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Chapter 8. Irradiation
BY:
M.SC. MOHAMMED SABAH
Chapter 8. Irradiation
 Ionising radiation takes the form of
-rays from isotopes
or, commercially to a lesser extent, from X-rays and
electrons. It is permitted in 38 countries to preserve
foods by destruction of micro-organisms or inhibition of
biochemical changes.
1- Advantages and disadvantages
The main advantages of irradiation are as follows:
 there is little or no heating of the food and therefore
negligible change to sensory characteristics
 packaged and frozen foods may be treated
 fresh foods may be preserved in a single operation, and
without the use of chemical preservatives
 energy requirements are very low
 changes in nutritional value of foods are comparable with
other methods of food preservation
 processing is automatically controlled and has low
operating costs.
major disadvantages
 the process could be used to eliminate high bacterial loads to
make otherwise unacceptable foods saleable
 there will be a health hazard if toxin-producing bacteria are
destroyed after they have contaminated the food with toxins
 the possible development of resistance to radiation in micro-
organisms
 loss of nutritional value.
 until recently, inadequate analytical procedures for detecting
whether foods have been irradiated
What Can Irradiation Do?
• Prevent Food Poisoning By Reducing
– E. Coli )157:H7 (Beef)
– Salmonella (Poultry)
– Campylobacter (Poultry)
– Parasites
• Prevent Spoilage by Destroying Molds,
Bacteria and Yeast
• Control Insects and Parasite Infestation
• Increase Shelf Life by Slowing Ripening
of Fresh Fruits and Vegetables
2. Theory
 x-rays and electrons are distinguished from other
forms of radiation by their ionising ability (that is they
are able to break chemical bonds when absorbed by
materials).
 The products of ionisation may be electrically charged
(ions) or neutral (free radicals). These then further
react to cause changes in an irradiated material known
as radiolysis. It is these reactions that cause the
destruction of micro-organisms, insects and parasites
during food irradiation.
3- Types of Radiation Energy Are Used for
Treating Foods
Two types of radiation sources are commonly used for food
treatment.
1- The first is a tightly sealed metal container of radioactive elements
cobalt 60 or cesium 137 that produce gamma rays. The rays are
directed onto the food being irradiated, but the food itself never comes
into contact with the cobalt or cesium source.
2- The second type of radiation source is a machine that produces
either X-rays or high-energy electrons. Because of the physical
characteristics of these sources, no radioactivity can be induced in
food thus treated, no matter how much energy (dose) is absorbed by
the food or how long the food is irradiated.
( L. Paisan. 2003)
.
Source = L. Paisan. (2003)
4. Effect on micro-organisms
 The reactive ions produced by irradiating foods injure or
destroy micro-organisms immediately, by changing the
structure of cell membranes and affecting metabolic
enzyme activity. However, a more important effect is on
deoxyribonucleic acid (DNA) and ribonucleic acid
molecules in cell nuclei, which are required for growth
and replication
Microbial destruction by irradiation: A, Pseudomonas sp.; B,
Salmonella sp.; C, Bacillus cereus; D, Deinococcus radiodurans; E,
typical virus.
5. Nutritional and sensory value
 A common consumer concern is whether irradiation
adversely affects the nutritional value of food. The fact is that
irradiation treatments do not change the nutritional quality of
foods any more than do other methods of food processing such
as cooking, freezing or canning.
Any changes in nutritional value caused by irradiation
depend on a number of factors:
1- radiation dose
2- the type of food
3- temperature and atmosphere in which irradiation is
performed (e.g., presence or absence of oxygen),
4- packaging and storage time
 Main components of foods such as proteins, fats and carbohydrates are
changed very little by irradiation, even at doses higher than 10 kGy. Similarly,
the essential amino acids, minerals, trace elements and most vitamins are not
significantly altered by irradiation.
 Some vitamins riboflavin, niacin and vitamin D are fairly resistant to
irradiation, but vitamins A, B1 (thiamine), E and K are relatively sensitive.
 Their sensitivities depend on
1- the complexity of the food system, whether the vitamins are soluble in water
or fat.
2- the atmosphere in which irradiation occurs.
For example, a solution of thiamine in water lost 50% of the vitamin after
irradiation at 0.5 kGy. In contrast, irradiation of dried whole egg at the same
dose caused less than 5% destruction of the same vitamin
 Results of studies on the effects of irradiation on vitamin C
in fruits and vegetables often are conflicting. Some studies
reported an effect only on ascorbic acid, while others reported
an effect on total ascorbic acid, which is a mixture of ascorbic
and dehydroascorbic acid, both of which provide vitamin C
activity.
6. Effects on Sensory Quality of Foods
Similar to other processes, irradiation causes certain
chemical changes in food that may, under some circumstances,
noticeably affect food quality
Some foods react unfavorably even to low doses of
irradiation. Milk and dairy products are among the most
radiation-sensitive foods.
 A dose as low as 0.1 kGy will impart an off-flavor to milk
that most consumers find unacceptable; thus milk and dairy
products are generally not irradiated. Irradiation of some fresh
fruits and vegetables may cause softening because of the
breakdown of cell walls.
 High-dose irradiation sterilization could induce “off” flavors
in many types of meat products if the process is not done
properly.