Microwave ovens can sterilise kitchen sponges
31 January 2007
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, according to University of Florida
(UF) engineering researchers.
|Note: 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 and people should be careful when
removing the sponge from the microwave as it will be hot.
“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
Bitton, an expert on wastewater microbiology, co-authored a paper about
the research that appears in the December issue of the Journal of
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
Bitton said the UF researchers soaked sponges and scrubbing pads in raw
wastewater containing a witch’s brew of faecal 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
The results were unambiguous: Two minutes of microwaving on full power
mode killed or inactivated more than 99% 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%
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.