Are microwave ovens safe? A comparison between conventional and
microwave cooking.
Introduction.
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Microwave ovens are now almost universally present in kitchens of
industrialized countries. They are used mainly for thawing and
reheating food, but also for cooking, even though organoleptic
qualities
of foods cooked in that fashion are considered as inferior by most
people. Although cooking with a microwave oven is often convenient and
time-saving, part of the population is concerned about its possible
health hazards and chooses to avoid it completely. The article below
briefly summarizes the various arguments against the use of microwave
ovens.
I. What is a microwave?
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Microwaves are electromagnetic in nature, like visible light, radio
waves and X-rays. Frequencies (in Gigahertz) are shown below
Gamma-rays: 10000000000 - 100000000000 GHz
X-rays: 10000000 - 10000000000 GHz
Visible light: 400000 - 800000 GHz
Microwaves: 2.45 GHz
Radio waves: 0.0001 - 1 GHz
Thus, microwaves are intermediate between light and radio waves. They
are slightly more energetic than radio waves.
Remarks:
-Microwaving differs from irradiation in that irradiated foods are
subject to gamma rays (see above), which are 10 billion times more
energetic than microwaves.
-Neither macrowaved nor irradiated foods are radioactive.
Microwaves have an interesting property: they are absorbed by water,
and to a lesser extent by fats and sugars. When they are absorbed they
are converted directly into atomic motion - heat. Most plastics, glass
or ceramics are transparent to microwaves. Metal reflects them, which
is why metal pans do not work well in a microwave oven. Microwaves
penetrate 1-2 centimeters inside food, heat diffusing to the center by
conduction.
Hot spots:
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When the microwaves bounce around inside the oven's cooking chamber,
they experience an effect called interference, which results in
uneven heating. When thawing a food in a microwave oven, part of it
may start to cook while another part is still frozen.
This effect is reduced, but not eliminated in ovens equipped with
a turntable.
II. Hazards of microwave ovens
--------------------------
See for instance http://www.chetday.com/microwave.html for more
details on some of the points listed below.
1. Heating a baby's bottle with a microwave oven is not recommended
because:
- risk of overheating: the bottle may seem cool to the touch, but
the liquid inside may become extremely hot and could burn the
baby's mouth and throat (it is difficult to control the
temperature
of the liquid inside the bottle).
- the buildup of steam inside the bottle may cause it to explode.
- the presence of "hot spots" may alter the nutritional quality of
milk (i.e. milk becomes partially cooked), but not more than
ordinary cooking would. An article published in the Journal
"Pediatrics" in 1992 says that frozen human milk heated at low
temperatures (20 degrees C to 53 degrees C) had significantly
decreased lysozyme and specific IgA [antibody] to E coli
serotype 06 [a particular bacteria].
Note that the hazards listed above are relevant for infants only, not
for adults, who don't get antibodies from food nor eat out of a
bottle.
2. Warming blood for transfusions:
Transfusion reactions, sometimes deadly, have been observed in
patients receiving blood heated by a microwave blood warmer. This is
likely due to overheating/hot spots. More recent "inline" microwave
bloodwarmers eliminate the problem of hot spots. A few studies show
that that method of microwave heating may be safely used to heat
blood; the future will tell if these studies are confirmed when the
practice becomes widespread.
3. Effect on the blood of human beings?
Hans Hertel, a Swiss scientist, carried out a study which showed that
microwaved foods cause significant changes in the blood. These changes
included a decrease in all haemoglobin values and cholesterol values,
especially the HDL (good cholesterol) and LDL (bad cholesterol) values
and ratio. Lymphocytes (white blood cells) showed a more distinct
short-term decrease following the intake of microwaved food than after
the intake of all the other variants.
On 7 August 1992 the Swiss Association of Manufacturers and Suppliers
of Household Appliances brought an action against the applicant in the
Canton of Bern Commercial Court. It produced an expert report by a
professor at Zürich Federal Institute of Technology from which it
appeared that the applicant's research was worthless and his findings
untenable.
As soon as Hertel and Blanc announced their results, the hammer of
authority slammed down on them. A powerful trade organization, the
Swiss Association of Dealers for Electroapparatuses for Households
and Industry, known simply as FEA, struck swiftly. They forced the
President of the Court of Seftigen, Kanton Bern, to issue a 'gag
order' against Hertel and Blanc: this was done in 1993.
However, in 1998 that decision was reversed. In a judgment delivered
at Strasbourg on 25 August 1998 in the case of Hertel v. Switzerland,
the European Court of Human Rights held that there had been a
violation of Hertel's rights in the 1993 decision.
To my knowledge, Hertel hasn't published his findings in any
peer-reviewed journal, and no other scientist has yet confirmed any
of his conclusions.
4. Vitamin losses?
Vitamins are equally or better preserved than with steaming or
boiling, because the temperature doesn't exceed about 100 degrees C
in a microwaved food.
5. Formation of toxic molecules?
There is no evidence that microwaving produces molecules that differ
from those produced by boiling or steaming. Overall, microwaving
denatures foods much less denatured than frying, baking or grilling.
6. Other
Eggs put in a microwave oven, even at low power may explode, even
seconds after you open the door, and cause thermal eye injuries. Even
if you wear glasses, you will need to clean up the mess in your hair,
your clothes and in the kitchen. Never use a microwave oven to cook
eggs!
Conclusion
----------
Three possible attitudes about microwaves seem rational to me.
1. You never cook anything, therefore have no more reasons to use a
microwave oven than a cooking pot.
2. Use a microwave at low or medium power (do not overheat food; it
shouldn't burst noisily). Do not use it to cook eggs or to heat a
baby's bottle.
3. Although there is no evidence that microwaved foods represent a
greater health hazard than ordinary cooking, you consider that not
enough research has been done concerning their safety, and prefer
more traditional methods "just in case".
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WWW pages on microwave ovens:
"The Hidden Hazards of Microwave Cooking"
<http://www.chetday.com/microwave.html>
"How a Microwave Oven Works"
<http://www.howstuffworks.com/microwave.htm>
"How Things Work"
http://landau1.phys.virginia.edu/Education/Teaching/HowThingsWork/microwave_ovens.html
--Jean-Louis Tu <[log in to unmask]>
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