Irradiation
Possible use in food preservation
Ionising irradiation is the only alternative to heat processing
for food preservation that has a lethal effect on micro-organisms. Further, it
is the only novel method of food preservation suggested for many centuries. It,
therefore, lacks the confidence of other systems derived from years of
experience. This is rightly so. There have been mistakes in the past and the
food processor has a responsibility to the consumer. As a result of this
cautious approach, the use of irradiation for treating foods is heavily
restricted in most countries, permission having to be sought before irradiated
foods are sold to the public.
It is obviously desirable that the irradiation process should in
no way affect the fitness of the food for human consumption. An important effect
of this consideration is that the radioactivity of the foodstuff should not be
increased appreciably above its natural level.
The two types of radiation considered for use are, therefore:
(a) particle or electron beam type: cathode ray and, B-ray
(limited energy and poor penetrative power).
(b) electromagnetic wave type: X-ray and '-ray (deeper
penetrative power).
The high frequency electromagnetic waves, high energy
electrons and beams of the heavier atomic particles, being capable of inducing
nuclear transformations in the atoms of the target food, are not acceptable.
Energy of radiation
Whether particle or wave type radiation is used, its energy is
measured in electron volts, eV, or more normally as MeV (106eV)
1 eV ~ 1.6x 10-12 ergs. (1 Joule = 1 x 106
ergs.)
A source of 5 MeV will induce radioactivity in food at a level
of about 0.3 per cent above the natural level; this is considered not to
represent a health hazard.
When the material which is being irradiated has absorbed 100
ergs energy per gram, it is said to have received a dose of 1 red.
Radioactive sources
(a) Spent fuel-rods from atomic reactors have been used for
experimental purposes but as their activity falls rapidly (about 97 per cent in
100 days), it is unlikely that they will be used industrially.
(b) Cobalt 60, an artificial isotype emitting '-rays at energy
levels of 1.17 and 1.33 MeV, has a half life of 5.3 years, losing activity at
the rate of about 1 per cent per month. (A typical layout of a cobalt 60
irradiation plant is illustrated in Figure 12.)
Figure 12 - Typical layout of
irradiation plant using cobalt 60
(c) X-ray machines have the advantages of being switched on/off
as required but are expensive in energy demand.
Chemical and biological effects in foodstuffs
A direct hit by a wave or particle beam on the cell nucleus may
cause total chromosomal disorder, or mutation in micro-organisms or food tissue,
but this effect is now considered to be less important in food preservation. Of
greater importance is the production of free radicals, the most significant of
which is the ionisation of water in the presence of oxygen to give the peroxide
ion. The oxidative effect of the peroxide ion no doubt plays a major part in the
inhibition of microbial spoilage, demonstrated by the fact that catalase
positive micro-organisms are least affected.
Unfortunately, the peroxide ion also causes many undesirable
changes in the composition of the food e.g. deamination of amino acids,
denaturation of protein and both deamination and dephosphorylation of nucleo
proteins. While carbohydrates are relatively stable, cellulose may be
depolymerised, resulting in a softening of texture. Fats are particularly
vulnerable to oxidation as are fat-based pigments susceptible to bleaching. Up
to 50 per cent of the vitamin C may be lost while vitamin A and E losses depend
on whether they are associated in protein or fat (the latter resulting in higher
losses). The ionisation effect also causes concern when considering the possible
hazards to the consumer. As many reactions are occurring in the food, it is
thought possible that toxic chemicals, e.g. carcinogens, might be produced;
therefore, considerable research effort is directed at testing irradiated foods.
Levels of treatment
Three levels are considered in processing foods:
Radappertisation (1 to 5 Mrads):
This level of treatment is the most severe and will destroy all
spoilage and pathogenic micro-organisms. C. botulinum spores require 4.5 Mrad
for a 12 D process (this requires that the process reduces a hypothetical C.
botulinum population by 12 decimal logarithmic cycles). Unfortunately, there are
no indicator micro-organisms that will survive such a process. Furthermore, it
would stimulate off-flavours and odours and possibly cause textural damage as
well. It has been claimed that such changes can be reduced either by blanching,
by including antioxidants or by irradiating at - 80 to - 180°C.
