Microwave zaps germs on sponges and scrub pads

According to a team of researchers at the University of Florida, two minutes of microwaving at full power can sterilize a kitchen sponge. This is good news, since kitchen sponges and plastic scrubbers are known to be common carriers of the bacteria and viruses that cause food-borne illnesses.

A University of Florida press release about this discovery says:

That means that the estimated 90-plus percent of Americans with microwaves in their kitchens have a powerful weapon against E. coli, salmonella and other bugs at the root of increasing incidents of potentially deadly food poisoning and other illnesses.

"Basically what we find is that we could knock out most bacteria in two minutes," said Gabriel Bitton, a UF professor of environmental engineering. "People often put their sponges and scrubbers in the dishwasher, but if they really want to decontaminate them and not just clean them, they should use the microwave."

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Bitton said the UF researchers soaked sponges and scrubbing pads in raw wastewater containing a witch’s brew of fecal bacteria, viruses, protozoan parasites and bacterial spores, including Bacillus cereus spores.

Like many other bacterial spores, Bacillus cereus spores are quite resistant to radiation, heat and toxic chemicals, and they are notoriously difficult to kill. The UF researchers used the spores as surrogates for cysts and oocysts of disease-causing parasitic protozoa such as Giardia, the infectious stage of the protozoa. The researchers used bacterial viruses as a substitute for disease-causing food-borne viruses, such as noroviruses and hepatitis A virus.

The researchers used an off-the-shelf microwave oven to zap the sponges and scrub pads for varying lengths of time, wringing them out and determining the microbial load of the water for each test. They compared their findings with water from control sponges and pads not placed in the microwave.

The results were unambiguous: Two minutes of microwaving on full power mode killed or inactivated more than 99 percent of all the living pathogens in the sponges and pads, although the Bacillus cereus spores required four minutes for total inactivation.

Bitton said the heat, rather than the microwave radiation, likely is what proves fatal to the pathogens. Because the microwave works by exciting water molecules, it is better to microwave wet rather than dry sponges or scrub pads, he said.

"The microwave is a very powerful and an inexpensive tool for sterilization," Bitton said, adding that people should microwave their sponges according to how often they cook, with every other day being a good rule of thumb.

In their report, the Florida researchers note that home kitchens are a common source of contamination, as pathogens from uncooked eggs, meat and vegetables find their way onto countertops, utensils and cleaning tools. Previous studies have shown that sponges and dishcloths are common carriers of the pathogens, in part because they often remain damp, which helps the germs survive.

Bottom line: Two minutes of microwaving a sponge on full power killed or inactivated more than 99 percent of bacteria, viruses or parasites, as well as spores.

No word on whether we should use the sponge to clean the microwave oven first and then nuke it, or do it the other way around!

UPDATE Jan. 26, 2007: Note -- in order to sterilize a sponge safely in a microwave oven, the sponge (or scrubber) has to be wet!

Apparently some people who saw news articles about this sponge-sterilizing technique did not realize this, and the result was disaster. Here's an excerpt from a Reuters article about what happened:

"Just wanted you to know that your article on microwaving sponges and scrubbers aroused my interest. However, when I put my sponge/scrubber into the microwave, it caught fire, smoked up the house, ruined my microwave, and pissed me off," one correspondent wrote in an e-mail to Reuters.

"First, the sponge is worthless afterwards so you have to throw it out instead of using it. And second your entire house stinks like a burning tire for several hours, even with windows/doors open," complained another.

Aaron Hoover, a press officer at the University of Florida, said several other news organizations received similar complaints, although no one had complained directly to the university.

"We figured, 'wow, we better let people know right away that the sponge should be wet,'" Hoover said in a telephone interview.

The university issued the following advisory: "To guard against the risk of fire, people who wish to sterilize their sponges at home must ensure the sponge is completely wet. Two minutes of microwaving is sufficient for most sterilization. Sponges should also have no metallic content. Last, people should be careful when removing the sponge from the microwave as it will be hot."

Okay, got that?? Carry on...