Stainless steel vs copper alloy
Preparing food on copper surfaces may significantly reduce the risk of spreading food borne pathogens, according to a study by researchers at the University of Arizona. The study, funded in part by the International Copper Association and published in the May issue of Food Microbiology, compared the ability of various Salmonella strains to survive on both copper and stainless steel surfaces. It comes on the heels of research by co-author Christopher Rensing demonstrating the antimicrobial properties of copper. Over time, copper reacts with oxygen in a process called oxidation, which produces a residue toxic to some bacteria, including some strains of Salmonella and E. coli. Stainless steel, the industry standard for cookware, has no antimicrobial properties. “Our idea was to see if copper could be used as an alternative to steel surfaces in food preparation,” said Sadhana Ravishankar, Ph.D., microbiology professor at the University of Arizona and co-author on the study. “We did not do anything related to cooking temperatures — we looked at other processes. When you wash and cut vegetables, for example, you’re not going to cook them, but you could prepare them on copper.” The researchers tested the survivability of three copper-resistant Salmonella strains and one copper-sensitive strain on copper alloy samples that varied in copper concentration from 60 to 99.9 percent. The copper-resistant strains were donated from a lab that grew Salmonella bacteria specifically for its ability to better withstand contact with copper. In every instance, both the resistant and sensitive Salmonella strains died off in significant numbers on copper, but were not affected by steel. Salmonella was exposed to copper and steel in both wet and dry environments, and in conditions with and without nutrients. The researchers found that copper-sensitive Salmonella could not survive on high-concentration (99.9 and 94.8 percent) copper alloys after 10 minutes of exposure, and couldn’t survive more than 15 minutes on medium-concentration (88.6 and 70 percent) alloys. They saw a 3-log reduction, or 99.9 percent kill-off, of sensitive Salmonella on the 65 percent alloy after 15 minutes, while again detecting no bacteria on the 60 percent alloy after that time. Under the same conditions, the researchers found that steel did not affect the Salmonella numbers at all. In fact, Salmonella survived more than a week on the stainless steel plates. The copper-resistant bacteria didn’t fare much better than the sensitive strain. In a few cases, the resistant strains completely died off after 15 minutes, much like their sensitive counterpart. Other times, the researchers observed between 2-log (99 percent) to 4-log (99.99 percent) reductions in bacteria on copper. The take-away? Copper, whether in the form of a cutting board, colander, bowl or container, has the potential to kill pathogenic bacteria and help prevent foodborne illness, Ravishankar told Food Safety News. Ravishankar grew up in India, where copper cookware is more common than it is in the United States. She shared anecdotal evidence from her childhood that might further support the study’s findings: “As a child, I consumed food cooked on copper,” she said. “We’d cook food and store it at room temperature all day in copper containers, and then we would consume it at night. Nobody got sick.” Considering that Salmonella is the leading cause of diarrheal illness worldwide, copper cookware could go a long way in preventing common illnesses in many countries, Ravishankar said. But cookware is not the only place copper could help prevent illness or cross-contamination, she added. Refrigerator handles, doorknobs, countertops and other surfaces prone to hosting bacteria could all be coated in copper to inhibit the spread of pathogens — an application that could be especially useful in hospitals and kitchens. Next, Ravishankar said her team hopes to secure funding to research the potential toxicity of copper residues on humans.