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

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Iron – Absorption, Transport, and Loss

0004 Iron is readily absorbed in the state Fe

2þ

, but most

dietary iron is in the ferric form Fe

3þ

. The gastric

secretions dissolve the iron and permit it to form

soluble complexes with ascorbic acids and other sub-

stances that aid its reduction to the Fe

2þ

form. In

humans iron-deficiency anemia is a relatively fre-

quent complication of partial gastrectomy. Heme is

also absorbed because of its solubility. Fe

2þ

contained

in heme is released into mucosal cells.

0005 Iron is absorbed from the upper part of the small

intestine, because duodenum and adjacent jejunum

contain most mucosal cells suitable for iron absorp-

tion. Various dietary factors influence iron absorption,

for example, phytic acid from cereals, which forms

iron-insoluble compounds in the intestine, as do

phosphates and oxalates.

0006 Nonheme iron binds to mucin. Cell adhesion mol-

ecule integrin transfers the iron to mobilferrin.

Mobilferrin carries iron into the cell and binds more

iron in iron-deficiency states. The mucosal cells

contain iron-binding protein, or apoferritin. Some

iron is utilized by mitochondria, and the remainder

is partitioned between apoferritin in the mucosal cells

and transferrin. Transferrin is a protein that carries

iron in the blood. Specific cell-surface receptors de-

liver the transferrin with its bound iron to endosomes.

Here the low pH induces the separation of transferrin

and the iron. Excess iron in the blood is deposited in

all cells of the body, especially in liver cells. Apoferri-

tin is also found in many other tissues, where, to-

gether with iron, it forms ferritin. Apoferritin is a

globular protein. Iron forms a micelle of ferric hydro-

xyphosphate. In ferritin 24 subunits of protein sur-

round each micelle. The ferritin micelle can contain

4500 atoms of iron. It is the iron storage form in

tissues. Molecules of ferritin may aggregate in lysoso-

mal membranes as deposits, containing up to 50%

iron. These deposits are called hemosiderin.

0007 Iron is also transported across the placenta from

mother to fetus. Maternal iron plasma concentration

is usually lower than fetal concentration. However,

the binding capacity of maternal plasma for iron is

much higher, even though iron is actively transported

into the fetal compartment. A daily transport

of about 1.8 mmol is necessary to meet total fetal

requirements.

0008 In adult humans, the amount of iron lost from the

body is relatively small. It is unregulated, and total

body stores of iron are regulated by changes in the

rate at which iron is absorbed. Men lose about 0.6 mg

day

1

, whereas women have a variable, larger loss

because of the additional iron lost in blood shed

during menstruation.

Biosynthesis and Significance of Heme

0009Evolution of higher organisms has been accompanied

by the development of oxygen transport protein. In

humans this protein – hemoglobin – is concentrated

in specialized cells – erythrocytes. Each erythrocyte

contains about 300 million hemoglobin molecules.

The organic structure of a protein is unable to direct

binding of oxygen. However, certain metals, in their

lower oxidation states (for example, Fe

2þ

) have a

strong tendency to bind oxygen. Thus, in the evolu-

tion of the hemoglobin–myoglobin family of proteins,

2þ

has been utilized in the O

-binding site.

0010There are a number of possibilities in which vari-

ous iron-containing proteins hold iron in the form

2þ

. Hemoglobin and myoglobin are a family of

proteins, in which iron is chelated by the tetrapyrrole

ring system – protoporphyrin IX. It is one protein in a

large class of porphyrins, which are also encountered

in chlorophyl, cytochrome proteins, and other nat-

ural pigments. The porphyrins are colored. The iron

porphyrin in hemoglobin accounts for the red color of

blood. The complex of protoporphyrin IX with Fe

2þ

is called heme. Ferrous iron is octahedrally coordin-

ated, which means it should have six ligands or bind-

ing groups attached to it. The nitrogen atoms of the

porphyrin ring account for only four ligands. Two

remaining available coordination sites lie along an

axis perpendicular to the plane of the ring. In myo-

globin one of these sites is occupied by nitrogen of

histidine (part of the protein helix).

0011When oxygen is bound, the O

molecule occupies

the vacant side. Hemoglobin has evolved from myo-

globin and forms a tetrameric structure. Each of the

four chains in hemoglobin has a folded structure

similar to that of myoglobin, and each carries a

heme. Hemoglobin contains four subunits: two

a-chains and two b-chains. Both chains are very simi-

lar but are distinguishable in primary structures and

folding. Each subunit has primary, secondary, and

tertiary structures. Amino acid side chains in hemo-

globin provide hydrophobic, salt bridges, and hydro-

gen bond interactions. These are necessary to stabilize

a particular quaternary structure.

