Researchers have found that defrosted meat can rapidly become contaminated with bacteria. The finding may partly explain the UK's food-poisoning epidemic.
Work at the Centre for Cell and Tissue Research at the University of York has shown that thawed tissue is riddled with capillary-like canals through which micro-organisms can travel.
Ashley Wilson, director of the centre and lead investigator, warned that the previously overlooked problem should be taken seriously by consumers, butchers, supermarkets and the Food Standards Agency after his results are published later this year.
"It is a potential risk," he said. "Our results suggest that you should use frozen meat the same day as it is thawed."
The team used novel microscopy techniques to reveal the ultrastructure of chicken breast. They found that it underwent a process of micro-canalisation.
Elongated ice crystals align themselves with muscle fibres as the chicken freezes. When it thaws, the crystals melt and leave behind a series of linked voids once filled by water, salts and proteins. This network is connected to the chicken's surface, which is often contaminated with micro-organisms such as salmonella.
"The micro-canals and their contents provide the perfect environment for the proliferation of bacteria," Dr Wilson said.
When the scientists exposed thawed chicken to non-pathogenic bacteria, they found that the bugs could penetrate more than 10mm into the meat in eight hours. Infection of fresh meat was confined mostly to its surface.
The scientists also discovered that meat subjected to high-speed cryogenic freezing - used on 10 per cent of frozen-food products in the UK - was more resistant to penetration by bacteria. This was because smaller ice crystals formed in the meat, which left behind a finer network of micro-canals that the bugs found more difficult to travel through.
The Food Standards Agency estimates that 5.5 million people in the UK suffered food poisoning last year. Regulations do not cover the shelf-life of defrosted meat, but the FSA recommends some simple steps to reduce risks, such as cooking meat thoroughly to destroy any bacteria present.
According to a study published in the latest issue of the journal Appetite , the British public regarded salmonella poisoning with as much dread as they reserved for vCJD.
Sara Kirk, a research fellow at the division of public health at Leeds University and one of the authors of the study, said Dr Wilson's research could be especially bad news for the elderly, children and people with compromised immune systems.