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), Background
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 Discussion
• 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 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 gov/fdac/features/2002/602_plastic.html.
100 degrees centigrade. At room temperature the PE film is quite (3). 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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