Proper Handling, Storage Can Keep Salads Safe, UF Researcher Says

GAINESVILLE—Vegetable salads served in supermarkets and restaurants generally are safe, although mishandling sometimes leads to contamination with bacteria, a University of Florida food safety researcher says.

Education for food workers, proper storage and washing can keep most fresh-cut vegetables free of bacteria, said Cheng-I Wei, a researcher for UF’s Institute of Food and Agricultural Sciences.

Wei cautions that the idea that fresh produce is uniformly safe is just not true in all cases.

“Salmonella can live on tomatoes, green onions can be contaminated with shigella and E. coli can thrive on cantaloupe,” Wei said.

In many cases, Wei said, a knife used to cut through the surface of a fruit or vegetable — watermelon, for instance — can carry surface bacteria into the flesh of the produce. Roadside stands that serve precut produce are a particular concern because many lack refrigeration and bacteria grow rapidly in warm environments.

In the study by Wei and doctoral researcher Chia-min Lin, 63 vegetable salads were purchased from supermarkets, fast food chains and family restaurants. The salads’ major ingredients were iceberg and romaine lettuce, red cabbage, carrot, tomato and cucumber, and the minor ingredients were broccoli, cauliflower, bell pepper, jalapeno pepper, onion, olive, mushrooms, celery and red hot pepper.

One of 63 salad samples contained L. monocytogenes, a pathogenic bacterium. Eight of 65 salad samples contained E. coli, a common inhabitant of the intestine of humans and animals, and an indicator of fecal contamination. The E. coli found in the salads was not the type that caused illnesses associated with Jack-in-the-Box hamburgers.

Health officials know that fecal contamination of food may mean that not-easily-tested-for pathogens could possibly be present. The nine contaminated salads were purchased at only five of the 31 establishments from which salads were obtained.

Wei said mishandling of the food items by workers could be responsible for the contamination.

Salads have become a growth industry for restaurants and supermarkets as consumers have become more health conscious. Freshly prepared salads are offered in more than 70 perent of fast food establishments and family restaurant chains. In addition, fresh cut fruits and vegetables are sold in supermarkets. Retail sales of packaged salad mixes have increased from $167.5 million in 1991 to $507 million in 1994. For 1995, the sales are estimated to be about $900 million.

But the same consumers who are buying the salads to protect their health, can also be made sick if the salad ingredients are not handled properly, Wei said. Washing vegetables and then refrigerating them, he said, may be the best way to keep them bacteria-free.

“Mishandling at the processing facilities, contamination of fresh produce during transportation, and cross-contamination at food service establishments from other contaminated food items or infected workers can all contribute to the occurrence and growth of bacterial pathogens on vegetable salads,” Wei said.

“Considering that many food-borne pathogens can survive and grow in fresh vegetables and fruits, they can easily be transferred to vegetable salads and pose a health hazard to consumers,” Wei said.

E. coli bacteria can live and multiply on shredded lettuce, sliced cucumber, shredded carrots and cubes of melon. In fact, the bacteria can survive up to 14 days on shredded lettuce stored at 5 degrees Celsius. E. coli was responsible for 26 illnesses in Waco, Texas, in 1994, and salad bar items were suspected to have been contaminated by raw ground beef. And in Nova Scotia in 1981, a listeriosis outbreak was traced to cole slaw prepared with cabbage grown in a field fertilized with raw manure from a flock of sheep known to have had listeriosis.

Professor Doug Archer, chairman of the UF Department of Food Science and Human Nutrition, said he was delighted with the outcome of Wei’s study.

“Fresh fruits and vegetables are being blamed for a lot of things lately, but this study shows that much of the blame may fall on people who handle the produce, and not the produce itself. That’s good news. If there were a high contamination rate in growing fields, it would present the “fresh cut” industry with a difficult problem,” Archer said. “If, as this study suggests, people are more to blame, intervention in the way of training is easier to accomplish.”

Wei said the Florida Department of Agriculture tested 206 raw field vegetables from 1991-95, and only 11 contained E. coli.

“Therefore, from a food safety viewpoint, the vegetables produced in Florida are relatively clean,” Wei said. “However, human errors during handling, transportation and preparation of vegetables can contribute to higher incidence of contamination with bacterial pathogens.”

The risk of food-borne diseases from vegetable salads depends on factors such as the types of vegetables and their production location, how the vegetables are prepared, handled, and stored, and the consumers’ resistance to the infectious agents. Often the source of contamination is not the vegetable itself, but processing, leading Wei and his graduate student to recommend better training for food-service workers.

“Workers need education about hygiene and sanitation. The problem is that the pay is low so the turnover rate is high so it is not easy to train everyone,” Wei said.

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