0012Each hemoglobin molecule can bind four oxygen

molecules, in four myoglobin-like sites. To simplify,

we can consider hemoglobin as having two states of

quaternary structure: one characteristic for the deoxy

form and the other for the oxygenated form. The oxy

form has a higher affinity for O

. The transition from

the deoxy to the oxy conformation involves changes

in the interactions between the subunits. A ligand

oxygen, binding to its side, tends to pull the Fe

2þ

very short distance down into the heme and flattens

the heme. Consequently, a molecular rearrangement

3380 IRON/Biosynthesis and Significance of Heme (Haem)

occurs, and pulling the Fe into the heme produces a

much larger shift in the surrounding structure, par-

ticularly at the ab interfaces. Cooperative binding,

oxygen transport, and allosteric transition between

structurally different high-affinity and low-affinity

states mean that hemoglobin behaves in an allosteric

way. Several factors, such as carbon dioxide, protons,

and other substances, promote these changes. They

are called allosteric effectors.

0013 For each polypeptide chain produced by an organ-

ism there exists a corresponding gene. The nucleotide

sequence in the gene dictates, via the genetic code, the

amino acid sequence of the protein. This process is

exemplified by the b-globin gene. In the gene for

human b hemoglobin, introns (noncoding regions)

alternate with exons (regions that are expressed in

the polypeptide sequence). From this gene is pro-

duced a primary transcript (pre-mRNA), which is

spliced to yield the final mRNA. This mRNA is trans-

lated into the b-chain, which then adopts its favored

three-dimensional structure. Very primitive animals

had only a myoglobinlike, single-chain globin for

oxygen storage (transport was not necessary; most

of these animals were very small). More than 800

million years ago the primitive globin gene was dupli-

cated. The one copy evolved into the gene for

an oxygen transport protein and to hemoglobins.

The evolutionary line led to mammals carrying

both a-andb-globin genes and capable of forming

tetrametric hemoglobins. Gene duplication has also

occurred in the hemoglobin line, leading to embry-

onic and fetal forms, and accounting for the special

hemoglobin, which is adapted to promote oxygen

transfer through the placenta from the mother to

the fetus.

0014Evolution of hemoglobin genes continues. This fact

is seen in the existence of several hundred recognized

mutant hemoglobins in the human population. Each

of these mutant forms exists in only a small part of the

total human population. Many give rise to recognized

pathologies, while others are referred to as neutral

mutations.

See also: Anemia (Anaemia): Iron-deficiency Anemia;

Ascorbic Acid: Properties and Determination;

Physiology; Iron: Properties and Determination;

Physiology

Further Reading

Ganong WE (1999) Review of Medical Physiology. New

York: Appleton & Lange.

Greger R and Windhorst U (1996) Comprehensive Human

Physiology. London: Springer.

Mathews CK and Van Holde KE (1988) Biochemistry.

London: Benjamin/Cummings Publishing.

IRRADIATION OF FOODS

Contents

Basic Principles

Applications

Processing Technology

Legal and Consumer Aspects

Basic Principles

P B Roberts, Institute of Geological and Nuclear

Sciences Ltd, Lower Hutt, New Zealand

Introduction

0001 The absorption of energy during irradiation can pro-

vide foods with desirable new benefits. Irradiation is

therefore analogous to more common processes such

as pasteurization or drying that use heat energy. The

energy absorbed during irradiation is high-energy,

ionizing radiation. (See Drying: Theory of Air-drying;

Pasteurization: Principles.)

0002Irradiation of food must not be confused with

the contamination of food by radioactive materials,

which themselves emit radiations that may harm a

consumer. Such contamination followed nuclear

weapon tests and the Chernobyl accident. Food

irradiation cannot make food radioactive since the

radiation used, though of high energy, is not powerful

enough to induce the necessary changes in atomic

nuclei. (See Radioactivity in Food.)

0003This article discusses the history and current status

of food irradiation. The radiation sources and

IRRADIATION OF FOODS/Basic Principles 3381

potential applications are outlined and the general

chemical, microbiological, and nutritional effects

summarized.

History

Before 1950

0004 Ro

ntgen discovered X-radiation in 1895 and

Becquerel observed natural radioactivity in 1896.

The radiations produced were soon found to have

many biological effects, including the ability to

inactivate bacteria. In 1904 Prescott suggested

that spoilage bacteria in food could be destroyed by

radiation. Patents were obtained in the US and France

in 1921 and 1930. However, no practical source of

radiation to treat significant food volumes existed

until the 1940s when high-energy electron acceler-

ators and nuclear reactors able to produce large

quantities of artificial radioactive nuclides became

available.

1950–1983

0005 The US Army Quartermaster Corps sponsored the

first cooperative effort to develop food irradiation

as part of research into light-weight, shelf-stable

army rations. By 1966 safety studies on 21 foods

were complete and sterilization of bacon and pork,

disinfestation of wheat and its products, and

inhibition of potato sprouting were uses of radiation

approved by the US Food and Drug Administration

(FDA). After this, safety studies in the US stalled for

10 years and commercial interest waned.

0006 European interest increased, however, and in 1970

the International Project in the Field of Food Irradi-

ation (IFIP) was initiated. IFIP compiled safety data

on many foods to be considered by expert committees

that were sponsored by the United Nations organiza-

tions responsible for secure, safe food supplies and for

the peaceful uses of atomic energy.

