FOOD IRRADIATION – The Lost Technology

| October 17, 2008

Irrad pic 1

It is more than 100 years since the discovery that ionising radiation could be used to destroy microorganisms. The potential applications for food safety and quality were realised early on, but despite this, food irradiation has made remarkably little progress in comparison with other technologies that have much shorter histories. Consumer attitudes, safety worries, regulatory restrictions, cost and other concerns have conspired to limit irradiation to a very small number of applications, especially in Europe. But now there are signs from the USA that this situation is at last beginning to change. Has the time come for the European food industry to re-evaluate food irradiation as a possible answer to some perennial food safety problems?

Ionising radiation is any form of radiation that carries enough energy to detach electrons from individual atoms and molecules, turning them into ions. It can be electromagnetic X-rays or gamma rays, or energetic alpha and beta particles. The first suggestion that ionising radiation could be used to preserve food appeared in a German medical journal in 1896, very soon after the discovery of radioactivity by Henri Becquerel. The first patents for using radiation to destroy microbes in food were filed in the USA and the UK as early as 1905. The idea behind the patents was a familiar one today – to preserve food without the use of chemicals. But commercial exploitation wasn’t a possibility at the time, since the only known source of radiation was radium, which was not easy to obtain. The next notable step was another patent, filed in the USA in 1921, which described the use of X-rays to inactivate Trichinella parasites in pork, but limited availability of X-ray sources and high cost prevented any progress. X-rays were also investigated in the 1940s as a means of preserving beef, but high cost mitigated against commercialisation once more.

It was not until after the Second World War that food irradiation began to look anything like a commercially viable process. The burgeoning nuclear industry in the USA and elsewhere provided a ready source of ionising radiation in the form of spent nuclear fuel rods, enabling scientists to carry out research on the effect of irradiation on foo. The US Army was at the forefront of this research from the early 1950s onward, and soon moved from the limitations of fuel rods to specially produced isotopes of cobalt and caesium, giving much better control of the radiation dose. Paradoxically, although the nuclear industry and the military provided the means to research and develop food irradiation, this association also served to restrict its commercial application as the technology became forever linked to atomic weapons.

Food irradiation research and development has continued steadily since the 1950s in North America, Russia and in Europe. A major milestone was a meeting of the Joint FAO/WHO/IAEA Expert Committee on Food Irradiation (JECFI) in 1980. The JECFI meeting concluded that, “the irradiation of any food commodity up to an overall average dose of 10 kiloGrays (kGy) presents no toxicological hazard; hence, toxicological testing of foods so treated is no longer required.” The Committee also concluded that this level of irradiation did not introduce any special nutritional or microbiological problems. This statement effectively gave an international green light for the introduction of low-level food irradiation without any further research being necessary, although things haven’t quite turned out as the Committee might have envisaged.

Applications and limitations

When ionising radiation penetrates food it produces short-lived, highly reactive free radicals. These free radicals, along with the ionising radiation itself, damage sensitive material within living cells, notably DNA and RNA. Cells with denatured DNA and RNA cannot function and are destroyed. Ionising radiation at different doses can be used to kill microorganisms, parasites, moulds and insects, and can also inhibit ripening of fruits and sprouting in bulbs and tubers. The doses needed to destroy insects and parasites and to delay fruit ripening and bulb-sprouting are generally low (less than 1 kGy) because the cells contain relatively large amounts of DNA and RNA. Microorganisms tend to be more resistant, having much smaller genomes. For example, doses of up to 3 kGy for chilled, and 7 kGy for frozen, have been recommended to reduce the numbers of Salmonella and Campylobacter pathogens in poultry meat. Much higher doses of up to 45 kGy can also be used to produce commercially sterile foods for special medical diets designed for people with a weakened immune system, such as transplant patients.

There are currently three methods of delivering doses of ionising radiation for food processing.

  • Gamma radiation – usually produced by radioactive isotope sources such as cobalt60 and caesium137. These are highly penetrative rays, but do not produce radioactivity in the material they pass through. Gamma ray sources produce radioactivity constantly and so need heavy containment and shielding for safety. Dosing is controlled by exposure time. Gamma ray facilities have been used to sterilise medical products for nearly 40 years.
  • Electron beams (e-beams) – these are streams of high energy electrons (beta particles) produced by an electron gun that can be turned on and off and does not produce radioactivity. Electron beams have limited penetration (3-4 cm) into food, and do not require such high levels of containment and shielding to protect workers. E-beam sterilisers have been used in medical applications for 15-20 years and the technology is seen as having great potential for food processing applications.
  • X-rays – the use of X-rays to process foods is more recent and involves a more powerful version of the X-ray machines used in medicine. Like the electron gun, these devices can be turned on and off and do not produce radioactivity. However, X-rays are much more penetrative than electrons and heavy safety shielding is required around facilities. There are only four commercial X-ray plants in existence worldwide.

