The 2011 outbreak of Listeria monocytogenesis in cantaloupe sparked a heightened level of interest in efforts to make cantaloupe safer, with fruit and produce trade associations developing safety guidelines and California cantaloupe growers pushing to set safety goals.
Bassam Annous, Ph.D., a microbiologist at the U.S. Department of Agriculture, might just have the solution the cantaloupe industry seeks. Since 2005, Annous has been developing a cost-effective treatment to kill pathogens on cantaloupe rinds by submerging the melons in hot water.
In Annous’ lab, a three minute bath in 168 degree Fahrenheit water has been shown to cause greater than 5-log reductions of Salmonella from the surface of cantaloupe — that’s a 99.999 percent elimination. By comparison, chemical washes on cantaloupe generally achieve a 2-log, or 99 percent, reduction — a significant difference when it comes to counting microbes.
“When you’re talking about percentages, the idea of killing 99 percent is misleading,” Annous said. “If you have a million microbes, a 99 percent reduction still leaves behind 10,000.”
A 5-log reduction? That would leave behind 10.
The hot water bath succeeds where chemical washes cannot because of simple physics, Annous said: The coarse surface of a cantaloupe contains many tiny pockets or “caves” with air bubbles that prevent penetration from chemical washes and essentially create a safe haven for pathogens.
Heat, on the other hand, kills pathogens whether or not water physically reaches inside the crevices of the rind. At three minutes of exposure, few bacteria have a chance to survive, but the thickness of the rind protects the fruit from damage or spoilage. In fact, tests have shown that heat treatment improves cantaloupe shelf life by reducing bacteria that cause spoilage.
Thus far, the treatment for three minutes at 168 degrees has been designed around eliminating Salmonella, a pathogen more commonly associated with cantaloupe than Listeria. But with some tweaking, the treatment might also effectively kill significant numbers of Listeria, known to be a more temperature tolerant bacteria.
Commercial-scale pilot tests in Annous’ USDA facility have proven the treatment can work and might add an additional cost of approximately 2 to 3 cents per melon, which comes from the time it takes to place each cantaloupe in a heated pump tank and then dry and cool them. And unlike chemical washes, hot tap water can simply drain into sewage systems without any additional treatment or cost.
Annous said that the treatment’s real-world applications should demonstrate levels of pathogen elimination comparable to the pilot test results, but bacteria in the wild may grow on cantaloupe differently than he can replicate in the lab. In tests, he places the bacteria on cantaloupe and gives it time to adapt to the rind and reproduce before treatment begins.
As the cantaloupe industry rallies behind food safety measures, Annous said he hopes his treatment will find a foothold among distributors.
“We are developing this technology to fit to the industry,” he said. “We’re not going to develop something that will cost $10 per melon. That’s great if it works, but who is going to implement it?”
While most other fruits cannot withstand the heat treatment long enough to kill pathogens, Annous said his lab would also like to develop a similar treatment for tomatoes. If that works, they might move on to mangoes.