Irradiation of food

Comment

Irradiation of food Bevan Moseley describes the process and its benefits On a global scale, somewhere between one quarter and one third of the world’s food supply is lost postharvest a:; a result of too rapid ripening, growth in storage, insect infestation and bacteria/ and fungal attack. To alleviate these problems, ionizing radiation, e.g. gamma rays, X-rays or electrons, can be used to extend the life of certain fruits and vegetables by delaying ripening or killing moulds, and for the control of insect infestation of grain and other stored products. Ionizing radiation also provides an effective alternative to chemical treatments for the prevention of sprouting in stored potatoes and onions and for the treatment of herbs and spices. More importantly, from a public health point of view, ionizing radiation can be used to kill, or reduce significantly, the number of pathogenic bacteria, e.g. Salmonella, Campylobacter, Vibrio, Listeria, in a variety of other foods, such as poultrymeat and shellfish.

The doses recommended for all these treatments are shown in Table 1 (see the Advisory Cclmmittee on Irradiated and Novel Foods Report on the Safety and Wholesomeness of Irradiated Foods (ACINF, 1986)). Not every country has the same problems with its food supply and the use to which irradiation is put will vary with each country’s needs. semi-tropical Tropical and countries would most likely use irradiation to inhibit the ripening process and to kill endemic insect pests, e.g. fruit fly, in their exportTemperate, able fruit crops. developed countries would use it to replace the use of the gas ethylene oxide in the treatment of herbs and spices and to reduce, or even in some cases remove, the most common food poisoning bacteria from foods in which they are endemic.

Sources of ionizing irradiation and their use The kinds of ionizing radiations which have been, or will be, used to irradiate food commercially include gamma (y) rays normally from a ?Zo source, X-rays radioactive with energies up to 5 MeV and electrons up to 10 Mev. M’Co y-rays are much more penetrating than 5 MeV X-rays or 10 MeV electrons and so can be used to irradiate thicker samples of food, e.g. a sack of spice or a whole Professor B. E. B. Moseley is Head of Laboratory at the AFRC Institute of Food Research, Reading Laboratory. Shinfield, Reading RG2 9AT. UK Presented at the Conference ‘Irradiation and Combination Treatments, London. l-2 March 1990

0956-7135/90/04020!%-02

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chicken carcase. The radiation cannot however be turned off so that the radiation plant needs to be operated 24 hours per day to be economic. The dose rate is also relatively low and this limits the throughput of material. Electrons and X-rays are not as penetrating as y-rays so that food to be irradiated has to be presented in a thinner form (- 10 cm thick). The dose rate is, however, very high and therefore allows a high throughput. In addition, the machines can be switched off. Thus spices would be expected to be treated with wCo y-rays, as for example occurs in Holland, and prepacked poultry meat with electrons, as occurs in France.

Sensitivity of bacteria to irradiation

Flavour changes in irradiated food

Bacteria are killed by irradiation because they contain, usually, only a single chromosome in the form of a very long circular molecule of double-stranded DNA which is vulnerable to the radiation-induced

Table 1

Dose ranges recommended

All foods and food ingredients will undergo flavour changes given a sufficiently high dose of irradiation but some foods are much more sensitive to this effect than others.

for various purposes

Process Inhibition of sprouting e.g. potatoes and onions Delay fruit ripening e.g. mangoes, papayas insect infestation e.g. grain and pulses Elimination of parasites in meat e.g. Trichinella spiralis Reduction of microbial load e.g. moulds on strawberries, bacteria in meat Elimination of non-sporing pathogens e.g. Salmonella, Campylobacter on poultry; Salmonella, Vibrio on shrimps and prawns

1990 Butterworth-Heinemann Ltd

formation of single and double strand breaks. Most bacteria can repair single-strand DNA breaks by enzymatic means but are unable to restore a significant number of double-strand breaks and thus remain unable to divide. The proportion of a bacterial population that survives a particular dose of radiation will depend on the intrinsic sensitivity of the species, at what stage of its growth cycle it is irradiated, the amount of radiation damage inflicted, its potential for repair and the relevant proportions of damage channelled into different repair pathways. In general, Gram-negative bacterial genera such as Pseudomonas, Campyiobacter and Salmonella, Vibrio are more sensitive to ionizing radiation than Gram-positive ones such as Listeria and Staphylococcus. Bacterial spores, formed mainly by the Gram-positive genera Bacillus and Clostridium, are at least an order of magnitude more resistant than those of vegetative bacteria of the same species. More resistant even than bacterial spores, are the vegetative cells of the Deinococcus and Deinobacter Cells of Deinococcus species. spp.appear to have multiple copies of their chromosomes and a DNA repair mechanism based on recombination between the copies. Luckily these bacteria are uncommon, not pathogenic and very sensitive to heat.

