Researchers at Rutgers University and the U.S. Department of Agriculture’s Agricultural Research Service (ARS) have found that chlorine dioxide gas is more effective at killing Salmonella on bean sprouts than chlorine wash — the industry-preferred decontamination technique.
The scientists found that the chlorine dioxide gas was able to reduce the presence of Salmonella on bean sprouts by 99.999 percent, compared to a 99-percent reduction achieved with chlorine wash.
Sprouted seeds have a long, and poor, contamination record. The warm, moist environment in which they are grown also provides the perfect growing conditions for pathogenic bacteria, most commonly Salmonella, E. coli or Listeria.
Since 1995, at least 51 outbreaks in the U.S. and Canada have been linked to sprouts. Of those outbreaks, 39 were linked to Salmonella.
The largest known E. coli outbreak in history was linked to radish sprouts in Japan in 1996. At least 8,500 people were sickened and 12 died in that outbreak.
National sandwich chain Jimmy Johns permanently dropped sprouts from its menu in 2012 after a series of outbreaks were linked to sprouts served at its restaurants.
Just last week, the U.S. Centers for Disease Control and Prevention said it is investigating an ongoing multistate outbreak of Salmonella Enteritidis in the Northeast linked to bean sprouts. As of Nov. 24, 68 people in 10 states were known to have been sickened.
While the sprouted seed industry has been searching for a surefire way to sanitize its product, cleaning sprouts is tricky because the food is sensitive and any harsh treatment could affect its color or taste.
Sprouts are also harder to treat because, as with other fresh produce, the porous, uneven surface of sprouted seeds provides many places for bacteria to tuck themselves away.
“There are areas we don’t see,” Dr. Bassam Annous, ARS research microbiologist and lead author of the paper, explained in an interview. “They look smooth, but really, if you go into the micro level, it looks like mountains and valleys.”
“It becomes harder to kill because it’s living in a protective environment now,” he said.
This is where, according to Annous, gas can be more effective than aqueous sanitizers because it is able to penetrate these protective air bubbles and expose bacteria. Water-based solutions, on the other hand, cannot penetrate these areas.
Annous likened it to a wine bottle. If the bottle is full of air, the air must be forced out in order for it to fill with wine.
The researchers found that applying the chlorine dioxide gas to sprouts in a tumbler was more effective than applying it to stationary sprouts laid out on the bottom of the chamber into which the gas is funneled. This way, the gas could reach all sides of the sprouts.
The sprouts used in the experiment were exposed to Salmonella in a manner meant to imitate what contamination would be like in the real world, Annous said.
The scientists used Salmonella strains collected from patients during three past U.S. sprout-related Salmonella outbreaks to make sure that the types of Salmonella they were using were indeed the type that could cause widespread Salmonella illnesses in humans.
They soaked the sprouts in a cocktail of the three Salmonella strains for five minutes and then let them sit overnight so the Salmonella grow and develop biofilm, a protective layer that shields the bacteria and provides it with nutrients.
The sprouts were then treated with either chlorine wash or chlorine gas, either in a tumbler or not, and for varying degrees of time.
The chlorine wash achieved a 2-log, or 99-percent, reduction in Salmonella levels, as opposed to the 3-, 4- and 5.5-log, or 99.99-percent, reductions achieved by the chlorine dioxide gas. Longer exposure times and being in the tumbler contributed to the highest log reductions.
“It gets the job done,” said Annous of the chlorine dioxide gas.
Chlorine wash is currently the recommended best treatment for reducing sprout contamination. The International Sprout Growers Association requires a treatment of chlorine wash and adherence to good manufacturing practices (GMP) in order for growers to be certified by its voluntary quality assurance program.
Other options for sanitizing sprouts include heat treatment of seeds, treatment with ozone, a highly reactive form of oxygen, or irradiation. All of these methods significantly reduce, but do not eliminate, pathogens from sprouts, according to the U.S. Food and Drug Administration’s sprout safety recommendations.
When asked whether or not chlorine gas treatment would be expensive for the industry to implement, Annous said it would be cost-effective in the long run. While it would be expensive to buy and install the necessary equipment, the cost would go down after implementation.
Plus, he noted, “It’s a lot cheaper than recalling your product, shutting down your plant and going through litigation if, god forbid, there’s an outbreak.”
Annous said he’s working on another research project to design a sealed bag that will release chlorine dioxide gas to treat a sprout once it’s been packaged, thus providing a kill step after the final stage of production.
Evidence shows that sprouts carrying bacteria are most often contaminated in the seed phase, according to FDA. But improper handling during or post-growing has also contributed to contamination.
Annous is also working with other researchers to use chlorine gas to sanitize seeds before sprouting. While the bean sprouts used in this study could be treated with gas after sprouting because they are white, he said that green sprouts, such as alfalfa sprouts, would be bleached by the process.
However, a seed is much tougher, and treating a green sprout in its seed phase will not compromise its color once it grows.
The other authors of the paper include Kit L. Yam and Vara Prodduk of Rutgers, The State University of New Jersey, Department of Food Science, and Linshu Liu of USDA ARS Dairy and Functional Food Research Unit.© Food Safety News