Summary and Conclusions
This writer is a polymer chemist with many years of
industrial experience. He has fol owed the debate
regarding the “release of toxic chemicals from plastic
packaging when exposed to microwave irradiation” with interest, and concludes that:
• Cling-wrap plastic films are usual y made from
polyethylene or plasticized polyvinyl chloride (PVC),
whilst rigid containers are usual y made from polystyrene, polyester, or polypropylene.
The focus of this note is to explore the safety aspects of food and drink containers which are exposed to microwave energy in order to heat their
• Al plastic packaging designed for use in direct food
contents. Stories of how dangerous, toxic, chemicals can ‘leach out’ into
contact must qualify against very stringent extraction
microwaved food have circulated on the internet for years. Such stories are
• There is nothing which would deter him from
Microwave energy is delivered to the food in the form of high frequency (2.5
using plastic ‘cling-wrap’, which is identified as
gigahertz) radiowaves (1). Radiowaves have the interesting property that
they are absorbed by water, fats, and sugars and converted directly into heat as they cause the chemical bonds holding these molecules together
• There is no toxic hazard arising from cling-wrap film
which becomes hot when used as a ‘splatter shield’ cover - when heating
Most plastics (see below for the exception of plasticized PVC), glass, and
food, such as baked beans, in an open ceramic dish
ceramics do not absorb microwaves - hence do not get hot. It is wel known
that metals reflect microwaves, which is why metal pans do not work wel in a microwave oven, and why plastic film containing metal particles is used for the
• He would not hesitate to microwave-heat food which
packaging of ‘microwavable popcorn’.
is wrapped in the plastic package ‘as purchased’ from the Supermarket (frozen dinners). Although he would
Manufacturers of cling-wrap films and food trays are wel aware of the rigorous
make smal holes in it to al ow any steam generated
FDA extraction testing requirements regarding ‘extractables’ from these articles
when used in direct food contact, and proudly display the term ‘microwavable’ or ‘microwave safe’on their products. One should always look at the cling-
• Plastics do not absorb microwave energy
wrap package for these terms to be sure that the film has been designed for
(radiowaves), hence do not get hot upon exposure to
• A plastic film wil pick up heat if it is in direct contact
with microwave-heated food, so he would try to
The basis for the conclusions listed above fol ows, using a series of questions
prevent the plastic cling-wrap film from contacting the
to clarify issues around the microwaving process.
food - because he wants it to stay cool enough to be removed by hand. General Questions:
• The reports that “. rigid plastic containers melt in
1. What are Microwaves?
the microwave” is an incorrect interpretation of the change in shape of a microwave-exposed food or
Microwave energy is delivered to the food in the form of high frequency (2.5
beverage container. In fact the observed dimensional
gigahertz) radiowaves. There is a wide range of dif erent types of radiation,
change are the result of the plastic undergoing a
cal ed the electromagnetic spectrum - ranging from low energy radiowaves,
phenomenon known as ‘stress relief’ when exposed
through infra-red, visible light, ultra-violet light, to high energy x-rays, gamma
to the moderate temperatures developed in a
2. How Do Microwaves Heat-up Food?
• He wil continue to cook his 13 kg Christmas turkey
The chemical bonds which make up the chemical structure of water, fats, and
using a nylon bags - to contain the fat splatter - in
sugars absorb the low energy of the radiowaves - causing them to oscil ate
a conventional oven set at 200 degrees centigrade
and generate heat. Most food contains a combination of water, fat and sugars
- much higher than the temperatures experienced by
- and wil respond to microwave energy by getting hot fairly quickly.
microwaved food. The nylon plastic is designed to survive this extreme temperature for a much longer
An acceptable level of heat generation is in the region of 60 degrees celcius.
time than required to cook the bird. 3. Do Plastics Absorb Microwaves (Radiowaves)?
Polyvinylchloride (PVC) is the rigid material used for the manufacture of guttering and downspouts. PVC shrink-wrap film
No. The chemical bonds which make up most plastics (carbon-
is flexible because the rigid PVC hs been ‘plasticized’ by blending
hydrogen bonds) do not respond to microwaves (radiowaves).
it with a liquid which is very compatible with PVC. This ‘plasticizer’
If you want to heat up plastics you need to use higher energy
acts like an internal lubricant by al owing the polymer chains to slide
radiation such as infra-red or ultra-violet rays.
past each other - making the film very flexible. 4. Why Does Cling-Wrap Film Become Limp and Get Hot?