Dose range (kGy)

0.05 - 0. I5 0.2 - 0.5 0.2 - 1.0 0.03 - 6.0 0.5 - 5.0 3.0 - 10.0

Food Control - October

1990

205

Comment

For example, low-fat foods, such as poultry meat, white fish, shrimps, prawns and spices can be given amounts of radiation required to eliminate or significantly reduce the numbers of food-borne pathogenic bacteria before flavour changes develop which would make the product unacceptable to the consumer. Until recently, a general rule was that fatty foods, e.g. dairy products and fatty fishes, were unsuitable for irradiation because the oxidation of fats leading to rancidity occurred before the desired effect, e.g. the of the food-borne elimination pathogen Listeria monocytogenes, was achieved. Recent experience suggests that this rule may now need to be reviewed. It is already known that the irradiation of meat in the frozen state can prevent the development of offflavours and that irradiation in the absence of oxygen can reduce the effect. Much research on the organoleptic qualities of irradiated foods remains to be done but a significant effort will only be made in the UK when food manufacturers can feel confident that the decision to allow food irradiation will be a permanent one.

Irradiation

of food worldwide

When the UK allows the irradiation of food for human consumption it will not be setting a precedent. More than 30 countries allow this procedure and 20 countries irradiate food on a commercial basis (Table 2). Many irradiate single items. e.g. Russia irradiates only grain and Japan only potatoes. On the other hand, the Dutch treat several food items. Most of the 20 countries irradiation commercially using irradiate spices. The tonnages of food treated per annum are small in proportion to the total consumed, but there exists a number of restraints. For example, in Holland, firms that export a great deal of their produce to countries that ban irradiated products, e.g. FRG, choose not to irradiate any of their products even when it is legal to do so for fear of destroying their export market. A list of the foods for which in 31 irradiation is approved countries and the foods currently 206

Table 2

Practical

Country

application

of food irradiation

Food irradiated (tonne year-‘)

Argentina

so

10000

Belgium Brazil

200 500 500 500

Chile China Cuba Finland

5 200 6(KNl 400

France GDR Hungarv

being treated are contained in Appendix A of a report on Irradiation of Foodstuffs by the House of Lords Select Committee on the European Communities (1989).

European

and UK legislation

There is a fundamental difference between the approaches of the European Commission and the UK Government to food irradiation. The latter’s approach is based on the conclusions of the Joint Expert Committee’s report on the wholeirradiated food someness of (JECFI, 1977, 1981) and the ACINF Report (ACINF, 1986). A general authorization would be given to allow irradiation of any food up to an overall average dose of 10 kGy. No maximum dose of irradiation would be set for any food class and reirradiation would be allowed, provided the maximum was not exceeded. The European Commission, in contrast, has accepted the view of its Scientific Committee for Food that irradiation should be allowed for a list of specific classes of food and that maximum doses should be established for each class. How the differing approaches of the European and UK authorities will be resolved is difficult to predict.

Countrv

Food irradiated (tonne year-‘)

Israel

120 20 o(K)

Japan South Korea Netherlands Norway South Africa Thailand USA USSR Yugoslavia

18000

600 3 300 400 000 100

that two thirds questioned were not against irradiation. Nevertheless, it raises the question of how the benefits, as well as the drawbacks, of the process should be communicated to the consumer - whether it should be the responsibility of government, the retailers or the consumer organizations.

Combination

So far, discussion has been on irradiation as a sole food treatment process. However, with the consumer’s requirement for minimally processed foods, irradiation could be used as one of a combination of treatments. Bacteria that have been irradiated become sensitized to mild heating and vice versa. The irradiation of some meat products at very low temperatures or in the absence of air avoids the development of ‘off-flavours’ which more than compensates for the enhanced microbial resistance to radiation. It could be in this area that the major benefits of food irradiation will eventually be seen.

References ACINF

(19X6) Report

Wholesomeness

Consumer

concern

There is a high level of concern among the genera1 UK public with regard to the introduction of food irradiation. In a recent survey by the Consumer Association (Which, 1989) over a third of those questioned thought irradiation should definitely not be permitted in the UK. This does, of course, mean

processes

HMSO.

of

on the Safety and Foods Irradiated

London

Consumer November

Association

(1989)

Which.

House of Lords Select Committee on the European Communities (1 YXY) Irradiation of Foodsmff~ HMSO, London JECFI (1977) Wholesomeness of Irradiated Food. WHO Tech. Rep. Ser. 604. World Health Organization. Geneva JECFI (1981) Wholcsomcncss of Irradiated Food. WHO Tech. Rep. Ser. 659, World Health Organization. Gcncva

Food Control - October 1990