0007 In 1980 a joint expert committee on food irradi-

ation stated that ‘the irradiation of food up to an

overall average absorbed dose of 10 kGy presents

no toxicological hazard’ and ‘introduced no special

nutritional or microbiological problems.’ This advice

prompted the Codex Alimentarius Commission,

the body responsible for world food standards, to

adopt a worldwide general standard for irradiated

food in 1983. The standard sets general conditions

for the irradiation of food. It does not imply that

irradiation is appropriate for all foods but that

irradiation should be regarded and controlled like

other physical processes such as canning, dehydra-

tion, etc.

Current Status

0008The Codex standard was issued at a time of increased

awareness of food losses in developing countries and a

growing aversion to chemical contamination of food

via chemical fumigation and preservation in developed

countries. By 1999, 41 countries had cleared one or

more foods for irradiation. There are over 50 irradi-

ation plants in nearly 30 countries irradiating food,

although the volume treated in most is small.

0009However, the 1980s saw a rise in public suspicion

about any technology associated with radiation.

Demand for ‘natural,’ minimally processed foods

has increased and official assurances on the whole-

someness of processed foods and additives are

increasingly questioned. There has been sufficient

public opposition to food irradiation to insure a

very cautious attitude to it within the food industry.

The annual volume of food irradiated is probably

only 500 000 tonnes worldwide. Foods that are

irradiated include dried herbs and spices, potatoes,

onions, garlic, dried vegetables, tropical fruits, and

chicken. Countries that regularly irradiate foods on a

commercial basis include the US, The Netherlands,

France, Belgium, Japan, South Africa, and China.

The Radiations Used

0010The changes in food or its contaminating organisms

achieved by irradiation processing are caused by

chemical reactions induced between the constituent

atoms or molecules. These reactions are initiated by

radiations that can strip electrons away from atoms

or molecules that are left positively charged (ionized).

The charged species rapidly split into fragments

called free radicals that have a free or unpaired elec-

tron. Free radicals react extremely rapidly with each

other and nearby molecules as they seek to become

more stable by gaining or losing an electron. It is these

free radical reactions that trigger the chemical effects

leading to the ultimate changes in the food.

0011There are two principal forms of ionizing radi-

ation: accelerated subatomic particles and high-

energy electromagnetic (EM) radiation.

Accelerated Particles

0012Particles travelling at speeds approaching the speed of

light can cause ionization. The only particle used for

food irradiation is the electron. An electron acceler-

ator works on similar principles to a television tube

(Figure 1). Electrons produced by heating a filament

are injected into a vacuum chamber and attracted to a

positive terminal. An electron beam is focused and

then accelerated within the chamber by the voltage

difference experienced by the electrons between the

3382 IRRADIATION OF FOODS/Basic Principles

start and finish of their travel in the chamber, a

difference generated electrostatically or by a radio-

frequency field. The beam emerges through a thin

window at a speed close to that of light. It is swept

from side to side by a changing magnetic field while

irradiating large targets such as food.

001 3 An electron accelerated by a voltage difference of

1  10

V acquires an energy of 1MeV (megaelectron

volt). Energies above 1 MeV provide sufficient

penetration in food to be useful. An upper limit of

10 MeV is recommended by the Codex general stand-

ard to avoid inducing radioactivity in the food.

Electromagnetic Radiation

0014Figure 2 shows that there is a range of EM radiations,

each characterized by different wavelengths or energy

and with different uses. Only X- and g-rays have

sufficient energy to cause ionization.

input

High-voltage

generator

and rectifiers

High-

voltage

DC power

Accelerator tube

Thin window

Scan horn

Scanning

magnet chamber

Electron

beam

High-vacuum

pumping

system

High-voltage

terminal

Filament (electron gun)

fig0001 Figure 1 Schematic diagram of an electron accelerator with the voltage difference obtained electrostatically. Reproduced from

Irradiation of Foods: Basic Principles. Encyclopaedia of Food Science, Food Technology and Nutrition, Macrae R, Robinson RK and

Sadler MJ (eds), 1993, Academic Press.

−10

−6

−2

−4

−8

−12

Wavelength (m)

Radiowaves

Microwaves

Visible light

Ultraviolet light

X-rays

γ-rays

Infrared

(heat waves)

Photon energy (eV)

fig0002 Figure 2 The electromagnetic spectrum. Reproduced from Irradiation of Foods: Basic Principles. Encyclopaedia of Food Science,

Food Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic Press.

IRRADIATION OF FOODS/Basic Principles 3383

0015 X-rays are produced when high-speed particles

strike a metal target and are rapidly decelerated. To

avoid inducing radioactivity in food, X-ray sources

must operate below 5 MeV. The efficiency of the con-

version of 5 MeV electrons to X-rays is under 10%,

making X-ray generators more costly to operate than

simple electron machines.

0016 g-Rays are emitted by many atomic nuclei under-

going radioactive decay. Sources for food irradiation

are limited to cobalt-60 and caesium-137, which emit

g-rays with energies around 1 MeV that cannot

induce radioactivity.