Although the potential to increase shelf life, control microbial pathogens and destroy insect and parasite infestations makes food irradiation very useful as a preservation and decontamination process, it does have its limitations. Some foods do not respond well to irradiation. For example, very fatty foods, such as meat products, oily fish and cheeses, suffer from taints and the rapid development of fat rancidity when subjected to quite low doses, and foods that continue to respire, such as fresh shellfish and fruits, may be damaged. Microorganisms also vary in their sensitivity to ionising radiation – as measured by the radiation dose in kGy needed to destroy 90% of the population (D10). Sensitive species like Campylobacter and E. coli have D10 values of about 0.2 kGy, but for Salmonella species and Listeria, typical D10 values are between 0.4 and 0.8 kGy. Bacterial spores are much more resistant and the spores of Clostridium botulinum have D<sub)10 values of up to 2.8 kGy. These spores could survive low dose irradiation and be a health hazard in some foods.

Significant restrictions on the use of food irradiation are imposed by legislation in many countries, but there is limited accord in regulations around the world. In the EU, a framework Directive (1999/2/EC) allows irradiation if it is technologically necessary, presents no health hazard, benefits consumers and doesn’t take the place of hygiene and good manufacturing practice. All irradiated foods also have to be labelled. A positive list of products that can be irradiated and traded in the EU has been drawn up, but so far includes only dried aromatic herbs, spices and seasonings at a maximum dose of 10 kGy. This list will eventually replace national lists in individual Member States, many of which currently have their own. For example, in France, the Netherlands and Belgium, other food categories can be irradiated, including certain fruits and vegetables, cereals, meat products and some shellfish, whereas the Danish government has been opposed to the process and allows no other categories to be irradiated. Eventually it is likely that the regulations in Europe will be harmonised, but there is little current pressure to do so. Elsewhere in the world, the USA, Thailand and South Africa have the lead in food irradiation, but about 50 countries have given some degree of approval for the process, including China. In the USA, the FDA has approved irradiation for the control of pathogens in poultry, red meats, fruits, vegetables, herbs and spices, shelled eggs and fresh and frozen shellfish.

Another important factor limiting the take up of food irradiation is the number of approved facilities available to process foods. Only 10 EU Member States have approved facilities within their borders, and most of these are in France (6), or Germany (4). Most are cobalt60 sources producing gamma rays. This means that a considerable investment in infrastructure would be required to make large scale food irradiation commercially viable in the EU. By contrast, there are estimated to be at least 100 licensed irradiation plants in the USA that could conceivably process food, including a commercial electron beam plant in Iowa designed for food applications. Nevertheless, the US food irradiation sector suffered a severe setback in 2004 when SureBeam, a spinoff company from the defence industry, which was operating electron beam plants in the US, had to file for bankruptcy after apparently overestimating the potential income to be gained from irradiating meat. Some of the SureBeam plants were closed, but one has since been taken over by the Sadex Corporation, which is now supplying a service to a number of US food processors.

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A schematic representation of a SureBeam plant

 

A controversial technology

There has long been a powerful, vocal and largely successful campaign by consumer groups and others against food irradiation, especially in Europe. This movement has raised a number of concerns about the process, including the following:

  • Production of potentially toxic radiolytic compounds during irradiation
  • Losses of vitamins and other nutrients during irradiation
  • Sensory changes affecting odour, flavour and texture
  • Resistance of pathogens to irradiation
  • Environmental and operator safety concerns
  • Misleading consumers by extending the shelf life of foods beyond ‘natural’ limits
  • Security risks
  • Use of irradiation to cover up deficiencies in hygiene and good manufacturing practice

Furthermore, many anti-irradiation campaigners question the technological need for irradiating foods. This attitude can be neatly summed up by the slogan of the UK-based Food Irradiation Campaign, “Good food does not need irradiating.”
But the main reason for opposition to food irradiation may be an emotional one. The word radiation is seen as threatening because of its associations with nuclear weapons and disasters like Chernobyl. Consumers are aware that radiation is a hazard and it is hard to accept that it might also be a benefit.

