COLUMBUS, Ohio -- Leafy green vegetables, power-packed with nutrients, are a growing part of the average American diet. Yet in 2009, leafy greens also made the Center for Science in the Public Interest's "Top 10 Most Dangerous Foods," due to a surprising number of foodborne illnesses linked to the seemingly innocuous salad staple.
But a team of researchers with Ohio State University's Ohio Agricultural Research and Development Center (OARDC) is working on an answer.
According to the Food and Drug Administration, 82 foodborne illness outbreaks between 1996 and 2008 were linked to the consumption of fresh produce. More than one-third of them were traced to leafy greens, accounting for 949 illnesses and five deaths. One outbreak alone, the 2006 contamination of spinach with Escherichia coli O157:H7, caused 204 illnesses, including 104 hospitalizations, 31 cases of hemolytic-uremic syndrome (a serious complication), and three deaths. On top of the human cost, the economic impact of that outbreak alone was estimated at anywhere from $37 million to $75 million.
"Leafy greens -- spinach, lettuce -- are the most difficult type of fresh produce to treat," said Ahmed Yousef, food microbiologist and professor of food science and technology at Ohio State. "The leaves are porous. That allows the plant to respire, but it also makes it easy for pathogens to enter into the leaf -- not just stay on the surface. And, leaves are much more sensitive to treatment than other types of produce. We have been trying to figure out how to kill microorganisms without damaging the leaf at the same time. There's not a large margin for error."
Freshly harvested leafy greens are often vacuum-cooled, washed and sanitized with chlorine solution before packaging. “But research shows liquid sanitizers are not always effective, and sometimes make the problem worse,” said Ohio State food engineer Gonul Kaletunc.
The problems are twofold, the scientists explain:
- Air bubbles can form in the liquid sanitizer, preventing it from ever reaching portions of the leaf surface.
- Even when liquid sanitizer does come in contact with the surface, it may not have enough time to penetrate the leaf deep enough to reach microbes.
"If you have a bacterium sitting 1 millimeter inside a leaf surface cavity, and your sanitizer doesn't touch the bacterium, it's not going to have any effect," said Sudhir Sastry, food safety engineer and leader of this effort.
That's why the team, armed with a $1 million grant from the U.S. Department of Agriculture, has turned its attention to studying the effectiveness of gaseous sanitizers, including ozone and chlorine dioxide.
"To get into the crevices of the leaf, it's got to be gas, not liquid," said Yousef, who is also holder of the Virginia Hutchison Bazler and Frank E. Bazler Designated Professorship in Food Science.
Although gaseous sanitizers have a much better chance of reaching the pathogens that can sicken and kill, they also need specific conditions to work. And that causes a challenge, Sastry said.
"At the processing stage, everything is geared to speed, speed, speed," Sastry said. "But sanitizers, especially the liquid sanitizers, are slow, slow, slow, which is not compatible with a high-speed operation. So, we looked at the whole process and thought, 'Why not apply the sanitizer when there is some time to let it work?'"
For example, larger processors use vacuum cooling for a half-hour or longer after the harvest and before the greens are transported for packing. "To use gas as a sanitizer, you need an airtight compartment," said Kaletunc, who also has an appointment with Ohio State University Extension. "You have that with vacuum cooling, and vessels that are currently being used can be adapted to use gas."
Another possibility is to subject the greens to a sanitizing gas during transportation.
"Greens are in transit up to 96 hours," Sastry said. "Why not subject them to the sanitizing gas then, when there's plenty of time for it to work? We need to be able to incorporate a gaseous sanitization procedure into the produce chain's existing operations -- that will be key."
One Ohio State study, published in 2009 in the Journal of Food Protection and conducted by Sastry, Yousef and two of their graduate students, found that a combination treatment using ozone gas during vacuum cooling and transportation reduced E. coli on spinach leaves by up to 99.999 percent. In the current project, the researchers are further studying possible combinations of gaseous sanitizer -- both ozone and chlorine dioxide -- and partners at Iowa State University are examining the use of liquid organic acids combined with surfactants to determine if those would be as effective. Additional partners at New Mexico State University are preparing training materials for growers and processors. A half-dozen produce companies from across the U.S. have attended project meetings and expressed interest in the work, Sastry said.
In Ohio, Bobby Jones of The Chef's Garden in Huron, said the work is an important contribution to the industry.
“Everyone knows that a new sanitizing technology is necessary," Jones said. This research will benefit growers of leafy greens nationwide -- but the fact that this work is being done in Ohio gives us a competitive advantage." Jones added that the project goes hand-in-hand with current work being done to develop the Ohio Produce Marketing Agreement, a collaborative food-safety standards effort for leafy greens between the Ohio Department of Agriculture, the Ohio Produce Growers and Marketers Association, and Ohio State.
Sastry said it all comes down to safety.
“If we apply the right techniques in the right place at the right time," he said, "we really can improve the safety of produce.”
OARDC and OSU Extension are the research and outreach arms, respectively, of Ohio State University's College of Food, Agricultural, and Environmental Sciences.