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.

  • I have been supervising the use of the 168ºF for 3 minutes technique since the 2008 Salmonella outbreak in Honduran cantaloupes. It is used in a retail catering operation. We have experienced no deterioration of organoleptic qualities. We have not performed micro testing.
    A presentation at the 2011 NEHA AEC detailed some experimentation with the same technique with tomatoes. The problem with the tomatoes was with the cooling process post the hot water bath. The tomatoes would absorb the cold water used for cooling. This introduced a new risk that the cooling water could be contaminated with a pathogen.
    Do cantaloupes exhibit this same absorbtion trait during cooling?

  • Yes, hot water works great to kill microbes, there is no denying that fact. However, it takes a lot of energy to heat water and there is no residual protection afterwards, and there is a potential for scalding workers. We feel there a safer chemicals (organic acids, for example) that provide a more worker and environmentally friendly option.
    Dana Johnson
    BIRKO Corporation

  • So, can consumers do the same at home? ie. place whole cantelopes purchased at supermarket in a hot water soak for three minutes to reduce bacteria numbers on exterior? Seems simple enough to recommend if this truly helps improve food safety and extend shelf life.
    I am a registered dietitian and consumer foods journalist.

  • Carolyn: I would not take the leap to recommend this process to the consumer yet. My question of absorption of the cooling water has not been answered. The consumer could easily increase the risk of a food borne disease by contaminating the inside of the cantaloupe. In the catering operation I described above the hot water bath is performed immediately before the melon is sliced for the catering order. If the melons are hot water bathed and then stored you run the risk of cross contamination during storage.


    What about a soak in a vinegar solution.
    Then rinsing well after that.
    Would the vinegar kill any bacteria?

  • Barbara Griffith

    I have been washing my fruit in hot water with a shot of dish soap for years. I never liked the idea of eating something that a stranger in this country or another picked. I leave them in the hot water mixture for at least 3 to 4 minutes. I wash cantaloups, apples, oranges and tomatoes and watermelon. Strawberries I wash for 3 minutes but I only buy enough to eat in a day or two because they turn soft. The only lettuce I buy is the heads that can have the outer leaves peeled off. I do not buy any melons that are being sold cut in half. Everything is rinsed several times and left in a rack to dry. I have never had any problem with any illness from any of it that was washed.