Although the anti-irradiation movement has attracted a good deal of support, there is little hard scientific evidence for many of its concerns. For example, while it is certainly true that radiolytic compounds are formed in irradiated food, most of these can also be found in cooked and other processed products and very few are unique. Toxicological and feeding studies suggest that there is little risk to health from the low levels of compounds produced by low dose processes, and even studies involving high dose sterilised products have not shown any significant toxicological effects. Irradiated foods are regarded as safe by a variety of public health bodies around the world, including the World Health Organisation. Concerns over nutrient losses also seem to be unfounded. Similar losses occur in canning and other preservation processes and are only considered to be a significant issue if the diet consists solely of irradiated foods.

Where the campaigners may have a valid concern is over the possible use of irradiation to conceal poor hygiene, or microbiologically contaminated foods. It is feared that unscrupulous traders could use the technology to introduce unacceptable or condemned products back onto the market. This is certainly something that the industry would need to guard against, since any reported incidents of this type would certainly undermine the case for irradiation. Most of the bodies in favour of food irradiation have stated very clearly that on no account should the process be used as a substitute for hygiene and proper GMP. Nevertheless, undeclared irradiated products, such as herbal supplements, spices and seafood, have been found on sale in Europe.

Only the most passionate supporter of food irradiation would suggest that the technology should be widely introduced throughout the food industry. Nevertheless, there are some specific instances where it might play an important role. The most obvious of these is in the control of microbial pathogens, especially in meat and poultry. Food poisoning outbreaks have regularly provided a stimulus for adopting food irradiation in the USA. For example, an outbreak of E. coli O157:H7 linked to undercooked burgers in 1993 led directly to the approval of irradiation treatments for red meat. More recently, product recalls and outbreaks of illness caused by this pathogen have sparked a lot of interest in the electron beam process as a means of decontaminating meat products. Control of E. coli O157 in red meat is very difficult once it is present in animals, and consumer safety relies very much on adequate cooking. Irradiation could provide another option that does not rely on the consumer. A similar argument could be applied to poultry with regard to Salmonella and Campylobacter. Both of these pathogens can be controlled by good husbandry and biosecurity measures, but at a cost, both in economic and animal welfare terms. Electron beam irradiation of carcasses during processing could reduce numbers of pathogens on poultry meat and help cut the number of cases of foodborne illness.

Where we are now

As things stand there is very little current European interest in food irradiation. Although some countries do irradiate foods, the amounts processed are very small. Even in Belgium, the biggest user of food irradiation in the EU, only just over 7,000 tonnes of foodstuffs were processed in 2005, and that included over 3,000 tonnes of frog’s legs. The large supermarket chains are always influential in these matters and most consider that there is so much consumer resistance to irradiated foods and so little demand that there is no point in developing the technology.

The situation in the USA is rather different. There, irradiated ground beef and other products are on sale and have been for some years. Sales are said to be slow and account for a very small section of the market. Nevertheless there are signs of growth and of greater acceptance among consumers uneasy about the number of recent food poisoning outbreaks linked to meat and fresh produce. Irradiation is being presented as one answer to this problem. For example, US Congressmen at a recent hearing on food safety were invited to taste samples of electron beam-irradiated and non-irradiated spinach – the cause of a major E. coli O157 outbreak in 2006 – by Professor Dennis Olson of Iowa State University. They were unable to tell the difference. “Popeye would approve”, one is reported to have said. “When we have widespread use of irradiation of our food supply, it will also be listed as a pillar of public health,” commented Olson, comparing the technology with milk pasteurisation. While this may be overselling food irradiation, it does have potential as a practical solution to some food safety problems that have yet to be overcome by other means.

It seems clear that food irradiation offers benefits, but that these are not yet sufficient to overcome consumer resistance to anything connected with the word radiation. Consumer surveys generally show that many people have serious misconceptions about the technology, but that their views could be changed if they were provided with accurate information about the benefits and given an opportunity to try irradiated products. Recent sensory studies have shown that the acceptability of irradiated foods can equal, or even better, that of conventional foods. But while irradiation slowly gains ground in the USA, the same cannot be said of Europe, and there are fears that the European food industry is being left behind. Whether or not this matters is a question that can only be answered by a reasoned debate, something that will be difficult to achieve in an atmosphere of polarised opinions, prejudice and fear.

Further information

The Sadex Corporation

The Food Irradiation Campaign

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