0017 Cobalt-60 is produced in a nuclear reactor by

neutron bombardment of the stable element. It is a

highly insoluble, high-melting-point metal. Caesium-

137 is one of the fission products produced in the

fuel rods of nuclear reactors from which it is

chemically separated as a potentially volatile, soluble

salt. Cobalt-60 dominates the g-processing industry

because of its more favorable production and

environmental features.

0018 Commercial decisions about the type and source of

radiation to use are dictated by cost, beam penetra-

tion, and the form of the food to be treated.

Most irradiation plants use cobalt-60. A few use

electron beams and the first plants using X-rays are

being constructed.

Dose and General Radiation Effects

0019 Radiation effects in food are caused by the energy

that is transferred to the food. The transferred energy

is called the absorbed dose (commonly, just dose). Its

unit is the gray (Gy) which is the absorption of one

joule per kilogram. An older unit is the rad; 1 Gy

equals 100 rad.

0020 As the absorbed dose increases, the number of

chemical reactions and the severity of the effects

caused will increase. Food irradiation usually

involves doses of 0.05–30 kGy, depending upon the

desired result, although the upper limit recommended

by the Codex Alimentarius is 10 kGy.

0021At low doses chemical reactions can disrupt bio-

chemical and hormonal processes in fresh produce.

As the dose increases, cell division is prevented as

chemical disruption of DNA becomes severe. In this

way microorganisms are inactivated and insects

sterilized. At the highest doses used, all the micro-

organisms are killed and the food is sterilized.

0022Such effects indicate that extensive chemical

changes are induced by irradiation. It may seem

surprising that any food can withstand irradiation

without being altered to an unacceptable extent. How-

ever, radiation deposits energy randomly throughout

the food while inactivation of microorganisms, for

example, requires that energy be deposited at a few

critical points in a vital molecule such as DNA or a

chromosome. Such molecules are relatively large

targets and present in very few copies. In contrast,

significant depletion or alteration of individual food

constituents (e.g., amino acids) requires energy to

be deposited in a large fraction of molecules that

are far smaller and far more numerous than DNA.

Frequently, therefore, food irradiation results in a

product that is virtually indistinguishable from

unprocessed food. However, as with conventional

processes, some foods will suffer from undesirable

changes in taste, smell, or texture.

Beneficial Uses of Irradiation

0023The major potential benefits of food irradiation are

summarized in Table 1. Each benefit requires a min-

imum dose to be effective. For a particular food the

maximum dose is limited by the onset of unacceptable

changes in quality. Before a commercial decision is

taken to irradiate a food, it is necessary to perform a

cost-versus-benefit analysis and to assess the benefit

to consumers against any potential loss in sensory

qualities.

tbl0001 Table 1 Major technical benefits of food irradiation

Benefit Doserange (kGy) Candidate foods

Inhibit sprouting 0.05–0.15 Potato, onion, garlic

Delay ripening 0.05–0.15 Some tropical fruit

Reduce parasites 0.1–0.3 Pork

Disinfest insects 0.1–1 Grain, rice, some fruit and vegetables

Delay spoilage (ambient) 0.5–5 Strawberry

Delay spoilage (refrigerated) 0.5–10 Meat, poultry, fish

Reduce pathogens 2–10 Meat, poultry, seafood, dried foods

Sterilization 10–30

Spices, herbs, special diets

Not a comprehensive list.

Doses above 10 kGy exceed a Codex recommendation and are permitted only in a few countries for special purposes.

3384 IRRADIATION OF FOODS/Basic Principles

Safety Considerations

0024 When considering whether any processed food is safe

and wholesome the chemical, microbiological, and

nutritional changes must be evaluated. Several expert

international and national committees have con-

sidered the safety of irradiated foods. The World

Health Organization (WHO) completed its latest

reviews in 1994 and 1999. All the reviews reinforce

the 1980 joint expert committee finding that,

provided good manufacturing practice is followed,

the irradiation of any food up to an overall average

absorbed dose up to 10 kGy presents no toxicological

hazard and no special microbiological or nutritional

hazards.

0025 The 1999 WHO report examined the safety of

foods irradiated above 10 kGy. It concluded that

foods treated above 10 kGy should also be considered

safe and nutritionally adequate when produced under

established good manufacturing practice.

Chemical Changes

0026 Chemical reactions induced by a 10 kGy dose lead

to about 300 mg of products per kilogram of food.

Radiation, therefore, produces a large number of com-

pounds but at very low individual concentrations. A

few products are species that were not present signifi-

cantly before irradiation. Some could conceivably be

unique to irradiated foods and of unknown toxicity.

0027 Any unique products must be present in extremely

low concentrations since modern analytical methods

can identify all the radiation products (or very similar

compounds) in other processed or unprocessed foods.

Several expert committees have considered a huge

database of toxicological experiments. Although a

small number of experiments initially raised some

questions, the committees are satisfied that any

doubts were due to inadequate experimental or

analytical methods and that any toxicological risk is

negligible.

