Preventing and controlling pathogens on produce
Spinach’s healthy reputation suffered a severe blow this fall. On Sept. 13, the Centers for Disease Control and Prevention in Atlanta learned that the raw leafy green was the prime suspect in a spate of virulent Escherichia coli infections. The next day, the Food and Drug Administration advised consumers not to eat any bagged fresh spinach. Two weeks later, the FDA announced that it had traced the tainted greens to one California company that bags spinach under several brand names. Fresh spinach from other suppliers soon began reappearing on store shelves and dinner plates. The outbreak’s toll, according to the CDC: 3 deaths and more than 200 people sickened in 26 states and 1 Canadian province.
Federal and state officials have found the implicated bacterial strain in cow feces, water, and wild pigs at sites near the four suspected spinach farms in California, but they still don’t know how the pathogen got to the greens. Officials continue investigating the incident, says Patti Roberts, a spokeswoman for the California Department of Health Services.
The spinach outbreak joins a growing list of health-related incidents tied to vegetables and fruits. According to the CDC, there’s been an increase in such outbreaks in the past few decades.
The rise in produce-related illnesses can be linked to several factors. With people becoming savvier about their health, fresh-produce consumption has grown, notes Robert B. Gravani, a food scientist at Cornell University. During this time, however, more-dangerous microbial strains have emerged, he adds.
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For example, the unusually virulent E. coli O157:H7 was first isolated in 1982, after an outbreak tied to contaminated hamburgers. A strain of that same bacterium was responsible for the spinach illnesses.
The food-distribution system also plays a role. “The production of fresh produce is much more centralized than it used to be, and [produce] gets distributed very widely and rapidly. Therefore, one contaminated field may lead to a multistate outbreak that affects a large number of people,” says Maria T. Brandl, a microbiologist with the U.S. Department of Agriculture’s Agricultural Research Service in Albany, Calif.
Finally, detection strategies have improved, notes Larry R. Beuchat, a food microbiologist at the University of Georgia in Griffin. He suspects that many outbreaks of illness of unknown provenance that occurred 20 or 30 years ago “would today, with the technology available, be confirmed or at least linked to particular types of [contaminated] produce.”
Preventing such contamination, from the farm to the dinner table, is the key to food safety, say many researchers. But farmers can’t stop all contamination, and once tainted, many fruits and vegetables are difficult to clean. So, for the rare times when unwanted microbes make their way onto a farmer’s crop, researchers are exploring new strategies and technologies to destroy these pathogens and to keep produce—and its consumers—healthy.
On the farm
A typical vegetable’s journey to market is full of potential contamination sources. Water that contains pathogens can come into contact with crops both during irrigation and in subsequent washing of harvested produce or its storage in ice. Animal feces can reach produce if domesticated or wild animals roam in the fields. Workers and equipment, such as bins or knives, can taint produce during the harvest or in later production steps.
To reduce the risk of contamination, the FDA in 1998 published recommendations for good agricultural practices (GAPs). This set of guidelines addresses issues that farmers must consider at various stages of the growing and harvesting process. For example, before applying manure to the fields, farmers must compost or treat it to remove pathogens.
“I honestly believe that if everyone was diligent about it, applying the principles of GAPs would … go a very long way to preventing outbreaks,” says Trevor V. Suslow, a plant pathologist and food-safety specialist at the University of California, Davis.
While “prevention is the best strategy we have,” says Gravani, “it’s not a simple task.” The guidelines don’t specify a single approach on how to achieve all the recommended practices because there’s huge diversity among farming operations.
“Every farm is different, and every situation is different,” says Gravani. The appropriate strategies, he adds, depend upon “the environmental conditions that beset you as a farmer.”
For example, a farm’s size, location, and even the time of year influence whether it accesses groundwater from wells or surface water from a river or creek, notes Suslow. Water from any of these sources can be dirtied by runoff from a dairy farm or other contaminated land surfaces, but strategies to maintain good water quality will differ according to the water’s source.
The needs of the crop also affect irrigation practices. Underground-drip irrigation minimizes contamination risks because the water, which may carry pathogens, isn’t applied directly to the edible portions of most plants that will be eaten raw. But drip irrigation isn’t suitable for all crops and environments. If growers use spray irrigation, which showers edible portions of many plants, they must take other measures to combat contamination.
“That’s why the guidelines are just that—principles of food safety,” says Suslow. “It’s incumbent on everybody to understand what it is exactly that they are doing … and what the risk factors are.”
In response to the spinach debacle, a few organizations, such as the Western Growers, an agricultural trade association in Irvine, Calif., have called for mandatory compliance with guidelines for spinach and leafy greens.
Much of the “controversy and anguish” on implementing mandatory guidelines, however, is “How do you set criteria in a way that is meaningful?” Suslow says. “You can’t just mandate, ‘You will have a deep well, and you’re only going to use drip irrigation.'”
Researchers have been searching for decontamination technologies that can back up preventive practices. An ideal treatment wouldn’t damage fruit and vegetables as it kills pathogens and wouldn’t leave a residue “that would cause any concern,” Beuchat says. The treatment should also be inexpensive. In terms of effectiveness, a 99.999 percent reduction of pathogens “is what we are shooting for,” says Richard H. Linton, a food microbiologist at Purdue University in West Lafayette, Ind.
Growers and processors today usually use chlorine as a sanitizer, adding it to the water in which they wash produce. The main role of chlorine is to prevent a contaminated piece of produce from spreading pathogens to other pieces during washing. The rule of thumb for chlorine, says Suslow, is that an effective concentration will kill 99.999 percent of the microorganisms in the water and 90 to 99 percent of the microbes on produce surfaces.
