September 24, 2008

4 Min Read

Fourteen food safety research projects underway at the Yuma Agricultural Center (YAC), Yuma, Ariz., are answering some tough questions faced by the Western leafy greens industry.

Concerns over the correlation between animal defecation in vegetable fields and potential E. coli infection has the entire vegetable industry searching for quick, yet accurate answers.

The outbreak of E. coli 0157:H7 in California-grown spinach almost two years ago cost the industry an estimated $100 million, according to the Western Growers Association. The E. coli find and the correlated human illnesses sharply decreased consumer confidence in the safety of eating fresh leafy greens.

About 80 percent to 90 percent of the nation’s winter vegetables are grown in Yuma County and across the Colorado River in California’s Imperial Valley (low desert) after summer vegetable production comes to a close in California’s Salinas Valley.

Jorge Fonseca, vegetable and post-harvest specialist with the University of Arizona, is a YAC researcher conducting field trials to evaluate leafy green vegetables and fecal contamination risks in the Yuma area.

The California and Arizona leafy green products handler marketing agreement requires growers to avoid the harvest of a crop within a 5-foot radius from the spot of fecal contamination. Fonseca says insufficient scientific evidence exists yet to back that up.

In Fonseca’s research sponsored by the California Lettuce Research Board, non-pathogenic E. coli was inoculated into collected fresh cow feces, placed in three acres of romaine lettuce at the YAC, and then captured with catch cans in the soil surface area. The area was watered with sprinkle irrigation. The water and nearby soil were later analyzed.

Results indicated that wind speed during the plant growth stage was the most important factor in spreading E. coli in vegetables. “If the wind blows more than five miles per hour in one direction, fecal particles will travel further, especially when animal wastes are between two sprinkler lines,” Fonseca says.

For winds less than 13 miles per hour, the maximum distance that fecal particles will travel is 18 inches, Fonseca notes. Higher winds can spread particles from 6 to 7 feet. Feces travel further during the plant’s early development stage without regard to wind speed.

Fecal matter from cows created the worst chance of E. coli contamination compared to lower-risk dog feces. Related laboratory analyses at the UA in Tucson have confirmed that cow waste in a field generally means double-trouble.

“If cow feces are contaminated with E. coli, or worse yet the E. coli 0157:H7 strain, the E. coli can survive for five months,” Fonseca says. The time period is shorter for dog, dove, and pigeon feces.

Fonseca detailed his research findings with growers, pest control advisers, and other agricultural leaders at the 2008 Preseason Vegetable Workshop in Yuma, sponsored by Yuma County Cooperative Extension.

Not all animals spread E. coli the same way, so the UA may conduct future research on feces from wild donkeys, jack rabbits, deer, and coyotes. Wild donkeys are a problem in Bard, Calif. (Imperial County), while deer present problems for growers in the Wellton area of Yuma County.

In other research initiated by the UA’s Kurt Nolte, non-pathogenic E. coli was inoculated into sprinkler, furrow, and drip-based irrigation systems to determine how each system might spread E. coli. Sprinkler-irrigated plants tested positive for E. coli for the next six days. After two weeks, furrow-irrigated soil was the only soil which tested positive for E. coli.

Fonseca says furrow irrigation is safer than sprinkler irrigation in terms of E. coli on the plants. Yet he warned that people walking in a moist furrow contaminated with E. coli can transfer the contamination when they walk into non-contaminated fields.

“This illustrates the importance of the timing of the last irrigation,” Fonseca explains. “To reduce risks, you don’t want the ground to be too wet even though it could decrease yield. What may be good for nice looking, high-yielding lettuce may not be good for food safety.”

Another research project included taking samples of irrigation canal water from four to five sites in the Yuma Valley. The water samples were tested for different coliforms and E. coli and then compared with such factors as insect and bird populations plus environmental data.

“The only factor that correlated well with the microorganism data is temperature, and the winter season has the lowest temperatures,” Fonseca says. “It is possible that lower temperatures may slow the development of bacteria in contaminated produce.” Low humidity may also reduce microbial populations.

All tests were carried out at the YAC, not in commercial grower fields.

Another research project by Fonseca and YAC Director Charles Sanchez is studying whether dog feces along canal banks pose a threat to food safety. So far, little E. coli has been found, but that doesn’t indicate that the feces don’t pose a threat, Fonseca says.

email: [email protected]

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