Lessen the risks of Salmonella contamination

Renee Raiden Boyer vacuum-packages tomatoes for processing.

Contamination of fruits and vegetables is a big issue, not only for consumers, but also for growers. During the production and distribution processes, current food safety surveillance programs and government alerts have been questionable at reducing the risk to contamination. Just go back to the government warnings and health scares of fruits and vegetables that have hurt growers in earning a profit. Consumers have been too scared to purchase them after these warnings.

For instance, there was the Salmonella outbreak associated with jalapeno and serrano peppers in 35 states, but the outbreak was initially attributed to tomatoes, according to Virginia Tech in Blacksburg. Tomato growers were afraid that the government regulators were going to request they destroy their crop.

What to learn from these types of scares is that more research, education and outreach are needed to keep our food safe. To minimize the risks, food scientists at Virginia Tech are studying ways to prevent foodborne illnesses.

For instance, two of the scientists are trying to determine the effect of high-pressure processing (HPP) on whole and diced red tomatoes that are contaminated with Salmonella. They are inoculating store-bought tomatoes with the pathogen at three different concentrations, then studying the results, such as microbial counts, to see if the safety is maintained after treatment.

“Although food safety is our primary objective, we aren’t really helping the fresh tomato industry unless we develop a process that can preserve the high quality and flavor of a fresh tomato,” says Rob Williams, an associate professor and extension food microbiologist. “A safe product that tastes bad is not a viable solution.”

Renee Raiden Boyer, an assistant professor of food microbiology and an extension specialist of consumer food safety, says tomatoes currently are not processed using any sort of pressure.

Their goal is to develop a process that will render the product safe and help preserve the quality and flavor of tomatoes. “Since many of the tomatoes that have been linked to Salmonella outbreaks were destined for the fresh market [for example, consumed in the raw form], there aren’t any sort of interventions set up along the way to remove contamination with Salmonella if it was present,” Boyer says. “I think there are several interventions in place that will reduce the likelihood of Salmonella contamination [including implementation of a food safety program, chlorine in dump tanks, worker hygiene, etc.], but there is no point where Salmonella is completely eliminated in fresh produce. There has been quite a bit of speculation as to where the contamination is coming from, and one of the ideas is that it may be originating on the farm. So, our goal was to develop a treatment for the tomato that would eliminate or significantly reduce Salmonella, if present, yet still maintain a fresh, high-quality tomato.”

There are few givens. Williams says high pressures are known to affect an enzyme’s functionality, but if HPP affected the enzymes negatively for tomato flavor and texture, then quality would be compromised.

After studying and analyzing the results of HPP to eliminate these outbreaks in fresh red tomatoes, Boyer and Williams says they “will apply high pressure to mature green tomatoes and assess its impact on Salmonella and subsequent ripening of the fruit.” For example, they will explore the question: will pressure treatment inactivate the enzymes that ripen tomatoes?

Once they fully evaluate the effect of HPP on the quality of tomatoes, they hope to ultimately develop a process that will improve the fruit enough to extend its shelf life. “HPP increases cost,” Williams says, “and we hope by improving safety and extending shelf life growers and processors will be able to get more out of the tomatoes they work so hard to produce.”

Effects of high-pressure processing

Tomatoes consist of loads of water, which is good because Boyer says HPP works better with products that contain lots of water. When she and Williams took the whole tomatoes and packaged them in water with a 1 percent salt solution, the fruit “came out of the machine looking virtually the same as when they went in,” Boyer says. “This was just a gross visualization though. There were no actual analytical or sensory characteristics of the tomatoes measured. That would be the next step in the process.”

They didn’t package the diced tomatoes in a solution because they weren’t concerned about their firmness. They just processed them as they were.

As part of their preliminary work on green tomatoes, Williams says he and Boyer will assess quality factors such as color and texture. “At this point, we don’t yet know how quality will be affected,” he says, “but based on preliminary work we are hopeful that the high-pressure process can be designed to yield a safe, high-quality product.”

Food scientists Renee Raiden Boyer and Robert Williams prepare fresh tomatoes for high-pressure processing.