Excessive chlorine damages produce and poses health and environmental concerns. Highly concentrated chlorine solutions can give off gases harmful to workers, and discharging large amounts of the chemical into waterways can affect aquatic life. The Environmental Protection Agency limits chlorine concentrations to 200 parts per million for the water used to clean produce that won’t later be rinsed in fresh water.
Some researchers are looking for alternative chemical sanitizers. In the March 2007 Journal of Food Protection, food microbiologist Alejandro Castillo of Texas A&M University in College Station and his coworkers in Mexico report on a spray that contains 2 percent lactic acid, a chemical used to sanitize carcasses in the meat industry. The researchers first contaminated cantaloupes and bell peppers with either E. coli O157:H7 or Salmonella typhimurium and then sprayed the lactic acid solution onto the produce for 15 seconds. The treatment reduced the bacterial populations on the cantaloupes by close to 99.9 percent and by slightly more on the smooth-surfaced bell peppers.
Linton has been conducting studies with chlorine dioxide gas, the sanitizer that was used to treat anthrax-tainted mail in 2001. In lab tests, his team placed the produce in a desktop-size chamber and then pumped in the gas.
The group has tested the gas on apples, green peppers, cantaloupes, strawberries, tomatoes, sprouts, and lettuce. “We find that it’s extremely effective for most products,” Linton says. For example, in a 2003 study, the researchers reported that treatment with chlorine dioxide gas at a concentration of 7.2 milligrams per liter for 10 minutes removed more than 99.999 percent of E. coli O157:H7 from apples’ skins. The produce industry would prefer a process that takes no longer than 15 minutes, he says.
Like the chlorine solutions currently used in industry, chlorine dioxide gas kills microorganisms by oxidizing them. But for leafy greens, some concentrations oxidize cut surfaces, turning them white or brown. Linton plans to explore whether modifications of the technique can make it applicable to the greens.
The chemical residues that remain on the produce after the gas treatment are within the range considered safe in drinking water, he says. The team is in the process of seeking FDA approval for the treatment, after which the researchers can test whether it alters the taste of produce.
The group has recently developed a 7-meter-long, 2-m-high, commercial-scale device. A conveyer belt moves the produce through three chambers. The first chamber rinses the food with water to remove dirt. The second chamber exposes the food to chlorine dioxide, and the third gives the food a final water rinse.
“It’s pretty easy to do things in a lab,” Linton says. “Now, we want to subject 500 strawberries in a real-life [commercial] processing situation.”
Baths and beams
Some scientists are looking beyond chemical sanitizers for decontamination options. Bassam A. Annous, a microbiologist with the USDA’s Agricultural Research Service in Wyndmoor, Pa., has developed a pasteurization technique for cantaloupes. It reduces salmonella populations on cantaloupe surfaces by 99.999 percent.
Annous and his colleagues built a commercial-scale tank that can process up to 360 melons per hour. A conveyer grabs a melon and immerses it in water heated to 76°C, which is hot enough to kill bacteria. In 3 minutes, the conveyer propels the submerged melon across the tank and out the other end. The researchers immediately seal each melon in a bag and then cool it in ice water. They are developing a cooling method that would work better on an assembly line.
The brief heat treatment isn’t detrimental to the flesh of cantaloupes because they have thick rinds, Annous says. The edible portion of the fruit begins about 5 millimeters below the rind. In the March Journal of Food Science, his team calculated that for the first millimeter below the surface, the heat rises rapidly enough to kill microbes. But the flesh of the fruit 10 mm below the surface stays below 36°C.
That’s cool enough to preserve the fruit’s quality, says Annous. In tests so far, fresh-cut pieces of pasteurized cantaloupes maintained their color, odor, and vitamin C content.
Annous says that he hopes that his group will soon team with industry to test the technique in production facilities.
Some researchers propose that irradiation, a technique that the USDA approved for poultry in 1992 and for meats in 1999 (SN: 1/15/00, p. 40: Available to subscribers at USDA gives nod to irradiating meats), may be useful to decontaminate some produce. Castillo and his colleagues at Texas A&M University have treated cantaloupes and tomatoes with an irradiation method that uses electron beams. Meat producers that irradiate their products employ either electron beams or gamma rays.
In the March 2006 Journal of Food Protection, Castillo and his team describe irradiation of fresh-cut tomato cubes infected with one of two strains of salmonella. The treatment reduced populations of one strain by 99 percent and the other by 90 percent. The group hasn’t yet conducted taste tests of the tomatoes. Castillo says that he’s currently trying the technique on spinach.
Like other treatments, irradiation isn’t appropriate for every type of produce. Castillo says that the method damages the texture of grapes and some other fruits and vegetables. Moreover, “some foods will lose nutritional power—for example, some vitamins are affected by irradiation,” Castillo says. He adds that fruits and vegetables need to be tested individually to see how each one fares under the treatment.
Irradiation also requires expensive equipment. Growers would have to send produce to regional centers for treatment, Castillo says, because it is unlikely that a single plant could afford the machinery.
Back to square one
Among the sanitizers and technologies under review, “there are promising developments,” says Beuchat, but “there’s still room for improvement.”
Rather than look to a single treatment, the most effective approach to sanitizing produce may be to combine several strategies that remove and kill pathogens, says Brandl.
Moreover, when more is known about how pathogens find their way onto produce, researchers may come up with new methods to prevent contamination. Brandl says that researchers need to determine, for example, the harmful bacteria’s preferred locations on plants and their interactions with normal microbial populations that live there.
“Once we have sufficient information about critical risk factors,” she predicts, “we’ll be able to come up with additional, specific guidelines for the safe production of fresh fruits and vegetables.”