The research may help plant managers better understand how pathogens escape irradiation treatments and cleaning chemicals. It could also lead to calls for minimum irradiation levels to be set by law.
US government regulations limit how high a dose of irradiation is allowed, but the rules do not set a minimum level for those processors who choose to irradiate.
Irradiation has proved to be effective at eliminating pathogenicbacteria from meat in a processing plant before aproduct is shipped out, says Aubrey Mendonca, an Iowa State University foodscience researcher. But irradiation can beless effective if plant personnel do not usesufficient doses and if they don't account for thestrength of the bacteria they are trying to kill.
Bacteria in plants can be tougher than the ones cultured in labs, he said.
Bacteria can adapt to thestressful conditions they encounter in a plant and become hardyenough to survive irradiation if the doseisn't strong enough, her research shows. Bacteria left for dead mayrise up and haunt the processors, he said.
"Many processors do not use the maximum level ofirradiation because of quality reasons," Mendoncasaid. "A maximum level of irradiation maydetract from the desirable sensory qualities of themeat. What they try to do is find a dose that wouldstill give them good food safety protection againstpathogens."
Food processors often determine the dose they need to irradiate meat by calculating the minimum needed to kill the pathogens without detracting from taste, aroma, appearance and other sensory qualities. Processors usually rely on studies that show how much irradiation is needed to kill pathogens such as E. coli O157:H7, Salmonella Typhimurium or Listeria monocytogenes.
The flaw in that approach occurs when processors use studies of pathogens that are cultured under optimal growth conditions in a laboratory, Mendonca said.
"The conditions that those microorganisms face in the laboratory are not as stressful as the situations encountered by the bacteria seeking to survive in a processing plant's environment," he said. "In fact, growth conditions in laboratory media rarely produce stress-hardened bacteria."
Starved bacteria competing in the more rugged environment of a processing plant develop greater abilities to resist adversity by adapting to the living conditions or die. The tougher bacteria may not die when targeted by irradiation treatments. They may merely be injured unless a higher dose is administered, her research shows. Irradiation dosages are measured in kiloGrays.
"For example, if I go to the literature right now it says you can eliminate 100,000 Salmonella cells in chilled ground beef with about three kiloGrays," Mendonca stated. "These data are not based on the highly resistant cells. They're based on cells grown in the laboratory. So the processors might have a false sense of security in using less of an irradiation dose to save quality and believing that they're actually eliminating the pathogen."
The solution would be for processors to base their irradiation levels on what it takes to kill stress-adapted organisms instead of laboratory-grown organisms.
Processors generally do not want to use the maximum dose when irradiating, but they might not need to do so, Mendonca believes.
Irradiation destroys bacteria exponentially - increasing in powers of 10 - so relatively small increases in irradiation dose can significantly raise effectiveness of the process.
Mendonca's results were gathered from experiments testing the ability of starved cells of Salmonella Typhimurium to survive following the irradiation of ground pork. Additional research needs to be done to evaluate the resistance capabilities of other stress-adapted foodborne pathogens.
Industry is becoming aware of the need to watch for the resistant bacteria. Some processors are starting to rotate sanitizers because the constant use of one sanitizer could enable microorganisms to become resistant.
"Many processors use two or three different sanitizers and will change every month," he said.
Mendonca's research is funded by the Food Safety Consortium and is described in the organisation's Winter 2006 newsletter.
Regulation of the use of irradiation in the food sector is inconsistent around the world and within the EU's borders, according to a legislative overview of the technology. The report was published by the UK's Institute of Food Science and Technology (IFST) early in February.
The process exposes foods to ionizing radiation that kills insects, moulds and bacterium. The technology, which can kill up to 99 per cent of pathogens, is seen by the industry as a means of ensuring food safety.
To date, about 50 countries have approved about 60 products to be irradiated. The US, South Africa, the Netherlands, Thailand and France are among the leaders in adopting the technology.
Currently regulations on food irradiation in the European Union are not fully harmonised. Directive 1999/2/EC establishes a framework for controlling irradiated foods, their labelling and importation, while Directive 1999/3 establishes an initial positive list of foods which may be irradiated and traded freely between member states.
So far the positive list has only one food category - dried aromatic herbs, spices and vegetable seasonings. Some countries, such as Belgium, France, the Netherlands and the UK allow other foods to be irradiated, whereas other countries, such as Denmark, Germany and Luxembourg remain opposed, the IFST reported. Within the UK seven categories of foods can be irradiated to specified doses.
Regulations across the world make provision for labelling to ensure that consumers are fully informed whether foods or ingredients within them have been irradiated.
The US National Centre for Policy Analysis estimates that if half the food at greatest risk consumed in the country were to be irradiated, food-borne illnesses would decline by 900,000 cases annually and deaths by 352. The centre estimates irradiation would cost about five cents per pound for meat and poultry products