This technique isn’t maybe is ready for home use. However, it would make a great science project for schoolchildren in a controlled lab.

The reason for the caution is two-fold–
1) dry sponges will burn up (combust). Recycled paper is also not recommended, for this reason. Use wet sponges only. Also, any use of a home microwave oven for small items needs to have a cup of water in the oven at the same time to prevent the microwave generator from damage [I believe this is still the case].
2) is sterilization complete? This must be confirmed before relying on the method. revised 2007-04-28. The comments below are about recent verification that 1 minute microwaving will kill the germs tested.

As a reader pointed out, boiling sponges and scrub pads for 15 minutes (at sea level; longer at altitude) in a pan on the stove also works.

Use clean sponges only. Do not use a sponge which contains chlorine bleach, as the chlorine will damage the oven’s steel. Although the article mentions the technique will “decontaminate” sponges, it does not. It can merely disinfect. It might even make chemical contamination of the items worse, by altering the structure of the sponge and what it has soaked in.

Other precautions for microwave ovens [I used to do research on laboratory ovens]–
The microwaves are generated in cycles (I believe one can hear the on and off humming; I don’t own a home oven). So, if too short a time is selected, there may not be enough microwave energy to activate the molecules and create the heat to sterilize. I suppose, depending on the organism, it may be possible the microwaves can disrupt the coating directly, much the way soap and other chemical disinfectants work. However, this, too must be tested.

It is certainly possible to heat containers past their melting point even if the water in the container does not boil. (Very little was known about these ovens and how they worked prior to Skip Kingston and Lois Jassie. For my lab experiments, I even bought a cookbook trying to figure out what food technologists knew–not much– and then how to extrapolate to environmental samples.)

Before working with them at the Bureau of Standards, I tried heating a hard plastic vial of distilled water to understand the relationship of time of exposure to temperature of contents. The polystyrene had a melting point above that of water’s boiling temperature. Imagine my surprise when the container started to lean over, even though the water wasn’t boiling. The water molecules touching the vial wall (outside the bulk of the water) overheated and conducted the energy faster to the plastic before the rest of the water could get warm.

This is true of foods and food containers, as many of you know, even in stovetop cooking. The outside of food can heat faster than the interior. (When storing cooked food, it is very important to stir foods or place into small containers so the middles will not continue to breed microorganisms and lead to food poisoning. The outside can cool properly while the inside does not. Check this yourself, using a kitchen thermometer to check on cooling rates in containers. Even at 30 below on the porch, a stewpot will not cool evenly.)

[With microwaves, which activate hydrogen molecules preferentially, it is possible to heat a frozen jelly doughnut so the interior will boil before the mostly air of the frozen breaded part. This is supposed to result in explosive, blistering hot contents all over the inside of the oven, but I never got a chance to try that.


Public release date: 22-Jan-2007
Contact: Gabriel Bitton, 352-392-4409, University of Florida
Microwave oven can sterilize sponges, scrub pads

GAINESVILLE, Fla. — Microwave ovens may be good for more than just zapping the leftovers; they may also help protect your family.

University of Florida engineering researchers have found that microwaving kitchen sponges and plastic scrubbers — known to be common carriers of the bacteria and viruses that cause food-borne illnesses – sterilizes them rapidly and effectively.

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.”

Bitton, an expert on wastewater microbiology, co-authored a paper about the research that appears in the December issue of the Journal of Environmental Health, the most recent issue. The other authors are Richard Melker, a UF professor of anesthesiology, and Dong Kyoo Park, a UF biomedical engineering doctoral student.

Food-borne illnesses afflict at least 6 million Americans annually, causing at least 9,000 deaths and $4 billion to $6 billion in medical costs and other expenses. 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 bugs survive, according to the UF paper.

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.

Spurred by the trend toward home health care, the researchers also examined the effects of microwaving contaminated syringes. Bitton said the goal in this research was to come up with a way to sterilize syringes and other equipment that, at home, often gets tossed in the household trash, winding up in standard rather than hazardous waste landfills.

The researchers also found that microwaves were effective in decontaminating syringes, but that it generally took far longer, up to 12 minutes for Bacillus cereus spores. The researchers also discovered they could shorten the time required for sterilization by placing the syringes in heat-trapping ceramic bowls.

Bitton said preliminary research also shows that microwaves might be effective against bioterrorism pathogens such as anthrax, used in the deadly, still-unsolved 2001 postal attacks.

Using a dose of Bacillus cereus dried on an envelope as a substitute for mail contaminated by anthrax spores, Bitton said he found he could kill 98 percent of the spores in 10 minutes by microwaving the paper – suggesting, he said, one possible course of action for people who fear mail might be contaminated. However, more research is needed to confirm that this approach works against actual anthrax spores, he said.


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