In their results, Boyer and Williams found out that the bacteria populations in the whole tomatoes they processed at 350 mega Pascal (MPa) were reduced by 96 percent. MPa is a pressure unit. A more familiar pressure unit is pounds per square inch (PSI), typically referred to when checking the air pressure in tires. For example, Boyer says 350 MPa equals 50,763 PSI, which equals 3,454 atmospheres of pressure.

At 450 MPa, the bacteria were reduced by 99.4 percent, and at 550 MPa, they were reduced by 99.95 percent.

At 350 MPa, bacteria populations in diced tomato were reduced by 65 percent. At 450 MPa, the bacteria were reduced by 96 percent, and at 550 MPa, they were reduced by 99.98 percent.

Reducing contamination

Williams and Boyer know they can find low levels of Salmonella contamination during the growing process, as other scientists have found Salmonella in the tomato-production environment. How tomatoes become contaminated is still unclear; however, the feces of birds and other wildlife might cause the contamination by transferring fecal material, Boyer says. Water contaminated with animal or human feces can also be a source of Salmonella. She adds that Salmonella can persist in soil, sediment and water sources, such as untreated ponds or other surface waters, once contamination occurs in the growing environment.

She says growers can reduce their contamination risks by following Good Agricultural Practices (GAPs), with Virginia training provided by Williams, and national training offered through Cornell University.

Williams says it is critically important that growers become knowledgeable about specific handling practices that minimize the risks to consumers. Those practice interventions should include the following:

  • Using good-quality, pathogen-free water for irrigation and any foliar applications.
  • Maintaining good water quality in the dump tank to avoid spread of Salmonella to uncontaminated product.
  • Stressing appropriate worker hygiene, including the use of proper toilet facilities and hand-washing in the field and the packinghouse.
  • Avoiding the use of raw manure when possible.
  • Excluding animals from the fields when possible and from the packinghouse at all times.
  • Participating in GAPs training and providing training to employees.

Williams says interventions to remove contamination from fresh produce are limited in their effectiveness, so prevention is essential. If HPP proves successful, “I would envision the high-pressure unit being placed in a packinghouse or processing facility,” he says. “Smaller operations that cannot afford to implement the technology may be able to form co-ops to share costs.”

The cost of such an HPP unit is significant, he says, but “if we see a substantial improvement in shelf life of fresh tomatoes in addition to improved safety, we hope that extended market time and reduced loss will add enough value to more than compensate for the high cost of high-pressure processing.”

For processing, Robert Williams loads a vessel with tomato samples.

Continuing research

He and Boyer completed the initial phases of their research on red tomatoes, funded in part by the Virginia Agricultural Council, in the spring of 2009; however, they are continuing to look at the effect of pressure application on green tomatoes. Boyer says they believe HPP has the potential to be a good way to eliminate Salmonella after harvest. They also will evaluate whether HPP will affect any subsequent ripening processes. In addition, they will continue to review the sensory characteristics of the process. To achieve these new goals, they will need further funding and are seeking support for their efforts.

Government action

On July 31, 2009, the U.S. Food and Drug Administration (FDA) published three draft guidances, designed to help growers and others across the entire supply chain, to minimize or eliminate microbial contamination in tomatoes, leafy greens and melons.

The guidances are, in part, based on those originally developed by the produce industry with assistance from FDA. They represent the first step in a fundamental shift in strategy for the agency in the prevention of foodborne hazards associated with fresh fruits and vegetables.

The comment period was supposed to be within 90 days of publication in the Federal Register but was extended to January 4, 2010. These guidances “will be made final as soon as possible after public comment and will be followed within two years by enforceable standards for fresh produce,” says FDA Commissioner Margaret Hamburg.

Key elements of the draft guidance:

  1. An acceptable baseline standard of industry practices that help both domestic and foreign firms minimize the risk for microbial contamination of their products throughout the entire supply chain.
  2. Recommendations regarding growing, harvesting, packing, processing, transportation and distribution of the product.
  3. Recommendations for recordkeeping, including some that will help the FDA determine more quickly the source of outbreaks that do occur.

To review the draft guidances, go to www.regulations.gov for more information.

Based in Danville, Va., Rocky Womack has written about agriculture and business for more than 25 years and currently serves as a contributing writer and correspondent for agriculture and business magazines, domestically and internationally. In the past, he has worked as a magazine editor and daily newspaper writer. For his efforts, Womack has won numerous awards for his interviewing, writing and in-depth reporting.