|Fish Handling, Preservation and Processing in the Tropics: Part 2 (NRI)|
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
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.
(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.)
(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.
Limiting doses of various pathogens:
- Shigella, Enteropathogenic Escherichia coli, Proteus vulgaris.
- Streptococcus faecalis, S. pyogenes, Staphylococcus aureus, Salmonella typhosa, S. paratyphi B, S. wichita, S. choleranius.
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.
Changes caused during treatment
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.
Changes occurring during storage
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:
(a) market expansion;
(b) better quality than iced fish;
(c) reduction of spoilage losses;
(d) easier distribution;
(e) easier handling.
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:
(a) ambient temperature;
(b) hygiene standards;
(c) water quality;
(d) equipping of vessels;
(e) scarcity of other processes;
(f) consumer acceptance.
Use of ultraviolet (uv) irradiation
UV of wavelength 2000-2950is 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.
Other general sources:
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.