Minimizing Microbes on Fresh-Cut
Left to right: Pathologist Ching-Hsing Liao, Vlasta Pilizota (visiting
scientist from Croatia), and microbiologist Dike Ukuku work on methods to
improve microbiological quality and safety of fresh-cut cantaloupe.
Fruits and vegetables are vital to our health and well-being, providing
essential vitamins, minerals, and fiber to our diet. But although U.S.
consumers have one of the safest supplies of fresh produce in the world, new
outbreaks of food poisoning linked to fruits and vegetables continue to occur.
These outbreaks come from produce grown both here at home and abroad. In the
past few years, outbreaks of foodborne illness have been traced to E.
coli O157:H7 and Salmonella found on lettuce, cantaloupe, and
sprouts; Shigella on parsley and lettuce; and Cyclospora on
"Concern about these outbreaks and their implications led the current
administration to propose a research strategy that enhances the safety of
fruits and vegetables," says James A. Lindsay. He is the primary leader of
the ARS national research program
directed toward the microbial safety of fruits and vegetables.
As part of his earlier National Food Safety Initiative, on October 2, 1997,
President Clinton launched the Produce and Imported Foods Safety Initiative.
Under this plan, USDA and other government agencies were charged with
developing a research plan that would help minimize risks posed by microbial
pathogens on fresh and minimally processed fruits and vegetables. Examples of
minimally processed produce are bagged salads, melon balls, and many other
peeled or slicedbut still uncookedfruits and vegetables.
"Working with the Food and Drug Administration's (FDA) Center for Food
Safety and Applied Nutrition, our new research plan focuses on reducing
diseases caused by foodborne pathogens," Lindsay says. "Our first
official response to the President's directive was to publish guidelines for
U.S. industry that provide voluntary, science-based information for improving
the safety of fresh produce as it moves from farm to table."
Titled "Guide to Minimize Microbial Food Safety Hazards for Fresh Fruit
and Vegetables," the publicationwhich is not a government
regulationaddresses microbial food safety hazards. It describes good
agricultural and management practices for growing, harvesting, washing,
sorting, packing, and transporting most fruits and vegetables sold to consumers
in fresh or minimally processed form.
Plant pathologists William Conway and Britta Leverentz sample fresh-cut apples
to determine survival and growth of the food-borne pathogen Listeria
"Another focus of the Food Safety Initiative is to identify and support
research priorities that will fill gaps in food safety knowledge," Lindsay
notes. "We plan to conduct a risk assessment for produce and incorporate
the information in a multiyear research plan."
Since 1997, ARS has increased research efforts to promote the safety of
fruits and vegetables.
"We're concentrating our research at three different locations:
Beltsville, Maryland; Wyndmoor, Pennsylvania; and Albany, California,"
Lindsay says. "This year alone, we allocated $6.6 million to eight
research projects devoted to enhancing the safety of fruits and vegetables and
have requested an additional $2.1 million for fiscal year 2000."
Understanding Microbial Miscreants
In Beltsville, Maryland, staffing is being increased, and positive research
results have already been achieved at the ARS Horticultural Crops Quality
"We're adding a microbiological safety team to our research unit,"
says HCQL leader Kenneth C. Gross. "It will include a microbiologist,
along with a plant pathologist and a plant physiologist."
This team will help deal with a critical aspect of food safety: the
physiological and biochemical factors involved in the interaction among
foodborne pathogens, their fruit and vegetable hosts, and associated beneficial
microbes, Gross reports.
In the lab, plant pathologist William S. Conway has already shown that the
Listeria monocytogenes bacterium can grow on fresh-cut apple slices. And
controlled-atmosphere storage had no effect on this bacterium's survival.
"Most consumers usually eat an apple whole," Gross says. "The
apple peel serves as a barrier to many foodborne pathogens. But demand is
growing for fresh-cut produce, a form that is open to potentially harmful
Cutting produce causes wounds that pave the way for pathogens to attack.
Gross and colleagues are studying relationships between these pathogens and
naturally occurring, beneficial organisms. There is only so much room and food
for organisms to exist on a given piece of producea case of survival of
the strongest organism.
Food technologist Alley E. Watada at the HCQL lab found that spinach
contains naturally occurring compounds and beneficial microorganisms that slow
down the growth of Listeria. "It's important that these helpful
organisms not be removed during sanitation treatments after harvest,"
HCQL scientists are also looking at the quality of fruits and vegetables.
"We're continuing our work on ensuring that we start with the best quality
possible," says Gross. This includes developing new lines of fruits and
vegetables that store longer and resist pathogens better.
Commodities being studied at HCQL to bolster their microbial safety include
fresh-cut spinach, celery, carrots, apples, bananas, sweet peppers, tomatoes,
squash, strawberries, grapefruit, grapes, plums, and melons.
Fruit and vegetable contamination is a primary concern of food safety
research at ARS' Eastern Regional Research Center (ERRC) in Wyndmoor,
"Our first priority is to eliminate the contamination, and we've been
very successful with irradiation," says Donald W. Thayer, head of the Food
Safety Research Unit. "Next year, we expect to have a commercial food
irradiator installed in our center."
Thayer's found that pasteurizing food by irradiation significantly reduces
the numbers of harmful microorganisms such as E. coli, Bacillus
cereus, L. monocytogenes, Salmonella, and Staphylococcus
aureus. Ionizing radiation is a safe and effective food preservation tool.
