Food safety in a microbial world

Food-borne illnesses strike more often in July and August than any other time of year. Whether it’s the warmer temperatures, outdoor eating, choice of foods or other factors that lead to the increase, UC scientists have recently documented techniques for consumers to cut their chances of getting sick from fresh fruits and vegetables.

“We live in a microbial world,” says Jim Cullor, the director of the UC Veterinary Medicine Teaching and Research Center in Tulare. While Cullor works on issues related to dairy and meat food safety, his assessment is true of produce as well. The fruits and vegetables purchased at grocery stores, farm stands or farmers’ markets – even those picked fresh from the home garden – may carry harmful microbes, such as e. coli and salmonella.

Mice rats, birds, human hands, trucks, boxes, shopping carts, check stands, counter tops, refrigerators, cutting boards, knives and serving plates can all harbor bacteria, even if they look clean. Every step produce takes from the farm to the fork could contaminate food.

“About two to four percent of reported outbreaks of food-borne illnesses are attributed to fresh fruits and vegetables,” said Linda Harris, UC Cooperatives Extension specialist in microbial food safety. “Although seafood, meat and poultry cause illness more often, recently fresh produce has also been recognized as a food-safety issue.”

Harris has evaluated produce washing methods to determine the best ways to clean fruits and vegetables. She used information from a survey conducted by Christine Bruhn, UC Cooperative Extension food marketing specialist, which found that consumers don’t want to spend a lot of time on the process.

For smooth-skinned fruits and vegetables, such as apples, Harris found that rubbing the fruit under running water for five seconds and drying with a clean paper towel can reduce the number of microbes 1,000 fold. This method works with a wide variety of produce, such as oranges, honeydew melon and tomatoes. Certain produce, which pose unique problems, garnered special attention in Harris’ evaluation.

Cantaloupe The netted rind on cantaloupe is more difficult to clean and cutting with a knife can transfer bacteria to edible fruit. Harris found that scrubbing the rind with a clean brush reduces microbes 100 fold. The brush can be kept clean for future scrubbing by putting in the dishwasher or soaking in boiling water for 20 seconds. Once melons are cut for serving, it is important to refrigerate or consume them within two hours. Harmful microorganisms can multiply rapidly on cut produce.

Strawberries Scrubbing strawberries’ tender surface is not practical. By rinsing the fruit under running water, consumers can expect a 10-fold reduction in microbes. Harris observed an additional 10-fold reduction after spraying strawberries with undiluted vinegar, leaving them for five minutes and rinsing in clean water. However, she said, consumers may not find this procedure acceptable.

Some consumers might consider cleaning products to cut back on microbes on fresh food. Although chlorine is widely used in wash water by the produce industry, Harris said she doesn’t recommend its use in the home.

“Chlorine bleach is not labeled for use on foods,” Harris said.

However, a chlorine solution of one-teaspoon bleach to one quart of water may be used to sanitize wood and plastic cutting boards, vegetables brushes, counter tops and other surfaces.

The experts also discourage the use of dish- or hand-washing soaps or detergents for cleaning fresh produce.

“Fruits and vegetables are very porous,” Harris said. “Using compounds not approved for food washing could leave harmful residues on the food.”

Specialized commercial produced designed for washing produce may safely be used. Although many claim to remove soils, none of the products currently on the market are labeled for reducing microorganisms.

Cooking vegetables is more effective at reducing pathogens than washing. Cooked fruits or vegetables are not a risk for human pathogens unless they are contaminated after cooking or left too long at room temperature.

For more information on food safety, see the UC FoodSafe Web site and the UC Center For Consumer Research Web site. Nutrition, family and consumer sciences advisors are available in most counties to answer individual inquiries related to food safety.

(March 2004)

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