Science News - December 16, 2006

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. Under-ground-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.’”

terial 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.”

CHEMICAL CLEANERS

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.

SUSLOW

Some researchers are looking for alternative chemical sanitizers. In an upcoming 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 bac-

PICK ME — A crew harvests lettuce in Yuma, Ariz. Good worker hygiene is among the prevention measures taken to keep the nation’s produce free of pathogens that can contaminate the food supply.

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.