Radurisation (0.5 Mrad)
This will eliminate most non-spore forming bacteria and give a
significant reduction in the number of spoilage micro-organisms, thus extending
the shelf life. Unfortunately, enzymes are not denatured and the ultimate
spoilage pattern is changed, requiring a reappraisal of spoilage criteria.
However, this process seems to offer the most promise in food preservation to
date.
Low dose (50 krad)
This inhibits sprouting in vegetables and cereals, and kills
tapeworms and insects.
Legislation and control
It has been proposed that the use of irradiation should be
prohibited unless specific approval is granted by Governmental authority.
Permission should define the food and the type and level of treatment. Plants
should be licensed and the regulations should cover plant design and
qualifications of the staff. There should be continuous records available
relating to the process, e.g. speed, load, period of irradiation etc.
International standards should be established covering the measurement of the
dose. Biological tests should be continually carried out to check the
effectiveness of the process. Labels should declare the treatment and give
sufficient detail to assist public health control and to inform the consumer on
handling and shelf life.
Irradiation of fish and seafood
In general, it is thought that, at doses higher than 0.3/0.5
Mrads, discoloration, production of off-flavours etc. would make irradiation
unacceptable. Russian workers have claimed favourable results, however, on
boiled fish using doses of 1.5/2.0 Mrads with a shelf life of 2 years; also the
Americans have obtained favourable results on treating shrimps at a similar
level. However, most work has centred on the milder radurisation of seafoods.
Such a mild treatment is ineffective against spores of C.
botulinum and there must, of course, be concern about the ability of type E to
produce toxin at 3.3°C. As radurisation only extends shelf life and does
not destroy C. botutinum spores, refrigeration is necessary, and possible
production of botulinum toxin is of considerable concern. Type E has a
widespread geographic distribution in temperate waters; little information is
available about tropical waters.
Some workers have claimed that toxin formation may be more rapid
in irradiated fish. However, it is also markedly influenced by the initial
number of spores and the actual foodstuff. The inclusion of 5 per cent (w/w)
sodium chloride apparently inhibits outgrowth of spores. While the doses under
consideration will not reduce numbers of C. botulinum spores, the normal
spoilage organisms Pseudomonas sp., which cause putrefaction and ammoniacal
odours, and Lactobacillus sp., occurring in shellfish, are significantly reduced
in number (thereby extending the shelf life). Therefore, such doses are liable
to change the apparent spoilage pattern. Radurisation could, therefore, increase
the botulinum hazard. Other pathogenic organisms have been shown to be resistant
to mild doses.
From the limited data available, it is clear that
generalisations should not be drawn. There are very definite species differences
in behaviour and, even when the species is the same, or similar processing
conditions pertain, the results are not easily comparable. Moreover, many
reports do not make it clear whether the changes they record occurred only
during irradiation, or whether they occurred only during storage after
irradiation.
However, it would seem that, even with the radurisation
treatment of 0.3 - 0.5 Mrad, there is some destruction of vitamins and of
cysteine, and a range of oxidative changes. Enzyme inactivation is far from
complete and many autolytic reactions continue.
Treatment of cod fillets and mackerel by doses of 50 head to 4.5
Mrad caused no changes in the Biological Value (BV) or Net Protein Utilisation
(NPU) of the fish protein. This does not indicate that no changes occurred, only
that changes did not alter BV or NPU. In fact, the non-essential amino acid
cysteine was destroyed at doses in the range of 0.1 - 0.5 Mrad; it was very
probably derived from a sulphur-containing amino acid (i.e. cysteine itself,
cystine, methionine) or vitamin (thiamine). Thiamine is known to be destroyed by
doses in the range 0.6 - 1.0 Mrad.
There have been reports that the lipids of the shrimp Peneaus
setiferus are unchanged by irradiation, and that the carotenoid pigments of the
shrimp Crangon vulgaris are not significantly altered during irradiation.
However, it is possible that free radicals produced during irradiation may cause
the loss of polyunsaturated compounds during storage after irradiation.
Trimethylamine oxide (TMAO) is degraded during irradiation by
doses of 0.6 Mrad upwards. The products are variously reported as trimethylamine
(TMA), dimethylamine (DMA), tetramethylenediamine, formaldehyde and water.