Microbiological Changes

0028 All physical processes have the potential to mutate

microorganisms and lead to increased resistance, en-

hanced pathogenicity, or changed physiological traits

important to their identification. If the process does

not sterilize the food the surviving organisms will

be those most resistant to the process. Subsequent

regrowth can lead to microbial populations and risk

different to those originally present.

0029 However, irradiation appears little different to

other physical processes in its potential for micro-

biological change. An exception may be its inability

to destroy toxins produced by microorganisms that

were present before processing, a useful property of

heat for some toxins. In practice the risks are small.

Following codes of good manufacturing practice

developed for other processes and for storage will

insure microbiologically safe irradiated foods.

Nutritional Changes

0030Irradiation up to 10 kGy does not alter significantly

the nutritional value of proteins, carbohydrates,

minerals, or saturated fats. Oxidation reactions can

lead to the loss of essential unsaturated fatty acids.

These reactions also induce rancid flavors and some

foods with a high unsaturated fat content will not

be suitable for irradiation. (See Oxidation of Food

Components.)

0031In common with heating, freezing, dehydration,

and storage, irradiation causes vitamin loss. Vitamin

E and thiamin are the most radiation-sensitive, with

vitamins A, C, and K also quite sensitive. Losses

below 1 kGy are insignificant. Above 1 kGy the losses

are comparable to those caused by other physical

processes and may reach 10–20% for the more

sensitive vitamins. If irradiated foods are to be

further processed or cooked, the combined losses

must be taken into account. (See Ascorbic Acid:

Properties and Determination; Retinol: Properties

and Determination; Tocopherols: Properties and

Determination.)

0032The overall nutritional consequences of irradiating

a particular food will depend upon:

0033whether the food is a significant source of particu-

lar nutrients

0034whether those nutrients are radiation-sensitive

0035the dose

0036the overall proportion of the food in the diet that is

irradiated

0037Although there may be consumers with particular

dietary habits or needs who could be adversely

affected, the nutritional consequences of food irradi-

ation will be insignificant for people eating a reason-

ably balanced diet.

See also: Ascorbic Acid: Properties and Determination;

Cheeses: Manufacture of Hard and Semi-hard Varieties

of Cheese; Drying: Theory of Air-drying; Oxidation of

Food Components; Pasteurization: Principles;

Radioactivity in Food; Tocopherols: Properties and

Determination

Further Reading

Goresline HE (1982) Historical aspects of the radiation

preservation of food. In: Josephson ES and Peterson

MS (eds) Preservation of Food by Ionizing Radiation,

pp. 1–46. Boca Raton, FL: CRC Press.

IRRADIATION OF FOODS/Basic Principles 3385

McLaughlin WL, Boyd AW, Chadwick KH, Mcdonald JC

and Miller A (1989) Dosimetry for Radiation Process-

ing. London, UK: Taylor and Francis.

Murray DR (1990) Biology of Food Irradiation. New York:

Wiley.

Ramler WJ (1982) Machine sources. In: Josephson ES and

Peterson MS (eds) Preservation of Food by Ionizing

Radiation, pp. 165–188. Boca Raton, FL: CRC Press.

Satin M (1993) Food Irradiation: A Guidebook. Lancaster,

USA: Technomic.

Thayer DW (1990) Food irradiation: benefits and con-

cerns.Journal of Food Quality 13: 147–169.

World Health Organization (1988) Food Irradiation: A

Technique for Preserving and Improving the Safety of

Food. Geneva: World Health Organization.

World Health Organization (1994) Safety and Nutritional

Adequacy of Irradiated Food. Geneva: World Health

Organization.

World Health Organization (1999) High Dose Irradiation:

Wholesomeness of Food Irradiated above 10 kGy.Re-

port of a Joint FAO/IAEA/WHO Study Group. WHO

Technical Report Series 890. Geneva: World Health Or-

ganization.

Applications

P B Roberts, Institute of Geological and Nuclear

Sciences Ltd, Lower Hutt, New Zealand

Introduction

0001 This article discusses for various foodstuffs the prac-

tical benefits of irradiation, including reduction of

pathogens, insect disinfestation, extension of shelf-

life, and delay of ripening or senescence.

General Principles

0002 Food presented for irradiation should be of good

quality and the process must not substitute for good

manufacturing practice (GMP) at any stage in the

production of food. When food is treated to reduce

pathogen or insect numbers, appropriate packaging

should be used to prevent recontamination.

0003 The maximum dose recommended by the Codex

Alimentarius, 10 kGy, causes a temperature rise of

under 2.4



C, resulting in an unchanged appearance

and texture in many temperature-sensitive foods.

Oxidative stress does occur and not all foods with-

stand the doses required to achieve the intended bene-

fits without a loss of quality. The doses suggested here

are guides only; the optimum dose varies with

conditions and varieties of food. (See Oxidation of

Food Components.)

0004Combining irradiation with another process may

result in an overall improvement in quality. For

example, fruit is less susceptible to softening when

lower radiation doses are combined with a mild heat

treatment, and insect disinfestation is still achieved.

