound science which is a prerequisite for regulations under WTO. Or as WHO worded it, the countries which need the new
technology most would also suffer most from the resistance of developed countries. There are provisions in national regulations for imports from third countries. However, when using such rules administrative obstacles must still be
overcome. The US is preparing to accept imports of irradiated fruits from its
southern partners; ASEAN member states are harmonising their national regulations; many countries are joining forces to develop the standards of the Codex
Alimentarius to the present state of the art. The parties to the Montreal Protocol
of 1997 agreed to a phase out (2005 and 2015 for advanced and developing countries, respectively) of several fumigants, with radiation processing being the technology that will take its place. Other fumigants such as ethylene oxide are already
banned in several areas because of their toxicological properties and radiation
processing has been demonstrated as an effective replacement for such fumigants.
The International Plant Protection Organization (IPPO) has decided that radiation processing is the broad spectrum quarantine treatment that has no specific
requirements regarding insect species or host commodities. Regional organisations such as the North American Plant Protection Organization (NAPPO), the
European and Mediterranean Plant Protection Organization (EPPO) and the Asian
and the Pacific Plant Protection Commission (APPPC) have endorsed this alternative technology. Furthermore, under such competent bodies certification
systems have been developed to facilitate international trade in commodities
carrying a phytosanitary risk. Similar efforts have not yet been undertaken for
sanitary purposes.

17.15

Future trends

Industrialised countries increasingly face the problem of providing their populations with safe food. As the tip of the iceberg, Escherichia coli type O157:H7
(EHEC) has become a major threat to the US food industry. The arrival of ‘electronically pasteurised hamburgers’ on the US market, i.e. treated with high-energy
electrons as ionising radiation and their acceptance by the consumer mark a new
era; the change in the public opinion occurred when consumers realised the
deadly risk of foodborne pathogen microorganisms. It would be too simple an
argument to state that only the diversion of the US food industry from natural

production to industrial mass production is the cause of this new challenge. The
increase in world population and the concentration of population in centres of
economic activity and wealth is unavoidable, and there is no way to return to the
days of our ancestors. Even under strict hygienic control and at the best level of
Good Manufacturing Practices there always remains a residual hazard. Radiation
processing of food can help ease this problem, to improve hygienic quality of the
food available, to save human lives, to save costs to the society and its social
system and to contribute to the wellbeing of everyone. Ionising radiation is one


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391

tool among many; it has specific limitations and advantages but is superior to traditional means in its hygienic applications.
Less industrialised and developing countries in particular face the problem to
secure the food supply; the growing population can ‘eat up’ any increase in food
production and 20 to 40 % (estimation by FAO) of the harvest can be lost during
distribution and storage. Ionising radiation is a tool here but must be combined
with substantial improvement of the logistics of the food production and distribution system. The usual improvement of Good Agricultural Practices and Good
Manufacturing Practices alone cannot alleviate the problem.
Furthermore, the application of ionising radiation can replace traditional
chemical treatments that are becoming more and more suspect. Fumigants which
were an upholder of agricultural production are likely to become unavailable in
the near future; several developed countries have banned their use because of the
toxicity to workers of some and of the ozone-depleting properties of others (as
detailed in the Montreal Agreement). Most advanced countries have even banned
the imports of raw materials produced using such chemicals and their long-term
availability is questionable because production is already dramatically reduced.
Because developing countries achieve a considerable part of their gross net

income from exports of food and agricultural raw materials this development is
a threat to their economies. Insect infestation is a major cause of such loss of
food exports, followed by spoilage through moulds, yeasts and other bacteria.
Once the harvest is stored in insect-tight silos and transported in insect-tight
sacks, re-infestation can be avoided and the contents are preserved for human
consumption, the insects can be prevented from proliferation by irradiation. When
the storage of grain, cocoa and coffee beans is under controlled humidity conditions, the outgrowth of pathogen bacteria is retarded and the formation of mycotoxins excluded.
Consequently, food irradiation is a tool that supplements traditional methods
of food preservation; it has already found its niche application. The total volume
of goods treated is still small, estimated at about 200 000 tonne per annum, one
half of which is spices and dry seasonings. Official statistics are unavailable for
other methods such as canning, cooling and freezing. As the development in the
US clearly demonstrates, the industrial implementation of radiation processing
and its acceptance by the consumer come at the time when awareness for such
needs has been established and the product is clearly labelled. This means that a
slow but steady growth of the amount of irradiated food is to be expected.

17.16

Sources of further information and advice

At present, 45 governments are members of the International Consultative Group
on Food Irradiation (ICGFI) with a secretariat at Vienna, Austria (c/o IAEA, P.O.
box 100, A-1400 Vienna, Austria; ICGFI has developed technical guidelines and Codes of Practice for radiation processing of food.
It is an international group of experts designated by Governments to evaluate and


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advise on global activities of food irradiation. A few very general and introductory publications are available (Satin, 1996; Murano, 1995; World Health Organization, 1991); renowned and competent organisation have published ‘position
papers’ (Olson, 1998; anon, 2000; anon 1999); full details of the technology are
covered in multipage compendia (Josephson and Peterson, 1983; Elias and Cohen,
1977; Elias and Cohen, 1983). The crucial question of the safety of irradiated food
is covered in full detail by Diehl (1995) and WHO has published the results of
expert evaluations (World Health Organization, 1981, 1994 and 1999). The actual
status of the technology can be determined from the proceedings of recent conferences (Loaharanu and Thomas, 1999) and from textbooks (Molins, 2001). Nor
must it be forgotten that in many countries national, competent bodies have published positive judgements of the technology. Through national and international
consumer organisations more information is available to the general public and it
completes the picture by analysing the technology from different aspects.

17.17

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