"We used the FDA-recommended dose of irradiation for fresh fruit on
raspberries contaminated with Cyclospora," Thayer says. "Not
only did we inactivate the parasite, we doubled the berries' shelf life as
Thayer and colleagues Kathleen T. Rajkowski and William F. Fett have
successfully used irradiation and chlorine to kill E. coli O157:H7 and
Salmonella on alfalfa seeds and sprouts.
Since 1995, raw alfalfa sprouts have been recognized as a source of
foodborne illness in the United States, with several outbreaks of both E.
coli O157:H7 and Salmonella. As a result, the FDA and the Centers
for Disease Control and Prevention have advised those at high risk, namely
children, the elderly, and persons with compromised immune systems, to avoid
eating raw alfalfa sprouts.
Because of their fragility, sprouts cannot withstand abrasive physical
washing. So the focus has been on cleaning the seeds, which are suspected as
the source of the pathogens. U.S. sprout growers are looking for an effective,
practical, and cost-effective way to ensure that sprouts are free of pathogenic
Microbiologists Rajkowski and Thayer used irradiation to treat alfalfa seeds
and sprouts. "We used a dose approved for irradiating meat and controlled
both Salmonella and E. coli O157:H7. The irradiation also
extended the shelf life of sprouts from about 5 days to more than a week,"
Both E. coli and Salmonella were more resistant on dry seeds
than on sprouts because of the lack of moisture in the seeds, although a higher
dose of irradiation did kill both pathogens. According to Thayer, the dose
level that eliminated E. coli had little effect on the germination of
the irradiation-treated seeds. This may not be true for Salmonella,
which needs a higher dose.
A Chemical Sanitizer
Microbiologist Bill Fett and Kathleen
Rajkowski compare alfalfa sprouts
grown from seeds that have been
irradiated to reduce bacterial
To decontaminate sprouts, Fett, a microbiologist in ERRC's Plant Science and
Technology Unit, has been investigating an alternative treatmentchemical
He subjected alfalfa seeds to 2-percent, 2.5-percent, and 3-percent
weight-per-volume concentrations of calcium hypochlorite (a chlorine source). A
3-percent concentration means about 20,000 parts per million of available
chlorine. At a neutral pH of about 7, Fett got a 99.99-percent reduction in
E. coli O157:H7 for the 2.5 and 3 percent concentrations.
"The pH is important because at a higher pH level, such as 10, the
chlorine would change to a form that would not be as effective in killing
bacteria," Fett says.
"And sprouts may be contaminated internally, which would prevent a
surface disinfectant from working effectively," Thayer adds.
"Therefore, in practice, the best way to eliminate pathogens might be a
combination of irradiation and sanitation treatments."
Food technologist Gerald M. Sapers, also in the Plant Science and Technology
Research Unit, is working with colleagues Ching-Hsing Liao, Dike Ukuku, and
Bassam Annous to study sanitizing of produce that has been contaminated with
human pathogens. These include E. coli O157:H7, Salmonella, and
This team is looking at apple cider and other fresh-apple products, as well
as fresh-cut fruits and vegetables including melons and sprouts. Kevin B.
Hicks, who heads the unit, says that they are identifying sources of
contamination and examining the processes that limit the efficacy of produce
"From these results, we hope to develop ways to prevent contamination,
kill or remove microbial contaminants, or suppress their growth on fruits and
vegetables," Hicks says.
"In collaboration with Pennsylvania State University, we're designing
and building a one-of-a-kind fruit and vegetable processing research facility
at ERRC. This facility will allow us to quickly develop methods to kill or
remove bacteria from fresh fruits and vegetables in surroundings more
reflective of an industrial setting," says Hicks.
We feel this is very important, since we've found that conventional
processing equipment and commercial sanitizers don't seem to be very effective
at removing bacteria from these commodities."
"We'll use commercial-scale conveying, washing, and processing
equipment with our newly developed sanitizing treatments," Hicks says.
"Our aim is to help meet the President's goal of developing cost-effective
ways to reduce the incidence of foodborne illness associated with fresh and
processed produce." By Doris Stanley Lowe, Agricultural
Research Service Information Staff.
This research is part of Food Safety, an ARS National Program described
on the World Wide Web at
James A. Lindsay is with the
USDA-ARS National Program Staff,
5601 Sunnyside Ave., Beltsville, MD 20705-5138; phone (301) 504-4674, fax (301)
Kenneth C. Gross is at the
Horticultural Crops Quality Laboratory, Bldg. 002, 10300 Baltimore Ave.,
Beltsville, MD 20705-2350; phone (301) 504-6128, fax (301) 504-5107.
Donald W. Thayer is in the
USDA-ARS Food Safety Research Unit, Eastern
Regional Research Center, 600 East Mermaid Lane, Wyndmoor, PA 19038; phone
(215) 233-6582, fax (215) 233-6406.
Kevin B. Hicks is in the
USDA-ARS Plant Science and Technology Unit,
Eastern Regional Research Center, 600 East Mermaid Lane, Wyndmoor, PA
19038; phone (215) 233-6580, fax (215) 233-6406.
"Minimizing Microbes on Fresh-Cut Foods" was published in
the June 1999 issue of
Agricultural Research magazine.