CORVALLIS, Ore. – An outbreak of E. coli O157:H7 in fresh strawberries sickened at least 10 people in Oregon in July and reminds us that food safety starts in the garden.
Sam Angima, a soils specialist, and Carolyn Raab, a foods and nutrition specialist - both with the Oregon State University Extension Service – offer these words of caution.
Fruits and vegetables can be carriers of pathogens that cause foodborne illnesses such as E. coli 0157:H7 and salmonella especially if untreated animal manure has been used in or is near the garden, Raab said. If animals have access to the garden, that could be a source of manure.
"The risk associated with garden produce is small, but it's there," she said. Foodborne illness outbreaks have been linked with many foods, including raw fruits and vegetables and unpasteurized apple cider.
What does this mean for gardeners?
Use particular care if and when you use animal manure in the garden. To avoid potential food safety risks, Angima and Raab recommend the following:
Use recommended food preparation techniques with garden produce. Always wash produce in clean water before eating it. Use a vegetable brush to remove visible soil. Peeling may also help reduce risk.
Keep fruits and vegetables and other raw food separated from cooked food. Wash your hands thoroughly with soap after handling raw foods, as well as before preparing food and eating it. Always wash hands after using the toilet and after changing diapers.
People who are more prone to foodborne illness include young children, pregnant women, older adults and those with cancer, AIDS and other immune-compromising diseases. "If a family member is at risk, serve cooked or canned vegetables and fruits for an extra margin of safety," Raab said. "Heating kills bacteria and parasites."
In the vegetable garden, use of compost rather than manure is preferred. However, if you use any kind of manure, ensure that the edible portion of the crop does not touch the soil. Use straw or mulch to separate the crop from the soil.
"If you do choose to apply fresh or partially composted manure to the vegetable garden, apply it to a crop with a low pathogen-contamination risk, such as sweet corn," Angima said. Plant crops whose edible parts contact the soil such as carrots, potatoes, lettuce and melons in a section of the garden where manure is not applied.
"Backyard composting can be an effective way to kill pathogens in manure," Angima said. "But the composting process must be carefully managed. To be certain of pathogen kill, the pile must reach temperatures greater than 130 degrees. The pile must be turned often to ensure that the cooler material on the edges of the pile gets into the hotter center of the pile.
"You'll need about five turns during the hot composting phase to assure pathogen kill," he said. "After each turn, temperatures greater than 130 degrees for three days are needed to kill human pathogens.
"We know that the microorganisms in manure that could be harmful to humans are not adapted for long term survival in the soil," he said. "After application to the soil, these pathogens are killed by unfavorable temperatures, pH, desiccation and by predation and competition from native soil organisms."
"My best advice for using manure in the home garden is: 'When in doubt, leave it out,'" Angima said. "It is best to keep manure out of a cool home compost pile that is not managed intensively."