Combination treatments are not discussed further.

Major Applications

0005Applications of food irradiation fall into several

major categories.

Improved Food Safety

0006Foodborne pathogens are a significant and increasing

cause of ill health and economic loss. Irradiation has

great potential to control pathogen numbers in at-risk

foods. For example, it is being considered for use as

part of Hazard Analysis Critical Control Point

(HACCP) systems for control of pathogens in meat.

Irradiation is a ‘cold pasteurization’ process suitable

for solid foods, especially for foods of animal origin

and consumed raw. Concern in some countries about

the increase in the presence of Escherichia coli 0157

H7 in foods has added considerable impetus to the

use of irradiation as a decontamination measure.

Improved Food Security

0007Irradiation can control insect numbers, inhibit

sprouting of bulbs and tubers, and delay maturation,

decay, and senescence of some fruits and vegetables.

Therefore irradiation can help to combat postharvest

food losses that may range from 10 to 50% for some

crops, depending on climatic conditions and available

storage technologies.

Quarantine Security

0008Most countries require that insect species not found

within their borders do not ‘hitchhike’ on imported

foods. The available chemical fumigants are being

phased out because of health and environmental

concerns. Irradiation is being accepted by plant

protection authorities as a technically viable, broad-

spectrum alternative to fumigation.

Reduction in Chemical Residues

0009Irradiation leaves no treatment residues. Consumer

demand for foods free of chemical residues is there-

fore an opportunity for irradiation to supplant chem-

ical fumigation and preservation methods.

Increased Trade

0010The four applications above provide increasing op-

portunity for increased international trade in foods.

3386 IRRADIATION OF FOODS/Applications

Irradiation can assist in meeting the hygiene, quaran-

tine, and quality demands of food-importing coun-

tries. This is likely to become economically important

for developing countries seeking to expand food

exports to markets in developed countries.

Raw Plant Products

Fresh Fruits

0011 Insect disinfestation The doses required to guarantee

quarantine security are far lower than the Codex-

recommended maximum (Table 1) and many fruit

suffer no loss of quality (Table 2) or vitamins. Fruit

still require proper storage conditions and ozone

produced during treatment should be vented to reduce

damage to the fruit. (See Insect Pests: Insects and

Related Pests.)

0012 Irradiation up to 1 kGy guarantees that insects

cannot reproduce. However, not all insect stages will

be rapidly killed. US authorities have a regulation in

place on the use of irradiation as a phytosanitary

treatment of imported fruits and vegetables.

0013 Extension of shelf-life

Reduction of spoilage Spoilage of fruit is usually

caused by fungal or yeast infections. These organisms

are inactivated by doses above 1.75 kGy, which ap-

proaches the maximum dose tolerated by most fruit.

Above the tolerated dose, softening and texture loss

occur because of degradation of pectin and cellulose.

Spoilage may even be accelerated since infection of

the weakened tissue becomes easier or natural antibi-

otic production is inhibited. (See Spoilage: Yeasts in

Spoilage.)

0014Sweet cherries and apricots may be protected from

fungal rots by a 4 kGy dose without loss of quality but

the only fruit generally considered suitable for such

treatment is the strawberry. Grey mold rot (Botrytis

cinerara) in temperate climates and rhizopus rot (Rhi-

zopus stolonifer) in subtropical areas can be con-

trolled. Treatment with 2–2.5 kGy and refrigerated

storage can increase strawberry shelf-life by a factor

of 2–4. (See Apricots; Cherries; Strawberries.)

0015Delay in ripening Nonclimacteric fruit are picked

ripe and then steadily deteriorate. Climacteric fruit,

which are mature but unripe at harvest, undergo an

initial decrease in respiration followed by a burst of

respiration associated with ethylene production and

the start of ripening. (See Ripening of Fruit.)

0016Irradiation can suppress ethylene production and

the ripening process in preclimacteric fruit. The time

between harvest and full ripening is an important

marketing consideration and irradiation could induce

a useful extension.

0017Doses above 1 kGy are required to delay ripening

of apples and pears and result in fruit of poor quality.

In peaches, nectarines, and apricots ripening is accel-

erated, not delayed. (See Apples; Peaches and Nectar-

ines; Pears.)

0018Ripening can be delayed successfully in some sub-

tropical fruits. A delay of a week (30



C storage) or

10–12 days (20



C) is observed in bananas after a

dose of 0.4 kGy. Higher doses cause softening and

discoloration. The fruit should be treated before full

maturity – an exception to the general rule. Mangoes

in the hard green state are delayed from ripening by

about 7 days (25



C storage) after 0.25–0.75 kGy.