It is possible for a minor portion of this liquid plasticizer to migrate
The film wil respond to direct contact with the hot food by
out of the hot PVC shrink-wrap film during microwave heating
becoming hot. It wil become less stif (see below) as the
- and come in contact with food which is in contact with the film.
temperature increases, but this does not indicate that it has
Because these smal molecules also ‘like’ fatty foods, there is
become ‘unstable’ to the point where it is breaking down
increased potential for their migration from the flexible PVC cling-
chemical y . There should be a one-inch or greater space etween
wrap if it is in contact with a microwave-heated fatty food.
the shrink-wrap plastic film and the food during microwave heating
7. Are the Plasticizers Used To ‘Flexibilize’ PVC Toxic?
(2). The film should be placed loosely over the food so that steam (generated by microwave heating of water in the food) can escape.
There are literal y hundreds of plasticizers - usual y smal molecules, which are very compatible with PVC. One plasticizer, diethyl hexyl
5. Why Do Plastic Films or Rigid Containers Appear to Melt
adipate (DEHA), has received a lot of negative publicity on the basis
When Exposed To Microwave Energy?
of (non-proven) potential toxicity. Because of this it was voluntarily
People assume that the distortion of the plastic film, bottle, or
withdrawn as a plasticizer for PVC in New Zealand. A similar
tray (containing food or beverage) which they observe when it is
unsubstantiated rumour regarding phthalate ester plasticizers led
exposed to microwave radiation implies that the plastic is melting.
New Zealand cling-wrap manufacturers to eliminate this family of
This is a common misunderstanding - the appearance of melting is
simply heat distortion of the plastic article - a phenomenon known
All plasticizers used for flexible PVC food contact cling film must
meet the stringent human toxicity safety limits defined by the
An example of this is when a PET bottle is partial y fil ed with
FDA based on animal testing. The cling-wrap film must also
water and exposed to microwave energy. It wil distort in a rather
meet these same regulations using extraction testing protocol
established by the FDA. Actual testing of cling-wrap film samples shows that levels of plasticizers extracted from cling wrap films
Why does this happen -:
based on plasticized PVC are well below these stringent safety limits established for the plasticizers.
During its manufacture, the film, bottle or tray is the molten plastic is forced through a shaping device - a film or sheet die, a bottle
8. Do Plastic Food Containers Release Dioxins When Heated
blowing operation, or a ‘thermoforming’ process in which an
in a Microwave?
extruded sheet is forced into the shape of a tray. These processes create regions within the plastic shape that are severely stressed
No. This was an email hoax. Plastics used for food contact/
(like a coiled spring). These stressed parts are dimensional y stable
microwaving do not contain the ‘building blocks’ which would
until they experience heat, through contact with the hot food or
be required to make dioxins. In fact, even with the appropriate
beverage - which al ows this stress to relax (the spring uncoils), and
ingredients the synthesis of dioxins requires temperatures
the part undergoes distortion - changes shape.
above 300 degrees centigrade - well beyond the capability of microwaving (3). 6. Which Plastics Are Used For Cling-Wrap Films?
There are two common plastics used for shrink-wrap film manufacture - polyethylene and plasticized poly vinyl chloride
(1). Marshal Brain, http://home.howstuf works.com/microwave1.htm
Polyethylene is a very thermal y stable plastic, and is ideal for any
(2). Edward Machuga Ph.D, FDA Centre for Food Safety, http:www.fda.
end-use application where the ambient temperature is below
100 degrees centigrade. At room temperature the PE film is quite
flexible but, as the temperature increases towards its melting point (~100 degrees C), the film softens and eventual y melts. A desirable
(4) Jean Weese, Extension Food Scientist, 334-844, 3296
temperature for microwaved food is ~60 degreesC, so the PE cling
Other Resources and google links;
film is wel below its melting point and, at no time - even above its
melting point - does the plastic exude any toxic components.
Plastics Division of the American Chemistry Council Microwaving Plastics American Cancer Society Plastics and the Microwave FDA Consumer, US Food and Drug Administration, November/December 2002
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