NSF International Study
Copper pots don't just lend a comforting gleam to the kitchen. They may also resist dangerous microbes better than stainless steel, two new studies suggest.
Cultures of dangerous Escherichia coli that survived for 34 days on a stainless steel surface lasted only 4 hours on a copper one in a British study. Meanwhile, research from the University of Chile suggests that copper halts the growth of Salmonella and Campylobacter strains1.
Although stainless steel looks hygienic, tiny scratches can trap harmful bacteria, says Bill Keevil, the microbiologist at the University of Southampton who conducted the E. coli study with collaborators at the Centre for Applied Microbiology and Research, Porton Down, UK.
"Under a microscope, scratch marks look like large valleys," he says. "If you wipe the steel with a cloth you'll get most of the bacteria off the surface, but it's very hard to clean them out of the cracks."
Transmitted through poultry, Campylobacter food poisoning strikes about two million people in the United States and 500,000 in England and Wales each year. About 40,000 Americans per year fall ill with Salmonella, and between 15,000 and 30,000 in England and Wales.
More than half of all chickens in the UK are contaminated with Campylobacter, according to Hugh Pennington, a bacteriologist at the University of Aberdeen. A similar proportion is infected with Salmonella.
E. coli infections, which come from cattle and sheep, are rarer than Campylobacter, affecting about 73,000 people each year in the United States and 1,000 in England and Wales. But they are also more deadly - 20 people died in a 1997 outbreak in Scotland in which 500 fell sick.
An old story
Copper's antimicrobial properties have been known for more than five millennia. The ancient Egyptians used copper pipes to transport water. French vintners have killed grapevine fungi using a 'Bordeaux' mixture containing copper sulphate for hundreds of years.
So a copper surface could provide extra protection from food-borne pathogens, but careful cleaning is still the best way to ward them off, says Pennington.
"The tried and tested way to get E. coli off a surface is to clean it with a detergent, then follow up with a bleach," Pennington says. "It would be dangerous to rely on copper's antibacterial property as the first line of defence."
More surprising than copper's powers is that E. coli should survive so long on stainless steel surfaces, says Ian Ogden, a microbiologist at the University of Aberdeen: "It's quite a worrying thought".
If consumers demand copper kitchenware, manufacturers will be ready to "rise to the occasion," says Hugh Rushing, executive vice president of the Cookware Manufacturers Association in Mountain Brook, Alabama.
But copper is not suited to many catering applications, Rushing warns. Acidic foods become discoloured and flavoured by the copper ions they leach from pots and cooking surfaces. "It's almost impossible to cook tomato sauce in a copper pan without it turning a horrible colour," he says.
Keevil is currently working on more practical alloys of copper. Brass - a mixture of copper and zinc - cuts down the survival time of E. coli to four days.