Higher doses cause skin darkening. Papayas have a

tbl0001 Table 1 Irradiation treatments to provide quarantine security against insect pests

Insect species Stage Minimum dose (kGy) Criterion

Bactrocera tryoni (Queensland fruit fly) Eggs/larvae 0.075 Nonemergence of normal adults

Tephritidae Eggs/larvae 0.15 Nonemergence of normal adults

Cydia pomonella (coding moth) All 0.25 Nonemergence of normal adults

All other All 0.3 Sterilization of any adults present, and nonemergence

of normal adults from preadult stages

Treatments and criteria extracted from the findings of an international task force meeting on irradiation as a quarantine treatment organized by the

International Consultative Group on Food Irradiation (IAEA) in Changmai, Thailand, February 1986.

tbl0002 Table 2 The ability of fruits to retain quality after doses up to 1 kGy

Quality retention Fruit

Good Apple, cherry, date, guava, mango, nectarine, papaya, peach, raspberry, strawberry, tamarillo, tomato

Reasonable Apricot, banana, cherimoya, fig, grapefruit, lychee, orange, passionfruit, pear, pineapple, plum, tangelo, tangerine

Poor Avocado, grape, lemon, lime, olive

Unknown Pomegranate, kiwifruit

Adapted from Kader AA (1986) Potential applications of ionising radiation in post-harvest handling fresh fruits and vegetables. Food Technology June:

117–121.

IRRADIATION OF FOODS/Applications 3387

ripening delay of 3 days at ambient temperature

induced by 0.5–0.75 kGy. Skin scald results from

higher doses. (See Bananas and Plantains; Mangoes;

Papayas.)

0019 Delay of senescence All fruit, even if not susceptible

to rots, eventually deteriorate as the carbohydrate

polymers responsible for firmness and texture break

down. Generally, irradiation accelerates breakdown

but senescence is delayed in sweet cherries, apricots

(3 kGy, 4



C storage) and papayas (0.75 kGy, 25



storage).

Fresh Vegetables

0020 Inhibition of sprouting in tubers and bulbs The sale-

able life of potatoes, onions, garlic, etc., is reduced

by sprouting. Low doses (0.02–0.2 kGy), allied with

cool, dry storage can inhibit sprouting for months.

The mechanism is uncertain but the likely causes are

interference with nucleic acid and protein synthesis,

chromosomal aberrations in meristematic cells, and

inhibition of the production of hormones that con-

trol dormancy and sprouting. At even lower doses

(< 0.01 kGy) sprouting may be stimulated. Irradiation

inhibits wound healing in potatoes which are there-

fore stored 3–4 weeks before processing. (See Onions

and Related Crops.)

0021 Texture and overall appearance are unaffected but

other features can be affected adversely depending

upon dose, storage conditions, and variety. A small

brown discoloration near the stem end may occur

in onions. A blue-gray discoloration that occurs on

cooking potatoes that have been stored for several

months may be increased by irradiation. Many potato

varieties display a temporary rise in sugar content

after irradiation. Some varieties are unsuitable for

processing into French fries or potato chips since the

sugar content increases permanently on storage.

0022 Inhibition of greening Greening of potatoes on ex-

posure to air is inhibited by doses that prevent

sprouting. There is conflicting evidence on whether

the production of the toxic alkaloid solanine which is

associated with greening is also prevented. Greening

of endive can be reduced by a dose of 2 kGy.

0023 Other effects Undesirable changes during storage of

mushrooms include opening and browning of the cap,

darkening of the gills, and browning and elongation

of the stem. Doses in the range of 0.05–0.5 kGy in-

hibit these changes with little effect on flavor. (See

Mushrooms and Truffles: Use of Wild Mushrooms.)

0024 Postharvest curvature of asparagus is inhibited by

0.15 kGy. Higher doses produce splitting of the ends

and a slimy, darkened appearance. (See Vegetables of

Temperate Climates: Stem and Other Vegetables.)

Cereal Grains

0025Dry cereals such as barley, wheat, rice, and maize are

subject to depredation by insects (e.g., coleoptera,

lepidoptera, and mites). Doses of 3–5 kGy are needed

to kill the insects within 24 h. However 0.5 kGy re-

duces their ability to feed and renders them sterile.

This is useful as the grains are usually stored for long

periods. Grains should be stored cool and dry to

avoid mold growth. (See Cereals: Handling of Grain

for Storage.)

Processed Plant Products

Dried Fruits and Vegetables

0026Low moisture content acts as a sufficient preserva-

tive measure in dried produce. The major benefit

of irradiation is to disinfest them of insects such as

the saw-toothed grain beetle, the fig moth, and the

Indian meal moth. Doses up to 1 kGy are required

and generally cause no detectable sensory changes.

Darkening may occur upon storage in some dried

fruits.

0027An additional benefit of irradiation is that rehydra-

tion and swelling properties tend to be improved and

cooking times reduced.

Dried Herbs, Spices, and Vegetable Seasonings

0028These useful food ingredients are usually dried and

ground, chopped, or finely divided. They may be sold

as such or used as an ingredient of other processed

foods. (See Herbs: Herbs and Their Uses.)

0029The initial microbial population is often high (over

1

) and irradiation at the recommended Codex

limit of 10 kGy is used as a decontamination measure.

Some countries permit treatment at 30 kGy because

these ingredients are of minor dietary importance.

0030The changes in aroma, color, and flavor caused by

irradiation are insignificant at 10 kGy and slight at

30 kGy.

