Some vacuum-packed chilled foods may not be as safe to eat as manufacturers and consumers think, researchers have warned.
The treatment used to kill E.coli 0157 bacteria is less effective than food manufacturers believe, tests at the Institute of Food Research in Norwich have shown.
New research findings suggest that E.coli 0157, the deadly bacteria that claimed the lives of 20 Scottish pensioners last year, is extremely oxygen sensitive when heated. This could mean that some vacuum-packed or modified atmosphere-packed foods are not being heated for long enough to kill E.coli 0157.
According to researcher Susan George, the key to the problem is how manufacturers test for the presence of E.coli bacteria in vacuum-packed foods. Vacuum packing in oxygen-free conditions is used to keep food fresh. Manufacturers often seal the food and then heat it to kill any E.coli 0157. They then test selectively for the presence of bacteria by taking food cultures and seeing if bacteria grow. Researchers say that as this is normally done in the presence of oxygen, E.coli does not grow. Thus manufacturers are satisfied that their heat treatments and safety margins are sufficient.
But scientists have obtained very different results from growing cultures in anaerobic (oxygen-free) environments, like inside a vacuum-packed container. They found that when cultures were taken anaerobically from heated food, E.coli very often flourished, suggesting that E.coli 0157 becomes oxygen sensitive when exposed to heat.
In one experiment, researchers had to heat food up to eight times as long in anaerobic conditions to achieve the same reduction in bacteria levels as in an environment with 2 per cent oxygen. In the kitchen of a normal house, the air would be about 20 per cent oxygen.
Ms George explained: "With some minimal heat treatments, previously unexpected survival of E.coli 0157 may occur. They may appear dead when manufacturers measure them in aerobic conditions. But if you look at them in anaerobic conditions, you find that they are not dead. They are injured and therefore sensitive to air, but they could recover in a vacuum-packed food where there is low oxygen.
"Manufacturers that do quality control and count bacteria aerobically may not have the safety level they think they have."
Alan Malcolm, director of the Institute of Food Research, said the results of the laboratory experiments demanded more research. Scientists are now turning their attentions to testing products off supermarket shelves.
"Most people think we know all we need about E.coli," he said. "But this is not the case. Increasing amounts of fresh food, particularly meat, are packed, wrapped and delivered under anaerobic conditions. But the amount of oxygen matters. We have not fully understood why this is the case. This is a very important issue that needs to be investigated."
A spokesman for the food industry said that they were not aware of the new findings but would like to hear more about them. He stressed that the industry followed strict safety procedures.