Formaldehyde is known to insolubilise protein and is thought to be partially
responsible for toughening of fish flesh during storage. In non-irradiated
material, DMA is only encountered in gadoid species during frozen storage; these
species possess an enzyme system capable of converting TMAO to DMA. In
irradiated samples, DMA seems to be formed also in non-gadoid species.
In Bangladesh, freshwater carp irradiated at a level of 200 -
250 head were found to have an extended storage life (8 - 10 days) compared to
untreated fish (1 day). During storage, the volatile acid number (VAN) was found
to be a better chemical index of quality than either TMA or TVB (total volatile
bases) nitrogen.
Similar findings have been reported from the Philippines: VAN
increased during storage of dried alakaok (plain croaker) and bisugo
(ribbon-finned nemipterid) irradiated at 50 head, alumahon (striped mackerel)
and banak (long-finned mullet) irradiated at 100 head, and tribe (shrimp)
irradiated at 300 head. VAN increased during storage at 6 and 30°C but TVB
and TMA contents did not change after irradiation. This procedure seems most
promising, as a quality assessment technique, at present.
Shrimp irradiated at 200 head had less than 3 ppm carbonyls
immediately after irradiation and this value remained essentially constant
during 28 days' storage. Non-irradiated samples contained about 3 ppm carbonyls
initially and showed a more rapid increase and spoiled after 21 days. Incipient
spoilage corresponded to 5 ppm.
Generally, irradiated samples show no increase in TMA or TVB
during storage, probably because the bacteria capable of producing these
substances have been destroyed.
With the risk of food poisoning, quality and methods of quality
assessment require careful and precise definition. This has not yet proved to be
possible.
Sensory tests are satisfactory is assessing consumer
acceptability. However, they cannot indicate the presence of pathogenic
micro-organisms and could be hazardous for panelists.
Chemical tests are of limited value for untreated fish. Also,
greater variance is found in irradiated fish, as indicated above.
Microbiological tests give the only reliable check but take too
long to complete. For fresh fish, a total count of about 1 x 10(6)/g coincides
with definite signs of spoilage, but for irradiated fish the level is 1 x
10(8)/g, thus increasing the chance of toxic effects if consumed. From the
above, it should be evident that there is a need for more research into quality
checks, particularly indicative tests.
While results will vary with species, time of year, etc., it has
been demonstrated that at any temperature a 100 per cent increase in maximum
shelf life can be expected for samples irradiated at 0.2 Mrad, or about 65 per
cent increase for a dose of 0.1 Mrad. Irradiation pre-rigor gives better results
(probably because the fish are more fresh). Other results indicate that combined
irradiation processes might also offer increases in shelf life, e.g. 0.05 Mrad
on board ship and 0.15 Mrad on shore after 3 - 7 days' storage in ice. No loss
in nutritional value up to doses of 0.6 Mrad have yet been demonstrated.
It is claimed that, if the problem of quality assessment can be
overcome, the following advantages would accrue from radurisation:
Overall, radurised fish will have to compare well with frozen
fish if it is to become acceptable. In adopting radurisation, the following will
have to be considered:
UV of wavelength 2000-2950Å is generally permitted. However, it
has very poor penetrative power and so is limited to treatment of surfaces
(e.g., packaging) or relatively transparent liquids. Furthermore, there is no
apparent lethal effect on spores; it is only bactericidal. In fisheries its main
use is in purifying water used to cleanse oysters.
For up-to-date information contact: INTERNATIONAL PROJECT IN THE
FIELD OF FOOD IRRADIATION, Institut fur Strahlentechnologie, 75 Karlsruhe
Postfache 3 640, Federal Republic of Germany.
1. AYRES, P. A. (1978) Shellfish purification in installations
using ultraviolet light. Lowestoft: Ministry of Agriculture, Fisheries and Food
Directorate of Fish Research, Laboratory Leaftet 43.
2. DESROSIER, N. W. and ROSENSTOCK, H. M. (1960) Radiation
technology in food. Westport, Connecticut: Avi Publishing Company, 401 pp.
3. KREUZER, R. (Ed.}1969. freezring end irradiation of fish,
Published by arrangement with the Food and Agriculture Organization of the
United Nations, by Fishing News (Books) Ltd, West Byfleet, Surrey, 528
pp.