Fresh Meats

Red Meats

0031Inactivation of Trichinella spiralis The infectious

cycle of the helminthic parasite Trichinella spiralis

in pork can be broken by a dose of 0.3 kGy. No

special handling procedures are required and meat

quality is retained.

Extension of Shelf-life and Pathogen Reduction

0032Meat is sterile at death but bacterial contamination

may occur during slaughter and processing. Adequate

3388 IRRADIATION OF FOODS/Applications

GMP should insure sufficient retail shelf-life, and

vacuum packaging technology can extend the shelf-

life of lamb and beef by several weeks. (See Meat:

Preservation.)

0033 If a further guarantee of product hygiene is re-

quired, irradiation may be used to reduce pathogen

numbers. This will coincidentally extend the shelf-life

by reducing spoilage organisms. The induction of off-

flavors limits the maximum dose applied. Highly

trained taste panels can detect flavor changes in

some meats at quite low doses but the threshold

dose for unacceptable changes in pork is 1.75 kGy.

Doses in the range 1–2.5 kGy are acceptable for beef

and lamb.

003 4 Such doses significantly reduce the population of

many pathogens (Table 3). The meat will not be

pathogen-free if the initial contamination is substan-

tial or if more resistant pathogens are present. Spoil-

age is usually caused by Gram-negative bacteria of

the family Enterobacteriaceae and genus Pseudomo-

nas, which are radiation-sensitive. Irradiation shifts

the population to Gram-positive Lactobacillus and

Achromobacter (Moraxella-Acinetobacter) sp. with

different spoilage characteristics.

0035 Other processes that reduce shelf-life are ‘drip,’ an

unsightly exudate, and oxidation which causes discol-

oration and flavor changes. Irradiation does not in-

hibit such processes.

Fresh Poultry

0036 The principles of decontamination and shelf-life ex-

tension are similar for poultry and red meats. Poultry

have generally been considered the more likely candi-

date for irradiation since poultry are a major vehicle

for the spread of foodborne pathogens. (See Poultry:

Chicken; Ducks and Geese.) However, concerns

about E. coli 0157 H7 have put more emphasis on

red meats.

0037Doses of 1–2.5 kGy significantly reduce pathogen

numbers and double shelf-life. Below 2.5 kGy, suffi-

cient spoilage organisms will survive to insure spoil-

age occurs prior to the possible, though unlikely,

production of botulism toxin. Other safety measures

include storage below 4



C and oxygen-permeable

packaging.

Frozen Red Meats and Poultry

0038Irradiation can provide extra protection against the

presence of pathogens. Doses in the range 3–7 kGy

are effective, require no special handling, and retain

quality.

Processed Meats

0039Processed meats are often vacuum-packaged to insure

a long shelf-life. However, they may be highly spiced

and have a high surface/volume ratio if sliced. During

long storage these factors favor pathogen growth

which can be inhibited by 2.5–3 kGy.

Seafoods

Dried or Salted and Dried Fish

0040Irradiation at 0.5 kGy disinfects the stored fish of

insects (Dermestes maculatus, Necrobia rufipes and

members of the family Sarcophagidae) but will not

inhibit spoilage due to mold growth. Sensory quality

is unaffected.

Fresh Fin Fish

0041Fresh fish has a relatively short shelf-life due to

microbial spoilage, enzyme action, and oxidation.

Irradiation (1–2.5 kGy) reduces microbial spoilage,

and the shelf-life can be extended safely by several

days. Other spoilage mechanisms are unaffected

and fish still lose quality during storage. Ideally the

fish should be irradiated on board the fishing

boat; alternatively, the fish should be put on ice

before irradiation on shore. (See Fish: Spoilage of

Seafood.)

0042Both marine and fresh-water fish can be treated

successfully. Accelerated rancidity is a greater prob-

lem in fish with a high oil content (herring, salmon,

tuna), though in some oily fish (mackerel) their strong

flavor can mask taste changes. Storage below 4



Cis

essential to prevent growth of Clostridium botulinum

and toxin production. This can also be inhibited by

oxygen-permeable wrapping which will, however,

not inhibit oxidation. Below 2.5 kGy sufficient other

microorganisms such as radiation-resistant Achromo-

bacter spp. will survive and cause spoilage before

toxin is produced.

tbl0003 Table 3 Sensitivity to ionizing radiation of some pathogenic

bacteria when present in red meat

Genus D

(kGy)

Campylobacter 0.08–0.16

Escherichia 0.30–0.55

Listeria 0.20–1.10

Salmonella 0.31–1.30

Staphylococcus 0.34

Streptococcus 0.69–1.20

Ye r s i n i a 0.04–0.21

Data adapted from Farkas J (1987) Decontamination, including parasite

control, of dried, chilled and frozen foods by irradiation. Acta Alimentaria

16: 351–384, and as published by the US Council for Agricultural Science

and Technology in Task Force Report no. 115, June 1989.

The dose required to reduce viable cell numbers to 10% of the original

numbers.

IRRADIATION OF FOODS/Applications 3389